Helium Optical Flat / Flatness Inspection Light

krap101 · Mar 28, 2018

Hi guys,

I've been looking for awhile for a light to screw around with optical flats with. Apparently low pressure sodium works, which is plan b, but I saw a bunch of forum posts where people bought a real one like a van keuren, kemet, etc for pretty cheap, but I've been looking for like a year and haven't ever seen anything on Ebay or elsewhere. Is there a good place to look?

Thanks,

Danny

cyanidekid · Mar 29, 2018

the subject has been covered a bit here, if you search "monochromatic light practical machinist" you will find some good info...

AlexO · Mar 29, 2018

I use something very similar to this :

Monochromatic Light Source - Kemet

In a pinch

you could improvise rubbing salt into a wick or a laser diode off ebay.

MattiJ · Mar 29, 2018

5-20 usd green laser and diffuser from ebay should work:
One example:
Very cheap unique wavelength light for using with Optical Flats... Using a 532nm. 5mw LED laser - YouTube

5 usd pointer style "10 mile green laser" that I just bought from ebay would be plenty of bright in dimly lit room (with some diffuser).

When working with lasers remember the safey rules: Never look in the laser beam with your remaining eye!

AlexO · Mar 29, 2018

MattiJ said:
5 usd pointer style "10 mile green laser" that I just bought from ebay would be plenty of bright in dimly lit room (with some diffuser).

Power word here is "dimly".

Depending on the surface one might want a proper light.

greif1 · Mar 29, 2018

A fluorescent lamp with a clear green Plexiglas filter (or similar) should provide a serviceable light source. The 546.1 nm mercury peak will predominate.

Type fluorescent lamp spectrum:
Fluorescent Lights : Appealing Fluorescent Light Spectrum 78 Compact Fluorescent Lights For Growing Plants When The Or Is Fluorescent Light Spectrum. Fluorescent Light Spectrum Data. Best Fluorescent Light For Growing Plants. Fluorescent Light Bulbs For Growing Plants. Full Spectrum Fluorescent Light Bulbs For Plants. Fluorescent Light Spectrum.

dian · Apr 4, 2018

MattiJ said:
5-20 usd green laser and diffuser from ebay should work:
One example:
Very cheap unique wavelength light for using with Optical Flats... Using a 532nm. 5mw LED laser - YouTube

5 usd pointer style "10 mile green laser" that I just bought from ebay would be plenty of bright in dimly lit room (with some diffuser).

When working with lasers remember the safey rules: Never look in the laser beam with your remaining eye!

should work? did you try it? i did and it didnt. worse than normal fluorencent. youtube b.s. imo.

MattiJ · Apr 4, 2018

dian said:
should work? did you try it? i did and it didnt. worse than normal fluorencent. youtube b.s. imo.

Didn't work? how?
Laser was dead? :crazy:

dian · Apr 4, 2018

i can see the friges better with regular fluorencent than with the pingpong contraption. at least i have a nice, strong laser to play around with.

MattiJ · Apr 4, 2018

dian said:
i can see the friges better with regular fluorencent than with the pingpong contraption. at least i have a nice, strong laser to play around with.

Fringes in all rainbow colors might show up better for the eye but afaik for "measurements" done with optical flats you want monochrome light source.

billzweig · Apr 5, 2018

I am using a 3 watt orange LED. Those have a narrow spectrum of about 600 nm.

dian · Apr 5, 2018

how did you set it up?

MattiJ · Apr 5, 2018

billzweig said:
I am using a 3 watt orange LED. Those have a narrow spectrum of about 600 nm.

Do you know the exact LED model /make? Or is the orange here the keyword?
I have been told that most LED's don't have enough narrow spectrum but I haven't played with the leds by myself.

Found related discussion:
http://www.practicalmachinist.com/vb/metrology/monochromatic-light-203295/

dian · Apr 20, 2018

i have acquired the following apparatus. i can see the fringes better than with fluorescent now, but its not great. do i need to add some kind of filter or what else can be done to improve the visibility?

Mark Rand · Apr 20, 2018

I have the same type of components as dian has got. Turning most of the lights off in the shop helps. It doesn't cause any problems, because the shop is lit by the sodium vapour light

. I have wondered about the fact that the sodium light has two frequency bands close together (588.9950 and 589.5924 nanometers or about 0.1% apart), but it doesn't seem to cause any problems.

SAG 180 · Apr 20, 2018

I have also used orange LED to get fringes. Mine were obsolete 1W Luxeon star LEDs and I used three close together mounted on a heatsink to get fairly bright fringes when used in dim lighting. A quick way to test for fringes is to get two small squares of float glass say 4" x 4" and clean them thoroughly before placing them in contact together and placing on a clean white surface like a sheet of copier paper. Even under a fluorescent light source you should see an interference pattern in green and pink kind of like oil slick on water colours. It should look like a relief map of the Himalayas. The orange LED illumination gave much better contrast than the fluorescent light.

Using LEDs that use the red or blue ends of the spectrum seem to have narrower colour peaks than the green LEDs which should give better contrast on the fringe patterns.

Obsolete LED that I used were the amber versions:
https://www.luxeonstar.com/assets/downloads/ds23.pdf

SAG 180 · Apr 20, 2018

I think you have a high pressure sodium lamp which has a wide, white spectrum, you want the orange low pressure sodium lamp with only two peaks of spectrum. even the low pressure sodium lamps have a long U tube inside the glass envelope with argon and neon gas inside which will give other colours when running normally.

dian said:
i have acquired the following apparatus. i can see the fringes better than with fluorescent now, but its not great. do i need to add some kind of filter or what else can be done to improve the visibility?

Mark Rand · Apr 20, 2018

SAG 180 said:
I think you have a high pressure sodium lamp which has a wide, white spectrum, you want the orange low pressure sodium lamp with only two peaks of spectrum. even the low pressure sodium lamps have a long U tube inside the glass envelope with argon and neon gas inside which will give other colours when running normally.

I think you may have spotted the problem. That part number seems to be SON (high pressure sodium/sort-of white). What's needed is SOX (low pressure sodium/traditional orange colour)

CalG · Apr 20, 2018

I use a metrologic He-Ne laser to illuminate a Davidson 305 interferometer.

There is a large core fiber as the input to the illumination port.

Crisp fringes, some speckle, and at times the fiber needs a wiggle to suppress speckle.
The fringes are crisp and dark.

Illumination of a work area suited to optical flats would require some beam expansion.

Miguels244 · Apr 20, 2018

The telescope geeks have some ideas for diy.
DIY Monochromatic light source - ATM, Optics and DIY Forum - Cloudy Nights

About the cheapest off the shelf unit I know about is Edmunds optics.
DIY Monochromatic light source - ATM, Optics and DIY Forum - Cloudy Nights

You can also use narrow band pass filters.
Optical Filters - Optical Filter | Edmund Optics

But really, a low pressure sodium lamp is pretty damn monochromatic.
Tighten it up with a band pass at 590 nm, or just a yellow filter, or film to clean the bluer bands out.

Helium Optical Flat / Flatness Inspection Light

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