Russian Optical Flat Testing

[@TheMix1017]

I can translate better than GL. The first value of 0.07 means exactly: " The total maximum deviation from absolute flatness of working surface is not more than 0.07 of micron at any place" Another value of 0.02 means: " The local deviation from flattness of working surface is not more than 0.02 of micron" The soviet industry was making two types of flattness measuring glass plates: PI (shown at your video and PM. The difference is that fist one have one working surface and second one have both sides with very very high grade of parallelism. Fist is designed to observe defraction lines pattern of air gape betveen tested surface and etalone glass surface fitted on it. Second can do the same, but also you can invesigate difference of opical way to front surface in monochrome light to obtaine absolute values of flatness deviation. This really very precise instrument to control flattness with acuracy exceeding most needs of tooling purpose.

[@movax20h]

What source of light would you usually use with it? sodium lamp maybe? They are pretty monochromatic.

[@ducatista1098s]

Sodium or helium. Any gaseous bulb with a narrow band of spectra. Not sure if led can work but seems possible.

[@BasementEngineer]

I've used Neon indicator bulbs successfully for this application. This is quite a small light source and 3 or more bulbs can be used in parallel with the right resistor in series. I didn't have an optical flat for my work at that time (30 or so years ago) and used a small piece of float glass instead. Worked quire well for my application: checking the flatness of mechanical seal surfaces. This was a custom made seal for a small shaft on a pressurized hot air engine.

[@peteferguson7024]

" you can take the boy out of Russia but you can't take Russia out of the boy". I quickly sketched that symbol on a piece of paper and showed it to a colleague who used to work in Russia, he was on a conference call and couldn't talk, and the message in his eyes was priceless :) Cool measurement system. Thanks.

[@johnsantos975]

Tom, I used clips of your videos to demonstrate the Fringe Patterns while using the optical flats we use here creating learning videos. I cited your channel on all of the captures I used. I captured 2 short captures, and a few still captures to demonstrate to students what they are expecting to see. If you have questions we talk offline. We can even do a zoom chat which a secure platform.

[@geckoproductions4128]

Tom, every time I watch one of your videos, it costs me money. I didn't know what an optical flat WAS, never mind that I NEEDED one until I watched your video! Well done OM, thanks for the info.

[@nickblaze49348]

Thanks a bunch for this video! I just took over our lapping area at work and have really no clue about what is all involved with it, but your video helped explain a lot without over complicating it.

[@juanrivero8]

Very interesting and informative. I have always found it very difficult to see interference fringes, but the He light made a huge difference. Your magic trick was mind-blowing. Translation of languages is one of the most difficult of all problems.

[@juanrivero8]

Erm, I referred to the translation app on the phone.

[@nothermarkgnomex]

Good to see you posting again! You do come up with some really interesting things.

[@gottfriedschuss5999]

One can use an inexpensive low pressure sodium (LPS) lamp from the local big box store. The wavelength is very nearly 589 nanometers. This will provide far better fringes than daylight fluorescent fixtures and is nearly an order of magnitude less expensive than a new helium light source with power supply. I get better results when I remove the plastic diffusser from the LPS fixture. Your mileage may vary.

[@GordieGii]

A coloured LED lamp should work well too. Red map lights (to prevent ruining your night vision) are pretty easy to find.

[@PlasmaX00000]

Led's are never as precisely monochromatic as gas discharge tubes - they have a variable electron gap, so they can produce a band of wavelengths instead of a single.

[@GordieGii]

Still, they have to be a lot better than 'daylight' florescent bulbs, no? I guess there's not that much price advantage over LPS. I seem to recall that years ago sodium was the lamp of choice for optical flats. When did helium take over?

[@Garganzuul]

Colorblind people might have a problem with non-monochromatic light, too.

[@nilzlima3027]

do we/ can we check this? that would be so much cheaper that and we could put those cheap lights in the lab so flats can just be used without turning on a separate source.

[@ramosel]

Tom Lipton: Collimating with the Russians to influence the refractions!!

[@stuartschaffner9744]

I'll admit that I have not made careful drawings, but I am pretty certain that I know what causes those regular interference patterns. The thickness of the optical flat, measured perpendicularly, is some value T+-70 nanometers anywhere over the flat. The perpendicular thickness varies by about 1/8 wavelength. In other words, this is standard 1/8 wave optics. However, the light you see is a reflection from the shiny reference surface. It goes through the flat (plus any air gap) twice at an angle theta. Theta varies from point to point in your image, just like with any mirror. If G is the air gap, then the effective "thickness" of the plate is (T+G)/cos(theta). It's the cos(theta) term that determines the spacing. Cool stuff! Thanks for showing us all this.

[@krazziee2000]

Glad to see you back.. thanks for the video.

[@hankus253]

Thanks Tom, you never cease to pique my interest.

[@fredgenius]

You can also use sodium lamps, they're commonly used to grow 'plants' so should be fairly easy to get hold of, and the wavelength is very close to that of helium. Some LEDs might also give better results than 'daylight' tubes. I'll be getting an optical flat sometime soon so I'll try a few out.

[@arduinoversusevil2025]

"In Soviet Russia, optics flatten you!"

[@MrSausageandeggs]

are you my real dad?

[@MrSidiox]

Always happy to see the other "mucking about in their shop" youtubers on each others videos.

[@VEC7ORlt]

No, he's your weird uncle.

[@billythebake]

VEC7ORlt that's Uncle Bumblefuć...

[@tbernardi001]

Troll alert.

[@Waffenschmiedinx]

One if the coolest things I've seen recently. Thanks for sharing it

[@m3rkwurdigliebe]

i believe the ПИ-80 is a bottom optical flat, so the working surface is the top one (marked). the bottom face is not guaranteed to be flat

[@uMuDDyu]

The fact that the optical flat floats on air is awesome.

[@aserta]

The translator app was made possible by everyone's authentication on captchas. It's a similar tech with ReadIRIS's OCR but in this particular case, the app has a library of words that have been already transplanted from image to actual text. Which from there it's far easier to translate. And, yes, it's much faster than letter to letter translation. The app grabs a word vs letters. For example, if a word has never been put in to text-from-image, like say, some WW2 German word of notable length, the app will have trouble with it (tho, i suspect that it might also have some point to point or at least half word function based on some of the half grabs i've seen).

[@eclecticneophyte2581]

Hi Tom, How much do surface scratches on the glass affect usability? Great video! Thanks for sharing all your tips on metrology (and magic too).

[@anguskangus8222]

Super super cool video ! Thankyou for posting !

[@pietzeekoe]

So how good are your diy flats?

[@MrRayopt]

That's funny :-)))

[@raidoenn8141]

I think you can use a sodium lamp also (as in street lights). They also have a sharp spectral line, which will surely produce better fringing than luminecent lamp.

[@simonp347]

Raido Enn I researched on that earlier. Seems like low pressure sodium light has a really good chromatic character

[@raidoenn8141]

Yes. It is by far not as clean as He-lamp, but it works in a pinch and is a lot cheaper. Also one could use a laser pointer, as that is also very monochromatic, but lighting a table with laser pointer gets difficult fast :)

[@simonp347]

Raido Enn Low pressure is the key. I'm not sure if you have ever tried the laser or not. I tested with the He-Ne laser we have, expecting to see some chrisp fringe, but result was quite disappointing. The fringe is quite fuzzy.... even with all the other lights turned off

[@raidoenn8141]

We have some optical lab work (practical course) in university and use Soviet era mercury lamps for the same reason. You would have to have a very powerful laser and diverge the beam to get some well visible fringing, but a lab He-Ne is usually not intensive enough (although being very monochromatic). So for all practical purposes, some atomic emission lamp is your best bet.

[@2lefThumbs]

Simon P it might be speckle due to coherence of hene (and many other) lasers

[@ccaissie113]

Neon light will work well. A flouroescent light with a green filter works, as does viewing thru green transparent film..or green glasses. Any curvature of the bands is easier to see with fewer fringes, so adjust the number of fringest 3-4 for the greatest accuracy.

[@James-fs4rn]

thanks for another interesting and informative post!

[@Swag-sw2li]

A darkroom light seems to work pretty well, as well. I picked up and old one for €30 and get some good fringes on gage blocks.

[@alext8828]

Good idea.

[@Aistlander]

About the Soviet quality mark from Wikipedia: The sign is a pentagonal shield with a rotated letter К (from Russian word качество, kachestvo - quality) stylized as scales below the Cyrillic abbreviation СССР, SSSR (USSR).

[@billdlv]

Good info Tom. The translator app reminds me of the sun glasses in the movie "They Live". Any special tricks on cleaning optical flats?

[@NoelBarlau]

Bill De La Vega in my laser technician classes we placed a new sheet of lens paper on the lens surface and wet it with acetone just to the point that the entire surface was wetted. Then you slowly pull the paper off in the same plane as the surface. Repeat as necessary.

[@jimwarakois2817]

Years ago we used to clean the flat in a laminar flow clean room with 190% ethanol (we also had great Christmas parties in that division of our company.

[@billdlv]

Yes when I was a graduate researcher we had optics that were part of a laser diode velocimetry rig and used ethanol (which was not denatured) to clean them. One of my lab mates got a little buzzed by the vapors after she cleaned a bunch of mirrors and lenses.

[@billdlv]

Hmm I thought that acetone would damage some optics which were coated? But I guess in this case that's not a problem. I'm wondering if hardware store variety is pure enough?

[@jimwarakois2817]

ETOH or Acetone. Would depend on the coatings. The etoh we used was Graves fine distilled. 190-200 proof. A few livers suffered greatly. (Available at select, exclusive "Spirits" vendors.

[@jamesrandalls6914]

that was awesome, thank you

[@rneal63]

Would LED light source be good for this? Usually the different colors are rated in their nanometer wavelengths. It was interesting that I had a 650nm laser and a 670 nm laser, to they eyes, 650 looked brighter, but to a camera with IR filter removed, the 670 was brighter.

[@hilltopmachineworks2131]

Thanks professor Ox. Learned something today.

[@NuclearHedgehog]

Very interesting stuff. I think that LPS lights should also work for that purpose as they put out monochromatic light. They are pretty affordable.

[@2lefThumbs]

Great tips Tom - and a timely posting -I just dug out my (ebay) optical flats earlier today to look at deformation under clamping pressure (I'm trying to machine an aluminium heatsink to an acceptable flatness and need to play with clamping pressure) , my flats work kind of ok by eye with fluroescent lighting, but I can't get my camera to see the rainbow fringes, I tried our external low pressure sodium floodlight (a light source suggested by Gottfried Schuss in an earlier comment) once it got dark enough outside , and it looks much more promising :)

[@oxtoolco]

Good to hear. I should try to find a HPS lamp to test. Cheers. Tom

[@2lefThumbs]

Hi Tom, the floodlight I have is LPS I believe (low pressure) aka SOX - these are yellow (two spectral lines around 590nm), the HPS lamps have lines from violet through to red, so may not be so useful - I might be wrong and the one I have may be HPS - I've ordered a SOX bulb and control gear so I can do this indoors - total cost around 50 GBP/70 USD from a lighting store, Cheers, Ray

[@ROBRENZ]

That toolmakers flat is begging to be lapped! Nice one Tom. ATB, Robin

[@MrSidiox]

Maybe a video for you to do :P You've been gone for too long!

[@ROBRENZ]

You are right!

[@JohnBare747]

In the realm of the minuscule things get very interesting Tom.

[@outsidescrewball]

Interesting...(grasshopper thought) curious what a piece ground from your surface grinder would have shown or would it even work

[@Birender100]

Very Nice Sir, please dwell upon the measurement/distance of optical bands in relation to the (given) lambda of the helium light. Thanks

[@penroc3]

what is the model of the He lamp? all i have are tube sources for He light and it is very pink compared to your orange

[@TABE-O]

That’s brilliant. The google translate I mean. Also insights into optics. V good!

[@mikemullenix6956]

What would happen if you tryed a granite surface plate?

[@lazyh-online4839]

What about a blue laser diode, is the light a narrow enough frequency band to make an effective light for viewing fringes when defocused? I have a 2 watt laser diode I was originally using for etching, but when I realized how little I used it I recycled it with an old overhead projector and some frosted glass to make a rudimentary optical comparator, so I'd be happy to keep reusing this little diode until I find it a permanent home where I find it's most useful.

[@williamsquires3070]

LazyH-Online - Some blue LEDs will work, but most are UV emitters covered with a yellow phosphor. Same for the white ones. If you look in the “business”-end of the LED, and it looks yellow, then it’s spectrum won’t be very narrow, compared to the true-blue LEDs where you see only the chip and (maybe) the fine, gold bond-wire. Also, watch out for LEDs with a blue tinted case; many of these are IR emitters, like in a TV remote!

[@glenking3337]

With the polished sample under normal florescent light what color of the fringes are actually in contact with it optical flat? The purple, green or yellow?

[@artetl]

1st: Whole degree of deviation from the working surface flat is: -0.07 micrometers. 2nd: Local degree of deviation from the working surface flat is: 0.02 micrometers. It's a second-class quality of accuracy so it's cheap but extremely useful. And yep! It's USSR made. Not in Russia. May be this stuff is 30 years old.

[@nebulousGemini]

On the back side of certificate it says it's made in 1981

[@886014]

Great video Tom, I've only ever used fluorescent lights with mine, and at times they can be hard to read. I've wondered if a diffused laser or even standard LED could be used, but haven't had a chance to try. Other light sources could make a good follow up video suggestion as this has come up before.

[@very1v]

just wondering how do you know that the high spot is a high spot and not a low spot ?

[@simonp347]

very1v You press on the optical flat. If the bands widens a lot where you push, you know that's low.

[@bilz0r]

Because the fringes get further apart when the gap between the surface decreases, and the fringes get closer together then the gap increases.

[@fefifofob]

Having principles is a good thing

[@ExON00]

If you get circles it can be hard to decide wether the part is concave or convex(high or low spot). By pressing with your finger, if the pattern moves towards your finger the part is convex, if it moves away from your finger its concave. Carbon parts can be tricky, they must be polished to a shine to be able to read. Getting both the shine and the correct shape i.e concave/convex (no you don't always want flat parts) is hard

[@Supraman007]

Hi Tom, do you think I could build a light source using LED's of the correct wavelength?

[@ChiralSymmetry]

Can you use a laser-pointer with a light-diffuser to look at fringes? I am thinking that laser-pointers might be an inexpensive monochromatic light source? Not sure how to make or set-up a light-diffuser to spread out the laser light. But might be worth a try? Maybe this: small lens to spread out the light onto a piece of paper which then diffuses/reflects the light?

[@kylejacobs1247]

Have you tried using a white LED light to see the fringes? They are typically a blue LED with some phosphors to balance out the color to white, but the spectrum typically still has a very strong blue peak compared to the much weaker and broader red peak. I'm curious if you get better results compared to a fluorescent.

[@ermennda]

Is there not a led color that approximates the helium light? It would be cheaper.

[@JekaMazurovskiy]

ermennda laser with some kind of defocusing lense may be a good substitute for sodium or helium lights. will try it as soon as laser diodes show up in mail from china

[@ronpeck3226]

Thanks Tom, The translator is amazing. Makes me wonder how much is "out there" that I don't have a clue how to access. Thanks for the share.

[@AddictedtoProjects]

Lol. None of this is top-secret. Get a smartphone, browse the internet and just show a general interest in new technology. You'll get there soon enough! ;)

[@eformance]

Would a red laser pointer with the lens removed, work for a light source?

[@1berkut]

how do you tell if it is a high spot or a low spot?

[@kylejacobs1247]

When using white light, discernable contrast fringes only appear for a distance of a few wavelengths and fade away as the gap gets larger. So with white light, high spots will have higher contrast fringes.

[@oxtoolco]

When you press on an edge of the flat the direction that the fringes move tells you if its high or low. Hope this helps. Cheers. Tom

[@luismaiorano2959]

Hello friend, your video is very interesting. I have a flat Russian optician. I need to get the monochromatic light. Can you tell me where to get it? Thank you very much for your help. Regards, Luis

[@teeess9551]

I had no idea Google Lens could do that. Thanks.

[@Ross_Dugan]

Will this work on a granite plate? Or does it need tone somewhat reflective? Thanks Tom. Really enjoy your videos

[@MrMa1981]

Doesn't works. I have one and I tried.

[@godfreypoon5148]

It doesn't work? Maybe try a can of fix-a-flat.

[@PatonHaus]

Explanation of what the heck an optical flat is, and what it does, starts at 10:00

[@mikekellam365]

How about looking at fringes on one of your collumater mirrors?? That would be cool!

[@andrewgreen9417]

How would this work with first surface aluminum and sliver mirrors? We are having issues with some at work and I wonder if this could help us see if the issue is the glass or surface coatings it's kind of a niche task we use them for.

[@dziban303]

Helium discharge lamps are pretty expensive. Much cheaper to use a laser pointer with a beam spreader or diffuser. A green, blue or purple laser would provide more accurate measurements than the yellow peak of helium emission, by virtue of their shorter wavelengths (typically 532nm, 447nm, and 405nm respectively for green, blue and purple, compared with the 588nm line of helium)

[@timmer9lives]

That Translate also blew me away. As a guy about your age who remembers needing quarters for a pay phone....You're right, we're living in the future.

[@Florreking]

Doesnt the frequency mean how quickly the gap changes over the surface? If the two surfaces were absolutely perfect and exactly parallel the frequency would be 0, ie no bands at all.

[@RumpLeINtiLINsKinnIN]

Hey Tom, kind of off topic but, you did a video a while back in which you made a template and followed it with an indicator keeping it as close to zero as possible as the lathe fed in. Ive been trying to find it as I have to do something similar for a job at work coming up and want to see how you did the job to refresh my memory. Could you tell me the name of the video if its still up?

[@zephyrold2478]

Hi Tom, If I am right it can only be used on a reflective surface.

[@oxtoolco]

Correct. The surface you are checking needs to be fairly reflective to show fringes. Cheers. Tom

[@Jloyd18]

Does the surface to have to be reflective to check with an optical flat?

[@63256325N]

Thanks for the video.

[@WayneOverbeck]

Off the subject but, is the building in the opening scene of your videos, as well as on the covers of your books part of the Mare Island Navel shipyard?

[@SuperSecretSquirell]

That Google translator is trippy.

[@MrRayopt]

Google is getting a little dangerous

[@JulieanGalak]

Great video as always. Out of curiosity, I looked on eBay, and there's still a bunch of these available. Had a question though: they are labeled as "bottom" and "top". Any idea what that means in practical terms? Does it matter which kind is used for this sort of analysis? In one of the lapping videos, you mentioned concave vs convex flats, wonder if that's what this refers to...

[@JoelHudson]

Tom, would a sodium light make for better contrast in your interference lines? Or is Helium also a pure color

[@brucewilliams6292]

Will any wavelength of light work? For example, a sodium "D" lamp produces a single wavelength of light; would that work?

[@forrestaddy9644]

Some colored LED's are rated in nanometer spectra. Red for example is commonly rated 620 Nm. An array might make a nice monochronomatic light source. There are over 6000 eBay hits for red colored LED flashlights and many are less than $10. The distance represented by fringe deviation will have to be figured in based on the actual color wavelength. Anyway monochromatic light can be affordable

[@oxtoolco]

Hi Forrest. Rick also sent along a special LED flashlight for looking at gemstones. Don't know the specifics of it but it was a dud with the optical flat. Good to hear from you. Cheers. Tom

[@forrestaddy9644]

oxtoolco Was that gem light monochromatic? Filtered? Do they make LED's to spectrum now? I've seen strong fringes in LED red on stacked Mylar sheets. Pack your flat and specimen out to the driveway and try it on the LED tail light of your radical ride.

[@allensandven0]

I just found a optic flat cylindrical test plate made by The VanKeuren Co. Watertown Mass. At a garage sale last weekend and they gave it to me with a few other stuff that didn’t sell at the end . I new it was a testing / calibration tool of sort. Thought it would make a good paper weight or ? As I watched your clip and the pitted flat needs a backup . ? Honestly I just stumbled across your channel and watched a few of the videos and realized that it wood be a shame to let it sit in my shot as a beer coaster ! So up fo grabs just take care of shipping cost , it feels like 10 lbs + and has nice case it stores in . 6” x 1” + /- surfaced both sides. OBTW. I did ask how they got it , they purchased the home a few years back from a elderly man that was a engineer for Hanford DOE site for over 40 years. I can send you pics and let me know if you or someone can use it . Allen S near Hanford Wa USA

[@allensandven0]

I posted a short clip of the plate on my page

[@techno_mesh]

Hi Tom, Interesting video. Can you share a link to where you can buy them? I searched at eBay and could not find this same one. Thanks

[@daveticehurst4191]

Hi there. You need to look on E-bay USA. I tried the Australian and British sites without success. I bought a 100 mm exactly the same according to the photo and it had the same data sheet. $ 85 + $ 20 postage from UKRAINE. He is still listing an 60 mm for $ 55 Here is the link. www.ebay.com/itm/Optical-Flat-Parallel-Top-60mm/222687471409?hash=item33d9352731:g:6coAAOSwu4BV0NTp

[@josepheirman4987]

i thought you had another video on optical flats. i cant find it right now.

[@Steve_Just_Steve]

Are there different coatings on optical flats for different purposes? Seems like I see different types for sale. Is there a certain one for metrology? Thanks to anyone willing to answer.

[@sleepib]

colored LEDs might be a good option for a monochromatic light source.

[@MrRayopt]

If you like to buy on ebay, just buy a vintage tanning bulb for $25 and aim it a white ceiling. Who needs helium or sodium light ? Also I was waiting to see you put it on something that would show us only 5 fringes.

[@bobthecannibal1]

So what I'm hearing is that you want to use a single wavelength light source to maximize the interference. I'd try a low pressure sodium vapor lamp (The kind used in cities, and the cheaper the better.) because they're common and have one dominant wavelength.

[@AmateurRedneckWorkshop]

Ah those Russians. This was flat out interesting though.

[@godfreypoon5148]

I felt the content was a little flat.

[@ДиДи-м3ю]

Russia is a country of contrasts. Where the greatest accomplishments drown in the swamp of poverty and madness of leaders. kzbin.info/www/bejne/d5jYgmuEfd6qeqs

[@benwood90]

Really loving these high accuracy based videos you and @robrens make. Is it actually a measurable means of flatness or just a representation of how flat something is or isnt and where high spots are?

[@BigBoss-rh7zq]

Hi Tom, do you know any supplier of lamp ?

[@christopherkann6385]

Could you use one of these to inspect a surface plate?

[@oxtoolco]

No. The surface you are checking needs to have a reasonably reflective finish. Granite just wont do it. Cheers. Tom

[@daveticehurst4191]

Tom, you kept saying Helium Lights, are you sure ? When I did optics at work we used a SODIUM Lamp, yellow glow as in street lights. This would also show up any stress marks in glass.

[@oxtoolco]

Hey Dave. Quite sure. Mine is made by Lapmaster and is a helium gas lamp that is diffused through a screen. Sodium would work as well but I don't know if the bands would be as crisp. Never tried one. Cheers. Tom

[@jesseruano88]

Sodium is best, monochromatic lamp is ideal

[@JBFromOZ]

my eyes my eyes!!! the babel fish is in my eyes!

[@derKarl_stp]

so with a lot of time and effort you might get some pretty interesting results on your surface plate I would say ;-)

[@IanZainea1990]

2:34 the CCCP symbol always reminds me of the Chrysler diamler logo the Pentagon thing lol. From the 90s at least

[@noneofabove5586]

Hey Tom, Did you ever consider making your own optical flat ?

[@minskmade]

great video. as usual.

[@Max_Marz]

Magic trick? What more magic do you need than being able to wring two flat things together? Pretty spectacular with that helium neon source though.

[@bcbloc02]

Cool stuff.

[@paulbadger6336]

I know you......”kind” of mentioned a way to calculate how far out of flat the plate is, but you didn’t show us how. Is there a formula that you plug in values , that you get from a chart that gives you the values based on the spacing of the lines established by the wave length of the light source. Sorry, it started as a simple question? Fascinating subject, thank you for giving us your time. -P

[@oxtoolco]

Check out Edmunds or Van Keuren on how to read optical flat fringes. Some good information. Basically its the amount of the bow that gives you dimensional information on the curvatures. It can get pretty complicated depending on where the flat is touching down and the bands curvatures that show. Plenty for an entire video. Cheers. Tom

[@beachboardfan9544]

12:45 how do you know its a high vs low?

[@ljguy300]

Because its teetering on the high spots

[@godfreypoon5148]

Wow. I'd like to live in an optical flat.

[@johnmckeon9577]

I have used my pocket laser pointer as a light source; try it.

[@josephwilson6651]

I find it interesting,you can acquire a precision device from the source stated,which would seem to be necessary for the former residents of the area indicated,to reproduce precision machining for their industries.Makes one consider why it becomes useless to them now? have they no need for such devices? when most government entities would highly covet them!!

[@brh4015]

The smallest strong wavelength wavelength from a Helium atom is at about 447nm. You can buy LEDs with their maxima at this wavelength (royal blue). Just google 447nm LED, cost you a fiver or so for a 2W one. That will be plenty.

[@daveanderson2316]

Skunge?

[@reiniertl]

Good LED in different colors can do the job because they emit mostly in a single wavelength, a LASER can also do the work very well, I suppose it should be scattered, but what matters here is that it is a monochromatic light. For sure any of these is cheaper than a Helium bulb and much better than fluorescent. And the best thing is to turn off other light sources while testing.