I was ready for my brain to let go of this whole light comparison thing when suddenly, from behind a bush along the sidewalk, the thought occurred to me: “What about strobe light?”
We know from last time on this subject that incandescents – or the sun or any black-body radiators – are true full-spectrum sources of light, unlike compact fluorescents which throw most of their light at certain peak wavelengths.
As photographers we love us our strobes and consider them ideal, if not necessary. So what kind of light do those babies put out? They work kind of like fluorescent tubes. The gas inside is Xenon, though, and there are no phosphors coating the tube as the light Xenon emits when excited by electricity is visible.
MORE SPECTROGRAPHIC ANALYSES
I did some more of that *extensive research* and found The Double Amici Prism Hand-Held Spectroscope page wherein Mr. Galidakis shares his spectrographs. He has some for incandescent and CFL bulbs as well as for Xenon, which I’ve included (below) for comparison to those from the previous article.
These analyses are slightly different. Last time we saw data plotted from a spectrometer. In these, a digital image is taken from a spectroscope showing the various light spectra, seen on the left. On the right we see a graph showing the intensity of the pixels representing the wavelengths of light seen in the image.
Although these are not calibrated to show wavelengths and may not be as accurate as the spectrometer output we saw earlier, they’re more than enough for us:
© I.N. Galidakis
© I.N. Galidakis
© I.N. Galidakis
As we can see, a Xenon strobe such as we have in most of our photographic lighting doesn’t make full-spectrum light either, though it comes a lot closer. We know that strobes make perfectly fine light sources for our photography needs, so if I wasn’t entirely satisfied that CFLs are fine for photography before, I am a lot more open to the idea now.
COLOUR RENDERING INDEX
I would like to draw attention to this Wikipedia article on the Colour Rendering Index. “The Colo[u]r Rendering Index (CRI) … is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source.” This basically describes what we’ve been looking for all along.
The CRI is a scale from 0 to 100. At the high end of the CRI we have incandescent lights (100) and other black body radiators that make great light. On the low end we have crappy sources of light like high-pressure sodium (24). Imagine trying to look at something colourful like a Gretag Macbeth colour chart under a sodium vapour streetlight. Not exactly what you want to be lighting your subject with, yes?
Typical fluorescent light sources score in the 70′s and 80′s on the CRI, according to the chart of typical values on Wikipedia. Xenon strobes aren’t listed but more *extensive research* quotes them at or above 95. I also found that, while your typical dollar-store CFL might have a low CRI, there are stores selling CFLs with CRIs in the 90s.
 Much thanks to Jim Talkington of ProPhotoLife.com, David Hobby of Strobist and others like them who teach that light is light, and that expensive gear is no substitute for thoughtfulness and creativity.
 I suppose a spectrometer works the same way, breaking the light apart with a prism, capturing it on a digital sensor and analyzing it. I assume that the spectrometer’s data is calibrated and more accurate than those from the spectroscope shown here.
 For those playing along at home who haven’t read the earlier CFL vs. Incandescent article, this is me obfuscating what is basically a bit of Googling and web surfing. Sounds more impressive this way, don’t you think?