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How monochromatic is the light from LEDs? I want a set of light sources which will emit light of different wavelengths, as narrow a spectrum as practical.

How wide are the spectrums of typical LEDs?

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    \$\begingroup\$ Depends how "monochromatic" you want. It's spectrum would be like a bell shape around the main wavelength. The question is how well the bell "width" works for you. If you want a really narrow one, you will need to look into lasers. \$\endgroup\$ – Eugene Sh. Jan 21 at 18:32
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    \$\begingroup\$ I suggest that you do some research, start at: en.wikipedia.org/wiki/Light-emitting_diode at the bottom of the page are some spectral pictures. Not all LEDs are the same. Look at some LED datasheets, some have a spectral plot. Search on Google for "LED spectrum". \$\endgroup\$ – Bimpelrekkie Jan 21 at 18:33
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    \$\begingroup\$ FYI, all light has a bandwidth, as true monochromatic sources do not exist. You need to define how small of a bandwidth counts as monochromatic for your purposes. For illumination purposes, LEDs are usually considered monochromatic, for telcom applications, they are extremely broadband. What is your application? \$\endgroup\$ – user1850479 Jan 21 at 20:48
  • \$\begingroup\$ How about RGB(W) LEDs? That's basically 3-4 different wavelengths bundled to get whatever colourtone you want. \$\endgroup\$ – Mast Jan 22 at 9:42
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    \$\begingroup\$ @ThePhoton absolutely sure, for several reasons, the main ones being that it's what I do all the time, and that it's a Fourier Transform spectrometer (UV-Vis-IR) so saturation doesn't look like that \$\endgroup\$ – Chris H Jan 26 at 9:41
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You will be getting a narrow band, many LEDs have width of about 20 nm at 50% down from the peak; other LEDs have quite a wide band. The datasheets for LEDs will give you a curve for those specific to a particular LED.

Here are the graphs of LEDs I chose for a scientific imaging project based on how narrow their bands were without resorting to exotic components. I wanted a selection of colours as narrow as possible (but cheap), and bought well-known manufacturers' parts with datasheets, which I summarised as follows:

enter image description here (The 400 nm line is dotted because it was an estimate. The others were traced from datasheets and normalised to give same height.)

If you need narrower:

  • a laser will be better than 1 nm, and can be much much better. (Given as 0.85 in datasheet of cheapest laser at a distributor). Some "broadband" lasers have a linewidth of a few to about 10 nm (Wikipedia)
  • some extrenely narrow bandpass filters are available, such as astronomical sodium filter, which can have a typical bandpass of 0.05 nm. Be warned they can be extremely expensive.
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    \$\begingroup\$ "a laser will be a few to about 10 nm" Sorry, but no. A laser will typically have a linewidth on the order of less than 1 nm, and rather closer to 0.1 nm. And you can get Single Mode lasers with linewidths in the .001 nm range. \$\endgroup\$ – WhatRoughBeast Jan 21 at 20:57
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    \$\begingroup\$ @WhatRoughBeast thanks for picking up on this, my mistake. I've edited the answer to show that my numbers were a worst case for some lasers, and clarify the normal case. \$\endgroup\$ – jonathanjo Jan 22 at 8:38
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I want a set of light sources which will emit light of different wavelengths, as narrow a spectrum as practical.

If you want as narrow as practical, you should use lasers. LEDs are intrinsically broadband light sources.

However you really should define a bandwidth you need. Too narrow will introduce problems such as speckle.

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Different WAVELENGTHS may be difficult. Different COLOURS, achieved by mixing the output of three LEDs, red, blue and green, is easy, and cheaply available in a single package.

https://www.rapidonline.com/truopto-ostama5b62a-full-colour-5mm-led-60-diffused-72-9647?utm_source=AffWin&utm_medium=Affiliate&awc=1799_1579713939_2a7737a0902bbca9c2076b48d027e40a

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  • \$\begingroup\$ The OP asked for a set of light sources of different colors, not a single light source of variable color. Furthermore, you only get different "colors" if you are using a human brain to observe them. Optical instruments and creatures with different kinds of color vision will not see distinct colors. \$\endgroup\$ – Elliot Alderson Jan 22 at 17:43

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