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[BOOK] Applied Photography, 1971, Chapter 8: Ultraviolet Photography

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#1 Andrea B.

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Posted 05 January 2015 - 16:25

This is an interesting book. Of course, it deals with film. But I found the UV and IR info interesting anyway. And some of it is certainly relevant to what we do digitally. The format is a bit messed up. I was thinking perhaps of copying Chapter 8 here and re-formatting it. I need first to check the copyright etc.

APPLIED PHOTOGRAPHY
C.R.Arnold P. J. Rolls J.C.J. Stewart
Edited by D. A. Spencer
THE FOCAL PRESS
LONDON and NEW YORK
1971 FOCAL PRESS LIMITED
First Edition 1971
ISBN 240 50723 1
http://archive.org/s...graphy_djvu.txt

Chapter 7 Infrared Photography
Chapter 8 Ultraviolet Photography.
Andrea G. Blum
Often found hanging out with flowers & bees.

#2 Andrea B.

    Desert Dancer

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Posted 05 January 2015 - 16:31

Apparently this book is fully downloadable, so here goes.
It will take me a while to clean up the formatting.

I want this here as a reference to some facts that is not from Wikipedia.

ADMIN NOTE:
The gear references (lamps, filters, etc.) are, of course, very out-of-date. However, this is still an interesting reference to UV photography and various topics which surround it.

All colour, bold text is my doing because I wanted to highlight some facts for future reference.

Safety information is bolded in red.

**********************************************************

Chapter 8

8.1 Introduction

8.1.1 Terminology

The ultraviolet spectrum extends from approximately 1 nm to 380 nm (see p. 113). It is possible to suggest several sub-divisions of this region note1, but in this book only three terms are commonly used:
  • Near UV (sometimes called 'black light'): the spectral band 320-380 nm,
  • Middle UV: the range 200-320 nm,
  • Vacuum UV (VUV): UV wavelengths shorter than about 200nm. This region can be studied only in a vacuum, because of a strong atmospheric absorption; it is usually quoted as extending from 200 nm to about 1nm, overlapping the region of soft X-rays.
The band from 120-200 nm is sometimes called the Schumann region, and is the shortest waveband in which refractive lenses (usually fluorite) can be used. The term extreme ultraviolet (XUV) is used to describe wavelengths from about 1-100 nm.

Photometric units (candelas, etc.) are not applicable to UV sources and special units (e.g. Finsen and E-viton) are used in medical and bacterididal work. The irradiance of UV lamps is normally expressed in microwatts per square centimetre (µW/cm2).

note1 A classification sometimes used by lamp manufacturers is as follows:
UV A (320-400nm) Glass transmission region
UV B (280-320 nm) Erythemal (sunburn) region
UV C (185-280 nm) Bactericidal or germidical region
The Joint Committee on Nomenclature in Applied Spectroscopy has recommended the terms Far UV for the 10-200 nm band and Near UV for the 200-380 nm band.

8.2 Sources of ultraviolet radiation
ADMIN NOTE: Eye protection must be used with ALL sources of UV light.

8.2.1 Incandescent sources.

The sun (colour temperature 6200K, peak emission at 480 nm) emits about 10 per cent of its radiation in the UV. However, only about 4 per cent of the solar radiation reaching sea-level is ultraviolet and this is subject to the atmospheric conditions and to seasonal variations. The sun is not, therefore, regarded as a useful source of UV (although there is often sufficient scattered UV to be a nuisance in exterior photography). The subject of solar UV radiation has been discussed by Koller.7

As indicated in Table 3.8, tungsten lamps are very inefficient as UV sources. Nevertheless, as shown in Table 8.7, tungsten lamps can be used for the UV photography of small areas if no other source is available. The main difficulty arises from the excessive heating of the subject and camera caused by a tungsten lamp at close range; the heat also precludes the use of a UV-transmission filter on the lamp.

Despite their inefficiency, tungsten lamps have been used by the US National Bureau of Standards as standard sources of UV because of their consistent operation.

Flash bulbs have been used successfully for direct UV photography. Kodak have suggested a Guide Number of 24 for a flash cube with Tri-X film and a Wratten 18A camera filter.6

High-intensity carbons usually have their peak emission at about 390 nm; they are a good source of near UV and are still used in some photo-mechanical work; core additives are sometimes used to enhance the UV output with intense emission bands.

Open arcs give substantial emission at erythemal wavelengths and may cause damage to unprotected eyes (see Section 8.8).

8.2.2 Gas discharge lamps.

Many discharge lamps, each giving UV lines characteristic of the filling gas, are available for spectrographic work, but the most useful source for general UV photography (and for many reprographic purposes) is the mercury vapour discharge lamp. The proportion of UV emitted by a mercury lamp varies considerably with current density and other factors, but the spectral quality is mainly governed by the operating pressure of the lamp. Low pressure MV lamps emit about 90 per cent of their output in a single line at 254 nm, whereas high-pressure MV lamps emit a continuous spectrum in the near UV, with dominant lines at 365 nm and certain blue and green wavelengths.

Several mercury lamps for UV work are listed in Table 8.1 below. In some cases a Wood's glass envelope (see p. 265) is used to confine the emission to the UV; these are often called black lamps and are given the designation W (see Table 3.11).

Germicidal lamps (e.g. Philips TUV) are low-pressure MV tubes emitting principally at 254 nm. In addition to their primary function of killing bacteria, these lamps are useful for exciting fluorescence in certain materials. Safety precautions are necessary when using these short-wave UV sources (see Section 8.8).

8.2.3 Electronic flash.

High pressure xenon lamps have a visible and UV emission closely matched to that of the sun; they have their peak emission in the region of 400-450 nm and they are a reasonably good source of near ultraviolet.

Electronic flash tubes normally have a xenon filling, which gives them a useful near-UV emission. Furthermore, electronic flash is free from the warming-up period required for mercury lamps and its portability and short exposure duration makes it a very convenient source of UV for biological and medical work. Some flash units (e.g. the Hico Corporation DP2 300 joule equipment) are designed with integral filters specifically for ultraviolet and fluorescence work. Perhaps the only disadvantage of a flash unit for fluorescence work is that the fluorescent areas cannot be assessed visually while the camera is set up.

Assessment of a flash guide number for direct UV recording can be made only by practical test, taking into account the UV response of the film and the UV transmission of the lens (see Table 8.7). The guide number for fluorescence work depends on the nature of the subject and its luminescence. note2

note2 Under the conditions relating to Fig. 8.9, the use of a Wratten 18A filter required an exposure increase equivalent to 7 stops (128:1). The guide number of the flash was thus reduced by a factor of V128, or about 11 x .

8.2.4 UV-emitting fluorescent tubes.

The domestic fluorescent tube is not an efficient source of ultraviolet, but actinic tubes with a UV-emitting phosphor are available for reprographic equipment. The emission peak of these tubes is in the region of 360 nm (see Fig. 3.13c), but there is also considerable violet and blue emission; in the 'blacklight' fluorescent tubes (MCFW) (Fig. 8.1e) the visible emission is absorbed by an envelope of cobalt glass (Wood's glass).

These fluorescent tubes do not match the total output of the larger mercury lamps, but they are cheap and efficient sources of ultraviolet and they give a very even illumination. In many cases the tubes can be used in standard domestic fittings. The Philips TW 6W lamp is unique in that it has a standard ES cap for direct use in a 200-250 volt supply without any ballast or starting gear.

Fig. 8.1. Typical SED curves for sources used in UV photography
Attached Image: Fig8.1.jpg

Table 8.1 Sources in Common Use for UV Photography and Reprography
Attached Image: Table8.1.jpg

8.2.5 Commercial UV lamp units.

All the UV units in common use employ mercury lamps (high-pressure for 365 nm radiation or low-pressure for 254 nm radiation) or UV-emitting fluorescent tubes (peaking at 360 nm). Examples are shown in Fig. 8.2.

Larger UV sources are used for water sterilisation, photo-chemical processes and
accelerated ageing tests on fabrics, but these units are not used for photography.

Fig. 8.2. Ultraviolet lamp units.
Attached Image: Fig8.2.jpg

Table 8.2 Ultraviolet lamp units.
Attached Image: Table8.2.jpg
Attached Image: Table8.2part2.jpg

8.2.6 Lighting technique.

UV sources are available which fulfil all the usual requirements for spotlights and floodlights. The latter are more often used for photography, as the intention is usually to irradiate the subject evenly with ultraviolet rather than to produce any modelling or cross-lighting. note3 In fluorescence work any unevenness in the UV beam must be avoided, otherwise there will be variations in the fluorescence which might be misinterpreted. The Hanau Fluotest Universal cabinet is claimed to have a flux variation of no more than 10 per cent over a 30 x 30 cm field.

If an intense narrow beam of UV is required, the best method is probably to use one of the compact-source mercury units (with a quartz condenser) that are designed as accessories for UV microscopy. A similar unit is used in the OptecUV Endoscope.

For many purposes the compactness and even illumination of tubular UV lamps is ideal. Large mercury discharge tubes (up to 7 kW) are used in diazo printing machines but for normal photographic UV copying the actinic lamps may be more suitable. The integrally filtered MCFW tubes are well suited for fluorescence work and may be
useful in home-made UV lighting units for special purposes.

UV-fluorescent tubes reach full output almost immediately, but the normal mercury discharge lamps require 5-10 minutes to reach full intensity. For consistent exposure it is important that photography should not be started until full output is reached.The time-intensity characteristics for a particular discharge lamp and circuit can be
plotted from a series of test exposures.

A small tungsten lamp is needed to set up the camera and subject; this lighting is often used for a supplementary exposure to show the subject outline or other general features.

Many white paints and white papers have a relatively low UV reflectance and, if fill-in reflectors are needed for UV photography, it is preferable to use aluminium.

note3 But see p. 280 for an instance in which UV texture lighting is useful.

8.3 Optical materials

Difficulties arise in recording short-wave UV because all materials have very strong absorption bands in this region. Conventional photographic techniques are used in the near UV, but the components of a photographic system, including the air, impose a series of absorption limits on recording at shorter wavelengths.

In the UV band below 120 nm there are no suitable materials for making transmission (dioptric) lenses, spectrographic prisms or diffraction gratings. Furthermore, the reflectance of all materials is very low in this region so that mirror (catoptric) lenses and reflection gratings are inefficient. These optical difficulties are a primary reason for our relative ignorance of this spectral band, despite its great interest to spectroscopists and physicists.

Optical performance in the extreme ultraviolet and soft X-ray regions is mentioned on p. 269.

Many textbooks give absorption figures such as those in Table 8.3 below; but confusion sometimes arises because different authorities quote different values. There are several reasons for this:

1. It is impossible to express complete curves such as those in Fig. 8.3 by a single value and there is no agreement on the absorption percentage figure that represents the cut-off figure. The 5 per cent, 10 per cent or 90 per cent transmittance levels are often quoted but other figures are equally permissible (and equally misleading).

For example, photographic gelatin may be quoted as having a cut-off wavelength at 250 nm or 210 nm. Both of these figures are correct in a sense, although they may relate to 90 per cent transmission, and 90 per cent absorption respectively.

2. The sample thickness also affects the quoted figure; in many cases the figure for a 2 mm sample is given, or the percentage absorption is quoted per unit thickness (e.g. 10 per cent per mm). A compound lens may have a total glass thickness of 20 mm and will reach a given absorption level at a wavelength 10-20 nm longer than for a thin glass sample.

3. Even with a very low transmission (say 1 per cent), it is still possible to produce a record if a sufficiently long exposure can be given. The effective cut-off wavelength may therefore depend on the experimental conditions, such as emulsion sensitivity or subject movement.

4. In the case of gases the extent of the absorption band depends partly on the temperature and pressure.

5. The term glass includes many hundreds of products, each having a different chemical composition. There is a wide variation in the UV transmission of camera lenses; a dense flint glass may have a cut-off limit of 380 nm, while crown glasses often transmit down to 310 nm.

6. Samples of an optical crystal can vary greatly in their spectral transmission; the figures quoted in text books are usually for the best specimens. These materials are invaluable in UV spectroscopy, but it should not be assumed that they are readily available in sufficient size or quality for large components, or that their physical properties are well suited for optical work. In the case of quartz (Si03), differences between synthetic quartz (e.g. Vitreosil and Suprasil) and the natural fused and crystalline varieties account for variations in quoted values from 160 nm to 185 nm; the latter is the most commonly accepted value.

7. Extended exposure to UV can induce darkening (solarisation) of many transparent materials; in the case of synthetic optical materials, variations in processing can cause additional discrepancies. Atmospheric ageing can also cause a drop in transmission; for example, lithium fluoride must be protected from the air because of its water absorption.

Table 8.3 The UV Transmission Limits of Various Optical Materials
Attached Image: Table8.3.jpg

8.4 Filters

The term UV filter covers a number of quite different products (see Fig. 8.3): it does not indicate whether the filter absorbs or transmits UV and must be qualified.

8.4.1 UV transmitting filters.

Most UV sources emit some visible light, so that a UV transmission camera filter (e.g. Wratten 18A or 18B) is used if the photographic record is to be confined to the ultraviolet. For UV-excitation of fluorescent specimens a filter must be fitted to the lamp house. The filters have a secondary transmission in the far red and infrared.

Gelatin filters are not available, although a solution of cobalt chloride can be used for some purposes. The generic term 'Wood's glass' is used for filters having spectral transmission of the form shown in Fig. 8.3a.

There are no dye-absorption filters for the middle UV, but interference filters are available (see Table 8.4). Thin-film filters and mirror coatings for the UV have been discussed by Hennes and Dunkelman.8, note4

note4 Low-reflectance (blooming) and high-reflectance coatings for work in the UV are offered by Optical Coatings Ltd., Hillend Estate, Dunfermline and by the firms listed with Table 8.4.

Fig. 8.3. Filters for UV photography.
Attached Image: Fig8.3.jpg

8.4.2 UV absorbing (barrier) filters.

To photograph fluorescence, the camera filter must completely absorb the exciting radiation. Filters made for this purpose (e.g. Wratten 2B, 2E) must not be confused with the UV haze-cutting filters (e.g. Wratten 1A) which are intended simply to attenuate the UV for pictorial purposes. Haze filters have only about 1 per cent residual transmission in the 310-380 nm band, but even this is excessive for fluorescence photography using high-power UV sources. Plain filter glass absorbs all wavelengths below about 330 nm and this imposes a lower limit on all conventional filters.

8.4.3 Accessory filters.

Conventional neutral-density filters begin to show strong absorption below about 390 nm and semi-reflecting aluminised niters (on a quartz substrate) may be used for the middle and near UV. Silvered filters would not be
suitable because of the sharp transmission band of silver at 300-330 nm (see p. 57).

Normal polarising filters absorb all wavelengths below about 370 nm and may be bleached by intense UV irradiation. The Polaroid Corporation markets a NHP1B polariser for use down to about 280 nm and Ealing Beck offer a UV polariser for use in the range 230-400 nm.

Table 8.4 UV-Absorption and UV-Transmission FIlters
Attached Image: Table8.4.jpg

8.5 Ultraviolet optical systems

8.5.1 Lenses.

Conventional UV photography normally uses the 365 nm emission from a mercury discharge lamp. Many lenses have good transmission (30-50 per cent) at this wavelength, although it should not be taken for granted, especially with multi-element lenses. Some lenses (e.g. the UV Topcor) are specifically designed to exclude the near UV, which causes a blue cast in normal colour photography.

A simple lens shows a progressive increase of focal length with wavelength (Fig. 8.4). However, most camera lenses have an achromatic correction for blue and green light; shorter wavelengths then come to a focus farther from the lens. With some lenses the error is relatively small and may be virtually eliminated by stopping down, but some form of focus pre-calibration is to be recommended for critical work. In other cases, such as the UV-Nikkor, the lens is focused visually and the distance setting is then transferred to a separate UV focus datum. A similar UV focus shift is automatically applied in the Caps microfilm diazo printers, avoiding the expense of a lens fully achromatised for both the UV and visible wavelengths.

Fig. 8.4. Effects of axial chromatic aberration on focusing for ultraviolet.
Attached Image: Fig8.4.jpg

There are certain advantages in using UV for photomicrography (see Chapter 5); because of the difficulties of achieving chromatic correction in the ultraviolet, some monochromat objectives are designed for use at one UV wavelength only (e.g. 254 nm and 546 nm).

The difficulty of focusing is overcome in some UV microscopes by use of a fluorescent focusing screen; this is excited by the image in the UV focal plane and permits viewing and accurate focusing even with uncorrected lenses. Alternatively, catoptric objectives can be used; these are free from chromatic aberration and may be used for work down to about 150 nm. The mirror lenses available for miniature cameras normally have some refractive elements and are not necessarily well-suited to UV photography.

Some enlarging and process lenses are designed to have good transmission in the near UV in order to give maximum speed with the majority of reprographic materials; these lenses may therefore be useful for direct UV photography. A similar requirement arises in oscilloscope recording with CR tubes having the P16 phosphor (peak
emission 385 nm — see p. 354).

Table 8.5 lists some of the camera lenses that are suitable for direct recording in the near UV with standard cameras.

Table 8.5 Ultraviolet Lenses for Conventional Cameras
Attached Image: Table8.5.jpg

8.5.2 Pinholes and Zone plates.

These two devices operate on quite different principles, but share the important property that they do not cause any spectral absorption. They may be used for work in the visible spectrum but have specialised application in the vacuum UV and soft X-ray regions, where normal lenses are useless. Pinhole collimators are used in gamma ray cameras (p. 301).

8.5.2.1 Pinholes.

Pinholes give images of a wide angular field and are free from any curvilinear distortions.

The small aperture (normally about f/100 to f/300) demands long exposures and also causes low resolution because of diffraction. The optimum pinhole diameter is given by the formula quoted on p. 459.10

A study of pinhole image quality using MTF methods has been made by Sayanagi;11 this paper also gives general references on the subject of pinhole cameras.

8.5.2.2 Zone plates.12

In some respects a Fresnel zone plate behaves like a normal refractive lens; divergent wave-fronts from the subject are diffracted by the concentric annular rings of the plate and are converged to a focus. A number of foci are produced, rather like the different orders of spectra given by a diffraction grating, but with the difference that the zone plate produces recognisable images rather than single spectral lines. The position of the focus varies with the wavelength, so that the zone plate suffers from marked chromatic aberration. This has been turned to advantage in some spectroscopic work, because an emulsion placed at the correct focus for a particular wavelength records other wavelengths unsharply; the image formed thus tends to emphasise one spectral zone.

The resolution and image illumination given by a zone plate are far superior to those of a pinhole. However, there is a practical limit to high performance, which demands a large number of rings: the diminishing size of the outer rings causes manufacturing problems. The pattern is normally produced by photographic reduction on to a high resolution plate,13 but metal plates with self-supporting rings must be fabricated for the shorter wavelengths. Pounds 14 has described a copper zone plate for producing solar X-ray images in the 5 nm region.

Baez15 has shown that the resolution given by zone plates (which can be comparable to that of a simple refractive lens) increases at shorter wavelengths, thus following the trend of Abbe's expression for lens resolution (Eq. 2.6).

Fig. 8.5. Enlarged plan of Fresnel zone plate.
Attached Image: Fig8.5.jpg

8.6 UV detectors

Ultraviolet radiation has a high quantum energy and produces a photo-electric effect in many materials ; the detectors available for UV work include photo-voltaic, photo-conductive and photo-emissive types.16

Most cadmium sulphide exposure meters have little response to the UV, but conventional selenium exposure meters have response down to about 300 nm; the half-peak band width is from about 400-650 nm. To use these meters for UV measurement, all visible light must be excluded from the cell; the exposure calculator must be re-calibrated for the UV source, emulsion and optical system in use. Wavelength conversion meters have been produced, using a normal light-sensitive cell with a phosphor coating which is excited by the UV waveband under study.

In certain fields, chemical UV detectors (actinometers) have been used; a bleaching or darkening takes place in proportion to the UV dose and a comparator strip is used for dose assessment.

For the measurement of visible fluorescence, normal spot photometers (for example, the SEI) can be used. With the photo-electric photometers it may be necessary to fit a UV-absorbing filter over the meter.

Shandon Scientific note5 offer UV meters for both long-wave and short-wave ultraviolet; they are used for checking that the output of UV sources is maintained and forstudying the efficiency of reflectors.

note5 'Blak-ray' UV meters: Shandon Scientific Co. Ltd., 65 Pound Lane, Willesden, London NW10.

8.6.1 UV image converters.

The general principles of electronic image tubes have been mentioned in Chapter 7 and the subject has been thoroughly covered by Soule.17

Samson3 has quoted performance figures for UV detectors sensitive down to a few nanometres and has given details of UV image converter tubes and television tubes for work in the vacuum UV. For work in this region, the glass face-plate of image tubes must be replaced with quartz or some other material that will admit the shortwave radiation to the photo-cathode.

The Hadland Imacon camera (framing rates up to 2xl07 /sec.) has an optional UV-sensitive photo-cathode.

The Wild M 500 image converter is available for macrophotography or photo-micrography. Interchangeable IR or UV image tubes allow the range 350-1200 nm to be recorded on the built-in camera unit.

Fig. 8.6. Wild M 500 IR/UV image convenor camera.
Attached Image: Fig8.6.jpg

8.7 Emulsions for UV photography

8.7.1 Spectral sensitivity.

The peak spectral response of silver halide emulsions is usually in the region 350-400 nm. Many published spectrograms do not show the full UV sensitivity; first, because the curves often relate to tungsten sensitivity, in which case there is relatively little UV emission from the source; secondly, because the spectrographic equipment usually has glass components, which strongly absorb UV radiation. Modern data sheets (Fig. 8.7) give a realistic idea of the UV response of the film, although this does not represent the effective spectral response in a camera
with normal UV-attenuating lenses.

Fig. 8.7 shows that the shape of emulsion spectral response curves varies considerably according to the density level used for film speed measurement. The natural tendency for reduced contrast in the UV often means that greatly increased exposure is necessary to give a high density and this curve therefore shows a markedly lower response than the low density curve.

The difference in UV speed between ordinary blue-sensitive emulsions and the dye-sensitised high-speed emulsions is less than might be supposed. For example, Ilford have stated that their HP3 and N5.31 films, which normally have a speed ratio of about 20:1, have a UV speed ratio of about 3:1.

The UV print-out paper used in instrumentation systems is sensitive over the range down to about 270 nm. The evaluation of these materials has been discussed by Jacobs and McClure.18

Fig. 8.7. Spectral sensitivity curves of photographic emulsions.
Attached Image: Fig8.7.jpg

8.7.2 Gamma-lambda effect (Gradient-wavelength effect).

Photographs taken with short-wavelength light are generally of lower contrast than those made with longer wavelengths (see p. 28). This is far from being a simple rule19 and there are considerable variations throughout the visible spectrum (see Fig. 8.8). There is nevertheless an inherent tendency towards lower contrast in the ultraviolet, although some of the UV spectrograph materials are especially designed to maintain reasonably constant contrast in the near UV. This gamma/lambda effect is an important matter in spectroscopy, where the line densities are sometimes measured for analytical purposes. Latham found that a number of Kodak Spectroscopic plates showed a 2 per cent increase in gradient for every 10 nm wavelength interval, over the near UV and blue region (340-490 nm).20

Time-gamma curves can be prepared for UV work by normal sensitometric methods, but the complete camera system should be used for exposure, because the spectral transmission of the lens and filters will determine how much of the low-contrast UV is used to form the image

Fig. 8.9 shows the characteristics of two films: apart from a lower D max, Film A shows little difference between exposure to the near UV (approx. 360-390 nm) and the normal photographic spectrum (approx. 360-670 nm). Film B shows the more common characteristics of reduced contrast in the near UV, as illustrated by the gamma/lambda curves of Fig. 8.8.

Fig. 8.8. Variation of gamma with the wavelength of the exposing radiation.
Attached Image: Fig8.8.jpg

Fig. 8.9. The effect of near UV radiation on sensitometric characteristics.
Attached Image: Fig8.9.jpg

8.7.3 Gelatin absorption.

Gelatin has a strong UV absorption band (beginning at about 235 nm and extending to the soft X-ray region) which inhibits the natural actinic response of silver halides in the short-wave UV. Some emulsions (e.g. the Kodak B10 plate) have a fairly low gelatin content and can be used for work in the 250-210 nm region, but this is the limit for any normal emulsion.

Schumann (1892) overcame the problem by depositing the silver halide on to a thin layer of gelatin, so that the crystals lay on the surface. Plates of a similar type are marketed by Ilford (Q plates), Kodak (Short Wave Radiation plates and films), Kodak-Pathe (SC-5 and SC-7 films) and Agfa-Gevaert (Schumann plates). They have been used to record wavelengths down to 7-5 nm, a figure probably limited more by the experimental apparatus than by the inherent sensitivity of the emulsion. All Schumann type plates require very careful handling both before and after exposure; Kodak 21 recommend the application of an anti-abrasion coating after exposure.

Vacuum-deposited layers of silver halide are completely free from gelatin and have been used as an alternative to Schumann plates for ion detection in mass spectroscopy.22

8.7.4 Fluorescent emulsion coatings.

The UV absorption of gelatin can be circumvented by coating a normal emulsion with a thin fluorescent layer, to convert the ultraviolet into a visible image which is readily recorded by the emulsion.

Mineral oils (Duclaux and Jeantet, 1921) can be used for this purpose, but must be removed with a suitable solvent before processing can take place. Sodium salicylate (e.g. a 0-5 per cent solution in methyl alcohol) is more commonly used because it dissolves in the developer and does not affect the processing.23 The spectral responseextends well below 100 nm and gives an emission band in the violet-blue region. A thin layer gives better resolution (up to 100 1/mm has been recorded) but a thicker layer gives a better fluorescent yield and requires less exposure.

Eastman Kodak will supply any of their spectroscopic films with a fluorescent coating which gives response below 100 nm and produces fluorescence in the 290-350 nm region; the company will supply a similar Ultraviolet Sensitising Solution for workers to treat their own plates.21 These coatings are highly efficient but are water-
resistant and must be removed after exposure.

8.7.5 Non-silver photo-sensitive materials.

Most of the diazo and photo-polymer materials used in reprographic and photo-mechanical work are sensitive to the near ultraviolet. They usually have a very low speed, but for some purposes the diazo materials for example, may offer a quick and cheap method of UV recording by contact exposure.

8.8 Medical and biological effects of UV radiation

Short-wave UV radiation has damaging effects on human tissue and most living organisms. All wavelengths below about 310 nm cause discomfort and possible permanent damage from conjunctivitis (inflammation of the eye) and erythema (reddening and blistering of the skin). Under medical supervision, controlled doses of erythemal radiation can have beneficial effects on the skin; however, in photographic work all unnecessary exposure to UV should be avoided.

Most photographic UV lamps are fitted with a Wood's glass filter (see p. 265) which transmits only the harmless near UV. However, unfiltered lamps are sometimes required (e.g. 254 nm mercury radiation for fluorescence work) and the manufacturer's safety precautions must then be observed, using goggles and gloves or UV-absorbent screens (e.g. glass or Perspex VA) to protect the eyes and skin.

The characteristic smell from mercury lamps with quartz envelopes is due to UV radiation in the 180-220 nm band, which produces ozone (03) from the air. This can have unpleasant effects in a confined space, but is not toxic in
the quantities produced in normal photographic work. Many mercury lamps are described as non-ozogenic; this is because the lamp envelope is designed to absorb the short wave radiation.

The near UV causes a slight fluorescence in the eye, which tends to give the disturbing effect of looking through a haze. For this reason, workers continually using UV lamps for fluorescence work sometimes wear UV-absorbing glasses, even though the filtered lamps are quite safe.

The physical effects of ultraviolet radiation have been discussed in more detail by Koller.24

Fig. 8.10. Biological sensitivity curves for ultraviolet radiation.
Attached Image: Fig8.10.jpg

8.9 Practical requirements in UV photography

8.9.1 Direct UV recording.
ADMIN NOTE: Eye protection must be used with ALL sources of UV light.

Normal laboratory and medical UV photography is concerned with the near UV band down to about 350 nm, which is the approximate transmission limit of normal camera lenses. The requirements for direct UV recording in this waveband may be summarised as follows:
  • Surroundings: A darkened room is not needed if the camera lens is fitted with a Wratten 18A or similar filter.
  • UV source: Mercury discharge lamps or electronic flash are normally used; a low power tungsten lamp is also required for setting-up.
  • Source Filter: Mercury lamps should be fitted with a Wood's glass screen to remove the dangerous short-wave UV.
  • Camera Filter: If the source is screened with a Wood's glass, a camera filter is not essential. However, such a filter is often used in order to exclude all visible light from the camera and allow work in normal lighting conditions. Both filters are sometimes used because this virtually eliminates the small visible transmission of the lamp filter and the contrast of the UV photograph may be slightly improved.
  • Camera: Any film or plate camera can be used. The lens must freely transmit the UV band in use and the focus setting must be correctly adjusted.
  • Film: All films are suitable for UV recording but it is often difficult to achieve sufficient contrast in UV records (see p. 273); for this reason the slower emulsions with higher inherent contrast may be preferred to the high-speed films.
  • Processing: If normal gamma values = 0-8 to 1-0) are required it may be necessary to use the more energetic developer formulae rather than fine-grain development.
UV transmission filters can be fitted either to the camera lens, or to the lamp, or to both, according to the circumstances. The possible permutations are summarised in Table 8.6.

Electronic flash units can be used without filtration for direct UV recording, but they are often fitted with a Chance OX-1 filter to confine the emission to the UV. This has the advantage that the unit can be used without further adaptation for either direct UV or fluorescence work, according to whether a UV- transmitting (Wratten 18 A) or UV-absorbing (Wratten 2B) filter is fitted to the camera.

Table 8.6 The Use of Lamp and Camra Filters in UV Recording
Attached Image: Table 8.6.jpg

8.9.2 Recording fluorescence.
ADMIN NOTE: Eye protection must be used with ALL sources of UV light.

Some specimens emit visible fluorescence under 254 nm mercury radiation and others are excited by blue light to emit yellow or red fluorescence. However, most fluorescence work is concerned with materials excited by the 365 nm mercury emission band; this normally gives a blue or green emission.

The requirements for this work may be summarised:
  • Surroundings: A darkened work area is essential to avoid uncontrolled visible reflection from the subject.
  • UV Source: Mercury discharge lamps, UV-emitting fluorescent tubes or electronic flash tubes are normally used.
    • A supplementary exposure with visible light may be necessary to show the general location of the fluorescent areas.
  • Source Filter (Exciter): The lamp must be filtered to produce only the required excitation band.
  • Camera FIlter (Barrier): A UV-absorption filter is always needed for fluorescence work. In some cases it may be helpful to adjust the relative contrast between the fluorescence and the background by the use of additional contrast filters.
    • It is essential that the transmission bands of the exciter and barrier filters do not overlap. Some UV lamps are fitted with filters that transmit a certain amount of blue light; a pale yellow camera filter is then required instead of the normal colourless UV-absorption filter.
    • In some subjects a red fluorescence is produced by a blue excitation; in this case a red barrier filter is used and a blue-violet filter is fitted to the light source.
    • The efficiency of any filter combination for fluorescence work can be tested by the photography of a specular metallic reflection from the source and exciter filter in use.25 Ideally, the specular shortwave reflection should be entirely absorbed by the barrier filter on the camera, but a small leakage of blue light is not normally objectionable.
  • Camera: No special problems arise in focusing, the fluorescent emission band falls within the chromatic correction band of most lenses.
  • Film: High speed films may be necessary for the weak fluorescence of some organic materials. Colour recording is often valuable in identifying a characteristic fluorescence and films such as High Speed Ektachrome and Anscochrome 500 are often used with forced processing to increase speed. Either artificial light or daylight colour films can be used, but according to Hansell26 the latter is preferable because of its lower sensitivity to stray ultraviolet. The contrast within the fluorescent areas is often rather low and care must be taken not to compress any subtle variations by incorrect exposure.
Fig. 8.11. Arrangement of equipment for UV and fluorescence recording.
ADMIN NOTE: Eye protection must be used with ALL sources of UV light.
Attached Image: Fig8.11.jpg

8.9.3 Exposure settings.

Subjects vary greatly in their UV reflectance and their fluorescence characteristics; exposure tests are always necessary when undertaking new work. The following figures may give some general guidance.

8.9.3.1 Direct UV photography.

The figures given in Table 8.7 were obtained primarily to compare the usefulness of the different sources. In each case a single unscreened lamp was used at a distance of 18 in. and at an angle of 45° to the lens axis: Pan F film was used. The lens was a Zeiss Pancolar 50 mm/2, with a Wratten 18A filter.

Table 8.7 Typical Exposure Figures for Direct UV Recording
Attached Image: Table8.7.jpg

8.9.3.2 Fluorescence recording.

The figures in Table 8.8 give a guide to the exposure necessary for recording a typical oil contamination. In other cases much longer exposures may be needed.

Table 8.8 Typical Exposure Figures for UV Fluorescence
Attached Image: Table8.8.jpg

8.10 Applications of UV photography

In addition to the examples mentioned below, UV and fluorescence photomicrography is covered in Chapter 5 and UV densitometry is mentioned in Chapter 1.

8.10.1 Medicine.

Pigmented areas of the skin (freckles etc.) and blood vessels have a high UV absorption and are given increased contrast in a UV record; certain skin conditions also stand out clearly. The near UV gives little penetration of the skin (0-1-1 mm) and surface detail is often shown with great clarity; this is the converse of the effect in IR photographs, in which penetration up to 10 mm is achieved27 and sub-surface features may be shown.

Some fungoid conditions have a characteristic fluorescence. Ring worm can readily be shown and some cancer cells can also be distinguished by their red fluorescence.

Artificial fluorescent tracers have been introduced into the retinal artery for study of the blood vessels in the eye; this shows up some of the fine capillaries that are normally of too low contrast to be seen clearly.28 In a similar way, injections into the carotid artery have been used to produce fluorescence in blood vessels of the head.

8.10.2 UV chromatography.

Paper chromatograms may contain substances that are colourless and yet have a characteristic absorption in the ultraviolet (often in the 254 nm region). Direct UV photography can be used to record these areas but a simpler method, which avoids the need for a quartz camera lens, is to make a contact print (using UV radiation) on document reflex paper.

Many chromatogram constituents are fluorescent and viewing cabinets (see Fig. 8.2 and Table 8.2) are available with UV tubes for excitation at both 254 nm and 365 nm; fluorescence records in colour or monochrome are then made with a camera mounted over the viewing window.

Barron 29 has described a simple apparatus using UV-filtered electronic flash for the standardised reproduction of chromatograms.

8.10.3 Industrial applications.

Fluorescent dye-penetrants are widely used for flaw detection in metals, plastics, ceramics and other materials. The basic technique is to clean the specimen and then to coat it with the penetrant oil ; the surface is then wiped clean, but the oil remaining in cracks and other surface defects fluoresces strongly under UV irradiation. This method is particularly useful for showing cracks in dark surfaces and in polished metallic specimens, where all the problems of harsh specular highlights are avoided. UV endoscopes (e.g. Optec and Shandon Blak-ray)
are available.

Fluorescent additives can be used in droplet studies to improve visualisation of the drop outline (see p. 413) ; Groeneweg et al have described the use of a pulsed UV laser to record droplets down to 10 /nm diameter at velocities of 50 m/sec.30

The natural fluorescence of most oils makes it easy to show oil splashes and similar contamination on fabrics. In other cases the cloth may contain artificial fluorescent brighteners and the effect of contamination may be to reduce the fluorescence in affected areas.

Tupholme has described photo-lofting methods in which fluorescent drawings or scribed markings on a fluorescent plate are used to transfer an image by contact exposure (using X-rays or UV to produce fluorescence) on to photo-sensitised metal.31

It can be very difficult to show the surface texture of high-temperature incandescent materials, because the intense red and yellow thermal emission from the surface swamps any cross-lighting from normal light sources. The extremely high intensity of lasers has led to their use in this application32 but an older idea is to use ultraviolet
cross-lighting and a UV transmitting filter on the lens, the principle being that most incandescent subjects emit very little UV. For the same reason, UV has been used in the high-speed photography of welding and other self-luminous subjects.33

UV recording has also been used to study the behaviour of high-voltage sparks; in the initial stages the discharge may emit more UV than visible light.34

8.10.4 UV Astronomy.

The atmosphere normally degrades optical images by turbulence and scatter, but one of the greatest difficulties to the astronomer has been the absorption of wavelengths below about 300 nm, owing primarily to an ozone layer in the upper atmosphere. The amount of ozone is very small, but the absorption is so strong that the solar and stellar UV spectra can only be studied from heights above 20 miles. In recent years, many rocket and balloon-borne experiments have been flown for X-ray and UV spectroscopy.14

There is great scientific interest in orbiting observatories which will permit a continuous study over the full electromagnetic spectrum, and Hutter 35 has described methods for television transmission of UV images from satellites.

Goldberg has summarised results from the Orbiting Solar Observatory OSO IV (1967) and the Orbiting Astronomical Observatory OAO II (1968) and has discussed plans for future work in UV astronomy.36

8.10.5 Forensic work.

The recording of fluorescence and direct UV photography can be useful in the study of forged documents and paintings. Superimposed brushwork or inks inserted on a document may be shown to be of a different origin and
chemically bleached areas may also be revealed. However, extended exposure to UV causes many pigments to fade and there is some risk that works of art could be irreparably damaged during a lengthy exposure.

All body fluids are fluorescent to some extent and can be detected by UV examination, even when present only in small traces.

The photography of fingerprints on patterned or polished surfaces can be assisted by the use of fluorescent powder. Any UV reflections from the surface are absorbed by the camera filter and only the fluorescent prints are visible.

8.10.6 Nocturnal photography.

After-dark photography of wild-life and some industrial processes can be carried out by direct UV methods. This work is perhaps more often carried out with infrared, but UV recording has the advantage that conventional films can be used. It may be noted that some insects have pronounced visual sensitivity to the near UV and may therefore react unfavourably to a series of UV flashes.

Aspden has mentioned an application of this principle to the recording of aircraft instruments; a 56 W/sec flash unit fitted with Chance OX-filters enabled standard recording cameras to be used throughout the flight without disturbing the aircrew's night vision by bright lights: a flash guide number of about 14 was found to be suitable
for Ilford HP3 film.37

References

General references in UV technology

1 Summer, W., Infrared and ultraviolet engineering, Pitman, London (1962).
2 Roller, L. R., Ultraviolet radiation, 2nd Ed, Wiley, New York (1965).
3 SamsoM, J. A. R., Techniques of vacuum ultraviolet spectroscopy, Wiley, New York (1967).

General references in UV photography

4 Hansell, P., Photography for the scientist, Ed C. E. Engel, Ch 8, Academic Press, London (1968).
5 Kodak Data Sheet SC-3, Kodak Ltd., London.
6 Ultraviolet and fluorescence photography, Eastman Kodak publication M-27 (1968).

Specific references

7 Roller, L. R., op. cit, pp 70-81.
8 Hennes, J. and Dunkelman, L., The middle ultraviolet, its science and technology, Ed A. E. S. Green, Ch 15, Wiley, New York (1966).
9 Lorant, M.,BJPhot, 111, 140 (1964).
10 Fjeld, J. H., JSMPTE, 74, 320 (1965).
11 Sayana.GI, K., / Opt Soc Amer, 57, 1091-99 (1967).
12 Ditchburn, R. W., Light, 2nd Ed, Appendix 6c, Blackie, London (1963).
13 Chan, H. H. M„ Appl Opt, 8, 1209-11 (1969).
14 Pounds, K. A., J Phot Sci, 13, 20-4 (1965).
15 Baez, A. V., The encyclopedia of microscopy, Ed G. L. Clark, pp 552-61, Reinhold, New York (1961).
16 Roller, L. R., op cit, Ch 8.
17 Soule, H. V., Electro-optical photography at low illumination levels, Wiley, New York (1968).
18 Jacobs, J. H. and McClure, R. J., Phot Sci Eng, 9, 82(1965).
19 Farnell, G. C, The theory of the photographic process, Ed C. E. R. Mees and T. H. James, 3rd Ed, Ch 4, Macmillan, New York (1966).
20 Latham, D. W. AstrJ, 73, 515-17 (1968).
21 Kodak plates and films for science and industry, Kodak Publication P-9, Eastman Kodak, Rochester, N.Y. (1967).
22 Honio, R. E„ Woolston, J. R and Rramer, D. A., Rev Sci Instrum, 38, 1703-13 (1967).
23 Samson, J. A. R., op cit, pp 209-11.
24 Roller, L. R., op cit, Ch 7.
25 Hansell, P., op cit, p 378.
26 Hansell, P., ibid, p 375.
27 Roller, L. R., ibid, p 8.
28 Rose, E. S., Fluorescence photography of the eye, Butterworths, London (1969).
29 Barron, J. C, B J Phot, 111, 216-7 (1964).
30 Groenweg, J., Hiroyasv, H. and Sowls, R., Brit J ApplPhys, 18, 1317-20 (1967).
31 Tupholme, C. H. S., Photography in engineering, pp 22-8, Faber & Faber, London (1945).
32 Christie, R. H., Perspective world report: 1966-69, Ed L. A. Mannheim, pp 175-84, Focal Press, London (1968).
33 Anderson, R. W., Proceedings of the 5th International Conference on high-speed photography, Ed J. Courtney Pratt, SMPTE (1962).
34 Waters, R. T. and Dyson, J., J Phot Sci, 10, 116-128 (1962).
35 Hutter, E. C, Light and heat sensing, Pergamon, Oxford (1963).
36 Goldberg, L., Sci Amer, 270, 92-102 (1969).
37 Aspden, R. L., Electronic flash photography, p 143, Temple Press, London (1959).
Andrea G. Blum
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#3 nfoto

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Posted 05 January 2015 - 18:30

I see only badly formatted text interspersed with rubbish ?
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#4 Alex H

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Posted 05 January 2015 - 19:48

Bjørn, there is an option there for "see other formats", linking, among others, to PDF files.

#5 OlDoinyo

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Posted 05 January 2015 - 20:08

It is interesting that zone plates are mentioned in the section on optics; one wonders where the zone plates of the day were obtained. No mention of Fresnel lenses, however. (The two are different--one works by refraction, the other by diffraction.)

#6 nfoto

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Posted 05 January 2015 - 20:22

Alex: Just saw that at the original link, so am downloading a pdf to store in my reference archive. Thanks.

However, my post that set Andrea off pertained to the UVP thread in which only garbage was shown. An hour or so later the text by some kind of magic sorted itself and now is readable including schematics. No idea why this happened?

I was a bit confused as to the reference to a UV-Nikkor as the book was published in 1971, more than a decade before our favourite UV-Nikkor 105 mm f/4.5 lens made its entrance. However, I now see the reference is to the extremely exotic UV-Nikkor 55 mm f/4 which hardly if ever got through the prototype stage. A handful of specimens might have been made though and I have seen a reprint of the leaflet allegedly accompanied the production lens.
Bjørn Birna Rørslett, Ph.D.
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#7 Andrea B.

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Posted 05 January 2015 - 20:55

Chapter 3
Page 130
Table 3.8 Proportion of Ultraviolet, Visible and Infrared Radiation Emitted by Photographic Light Sources

Attached Image: Table3.8.jpg
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#8 colinbm

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Posted 05 January 2015 - 23:46

Excellent thanks Andrea.
Interesting comparisons between the Sun, Xenon & Mercury lamps.
I was beginning to doubt the Mercury lamps usefulness for UV output, I guess I had better be grateful for small mercies.
Col

#9 Damon

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Posted 06 January 2015 - 00:56

Good to see the Gods of Mount UV are showing some spirit today.

There is loads in there I didn't/don't know. Maybe if I read through this I won't have so many stupid questions--don't hold your breath on that one though.
Thanks for sharing this resource.
Blak-rays apparently have been around for awhile.
Fig 8.11 is cool.

Did you read any of that Col or just look at the pictures again?

-D

#10 colinbm

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Posted 06 January 2015 - 02:48

I was hoping you can read it to me before I go to sleep please...
Col

#11 JCDowdy

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Posted 06 January 2015 - 15:56

Andrea,

Thanks for this historical perspective. It is very important we not forget lessons of the past, even the recent past. This internet age sometimes seems like the digital equivalent of the proverbial destruction of the Ancient Library of Alexandria. Nowadays if people can't Google it, look it up on Wikipedia, PubMed or Chem Abstracts it is like it never existed.

If I may recommend a minor editorial revision, please put a note on figure 8.11 that eye protection should be used under all conditions. The figure incorrectly instructs it is only needed in panel b.

#12 Andrea B.

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Posted 06 January 2015 - 16:21

Will do, JD !!!

I'm going to also remind folks that while the UV facts are correct (insofar as I can determine), the gear info is - naturally - quite out of date.
Andrea G. Blum
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#13 Andrea B.

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Posted 06 January 2015 - 16:56

There are two lenses there that I've never seen anywhere: the UV-Nikkor 55mm f/4 and the Zeiss 50mm f/2.
I've heard about the 55mm UV-Nikkor.
Never before heard about that Zeiss though.
Andrea G. Blum
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#14 nfoto

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Posted 06 January 2015 - 17:08

Rumours had it NASA ordered a 55 mm f/2 UV Nikkor with fixed infinity focus (no focusing). This is mentioned in P. Braczko's compilation handbook and there even is a photo of such a lens in it. I don't know anything else about that design.
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#15 Andrea B.

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Posted 06 January 2015 - 17:59

thx, B.

googled around and also found this: http://www.mir.com.m...cial/55mmUV.htm
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#16 nfoto

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Posted 06 January 2015 - 23:00

From experience, I can tell serendipity plays a MAJOR role in locating any lens produced in say < 100 copies. Still, it is not impossible.
Bjørn Birna Rørslett, Ph.D.
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#17 JCDowdy

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Posted 07 January 2015 - 05:44

Serendipity - one of my favorite words!

Edited by JCDowdy, 07 January 2015 - 05:44.


#18 Adrian

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Posted 07 January 2015 - 09:27

I was interested in this post as I did a three year course in scientific photography in the early '70s (at the Harrow School of Photography in the UK), and Arnold Rolls and Stewart was my bible for the course. I still have my much thumbed copy in a prominent place on my bookshelf. There was another we used extensively - Photography for the Scientist by Engel (I think) but I no longer have this one. I seem to remember it had a good section on UV.
Adrian Davies