I know, what could be more exciting than to take a black and white photo of a rainbow. But, there's actually much to be learned from this, for example what wavelengths (colors) does the film actually record? To measure the spectral response of a collodion film, we first need to collect a solar spectrum using that film. The resulting image is then scanned, and with some spreadsheet magic, we generate an intensity profile for the pixels across that image. This profile describes the response of the collodion film to the various wavelengths of light found in the solar spectrum.