Light dependent resistors can be used to measure quantatively the intensity and the colour of visible light, as their spectral sensitivity peaks somewhere in the green region, where also the human eye is most sensitive. The resistance of the semiconductor material decreases with the intensity in a non-linear fashion, which is close to a power law:(I/I0) = (R/R0)^(-gamma)where R0 is the resistance at the intensity IO. The constant gamma is given in the data sheet of the device, and is usually around 0.6 ... 0.8.For our application, it is not necessary to have an absolute measure of the light intensity. We want to know how much the light at heighthhas diminished with respect to the value on the Earth's surfaceI(h)/I(0) = (R(h)/R(0))^(-gamma)Since gamma is a positive number, a drop in intensity always means an increase of resistance. And once we know gamma, we can compute intensity ratio from the resistance ratio.How does one measure colour? By comparing the light viewed through a colour filter, say green (G), with the intensity obtained without the filter, i.e. in white light (W):I(G)/I(W) = (R(G)/R(W))^(-gamma)If the light source emits only green light, all of which will be measured both without and with the green filter, and the green resistance will be nearly the same as the white resistance, or a bit higher, because the green filter may absorb some light. If the light source emits in the red, the green filter does not pass anything, so the resistance with filter will be much higher than without... So the ratio of the green and white resistances gives a measure how green the incoming light is. And so on with other filters.Again, if we know gamma, we can express this in intensity ratios. But as the passbands of the various filters are not perfect and absorb in a certain wavelength range, such knowledge would only be of use, if we knew the exact wavlength dependences of the filters, and if one wanted to compare the measurements quantitatively with model predictions. However, for a relative judgment of how the colour of the sky changes with height or with conditions, it suffices to calibrate such a photometer with light of various colours and keeping a record of the resistance ratios.