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Definitions of Transmission Summary Parameters

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#1 UlfW

    Ulf W

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Posted 13 January 2021 - 07:48

Transmission Summary parameters found in UV-Lens Data entries
As graphs can be difficult to interpret and are too big to be included in texts, digested numeric values are needed for use elsewhere. They also provide a quick, useful summary of a particular lens's UV transmittance capability.

Range.
The approximate wavelength range within which the lens transmits any UV.
See remarks below about the left-hand endpoint, λUVzero(nm), of the range.

TVISmax(%)
The highest transmission found in the visible wavelength range (400 - 700 nm).
This value, when compared to the next two values, gives a general idea of the quantitative losses in the UV passband. Generally, we like to find UV-capable lenses which require only +3 stops more exposure length than the corresponding visible exposure of the same subject.

T365nm(%)
Transmission at 365nm.
This value is a good indicator for how much the exposure time is affected for a UV shot when using standard UV-pass filtration around 360 nm and/or UV-LED illumination peaking at 365 nm.
(This value might be obtained with simpler tools than a spectrometer by comparison to a well known lens.)

T400nm(%)
Transmission at 400nm.
For most non-specialized lenses this is the peak UV transmittance point. This value is a good indicator for how much the exposure time is affected for a UV shot for UV-marginal lenses or filtration peaking close to 400nm.
(This value might be obtained with simpler tools than a spectrometer by comparison to a well known lens.)

λUV HMvis(nm)
The wavelength where the UV transmission below 400nm is 50% of the visible max, TVISmax(%).
This value is similar to the FWHM (full-width half-maximum) for bandpass filters.
Observe this is not at 50% total transmission, making it more meaningful as it relates to the passband level.

λUV HM400(nm)
The wavelength where the transmission below 400nm is 50% of the (max) UV transmission, T400nm(%).
This value gives us a feel for how quickly the lens's UV transmittance drops.

λUV Zero(nm)
The wavelength where the transmission asymptotically goes to zero%, passing 1%.
A lens having a deep UV "reach" may not necessarily be useful near that zero point. So we must look for those lenses having a steep shoulder just past their zero cut-in points. For normal UV-photography the 0% cut-in alone gives an inflated view of the usable wavelength range as the light from longer wavelengths will normally dominate the image when using broadband UV illumination.
(The limit somewhere near 0% is sometimes seen in the UV-Lens Sticky.)
Ulf Wilhelmson
Curious and trying to see the invisible.