On Oct 11, 2014, at 7:01 AM, Brad Funkhouser <brad.funkhouser@xxxxxxxxxxx> wrote: > So, is there any realistic way to approximate my camera's spectral response > curves? I have a lot of mixed news for you. First, the good: you're groping towards the right answer, and, indeed, once you're there, your immediate concern becomes moot. And it's all perfectly doable. The bad: it's not particularly well-trodden territory. The not-so-bad: I'm pretty far down the trail and hope, before too terribly long, to have some straightforward solutions. Basically, what you're aiming for is to photograph a known spectrum. The common way to do this is with a monochromator whose output you measure and photograph at each slice of the spectrum at whatever resolution you're looking for. I'm pretty close to an easy and cheap way to do it with the entire spectrum in one photograph, provided you have an i1 Pro or equivalent instrument. You'd build a basic spectroscope and photograph the spectrum it projects...but the devil is in the details. Specifically, you need to know the distribution of the spectrum that gets generated, which is either going to involve equipment that costs more than your mortgage or some cruder measurements and math...and it's that latter part I'm working on now. Specifically, I'm building a monochromator so I can measure the transmission efficiency of the grating film I'm using. But, once I've got that, a bit of spreadsheet work should be the only other remaining piece of the puzzle. The final workflow will basically be getting an ambient measurement of the illuminant and a reflective measurement of the diffuser you're bouncing your illuminant off of and into the spectroscope...and then photograph the spectrum, treat it like a chart with Argyll with a custom set of reference files, and feed the .ti3 to a spreadsheet for a bit of mangling before handing it back to Argyll to generate your profile. When it's all put together, you won't need to worry about how good a chart you can make with your printer; you'll already have the ideal chart, limited only by the quality of your process control and your equipment. Though I've spent far more on experimentation, the final bill of materials for those who use the same diffraction grating and who trust my measurement will be all of about $10; those who want to build a monochromator to get their own measurements would be looking at three or four times that cost. And the equipment, though far from laboratory grade by any modern sense of the term, surpasses the state-of-the-art from a century or so ago when all the famous groundbreaking work was done in the field of spectroscopy. Also, in practice, high-end modern DSLRs have spectral sensitivities that are "good enough" that you generally don't have to worry about different metameric interpretations between the camera and the standard observer. They're not perfect, of course, but the differences between the camera and the standard observer are less than other errors in the system. And, if you _really_ need to worry about that sort of thing, then you can get into the realm of multi-spectral imaging by shooting your work through various carefully-selected color filters and applying some high-powered math; see the work of Dr. Roy Berns at the Rochester Institute of Technology for all the necessary peer-reviewed publications. Hope this helps, and that you don't mind the teaser...and, now, if you'll excuse me, I need to figure out how I'm going to mount the diffraction grating in the monochromator.... Cheers, b&
Attachment:
signature.asc
Description: Message signed with OpenPGP using GPGMail
