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Fluorescence
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#1 Andrea B.

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Posted 11 July 2015 - 15:06

Sticky :: UV Induced Visible Fluorescence
by Andrea G. Blum
for UltravioletPhotography.com

Last Update: 03 June 2017
added reference to fluorescent wb target

Sticky List:
<> Sticky :: UV-Capable Lenses <>
<> Sticky :: UV Induced Visible Fluorescence <>
<> Sticky :: UV/IR Books <>
<> Sticky :: UV Photography: Cams, Mods, Lights, Links <>
<> Sticky :: UV/Vis/IR Filters <>
<> Sticky :: White Balance in UV/IR Photography <>

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Note from Editor:
This is a joint effort by the members of various forums who enjoy UV induced visible fluorescence photography. Thanks to everyone for their input.

Please PM Andrea B. on UltravioletPhotography.com with any corrections, additions or suggestions.


UV Induced Visible Fluorescence (UVIVF)

The type of fluorescence most often encountered in UV photography is the visible light emission, or luminescence, from an object caused by its absorption of high energy ultraviolet light (shorter wavelengths). The absorbed UV photons excite the object's electrons to a higher quantum state. The electrons immediately relax to a lower quantum state and emit photons of lower energy in the form of visible light (longer wavelengths). This visible fluorescence may be successfully photographed.

Not every subject fluoresces.

Note that it is possible for UV light to also induce Infrared fluorescence in some objects. Similarly, Visible light may induce IR fluorescence.


Equipment for UVIVF Photography
  • UV Light Source
    • UV-LED Flashlight (aka Torch)
      It's easier to add a UV-pass filter to a torch than it is to a mercury lamp.
      Example: MTE U301 Flashlight
    • Mercury Lamp
      Lamps are typically filtered by the manufacturer. If not, it may be difficult to find excitation filters large enough to fit a lamp.
      Example: Blak-Ray B-100 Lamp
    • Flash
      It is possible to induce visible fluorescence with UV-flash. However, if the fluorescence is dim, then a longer exposure under a steady UV-lamp or UV-torch may be necessary.
    • UV Lighting Tag
      For more information, find all tagged posts: UV Lighting
  • UV-Pass Excitation Filter on the UV light source.
    The purpose of the UV-Pass excitation filter is to block all non-UV light leaks commonly seen with UV light sources. Such non-UV light leaks could cause non-fluorescent visible or infrared light reflections off the subject that might contaminate the fluorescent light emitted from the subject. And any non-UV light leaks could also potentially cause additional fluorescence depending on the subject being photographed.
  • Note that ALL UV-LED flashlights tested so far do leak some visible light and need to have some UV-only filtration. For the better Nichia UV-LED chips, the leakage seems to be confined to the violet end with maybe some blue. Other UV-LED chips seem worse with some having minor leakages further up the spectru.
    • Example: Baader UV-Pass Filter
    • See more about excitation filtration in the Shiny Metal Test below.
    • For more information, find all tagged posts: Filters
  • UV/IR-Cut Barrier Filter on the camera lens.
    The purpose of the UV/IR-cut barrier filter is to exclude all non-visible light from reaching the sensor. Sometimes newer digital cameras may have an internal UV/IR-blocking filter which is strong enough to act as a barrier filter but there are a few conterexamples. To ensure you are recording only visible emitted fluorescence, add a good UV/IR-blocking filter to the lens. Do carefully test any stock camera which is to be used for UVIVF work.
    • Example: Baader UV/IR-Cut Filter
    • See more about barrier filtration in the Shiny Metal Test below.
    • For more information, find all tagged posts: Filters
  • Camera and Lens for recording a visible light photograph.
    No modifications needed, of course.
    Do see the preceding comment regarding the camera's internal UV/IR-blocking filter.
  • A Dark Room in which to photograph UVIVF.
    Any ambient light will cause unwanted reflections off the subject which will contaminate or wash out the emitted fluorescent light. As a practical consideration your Dark Room (or dark closet, perhaps) should contain a lamp or a handy flashlight to allow you actually see the equipment and subject while setting up for the UVIVF photograph.
    Remember to take your UV-blocking goggles with you into your Dark Room.
*
Shiny Metal Test in UVIVF Photography
from UVP member Shane Elen

Capturing fluorescence (spectrally or photographically) works on the principle that the combined effect of the excitation and emission filters allows only fluorescence emission to reach the camera sensor. Excitation wavelengths from the excitation illumination must be totally excluded. Typically very narrowband filtration is used in laboratory fluorescence photography. Here on UltravioletPhotography.com most fluorescence photography makes use of rather broadband filtration because narrowband photographic filters are not readily available.

For UV induced Visible Fluorescence photography, the UV illumination must be filtered for UV-only output, and the lens must filtered for Visible-only input. To test that the filtration is working properly, photograph some shiny ball bearings or a shiny spoon (which are non-fluorescent, of course) in total darkness. If the filtration is working properly, then the ball bearing is not visible. If the ball bearing is visible (shows a shiny reflection), then there is leakage in the filtration which must be remedied in order to claim proper UVIVF photography.

I don't have a statement to make at this time about whether the length of the exposure time might mitigate the effects of any unwanted stray light output from the UV illumination source.

When using a broadband camera or a camera with a weak internal filter, it might be difficult to determine whether filtration leakage in the Shiny Metal Test is from stray Visible or Infrared output from the UV illumination source or whether it is from some UV getting through a mediocre UV/IR-blocker filter on the lens. Further tests using additional filtration might be useful to sort this out.

Examples: Shiny Spoon Test

http://www.ultraviol...ch__1#entry7172

http://www.ultraviol...h__1#entry16181

http://www.ultraviol...__fromsearch__1

Here are two photos from the third link.

D600-broadband
Zeiss 60/4.0 UV-Planar with Baader UV/IR-Cut Filter
Unfiltered Nichia 365 UV-LED Flashlight
f/4 for 1.6" @ ISO-400 in total darkness
The lens is filtered to permit only Visible light to be recorded.
The unfiltered UV flashlight produces visible violet leakage.
Attached Image: leak.jpg

D600-broadband
Zeiss 60/4.0 UV-Planar with Baader UV/IR-Cut Filter
Nichia 365 UV-LED Flashlight with Baader-U UV-Pass Filter
f/4 for 1.3" @ ISO-400 in total darkness
Again the lens is filtered to permit only Visible light to be recorded.
The violet leakage from the UV flashlight is reduced to almost nothing
when a UV-pass filter is used on the source illumination.
Attached Image: noLeak.jpg


*
White Balance in UVIVF Photography

A Brief Review:
For our reflected UV or IR photography it seems reasonable to consider 'white' to be a reflection of all of UV, Visible and IR wavelengths. So we therefore use uniformly UV/Vis/IR-reflecting materials such as PTFE or Spectralon as white balance targets in reflected UV/IR photography. Given that both reflected UV or IR photography produces false colours, then, strictly speaking, white balance is not a necessity. But we do it anyway in documentary UV or IR photographs in order to standardize the false colour outcome over differing gear platforms.

Analogously, in order to perform white balance for the visible fluorescence induced by UV or IR light, we need some material or pigment which fluoresces white or grey under filtered UV or IR illumination (in the dark). Finding such a material or pigment has been a problem. Clean, uncontaminated Spectralon is not fluorescent (as per the manufacturer Labsphere) so we cannot use that. PTFE does often fluoresce but not in a neutral colour, so it too is unusable for white balancing UV- or IR-induced visible emissions.

However, recently UV Innovations has developed the UV-GreyTM fluorescent target for use in white balancing fluorescence photography where emissions are induced with UV illumination around 365nm. UV Innovations also offers the Target-UVTM with both fluorescent neutral and fluorescent RGB patches for use in determining the intensity of a subject's fluorescence or for white balance and color checking. The targets use a proprietary coating to produce the fluorescence and are calibrated to certain standard filtration.

[Disclaimer: UltravioletPhotography.com is an independent website which accepts no advertising and does not solicit products for testing. We are not affiliated with any manufacturers or vendors.]

Here are some tests with the Target-UV and the UV-Grey. From UVP member John Dowdy:
The long established standard for film fluorescence photomicrography was daylight balanced film. Seems to me that for best reproducibility that should likewise be applicable to digital fluorescence macrophotography.

References
from UVP member Shane Elen
The links are for your convenience only. UVP does not receive anything if you click on them.
Andrea G. Blum
Often found hanging out with flowers & bees.

#2 Andrea B.

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Posted 29 December 2016 - 18:05

Added a section: White Balance in UVIVF Photography
Andrea G. Blum
Often found hanging out with flowers & bees.

#3 Andrea B.

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Posted 07 January 2017 - 17:40

Added 3 references and comment about daylight film.
Andrea G. Blum
Often found hanging out with flowers & bees.

#4 Andrea B.

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Posted 03 June 2017 - 06:06

Rewrote the White Balance section.
Andrea G. Blum
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#5 Andrea B.

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Posted 24 February 2019 - 01:54

SHORT VERSION:
Filter the lens to block everything except visible light.
Filter the UV-source to pass only UV.
Shoot in complete darkness.

Test for leaks using the Shiny Metal Test described above.


EVEN SHORTER VERSION: Filter, filter, dark. :lol: :rolleyes:


.
Andrea G. Blum
Often found hanging out with flowers & bees.

#6 Andrea B.

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Posted 24 February 2019 - 02:19

UPDATE: Added example photos for Shiny Metal Test.
Andrea G. Blum
Often found hanging out with flowers & bees.