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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.
Ulf Wilhelmson
Curious and trying to see the invisible.