How can one deterine the calibrated white point of the image sensor of a digital color camera? That one certainly exists is evident from the following considerations—
that the temperature control in raw processors sets an absolute termperature value (in kelivn), rather than a relative one.
that there should seem to exist a light spectrum producing voltage of equal magnitude on red, green, and blue sensor cells. The temperature of this spectrum will be the one I seek.
that color film is calibrated to a certain white point, and so by analogy should be digital sensors.
that exposure adjustments in both film and digital affect the amount of total light energy admitted to the medium, while retaining the spectum, and consequently the relative proportions between the red, greeen, and blue components. The said proportions should be available in the raw image without specifying the temperature—the values are already there, whereas the temperature setting specifies their interpretation in the flat raster image.
dcraw, the most honest and straighforward raw processor out there, can accept white balance not as an absolute temperature value, but in relative form, as a set of per-channel multipliers, the unity values yielding sensor bulit-in WB—what is it?
I, however, have found nary a mention of this fundamental parameter of a color CCD sensor.
A sensor's whitepoint (including both temperature and tint) can be determined in a simple experiment, by taking a picture of a gray card illumined by a light source with known characteristics, e.g. direct sunlight or a high-quality photographic light. The sensor's native rendition of the light can be generated by demosaicing the raw file with
dcraw -r 1 1 1 1
Now that both the input spectrum and the (unadjusted) output color are known, the whitepoint can be easily calculated. Another option is to use a high-quality adjustable light source to determine the parameters that make the demosaiced photo netural gray.