#diffraction grating

holo eclipse with diffraction grating! 🌈🌙☀️💥✨

[This is what the refrance]

Two categories here:

Pairing source and spectrum

The simplest trick here is that the blue end will point towards the source because blue light is diffracted least and red most. If you draw an imaginary line from the red end through the blue end and extend it out, it'll hit the source. You can also combine this with a small head-tilt, which will give you two imaginary lines that intersect at the source.

At first this will be very time consuming to path out, but you'll gradually internalize how the spectra move at varying distances and it'll become pretty quick, a lot of this is just about wearing it out and training your brain passively. I had a bit of a time driving with this in the first two or three days but it was completely fine thereafter.

For bigger sources the spectrum will also just look like the thing itself, so you can just go on shape.

Identifying lights

Easiest start here is just looking at some lights you know. Orange sodium vapour street lights, LED lights in your house, fluorescent bars in offices, etc. You'll learn their spectra pretty quickly, and if you see one you aren't sure about searching for "X spectrum" online will usually turn up graphs like below.

LED's are now most common and are easy to spot by the huge drop around cyan-ish blue:

Orange high-pressue sodium vapour is really easy to see on the road, and it's easily identifiable because it has a massive gap between yellow and orange.

In general you won't see absorption lines in things like sunlight, they're just too small and your eyes aren't high resolving enough.

If something looks really spiky and has a few very distinct lines, it's probably a gas emission like xenon or neon, or a metal halide. You can tell real neon signs apart from fluorescents and LED's cosplaying as neons by looking for very, very sharp edges to the spectrum. Phosphors in fluorescents and LED's have smoother rolloff.

Complete spectra with no holes are either the sun or incandescents or a handful of exotic specialist lighting solutions. A good place to look for exotic lighting solutions is big sports stadiums, which may have carbon arc lamps or interesting vapour lamps. If you're ever at the Smithsonian Air and Space Museum or EPCOT, look at the overhead lighting, some of it is 3M light tubes fed from three extremely bright sulphur microwave plasma lamps, which almost perfectly imitate the visual spectrum of natural sunlight.

Three very distinct red, green and blue spectra means RGB lighting, either directly or because you're looking at a modern computer screen. Older screens may still be fluorescent illuminated, in which case you'll see five or six phosphor colours like this.

That's also what fluorescent tubes and CFL bulbs will do.

My brain has wired in tracking lights with the grating hard enough that I keep instinctively trying to pick up emission spectra for bright objects and the lack of feedback makes me go through almost a quarter turn before I catch myself. Fortunately the lines come in at the same depth of focus as the light in question so I doubt it’s putting any undue strain on my eyes.

I do want to go get a near-UV laser pointer for getting emission spectra off common objects but probably gonna have to wait until shipping and post get back to normal.

Here’s a photo I took through it a while back showing how a setting sun has more or less the full spectrum, but with all the blue stripped out because it’s all going to become daylight closer to the horizon. (the thin spectrum on the right just below the horizon is the one that matches the sun)