Light box

[Rich G.]

What is the best light box for negative and transparencies.

[Bob Salomon]

A properly color balanced one whose output is designed for photographic results and not something designed for other fields, like medical!

[Drew Wiley]

If it's too cheap to be true, it is too cheap to be true. Just Normlicht was a good brand. I made my own light boxes.

[LabRat]

A Macbeth is a photo lab standard, but not sure if still available. ..

Steve K

[Rich G.]

Thank you all.

[Drew Wiley]

Macbeth is still in business, but has been bought out by XRite. The keyword would be "Macbeth Proof Light". Pricey, but the
real deal.

[Rich G.]

thank you , I found a used McBeth pl-214 on EBay

[jim10219]

You can also build your own. That’s what I did. I made mine our of a project box, a piece of plastic that acts as a diffuser, and lined the box with aluminum foil attached with spray glue. Then you just get some LED strips and line over that, being careful not to let them short out against the foil. I used liquid electrical tape for that. Lastly I ripped up an old DC converter and hooked it up to a switch to power the thing. I used a combination of warm and cool LED’s for better color. I basically made it for free using spare parts I had on hand. If I had to buy the parts and wanted to get really color accurate, I would probably have used a mixture of red, green, and blue LED’s, like what they use in high end scanners these days. If you wanted to get really technical with it, you could use a DSLR’s histogram to calibrate the color balance.

I’m not saying you have to go the DIY route, but it is a fairly easy project.

[Bob Salomon]

You can also build your own. That’s what I did. I made mine our of a project box, a piece of plastic that acts as a diffuser, and lined the box with aluminum foil attached with spray glue. Then you just get some LED strips and line over that, being careful not to let them short out against the foil. I used liquid electrical tape for that. Lastly I ripped up an old DC converter and hooked it up to a switch to power the thing. I used a combination of warm and cool LED’s for better color. I basically made it for free using spare parts I had on hand. If I had to buy the parts and wanted to get really color accurate, I would probably have used a mixture of red, green, and blue LED’s, like what they use in high end scanners these days. If you wanted to get really technical with it, you could use a DSLR’s histogram to calibrate the color balance.

I’m not saying you have to go the DIY route, but it is a fairly easy project.
How,do you correct for the overly blue color of the aluminum? For photo you should be around 5500K,with a CRI,of 95,or,higher,how do you measured that?

[jim10219]

How,do you correct for the overly blue color of the aluminum? For photo you should be around 5500K,with a CRI,of 95,or,higher,how do you measured that?

You don't worry about the blue of the aluminum. Balancing the RGB LED's takes care of that. And achieving a high CRI is easy with RGB LED's. 95 is very achievable with 99 being possible. CRI basically just measures RGB light, so by creating white light our of RGB's, you're essentially cheating the test. In fact, that's why TLCI was invented. The LED's that get a bad CRI rating aren't made from an RGB array. They're made from blue LED's that are mixed with yellow and red producing phosphors to mimic white light to our eyes. That's why the RGB method is what's used in scanners and such. True white LED's don't exist.

If you want to get a little more technical with it, you can put each color of LED on it's own circuit leg, and control the voltage (and thus intensity) applied to each color through a set of resistors or a potentiometer. As for the color temperature, you just take a photo of the light box with your digital camera and view the histogram. With the white balance set to 5,500K on the camera, all of the peaks in the photo's histogram will line up with each other when the temperature of the lightbox output reaches 5,500K.

[Drew Wiley]

All kinds of homemade light boxes are fine for sorting out transparencies etc. But critical color evaluation or backlit copy work just isn't possible with current LED illumination. The spectrum is far too uneven; and attempting to fool the eye by mixing the bulbs won't save a person from serious metamerism errors. Appropriate liner paints can be purchased or made by an expert who knows how to correctly use a spectrophotometer, and people like those (including me) are far and few between. Then one has to factor in the inherent bias of any diffuser and surface glass. Then finally, a good color temp meter is invaluable. Way more to it than just buying properly rated bulbs, of which the real deal are specialized, uncommon, relatively costly, and
of mature technology rather than of an adolescent LED nature. I don't like raining on your parade, Jim, but you're apparently unaware of what kind of equipment and training serious color evaluation actually involves. Outfits like Macbeth and XRite specialize in it. Your concept of RGB producing white light is theoretically correct, but like I hinted, there's a lot more to it
than just fooling the eye. CRI is based upon the percentage of specially selected color samples that can be rendered without
noticeable error or metamerism, not just the approximate RGB peaks, which are probably not accurate with current LED options anyway. You need something much more closely mimicking a blackbody continuous light source. And correctly measuring such things requires a helluva lot more than digital camera histograms can provide. Incidentally, the industry standard for color evaluation, as used by Macbeth etc is 5000K. The option of 5500 is given by Kodak in relation to mean
daylight color temp for film EXPOSURE purposes, though there seems to be some deviation from this among film manufacturers. But the problem in question here is not unexposed film, but how to best analyze already exposed and developed film. You're confusing these two kinds of problems. Simple tri-color breakdown of color is basically WWII technology and terrible inaccurate. True continuous spectrophotometers read the entire spectrum but were fussy to maintain.
Modern spectrophotometers typically use xenon flash tubes for an evenly distributed range of measurements (typically 12, 16, or 24 points), then interpolate. Hypothetically, the more the better; but engineering-wise and cost-wise, certain compromises are inevitable. The first spectrophotometer I worked with cost around $55,000. My wife operated a custom X-Rite model in biotech that cost six million; but at 40K per ml of end product, that was well justified.

[Bob Salomon]

All kinds of homemade light boxes are fine for sorting out transparencies etc. But critical color evaluation or backlit copy work just isn't possible with current LED illumination. The spectrum is far too uneven; and attempting to fool the eye by mixing the bulbs won't save a person from serious metamerism errors. Appropriate liner paints can be purchased or made by an expert who knows how to correctly use a spectrophotometer. Then one has to factor in the inherent bias of any diffuser and surface glass. Then finally, a good color temp meter is invaluable. Way more to it than just buying properly rated bulbs, of which the real deal are specialized, uncommon, and relatively costly.

+1

[Rich G.]

Thanks