Originally posted by ProfessorBuzz a good old 18% grey card?
Not sure how "grey"! Read on:
Perhaps this begs the question: What, really, is “white” balance for an IR-converted camera?
As we all (should) know, our standard color digital cameras use a sensor chip (CCD or CMOS) with an array of R, G, and B filters deposited just on top of the actual sensor pixels.
In order to get a proper-looking image, the camera must adjust the relative proportions of the signals from the various filtered sensors. Selecting a WB setting in the camera sets up the expected ratio, depending on the spectrum of the light source (how much R, G, or B light there is coming from, e.g., the Sun). The actual application of the relative factors can be done internally by the camera for the creation of jpeg files, or be left to PP software in the case of RAW files.
One can do a “custom white balance” by taking a shot of a “gray scale” card and then having the camera adjust the amount of R,G,B from the sensor so that the card looks “gray” (or white - a sheet of white paper works just about as well). When we do this we are relying on the maker of the gray card to guarantee that over some range of light wavelengths, the card is truly gray: it reflects the same fractional amount of R, G, and B at all wavelengths. For normal color photography, the wavelength range will be from around 400 nm (very deep blue/violet) to somewhere between 650 and 700 nm (very red). And, I suspect, the average gray scale maker does not worry about the reflectivity of their product outside this range of wavelengths.
You can readily find the color response of cameras on the web. Look here:
RGB: All About Digital Camera Color Bin space: Spectral response of Nikon DSLRs (D90 and D300s) Nikon CFA Spectral Power Distribution | Strolls with my Dog
(Sorry about all the Nikons!)
Other cameras have quite similar responses, including our Pentaxes. Notice that sharp falloff at the red end of the spectrum - this is due to that IR-Cut filter.
What happens when that IR filter is removed? Colors from any standard processing program may well look bonkers!
You can find the intrinsic sensitivity of many camera sensors with a bit of web searching (i.e “Sony CCD sensors spectral response”). Here’s something you might find: This plot shows the non-IR-filtered behavior of a Sony IMX250 chip - note that the Blue and Green filters on the sensor allow a lot of leakage out in the IR and the overall sensitivity is quite high out to almost 1000 nm! The concept of blue or green or red color response is not so clear cut when that IR filter is removed.
Of course, the response of your modified camera will depend on the external filter you use, and the transmissivity of your lenses, along with the idiosyncracies of your sensor. Processing with RAW and choosing the behavior of various color weighting / temperature etc. may well give you some very strange looking “white”balance effects.
I think you will just have to play around to find what you like, and might even consider software that will allow you to pull out, say, just the Red part of the RAW data. This will correspond most closely to what you would image if your were using a true IR camera - i.e. something that responded only to wavelengths longer than ~700 nm.