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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.
    Commented Jan 21, 2020 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\$ Commented Jan 21, 2020 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\$ Commented Jan 21, 2020 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
    Commented Jan 22, 2020 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
    Commented Jan 26, 2020 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\$ Commented Jan 21, 2020 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
    Commented Jan 22, 2020 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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Just going to dump some datasheet extracts here as I find the patience for fun. E.g. at https://uk.rs-online.com/web/c/displays-optoelectronics/leds-light-emitting-diodes/leds/ we can filter by brand, sort by increasing price, and then quickly access the datasheets of sellers that give the information we want easily:

Component Price (£) Luminous Intensity (mcd) Peak (nm) Half width (nm) Test condition (mA)
Lite-On2.6 V Red LED 3mm Through Hole, LTL-4221N 0.033 8.7 635 40 10
Lite-On2.6 V Yellow LED 5mm Through Hole, LTL-307Y 0.033 29 585 35 10
Lite-On2.6 V Green LED 3mm Through Hole, LTL-4231N 0.033 12.6 569 30 10
RS PRO 3mm Yellow green LED Round with Flange 0.053 30 565 20 20
RS PRO 3mm Deep red LED Round with Flange 0.057 30 660 45 20
RS PRO 5mm Yellow LED Round with Flange 0.058 30 590 35 20
RS PRO 3mm Blue LED Round with Flange 0.081 3800 (?) 468 25 20

Legend:

  • "Peak": Peak wavelength, wavelength that has the greatest intensity
  • "Half width": Spectral Line Half-Width, wavelength delta at which we have half of all intensity. Smaller means "more monochromatic"

So between 20 and 45 seems typical for these cheap LEDs in our small sample size.

Related:

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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\$ Commented Jan 22, 2020 at 17:43

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