Exposing PseudoAstronomy

December 12, 2012

Being Pedantic Over Laser Colors on Law & Order: SVU


This is a quick post about me being a crotchety young man. But, one of the founding ideas of this blog was to point out not only bad astronomy/physics/geology that I see out there on crazy blogs and Coast to Coast, but also what I see in the media, on TV, and in movies.

I’ve been going through and watching old Law & Order: SVU episodes. I enjoy the series and watched it with my mom during part of high school and sometimes when home from college. Now in it’s 14th season, I have a lot of catching up to do.


I was watching season 5 episode 1 last night while doing some other work on the side (you can decide for yourself if that’s an intended or unintended dangling participle). About 31 minutes into the episode, a lab tech makes a big deal about restoring a receipt and the detectives are hoping it can lead them to a woman who had been kidnapped. Only problem is the receipt is saturated with blood and unreadable … under normal lights.

Good Premise

The lab tech makes a big deal about how it’s illegible “to the naked eye, that’s why God invented lasers. Different frequencies reveal different inks.”

This premise is true. In fact, over Thanksgiving, I was back in Ohio visiting with my parents and went to the Cincinnati Museum Center’s special exhibit on the Dead Sea Scrolls. They had a side room on their digital imaging process for documenting the scrolls and, “using technology developed by NASA” (people who don’t think the space program has any practical applications …), they showed how the fragments are all being imaged with 12 different colors of light: seven visible, five IR. Here’s the video that I had seen on it, and here’s a shorter version that just shows one scroll under the different wavelengths.

What the video pointed out is that several letters were not visible due to burns or dirt. But, under different wavelengths of light, the dirt becomes transparent while the ink remains opaque and you can read them.

This is exactly why astronomers use different wavelengths of light to study different things. Anyway …

Law & Order: SVU -- Laser Colors Mistake

Law & Order: SVU — Laser Colors Mistake

Bad Science

In what was obviously done for visual interest for television, the lab tech then goes through and supposedly illuminates the receipt under three different laser colors to try to bring out text. First, she says she’s using 400 nm, which shows up as bright blue, tinging on violet. Then 500 nm, which shows up as a ruddy orange. Finally, 600 nm, which is a brilliant green and they can read it and go and find the girl.

Anyone shaking their head right now?

If not, let me explain the first issue: Those colors are wrong for those wavelengths. I should know — I just purchased four lasers, one each at 405 nm, 460 nm, 532 nm, and 650 nm. The colors for those are deep purple, bluish-violet, green, and deep red.

As in, if those wavelengths she stated were the actual colors, 400 nm would have been a very deep purple, bordering on invisibility to the human eye (edge of human vision is somewhere around 380-400 nm). 500 nm should have been a turquoise blue-green. 600 nm would have been a yellow bordering on red (yellow sodium lights in parking lots are at 589 nm).

A second problem is that lasers are not made at those wavelengths. I guess it’s theoretically possible, and there might be some very rare laser of which I’m not aware, and while there are a large variety of lasers out there, 400, 500, and 600 nm are not among them.

My third of three problems with this is that lasers generally make a dot. You can have spreaders and put in gratings and whatever to make broader patterns, but generally speaking, they’re dots. This was basically like a broad light. What she showed, and what should have been used, and what she should have simply said, are that she was using a diode. Speaking as someone who’s been kept awake by the diode power lights on a computer case, diodes create a broad illumination, not a tiny point of light.

But I guess lasers sound cooler.

Final Thoughts

Yes, pedantic, nit-picking, etc. Does this have any bearing on broader society? Probably not.

But, then again, there are two issues here. One is that she was simply wrong. Portraying bad science or getting the science wrong is … wrong. It shouldn’t have happened.

The second issue is that someone might pick up on this and think that’s the way things work — that those are the colors that correspond with those wavelengths. And then it could take a long time for them to unlearn it. For example, I had an 8th grade science teacher who claimed a kilometer was longer than a mile (among other things). Three years later, I was in AP music theory class and, as usual, we were doing nothing, so I was complaining with a senior about incompetent teachers at our school. And I brought up the 8th grade teacher and units of length. And she exclaimed that because she had been taught that, too, by the guy the year before I had him, it screwed her up for two whole years. It took 10th grade chemistry and a sit-down with the chemistry teacher before she got everything straightened out such that she now knows a centimeter is shorter than an inch.

Perhaps an extreme example, perhaps an example that further illustrates the tiny effect that this doesn’t have, but it’s stuck in my mind.

And there are a lot less useful blog posts out there, and I’m tired about hearing about 12/12/12 and 12/21/12.

P.S. If I had to guess, I’d say that the “400 nm” was around 470, the “500 nm” around 600 or so, and the “600 nm” right on that classic green laser color of 532 nm.

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