Circling the color wheel

 kw: color studies, spectroscopy, colorimetry, spectra, photo essays

Recently I was cleaning out an area in the garage and came across an old lamp for illuminating display cases. The glass bulb is about a quarter meter (~10") long. It's been hiding in a box for decades, ever since I stopped trying to keep an aquarium. It has a long, straight filament, which makes it a great source of incandescent light for occasional spectroscopic studies I like to do.

(The metal bar seen here is the filament support. The filament itself is practically invisible in this photo.)

This prompted me to rethink the way I've been setting up spectroscopy. Before, I had a rather clumsy source-and-slit arrangement. I decided to try a reflective "slit", that is, a thick, polished wire. As a conceptual test I set up a long-bladed screwdriver with a shaft having a diameter of 4.75mm. It isn't as badly beat up as most of my tools, and the shaft, some 200 mm long, is fresh and shiny. Based on these tests, I can use a thinner wire, in the 1-2 mm range, for a sharper slit. Later I may set up a lens to focus light on the wire for a brighter image.

I threw together a desk lamp and baffle arrangement, put the camera on a tripod with a Rainbow Symphony grating (500 lines/mm) mounted in a plastic disk that fits inside the lens hood, and produced these spectra. I also made a test shot with my Samsung phone and the grating, to see if I had sufficient brightness. Then I put various bulbs in the desk lamp and shot away. Here are the results. Each of the photos with my main camera shows the "slit" (screwdriver) along with the spectrum, to facilitate calibration and alignment of the spectra. Not so the cell phone image, which I fudged into place for this montage. The montage was built in PowerPoint.

The first item I note is the difference in color response between my main camera and the cell phone. The camera's color sensor cells have very little overlap between the three primary color responses, red, green and blue, so the yellow part of the spectrum is nearly skipped. The rapid fading in blue is a consequence of the very small amount of blue light an incandescent lamp produces. The cell phone sensor has more color overlap, more similar to the eye.

The two spectra in the middle of the sequence are both of mercury-vapor compact fluorescent bulbs. The white light bulb takes advantage of a few bright mercury emission lines, and adds extra blue, yellow, and orange colors with phosphors, which are excited by the ultraviolet (filtered out and not seen) and by the deep blue mercury emission line that shows as a sharp blue line. In the UV lamp, a "party light", the ultraviolet line at 365 nm is the point, and visible light is mostly filtered out; just enough is allowed out so that you know the lamp is on. There is also a phosphor inside that converts shortwave UV from mercury's strongest emission line as 254 nm to a band in the vicinity of the 365 nm and 405 nm lines; it shows as a blue "fuzz" here. The camera sensor has a UV-blocking filter, which doesn't quite eliminate the 365 nm line, so you can see a faint violet line where I marked it with an arrow. The emission lines visible in this spectrum are:

• 365 nm, near UV
• 405 nm, deep blue
• 436 nm, mid-blue (barely visible, directly below the mid-blue line shown in the Compact Fluorescent spectrum)
• 546 nm, green
• 577 & 579 nm, yellow, a nice doublet, and I'm glad the system could show them both
• 615 nm, red-orange, quite faint

I was curious to see if my "bug light" was really filtering out all the blue and UV light, and it seems that it is. There are still insects that get attracted to it, probably because they see the green colors. The "warm white" spectrum shows that the blue LED excitation wavelength is at about 415 nm, with a width of about 20 nm. Modern phosphors used in LED bulbs are quite wide band, as we see here, which makes them much better for showing true colors than the CFL bulbs we used for several years.

With a bit of careful looking, we can see that the LED bulbs don't have red emission quite as deep as the incandescent lamp does. That is the reason that for some purposes specialty lamps such as the CREE branded bulbs have a special phosphor formula with a longer-wavelength red end.

I also got to thinking about the way most of us see colors these days, on the screen of a computer or phone. The digital color space contains exactly 16,777,216 colors. Each primary color, R, G, and B, are represented as a number between 0 and 255, although they are very frequently represented as hexadecimal numbers from #00 to #FF, where "F" represents 15 and "FF" represents 255. The fully saturated spectral colors, also called pure colors, for which at least one of the three primaries is always #00 and at least one is always #FF, are then comprised of six sets of 255 colors, for a total of 1,520 virtual spectral colors…except that 2/3 of them are red-blue mixes that are not spectral colors. They are the purples. Note that violet is the bluest blue and is not considered a purple color, at least in color theory. The rest of the sixteen million colors have values "inside" the numerical space defined by the "corners" of the RGB space.

I prepared a chart of the pure colors, a dozen sections of the full "color wheel", which we will see is actually a color triangle. The RGB values for the end points of each strip are shown at their ends. "7F" equals 127, halfway from 00 to FF. They are separated as to spectral colors and purples.

To name the twelve colors at the ends of these sections, in order, with full primary colors in CAPS and the halfway points in lower case:

RED - orange - YELLOW - chartreuse - GREEN - aqua - CYAN - sky blue - BLUE - purple - MAGENTA - maroon - and back to RED.

To see why I spoke of "color triangle" let us refer to the CIE Colorimetry chart, based on publications in 1931 that are still the definitive work on human color vision. I obtained the following illustration from Wikipedia, but it was low resolution, so I used Upscayl with the Remacri model to double the scale.

There is a lot on this multipurpose chart. Careful work was put into the color representations. Though they are approximate, they show in principle how the spectrum "wraps around" a perceptual horseshoe, with the purples linking the bottom corners. The corners of the white triangle are the locations in CIE color space of the three color phosphors in old cathode-ray-tube TV sets. The screens of phones or computers or modern television sets use various methods to produce colors, but all their R's cluster near the Red corner of the diagram, all the B's cluster near the Blue corner, and all the G's are in the region between the top tip of the white triangle and the tight loop at the top of the horseshoe. Getting a phosphor or emitter that produces a green color higher up in that loop is expensive, and so it is rare.

I added bubbles and boxes to the chart to show where the boundaries of the colored bars are in the upper illustration:

 

I think this makes it clear that the "color wheel" we all conceptualize turns into a "color triangle" when it is implemented on our screens. All the colors our screens can produce are found inside the triangle anchored by the R, G, and B color emitters.