Edwin Blenkinsopp All leds are not the same. I looked at the spectrum of some commercial white leds and the spectrum is very non-linear Those used for video photography should be better edwin colour densitometers use filters, which I found non linear, and some which have aged poorly - yet the daily calibrations step seemed to negate any ill effects of such filtration. For the purposes of monochromatic sensitometry even a film base would add colour filtration.. but they was never taken into account - since density is really only considered in a relative sense.. so a white light LED, even a poor one, would be suitable for the purposes of making a densitometer. another way to look at it for color sensitometry is most of the older models used an incandescent globe sheathed in a blue filter to shift it to a 'whiter' source of light.. filters were then introduces to block as appropriate, to yield red, green and blue light - but it didn't matter really where the transmission curves actually occured for reference, only that they remain the same against calibration points. They could have had a 2 point system, 3 or 5.. 3 is just a good seperation .. After all, the light sensor most often used is a CdS photoresistor- which has a very non-linear response curve < see http://www.sunrom.com/files/3190-datasheet.pdf > so in normal systems you can see the blue and green responses would be way higher than the red - but again, calibration takes care of all that :) karl
