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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1\$\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, 2020 at 18:32
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4\$\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\$– BimpelrekkieJan 21, 2020 at 18:33
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3\$\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\$– user1850479Jan 21, 2020 at 20:48
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\$\begingroup\$ How about RGB(W) LEDs? That's basically 3-4 different wavelengths bundled to get whatever colourtone you want. \$\endgroup\$– MastJan 22, 2020 at 9:42
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1\$\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 HJan 26, 2020 at 9:41
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3 Answers
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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:
(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.
answered Jan 21, 2020 at 18:38
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1\$\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\$ Jan 21, 2020 at 20:57
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1\$\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\$ 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.
answered Jan 25, 2020 at 19:34
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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.
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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\$ Jan 22, 2020 at 17:43