Do neon signs emit x-rays?

Question

I'm interested about whether or not a conventional neon sign such as this one could emit x-ray that could be harmful.

The sign that you can see in the picture is just below my office window so I'm basically sitting like 8 hours a day about 2 meters away from the sign which is functioning the whole day. Just curious if this can cause any harm of any kind because of the long exposure.

If you are concerned, a Geiger counter does respond to x-rays and you can buy them where they are pretty sensitive. They won't tell you what energies -- they just "click." But they can provide a "measurement" of sorts. You could set one up where you sit and let it count for you for a day or so. Then, the next day, move it twice as far away from the sign (or further) and let it count for exactly the same period of time. If you see a significant change between the two summed count readings, and the closer one is much larger than the distant one, then you may have a more valid concern. — jonk, May 19 '22 at 19:11
Thank you everybody. So I guess that if the sign doesn't emit xrays is some other kind of radiation that it can produce that I should be concerned about? Just asking because I sit close to it for a long time every day — Raaas, May 19 '22 at 22:38
Light is a form of radiation. But it's ionising radiation you need to be concerned about, and the background radiation you get exposed to all the time dwarfs anything you'll get from a neon sign. — Finbarr, May 20 '22 at 09:17
You are probably closer to neon tubes on the office ceiling that are the same, just white in color not green. Millions of people spend many hours a day for years with neon lights over their heads and there is no difference with others living in other conditions. — Joan, May 20 '22 at 14:26
@Joan green "neon" tubes are probably argon-filled, while the lamps in an office ceiling use tiny amounts of mercury (mixed in to other gases) that then excites a phosphor coating on the glass. So chemically and spectrally very different, even though your conclusion holds — Chris H, May 20 '22 at 15:46

Math Keeps Me Busy · Answer 1 · 2022-05-19T18:39:01.363

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Neon signs do not produce X-rays. Although they operate at high voltages, the gas within the tubes prevent electrons from gaining enough energy to produce X-rays.

However, if one wants to split hairs, all emission of electromagnetic waves is probabilistic. Even an incandescent bulb will produce a negligible amount of X-rays. Maybe one photon per second, or maybe one photon per year. I don't know the rate but it is very small compared to the more or less astronomical number of photons produced by an X-ray machine. I would guess that the amount of X-ray radiation you would receive from a neon light might well be less than what you would receive sitting under a beach umbrella on a sunny day.

Edit: I just did some back-of-the-envelope calculations to estimate the power of the X-rays emitted by an incandescent bulb.

Assume an incandescent bulb emits black body radiation according to Planck's Law, and is at 2700 Kelvins. Assume further that X-rays consist of photons with frequency 3 x 10^16 Hz.

To find the relative intensity of X-rays to yellow light, I divided the Planck's Law result for X-rays by that of yellow light and get this approximation.

$$B_{rel} = e^{\left(\frac{h}{k_{B}T}\right)\left({f_{yellow}-f_{x-ray}}\right)}$$

where \$h\approx 6.6 \times 10^{-34} joules\$ is Planck's constant. \$k_{B}\approx 1.38 \times 10^{23} [complicated-units]\$ is Boltzmann's constant, \$T\$ is the temperature in Kelvins.

$$\frac{h}{k_{B}} \approx 4.8 \times 10^{-11} sK$$

$$\frac{h}{k_{B}\times 2700K} \approx 1.778 \times 10^{-14} s$$

$$e^{\left(1.778 \times 10^{-14}\right)\left(3 \times 10^{16}\right)} = e^{\left(5.333 \times 10^2\right)} \approx 4 \times 10^{231}$$

So, (with a bit of further approximation) there is somewhere around \$4 \times 10^{231}\$ times more yellow light emitted from an incandescent light bulb than x-ray radiation. AFAIK, that is more than the number of atoms in the visible universe.

This was just a fun calculation and not meant to be precise.

edited May 19 '22 at 18:39

answered May 19 '22 at 17:23

Math Keeps Me Busy

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Come on, the second paragraph is on par with a probability that vacuum can spontaneously generate pairs of particles, including X-ray level photons. – Eugene Sh. May 19 '22 at 17:26
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This calculator says that a 1 cm^2 5000K thermal source generates an X-ray photon about once every 10^100 seconds. Not sure I trust the floating point precision there, but seems very unlikely you will ever see an X-ray photon from a light bulb. Link: https://www.opticsthewebsite.com/OpticsCalculators – user1850479 May 19 '22 at 18:02
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@EugeneSh. Lol. I just did a back of the envelope calculation for the x-rays emitted by an incandescent light bulb compared to the yellow light. The amusing result is found in an edit I just made. – Math Keeps Me Busy May 19 '22 at 18:42
@user1850479 I agree. I just did a back of the envelope calculation. Result in edit. – Math Keeps Me Busy May 19 '22 at 18:43
Thank you so much for such a detailed answer! I assume there's nothing to be worried about spending so much time near the sign then? – Raaas May 19 '22 at 19:00
@Raaas If it is not making you physically uncomfortable. I remember sitting in an open-space office, and then one day they brought a photographer to make some promotional materials, so this guy was taking photos all day with a flash. I got some serious pain in the eyes and redness for couple of days afterwards. – Eugene Sh. May 19 '22 at 19:04
No need to worry about X-rays. Fluorescent lights, however, do emit ultra-violet light. In "white" fluorescent lights, most of that UV is absorbed in the white coating in the tube. Neon tubes are a little different. They are not made to emit UV light, but it is possible they do emit some anyway. I haven't done the calculations, but a neon bulb emitting a UV photon is not as ridiculously improbable as a neon bulb emitting an x-ray. My guess, and it is only a guess, is that neon bulbs emit less UV than the white fluorescent's. – Math Keeps Me Busy May 19 '22 at 19:09
Thank you everybody. So I guess that if the sign doesn't emit xrays is some other kind of radiation that it can produce that I should be concerned about? Just asking because I sit close to it for a long time every day – Raaas May 19 '22 at 21:24
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@Raaas Although I don't know the science well enough to make an airtight case that the UV radiation from a neon sign is nothing to worry about, I am neverthess confident that the UV radiation from neon lights is negligible. If you want an airtight case, I suggest you ask about UV radiation from Neon lights at Physics Stack Exchange. There are no other forms of harmful radiation that could come from a neon light. Some people, however, find electrical hum and the flickering of neon lights (not always noticable) to be low grade psychological stressors. – Math Keeps Me Busy May 19 '22 at 22:38
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Thank you. Lets say I cover the window so the neon light doesn't get to me. Will that stop the UV radiation coming to me? I mean, the only way to receive UV radiation is to being directly exposed to it as we are exposed to sunlight? – Raaas May 19 '22 at 22:48
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Yes. If there is any UV to begin with, pretty much anything opaque will block it. I think even glass blocks some, because I have heard sunlight through glass is not efficient for vitamin D production. But that may be viral misinformation. I haven't researched it. – Math Keeps Me Busy May 19 '22 at 23:08
@Raaas Almost everything blocks UV to some extent. That's why UV-pass filters are so expensive. – DKNguyen May 20 '22 at 00:12
A thermal model is not relevant to looking at the photon energies of a discharge lamp. – Loren Pechtel May 20 '22 at 04:28
@LorenPechtel Agrred. That is why when I referenced Planck's Law etc. I referred specifically to incandescent bulbs, and stated in a comment that I lacked the scientific expertise to answer whether neon signs emit UV. – Math Keeps Me Busy May 20 '22 at 10:50
Black body radiation is a sidetrack not closely related to the question. I've expanded my answer. – Perry Webb May 20 '22 at 13:43
Thank you everybody for such detailed responses and help. Does the fact that the neon sign has a lightning pattern going on similar to what you can see here: https://www.youtube.com/watch?v=n0zPR8Yd31A&ab_channel=NeonRoyke makes anything different? – Raaas May 20 '22 at 19:31
Why would the lighting pattern make any difference at all to the emissions? – Cody Gray - on strike May 21 '22 at 00:41
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To everyone upvoting my answer. It's really not that great. The answer by @PerryWebb is, in my opinion better. I just happened to be first. Please upvote that answer. I did. – Math Keeps Me Busy May 21 '22 at 12:26

Perry Webb · Answer 2 · 2022-05-21T17:45:48.083

High energy electrons produce x-rays when they strike atoms, usually metal. That's how a scanning electron microscope can detect elements. Basically, you need electrons accelerated with a high voltage and a long mean free path to get x-rays. Cathode ray tubes (CRT) produce x-rays when the electron beam strikes the phosphor. As Math said the noble gas in neon lights makes the mean free path of the electrons too short to gain the energy for the electrons to produce x-rays.

As shown with the black body radiation equations given by Math, to produce x-rays based on the temperature of the object takes temperatures well above our normal capabilities. That is not how we produce x-rays, nor is it how light is produced by neon lights.

Neon lights produce light by electrons interacting with the electron shells of the noble gases atoms. When the electrons lose energy, the interaction with the electron shells produces a photon with the energy equal the amount of energy lost by the electron.

We produce x-rays in a similar fashion, but consider this.

An electron accelerated across a field of 1V has an energy of 1 electron volt (eV). Thus, the upper limit of the energy in eV produced is the voltage of the power source.
Electrons cannot lose more energy than they have. Thus, cannot produce photons with a higher energy than they have.
X-ray photons have energies in keV (>1keV).

Figure 1. Photon energies of the electromagnet spectrum (Taken from "Μελέτη της διακριτικής ικανότητας μηχανής ακτίνων-Χ με τεχνικές προσομοίωσης Monte-Carlo," p. 16)

Thus, the power source to produce x-rays needs to be greater than 1000V.

Neon signs can be operating with 2V to 15kV, but for an electron to gain the energy of the power source, it needs to be accelerated from the negative electrode and strike the positive electrode without colliding with atoms, causing energy loss. In a neon sign electrons collide extensively with noble gas atoms. As the electrons lose energy in the atom's electron shells, they produce visible light photons (1.75 to 3.1eV). The unobstructed path between the electrodes to produce x-rays isn't there for neon signs.

Do neon signs emit x-rays?

2 Answers2