1
$\begingroup$

I have an LED lamp from IKEA very similar to "JANSJÖ" (see https://www.ikea.com/ca/en/p/jansjoe-led-usb-lamp-black-70291232/). One day, I noticed that white paper illuminated by the lamp appeared mildly violet. To me, this would indicate that there is UV-radiation coming from the lamp, that gets converted to blue and violet light through the optical whitening agent of the paper. Need I be concerned about this UV-radiation? I decided to wrap a printer paper around the lamp to further investigate this. The image shows the result of this experiment, and there is quite a lot of violet light. Of course, one would need to make a spectroscopic measurement for a more quantitative investigation, but to me, even this qualitative result looks quite concerning.

enter image description here

$\endgroup$
10
  • $\begingroup$ It is hard to say much without knowing what wavelengths are present in your lamp. But in general, you should protect yourself from UV. Here is some info. cdc.gov/nceh/features/uv-radiation-safety/index.html $\endgroup$
    – mmesser314
    Jan 18, 2021 at 20:28
  • $\begingroup$ Are you aware of the effects of UV outdoors? At the beach? When skiing? $\endgroup$
    – user137289
    Jan 18, 2021 at 20:28
  • 1
    $\begingroup$ @Pieter I am aware of that, yes. However, I'm not outdoors very much. I am not concerned about getting skin cancer from the lamp, but I am concerned it might hurt my eyes in the long term, if I use the lamp daily. See e.g. en.wikipedia.org/wiki/Pterygium. $\endgroup$ Jan 18, 2021 at 20:33
  • 1
    $\begingroup$ Even non-UV blue light can cause fluorescence in high quality paper. Good business cards will show that. $\endgroup$
    – Jon Custer
    Jan 18, 2021 at 20:39
  • 1
    $\begingroup$ @QuercusRobur Then maybe one should get concerned about vitamin-D deficiency instead. $\endgroup$
    – user137289
    Jan 18, 2021 at 20:56

1 Answer 1

1
$\begingroup$

I will assume that the color that I see on my computer screen is a fairly good representation of the real life color. (This is assuming that if the color in the picture would look off on your own computer screen you would not have uploaded.)

The tint that you refer to as violet is a shade of magenta.

The human color perception recognizes a color as (a shade of) magenta when in the retina both the blue sensitive and the red sensitive pigments are exited, but not the green sensitive pigments. (The actual story is several levels more subtle; this is a simplification, but for the purpose of this answer this level of information is sufficient.)

You describe that the optical whitening agent in the paper, when lit by the LED light, gives the paper the appearance of a tint that you describe as 'violet'.


I submit that that tint is in fact a shade of magenta. That would mean that the optical whitening agent emits a bit less light in the green part of the spectrum, thus giving rise to a magenta tint.

Therefore the observation of the tint of the illuminated print paper doesn't give information as to whether or not the LED light also produces UV-light.


About producing light with LED's:
An early example of using LED's was for the number display of pocket calculators. Those LED's produced red light. For many years red LED's were the only form available; the technology does not lend itself to producing short wavelength visible light.

It took decades to develop LED technology capable of emitting blue light. Once LED's could be manufactured that emit in the blue part of the spectrum there was a path to manufacture LED's that produce multiple wavelengths of light, in such a ratio that the human eye experiences the incoming light as white.

In a comment it is mentioned that nowadays germicidal (UV-producing) LED lights are manufactured.
I think it's implausible that the LED is also producing UV-light, given that it was exceedingly hard to get to LED's that emit in the blue part of the spectrum.

$\endgroup$
4
  • $\begingroup$ Thanks for your answer (due to my low reputation, I cannot upvote it). You may indeed be right that the color is magenta. I will hold off on accepting your answer, in order to see if there is further input. $\endgroup$ Jan 18, 2021 at 21:22
  • 3
    $\begingroup$ Re, "LEDs that produce multiple wavelengths." There ain't no such animal. A so-called "white" LED actually is a pure, deep blue LED hidden behind a phosphor layer that transmits some of the blue light, and converts the rest to a broad spectrum of lower wavelengths. If you look into the business end of a "white" LED (While it is turned off! Please!) and you see yellow, that's the phosphor layer that you're looking at. Some other LED lamps, especially those with adjustable color, use a trio of narrow-band red, green, and blue emitters. $\endgroup$ Jan 18, 2021 at 21:28
  • $\begingroup$ Re, "it's implausible that the LED is also producing UV-light." You can now buy "blacklight" LEDs (cheap as dirt), and germicidal LEDs (still pretty expensive). Search Amazon, but beware! if you actually want to buy a germicidal LED lamp. There's a lot of fakes out there that claim to emit UV-C, but which actually contain the cheap-as-dirt "blacklight" LEDs. $\endgroup$ Jan 18, 2021 at 21:35
  • $\begingroup$ @SolomonSlow I used the expression 'multiple wavelengths' for the adjustable color class of LED lamps. But yeah, expressing it that way is too loose. Three is more than two, so technically it's "multiple", but in the vast majority of cases 'multiple' refers to way more than three. So in any future answer I will write explicitly: a trio of narrow-band red, green, and blue emitters $\endgroup$
    – Cleonis
    Jan 18, 2021 at 21:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.