How to make a monochromatic light?

[RainbowsParticipant @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?

[RainbowsParticipant @rainbows]

[Jon GibbsParticipant @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

[Michael GilliganParticipant @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

[Mark RandParticipant @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

[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

[Michael GilliganParticipant @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.

[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

[RainbowsParticipant @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

[not done it yetParticipant @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

[Michael GilliganParticipant @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.

[Neil WyattModerator @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.

[jimmy bParticipant @jimmyb]

You can still use optical flats in normal light. The “interference fringes” should still be visible, the surfaces must be totally clean.

[john carruthersParticipant @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.

[Author]

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