Can anybody here point me in the direction -- even of a textbook -- of good ways to measure the efficiency of a transmission diffraction grating? I've been trying to do it by comparing photographs of spectra from a single grating with that through a second grating. But if the second grating continues the same angular direction, the first order of the second grating perfectly overlaps the second order of the first grating. If I reverse angular direction, the spectrum gets linearly compressed to the point of becoming unusuable. I'm trying to do this on a shoestring budget -- not just for myself, but in the hopes that what I'm working on will be useful to a general audience. I'd ideally like to limit equipment to the sorts of things a photographer is already likely to have (such as a camera or a photographic light meter) or that you can cheaply buy at an art supply or hardware store or online, plus a typical graphic arts spectrophotometer (presumably not a colorimeter) such as the i1 or ColorMunki. The grating in question is cut from one of those 6" x 12" "novelty" plastic sheet gratings that, for example, Edmund Optics sells for $5 / each. The ideal end result would be a lookup table (or other function) that can be used to "idealize" the diffraction grating -- that is, take measurements, multiply the results by the factor in the table for the wavelength (or do whatever), and get what the measurements would have been if the grating were 100% efficient. High precision and resolution aren't necessary. The same sorts of tolerances used in graphic arts are fine. I'd like to get to ~3 nm and ~1%, but 10 nm and 5% would probably be "good enough." The setup I currently have clearly shows the separation between the sodium 577 / 579 doublet, which I'm sure is overkill. And, if this isn't the sort of thing that's practical to do without an expensive optical bench setup, I'd be interested in pointers to how to find somebody with such a setup. But I really would like to do it myself, if only for the same reasons kids do science fair projects.... Thanks much, b& P.S. After giving up on the transmission grating a while back, I spent a bunch of time trying to use a CD as a reflection grating. But I finally got to the point today that I could take pictures "for real"...and the rotary nature of the grating just causes much too much distortion. Thus, the circling back to the transmission grating. b& P.P.S. If a prism would be a better bet than a grating, I'm willing to go that route, too...but the ones I've played around with haven't had much dispersion. b&