using a optical flat

dian · Apr 18, 2017

i got myself my first optical flat. its 100 mm in diameter and specified at 0.07/0.02 mu. i have two questions:

1. what light source to get? i can see faint rainbow fringes under normal light. i see nothing with a led flashlight or a yellow incandescent bulb. this is just for fun, maybe i have it already and just dont know it could be used?

2. how long do i let the flat wring? im afraid to not be able to get it off if too long.

Gordon B. Clarke · Apr 18, 2017

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How about a picture of what you bought?

CalG · Apr 18, 2017

Ahh. using an optical flat...

dian said:
i got myself my first optical flat. its 100 mm in diameter and specified at 0.07/0.02 mu. i have two questions:

1. what light source to get? i can see faint rainbow fringes under normal light. i see nothing with a led flashlight or a yellow incandescent bulb. this is just for fun, maybe i have it already and just dont know it could be used?

2. how long do i let the flat wring? im afraid to not be able to get it off if too long.

2) Don't worry about time, there is always enough dust in the air to prevent fatal contact ;-)

1) An illumination "box" fitted with a Hg light source and a "green" filter glass is the established method. Fringes at 546.1 nm are well documented and universal.

ps
If one could remove the phosphorous material (ZnS) from the inside of a common fluorescent lamp, the Hg lines would be prominent. Still, a fluorescent bulb can be useful as is.

PeteM · Apr 18, 2017

Monochromatic light source is best to get well defined fringes. This is achieved using a gas lamp (helium etc.) with energy in just a few bands and a filter to get the band of interest. As Cal suggested a green filter (but in my experience with a helium or sodium rather than mercury source) was common. Use care if you try a source with strong lines in the UV spectrum.

Lasers were/are another monochromatic source. These days someone probably makes a suitable LED source?

dian · Apr 19, 2017

if looking for gas lamps i come across stuff from this:

8 x 8W OSRAM HQL MERCURY VAPOUR MBF-U LIGHT LAMP BULB E27 BRAND NEW | eBay

GE LU25 25-Watt S5 High Pressure Sodium Lamp Light Bulb 25W E39 Mogul 4447

to bulbs starting at $1000. what do i need?

also the geen filter, would that be a piece of green glass or again something special? i also wonder about the "box", would that be a white box reflecting the light? because i thought the light was supposed to be directional.

the flat was $60, so i dont see myself spending big $$ on this.

M Stewart · Apr 23, 2017

dian said:
i got myself my first optical flat. its 100 mm in diameter and specified at 0.07/0.02 mu. i have two questions:

1. what light source to get? ...

I soon got fed up with "natural" light (mainly compact fluorescent, and after seeing the prices for dedicated sodium lamps as advertised by UK optical flat suppliers, I made my own using a redundant street lamp module as the source. It uses a Philips SOX 35W lamp, and takes a few minutes to warm up. Ebay (UK) was my source, and I made a vertical housing with a diffusing plastic surface to hide the grill on the street lamp's glass cover. The lamp included the necessary choke and starter gear. Cost was around £25, plus a little for timber frame etc.

dian · Apr 24, 2017

thanks. so that is a "low pressure" lamp. i can only find it in the states for some reason around $30 plus shipping. the one i listed is "high pressure". and can be had for $5 locally. whats the difference? and yours needs a special power supply? i saw something about 70v somewhere.

M Stewart · Apr 24, 2017

dian said:
thanks. so that is a "low pressure" lamp. i can only find it in the states for some reason around $30 plus shipping. the one i listed is "high pressure". and can be had for $5 locally. whats the difference? and yours needs a special power supply? i saw something about 70v somewhere.

Yes, ideally you need the low pressure version because it has a narrower spectrum. Being a gas discharge type of lamp, it needs special electrics to stabilise its operation. In my case, it's a choke and a starter (as in old-style fluorescent tubes) to get it going. It takes about 10 minutes to produce its yellow light.

I was lucky in that I'd bought several optical flats, and when I realised that I needed a proper light source, the UK was busy changing over from old style street sodium lamps to more modern, so ebay was a good source. I've had to cover the "illumination level" switch to fool it into allowing the lamp to operate. (Much easier than trying to decode the wiring.)
Mine uses the old "street standard" 35W tube. Lower wattage tubes were very expensive. PM me your email address, and I'll send some jpegs of my lamp.

Mark Rand · Apr 24, 2017

New SOX/low pressure sodium lamps, ballasts and starters appear on EBay. That's where I got mine from. The price is quite low and postage to Ch from GB, De, Fr etc. should not be a problem, since the weight is small.

CalG · Apr 24, 2017

This
http://www.engis.com/flatness-measurement-devices.php

HeNe lasers work fine with the beam sent through an expander.

LEDs don't work at all. The emission band is much too wide.

dian · Apr 25, 2017

i had to go to the uk site to find the stuff:

sox low pressure | eBay

so i get the the £24 lamp, £12 supply and pay £23 shipping. well, not so bad. i dont see any combined deals.

M Stewart · Apr 25, 2017

Bear in mind that the UK and European domestic mains electricity supply is around 230V, and that's used for local street lighting in the UK. No idea about the practice in Europe.
(I've had to pay eye-watering Postal charges for some stuff I've imported from the US via ebay, and my optical flats from Melles Griot were a good example.)

Nobby · May 6, 2017

Seeing fringes, is not just a matter of a decent monochromatic lightsource. The surface you inspect needs to be reflective. Let's say Ra 0,1 micrometer of better. (Ra 4 micro inch)

I use optical flats for checking flatness of parts after flat lapping. Lapping on a conventional cast iron lapping plate gives dull (but very flat) surfaces - unsuitable for inspection by optical flat. Trick of the trade is to make the surface a bit more reflective by fine sandpaper.

Lapping with more modern composite lapping plates and diamond fluid gives a reflective (and flat) surface at once.

You can see flat lapping and flatness measuring by optical flat in my video about flat lapping. With some basic knowledge of Youtube you can get subtitles in your own language.

CarbideBob · May 6, 2017

Nobby said:
.... Trick of the trade is to make the surface a bit more reflective by fine sandpaper.

Won't this step affect the flatness?
Well under 4 is doable on a Blanchard at much higher speeds than a lapper but the optics and mirror guys are down in crazy stuff.
Often check 15+RMS grind with optical flats without going to a hand polish step. Ultrasonic clean is cheap and helps much for me.
Not sure you need reflective as much as you need Intel level clean room clean.

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