The other night I was setting up my two cheap compact fluorescent lights for taking photos of my paintings. With the lights reflecting off our white wall, it was obvious that one appeared to have a pinkish cast and the other had a greenish cast. The lights are from two different manufacturers, one is 5000 K, 82 CRI, and the other 5100 K, 84 CRI. Even though the color temperatures are very close, with the CRI being so low, we figured that's why you could see the difference in color. My husband and I were talking (both of us have an electrical engineering background), and wanted to make a spectrometer for artists to measure the color of their light source. (They do exist but they're upwards of $1500.)
Then we realized that we had a diffraction grating, which separates white light into visual wavelengths. We started playing, and were excited at what we found. With the grating in between the camera and our light source, we were able to record a visual spectrum of different light sources. An incandescent bulb, which I think is generally around 100 CRI, showed a smooth spectrum of color on the grating, like a rainbow. A fluorescent, whose coated phosphors emit varying amounts of light at different wavelengths (i.e. lower CRI), showed discrete images of the bulb corresponding to each of the red, orange, green, cyan, and violet phosphors. An image of the two is posted.
I should note that the digital camera images are a bit different than what our eyes see, as they are also subject to the filters in the digital camera sensor. In the left side of image #1, the pictured spectrum of the incandescent source shows bands of red, green and blue wavelengths (the camera's filters). In reality its a beautiful continuous rainbow. Similarly in the right side of image #1, the spectrum of the fluorescent bulb in reality also clearly shows orange and purple discrete images (that don't show up in the digital image since they are between or beyond camera filters). If you look at image #2, and wonder why you can see orange and violet in the images, it's because the grating was close to the camera (to take our setup shot), and the camera is mixing the bands together.
You can probably guess that if a paint color has a high reflection in one of the dark areas of the spectrum of your light source, it will look substantially different than natural light. When I get some higher CRI bulbs, I'll post some new pictures. For now, I'll take pictures with incandescent bulbs, and color correct the images with a gray card in PhotoShop.
The upshot to all of this is, if you want an inexpensive qualitative way of measuring the CRI of your light source, all you have to do is hold up a diffraction grating in front of your light. A higher CRI should have more continuous colors. By the way, you can order diffraction gratings from
www.edmundoptics.com, part # 40267, two 6" x 12" sheets for $9.60. It's pretty cool.
Holly
Threaded Mode
