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White Balance
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#1 Andrea B.

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Posted 11 July 2015 - 15:11

Sticky :: White Balance and Color Correction in UV/IR Photography
by Andrea G. Blum for UltravioletPhotography.com

Last Update: 2018 12 April
Completed paragraph about visible color correction.

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

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




Three UV White Balance Tutorials .
False Colour is False Colour
Do we absolutely have to white balance a UV or IR photograph? No, of course not. It is your choice as the photographer.

Any colour seen in a UV or IR photograph is false regardless of what processing is applied. That is to say, the UV or IR colour we see is an artifact of the particular combination of Bayer dyes used in the camera's internal filtration pack as well as an artifact of the particular filter, sensor, lens and lighting in use when the photograph is made. Then the demosaicing algorithm of either the camera or the raw converter further affects the colour outcome. There is no "correct" false colour for UV/IR work.

However, most UV/IR photographers do like to set white balance either in-camera or in the raw converter because it usually gives a photograph with more clarity and detail. And, with the current Bayer sensors, a certain standardized false colour set appears after the white balance step.


Ordinary White Cards for UV White Balance: NO!
The typical white balance card used for setting white balance in an unconverted digital camera almost never remains fully reflective in Ultraviolet or Infrared light so should not be used to set a white balance in non-Visible wavelengths. Use instead the materials suggested below.


White Balance Materials for UV/IR: Why?
With some converted digital cameras an in-camera white balance can be achieved under the dark external filters used for UV or IR by performing the white balance measurement against a white target with stable reflectivity under UV or IR light. For example, Sony or Panasonic Lumix bodies are good at UV/IR in-camera white balance against such targets. However, converted Nikon bodies, for example, cannot achieve an in-camera white balance through UV filters. So for those cameras, the proper UV or IR white balance must be set in the raw converter by measuring a photograph of an UV/IR stable white target made under the desired filtration. Also, even those cameras which can set in-camera white balance under UV/IR filters are not always perfectly accurate under all lighting conditions or filter types. So always making a photograph of a white balance target at the beginning of each shooting session with a particular filter is a good practice whieh enables later colour corrections or white balance setting in a raw converter or photo editor.


White Balance Materials for UV/IR
These materials reflect light uniformly (are spectrally flat) over a large range in Ultraviolet, Visible and Infrared wavelengths. Unbranded PTFE is the least expensive of these materials. PTFE or Teflon can be more prone to blowout from specular reflection. Adjusting the shooting angle slightly should help with that.
  • Teflon® = brand name PTFE made by DuPont®.
    Note that the Teflon name is sometimes also applied to PFA and FEP.
  • Fluon®, Dyneon® = other brand names for PTFE.
.
Examples:
[UltravioletPhotography.com does not endorse any specific products as a website. We simply offer reports, reviews and lists for your further investigation. Any opinions in such reports/reviews/lists belong solely to the poster writing them. UltravioletPhotography.com as a website receives no compensation or income from any source.]
  • PTFE Sheet from ePlastics
    Just one example for a vendor of natural virgin PTFE sheets in various thickness and dimensions. There are many vendors to choose from.
    Don't buy a sheet which is thin enough to transmit any stray light unless you are using it over the lens. (See next.)
.
White Balance Tools in Raw Converters
Most raw converters (or editors) feature a white balance tool in the form of a dropper or rectangle which may be clicked or dragged over the photographed white standard to set the white balance in a false-colour photograph. This is the only white-balancing method available for those cameras which cannot record an in-camera white balance preset through dark external UV or IR filters.

But not all such tools are able to fully white balance a raw ultraviolet file. We think this is because some converters cannot push the temperature setting to the needed low between 1700-2000°K. As one example, the most widely used raw converter, Adobe Camera Raw, was not very good at white balancing Nikon D700 files made under a broadband UV-pass filter. (Editor's Note: I am no longer using Adobe products, so cannot speak to current ACR capabilities.) So such files had to be first converted in a Nikon converter (NX-D, View NX) or in a converter like Photo Ninja (or others) before being taken into CS6 for further editing. Two step conversion and editing makes for a slightly longer workflow, but typical UV photography does not produce large batches requiring overnight editing. Creating preset conversion profiles for quick application of white balance and colour correction is the way to speed up this step and keep workflow manageable for any camera used for UV photography.

Visible Color Correction: White Balance and a Color Profile
The white balance step alone is not always enough to produce proper colours in a converted camera when the converted camera is used to record a Visible scene under a corrective BG filter or UV/IR-cut filter unless you have a filter identical to the internal blocking filter which was removed during conversion. Given that not all the camera manufacturers use the same internal blocking filters, it should be no surprise that color profiling is a necessary step for using a converted camera for Visible work. Typically software color profiles can be built from photographs of color patch cards like the CC Passport (as just one example) using its software. Some converters provide their own tools for creating color profiles. As just one example, Photo Ninja can create a color profile from a photograph of a CC Passport.
Andrea G. Blum
Often found hanging out with flowers & bees.

#2 Andrea B.

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Posted 12 April 2018 - 17:04

Update 12 April 2018: Completed paragraph about visible color correction.

Comments and suggestions always welcomed.
Andrea G. Blum
Often found hanging out with flowers & bees.