Hi All,
Holly Snyder introduced me to the Diffraction Grid method of visually comparing the CRI (color rendering index) of light sources, and I got a grid sheet of my own to compare my findings with hers.
I use
JUST Normlicht Color Control Daylight 5000 fluorescent tubes in my studio. They are 36 watts, 5000K, and 48 inch T-8 tubes, and are rated at 98 CRI. This is as good as a fluorescent light can get.
I don't have a north light window to match, so 5000K is comfortably cool for my purposes. Marvin's argument for 6500K illumination seems valid for the purpose of matching a north light, which would be cooler due to the dominant illumination coming out of the blue sky.
For comparitive purposes, I've grouped four diffraction grid spectrums into one chart. From the left, the first two strips are from my camera, and the last two are from Holly's camera. I believe we both have the same camera (Nikon D100), and the same brand diffraction grid sheets. I shoud note that the spectrum looks very different to a camera than it does to the naked eye. Cameras have their limitations.
The 5000K Just Normlicht, and the 6500K Lumichrome appear to be about equal and genuinely 98CRI each. The 5000K Lumichrome, is not as good as the JUST Normlicht, as it shows more gapping in the spectrum. A Standard cool Philips tube is clearly way down on the CRI scale as there are wide black gaps.
So if you need to match north light, the 6500K Lumichrome tubes That Marvin uses look best. If you prefer 5000K, then JUST Normlicht is your best option.
Garth