How to make a monochromatic light?

This topic has 13 replies, 9 voices, and was last updated 9 October 2017 at 08:33 by john carruthers.

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8 October 2017 at 12:36 #320499
Rainbows
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@rainbows
I have a chance to buy some optical flats for pretty much no money. Seen some people on youtube use them and though I probably don't really need them it still looks like a cool thing to have.

Issue is no one appears to be selling monochromatic lights willy nilly and new ones are >£2000.

Anyone know a way to cheaply bodge a monochromatic light source?
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8 October 2017 at 12:36 #25546
Rainbows
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@rainbows
8 October 2017 at 12:41 #320502
Jon Gibbs
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@jongibbs59756
A cheap laser diode or any non-white light LED would be pretty close to monochromatic and as cheap as chips I'd have thought.

[Correction, LEDs are not monochromatic but I think Laser diodes are]

Jon

Edited By Jon Gibbs on 08/10/2017 12:49:30
8 October 2017 at 12:43 #320503
Michael Gilligan
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@michaelgilligan61133
Laser diodes and LEDs should be near enough …. Just bounce it round in a frosted sphere [small lampshade] if you want a diffuse light.

MichaelG.

.

John beat me to the post

Edited By Michael Gilligan on 08/10/2017 12:43:55
8 October 2017 at 13:02 #320509
Mark Rand
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@markrand96270
A low pressure sodium lamp (SOX type), ballast, ignitor and an Eddison screw socket are cheap on EBay and simple to wire up. making a box with a diffused cover over it is simple woodwork. That's what I did, except that I haven't bothered with the box.

According to my EBay purchase history, it cost me £47 in 2015 for a 35 Watt setup.

Edited By Mark Rand on 08/10/2017 13:06:14
8 October 2017 at 13:55 #320521
Anonymous
Single colour LEDs (red/amber/green) are pretty close to monochromatic output, bandwidth is usually a few 10s of nanometres. However the output is not coherent. For that you need a laser diode, rather more expensive and may need some heatsinking.

Of course many lasers don't output a single frequency, just a really narrow bandwidth. The bandwidth is determined partly by the Q of the laser cavity. If the laser was purely monochromatic it would need to be on for an infinite time.

Andrew
8 October 2017 at 14:27 #320527
Michael Gilligan
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@michaelgilligan61133
Posted by Andrew Johnston on 08/10/2017 13:55:17:

… However the output is not coherent.

.

True, but does that matter ?

I don't recall needing coherent light for interference fringe tests.

MichaelG.
8 October 2017 at 14:42 #320530
Anonymous
Posted by Michael Gilligan on 08/10/2017 14:27:35:

Posted by Andrew Johnston on 08/10/2017 13:55:17:

… However the output is not coherent.

True, but does that matter ?

Nobody said it was a requirement, simply a statement of fact for completeness.

Andrew
8 October 2017 at 14:47 #320532
Rainbows
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@rainbows
Could go for the 35W sodium bulb or a 7W array of lasers (based on lumen per watt). Think they will end up approx same cost/ £26

If I use yellow/amber LEDs I can get away with just £5 and have the sodium colour. Thats using 100 of the 5mm round LEDs. Don't know if the type you would see in say a spotlight would work as well.

Edited By Rainbows on 08/10/2017 14:47:34
8 October 2017 at 14:50 #320533
not done it yet
Participant
@notdoneityet
Gelatin filters were good enough in the old days. Depends on how monochromatic you need the light to be.

Or buy a prism and make your own spectrum.

Edited By not done it yet on 08/10/2017 14:51:34
8 October 2017 at 15:00 #320537
Michael Gilligan
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@michaelgilligan61133
Posted by Andrew Johnston on 08/10/2017 14:42:08:

Nobody said it was a requirement, simply a statement of fact for completeness.

.

Thanks for the clarification, Andrew … Sir Isaac will be relieved.

MichaelG.
8 October 2017 at 15:38 #320545
Neil Wyatt
Moderator
@neilwyatt
If you want bright mono light on the cheap, buy a 1.25" 7nm Ha filter for about £50 and fit it to a bright torch.
8 October 2017 at 16:59 #320553
jimmy b
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@jimmyb
You can still use optical flats in normal light. The “interference fringes” should still be visible, the surfaces must be totally clean.
9 October 2017 at 08:33 #320650
john carruthers
Participant
@johncarruthers46255
The old school kitchen table method was to chuck common salt into a spirit lamp flame to give a sodium light.
Albert Ingall's 'Amateur Telescope Making' (vols 1 to 3) shows simple methods for optical testing of flats and various conics.
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