Visually measuring CRI

[Garth Herrick]

Holly,

Thanks for your thorough and enlightening report on the Lumichrome fluorescent lamps. You have given me the courage to share my own light tests for comparison.

I am including a similar shot of my new Just Normlicht Color Control daylight 5000 fluorescent lamps, and for comparison the earlier Phillips lamps I had installed before. I was not able to get as far away from the lamps with my camera as you. My camera was about eight feet away, and the diffusion screen was about two feet closer. For the Just Normlicht shot, I had only one lamp lit, while for the Phillips shot, two were lit, which is obvious in the image. I too am surprised that the digital camera can only see the red, green and blue light in the spectrum. To my eye there was much more. What the camera cannot see are the deep intense blue-purples, the brilliant turquoise, the yellow range, and the deepest garnet reds.

Despite the shortcomings of my photography, the Just Normlicht T-8 5000 K lamp seems roughly comparable in its CRI index to the Lumichrome T-8 lamps in your test. The salesman claimed the Just Normlicht has a CRI of 98. Compared to the Lumichrome 5000 K lamp, it appears to be at least as good, as it looks to have the smoother spectrum graduations like the Lumichrome 6500K lamp.

Again thanks,

Garth

[Holly Snyder]

Garth,

Cool. The Just Normlicht spectrum does look quite similar to the Lumichrome's, but a bit better (smoother). Could it be that the spectrums are just a bit out of focus? If you used autofocus and focused on the diffraction grating edge, the image would be slightly out of focus, since these are kind of virtual images that exist somewhat behind the actual grating. I manual focused on the images as best I could.

Interesting comparison to the Philips. The Philips is much less smooth, that is, a lot of black gaps. It would be a fairer comparison however if you only had one Philips light on. Maybe we shouldn't have done this, but our ballast is for two lights, and my husband just rotated one light off briefly while I took the Lumichrome pictures.

In any case, these images don't do justice to the spectrum in reality, as the blue-purples, turquoise, yellows and deepest reds are missing. I think we have the same camera Garth, the D100? It is disturbing the camera can't capture these colors, but not completely surprising. The camera separates the light through red, green and blue filters before reaching the CCD. Maybe someday the technology will change and the digital filter system will be redone and based on the Munsell system? I'm not sure how these images would look if taken on a film-based camera?

For comparison's sake, I took a picture of the spectrum of northeast light (not that is really matters the direction of light) seen through a pinhole. The pinhole was just a piece of cardboard with a hole from a hole punch. You can see the smoothness of the gradient in the image below, but once again, in reality the image is a smooth (and perfect) rainbow of colors. At least this allows a benchmark comparison of the artificial lights against northlight, seen through the skewed eyes of a camera. Below is also a picture of the setup, with the pinhole in the cardboard surrounded by several pieces of dark cloth held in place with clothespins, so no light will interfere with the image. The grating was 5'4" from the point source of light and the camera ~9' from the light source.

The other artificial light images probably should have been taken using this point source method, as you can see the gradient better. Maybe I'll redo all of them, and place them side by side along with the northlight picture for comparison's sake.

Holly

[Garth Herrick]

Holly,

This is interesting and revealing. Even the pinhole spectrum of natural north light seems to lack yellow in the digital camera capture.

Holly, I was afraid to show my diffraction grid pictures until you showed yours first. The camera really didn't do the spectrum any justice. Seeing that your pictures looked about the same as mine made me realize the fluorescent lamps I bought really weren't all that bad after all.

Your photos are better because you were farther from the light source than I was and you had a black backdrop to photograph the spectrum against. Last night I did't have a handy black background, and I took the left half of the photo against a foil insulation ceiling in the shadow of the light. I took the right half several weeks earlier before I changed the lamps.

Actually I did manually focus on the spectrum, but for some reason it never quite appeared to be able to be in focus as well as my eyes. I was in a rush to add a post to your thread last night, so my photo was quick and dirty. I held the loose diffraction grid in one hand slightly arched so it would not collapse, and tried to focus and shoot the picture with my other hand. all while laying on the floor. So no wonder my photo looks worse than yours. I also slightly rotated one tube to shut it off for the photo. I hope that in the short term the dual lamp ballast is not harmful to just one lamp lit.

Thanks for initiating this spectrum analysis of (nearly) full spectrum lighting.

Garth

[Richard Monro]

Has anyone done any spectrum work on the Philips F32T8 TL950 fluorescent bulbs? They have a CRI of 98 and seem to be used extensively in the print industry. They also appear to be less expensive than other high CRI lights.