Scoping out lamp spectra

kw: analytical projects, spectroscopy, photographs

Technical photography of biological subjects has its tricky aspects. An important one is the quality of the lights. This picture shows one setup I have used to avoid the use of the compact fluorescent lamps (CFL's) that were brought in to replace the incandescent flood lamps they had been using, but which burn out on a regular basis…and they are hot. CFL's have serious drawbacks for color photography, which we'll see later on.

The little blue lamps, wrapped in tissues, are "work lights" from Harbor Freight, with 18 small LED bulbs each. They have a pretty good spectrum (I'll point out a similar one below).

We have been considering a more quantitative approach, particular for photos of birds. Few mollusks have "interesting" colors, but nearly all birds do. Birds have an extra color sensor in their eyes and can see ultraviolet light, so it is also of interest to be able to photograph birds in UV light.

I decided to explore the various lamps available, not only for these reasons but a few extra ones. In recent years I used pieces cut from a page-size sample of diffraction grating, obtained from Edmund Scientific many years ago, to make two spectroscopes. One, about a foot long, is for handheld use, and another, about twice the size, is for use with a camera.

I originally made them to investigate which yellow lamp would be the best "bug light". Night-flying insects see UV even better than birds do, and many of them cannot see yellow or red light, and have low sensitivity even to green. The ideal bug light would have a cutoff in the yellow-green range, and would be rather orange. In order to keep people on your doorstep from looking too weird, all commercial bug lights include some green and look distinctly yellow.

This shows a test setup with the large spectroscope on my workbench, using a black light as the source. Not only does the camera "see" UV, it is also being overloaded by the bright blue and violet lines of mercury (Hg). That black light bulb is not nearly this bright to my eyes. The camera at the left is set back a little farther than usual; I usually use it with the front of the zoom lens about two inches from the grating, which is at the end of the white bell-shaped "front" of the spectroscope. The "slit" is currently a thin slot sawn into a PVC cap at the end where the lamp is.

Initial tests verified that the camera's sensor can record UV and that the lens passes it, and also that none of the lens elements is fluorescent (a problem with the lens of another camera I have!). I also learned that the "UV filter" I was sold with the camera does not block UV-A (longwave, or near-UV). It does block short-wave UV, that is UV-B and UV-C, pretty well. So I bought a better filter, a UV(0) filter from Hoya. Now to the recent batch of tests, summarized in this image, a screen shot from PowerPoint:

Depending on your monitor, this might be hard to read. To summarize, the top shows the spectrum of a "black light" CFL (a "party light"), at two exposures; the next 11 sets are at three exposures each. From the top, then, we have

• two sets for different hues of fluorescent tube, 
• two sets for incandescent bulbs, 
• two sets for different hues of CFL, 
• a yellow CFL "party light", 
• two commercial "bug light" bulbs, and 
• two sets for LED bulbs from different manufacturers; the spectrum of the work lights from Harbor Freight is a little bluer than the bottommost set.

Only the black light has a spectrum that includes a strong UV line at 365 nm. Most of the lamps have a cutoff near 420 nm, though a couple of the CFL's let through a little deeper blue and the UV line. The spectra of the CFL's show very strong lines with darkness in between, which is why these lamps have poor "color rendering", as it is called. LED's, as shown at the bottom, come the closest to mimicking the spectra of incandescent lamps.

My preliminary conclusions are (1) that for ordinary color photography, LED lamps are the best choice among the "non-incandescent" ones, and (2) to get good UV images we'll need to use black light CFL's, probably with a visible-blocking filter. It may also work to use UV LED flashlights like the ones used by TSA at airports, though they are a bit costly, because they don't produce any visible light.