Non-Reflective Surface for Optical Housing

You will either need to anodize the metal or some form of zinc chromate primer to prevent oxidation.

Simply spray painting it will not hold up over time.

You might try searching for DIY telescope building supply stores or even a local club for ideas.

You can get black matte vinyl (or other plastic material), with a pressure sensitive adhesive backing, die cut to the size you want in just about any shape you can imagine. Peel and stick onto the surface you want to coat. Here's a link to an example of a material made by 3M. They make a wide variety of materials with various PSAs for a wide range of interior and exterior uses. It's much less labor intensive than painting. I've tested 'electricians tape' -- the black vinyl stuff - and it is strongly absorptive out to 1000 nm.

http://3mcollision.com/products/masking/tapes?color=24

If you've got a complex contoured surface where a decal might not work (like I mentioned in my previous reply), you might consider a powder coating rather than spray paint. It's an extremely durable finish that can be applied with much more control than spray paint.

http://www.innovativealuminum.com/powdercoating/index.php

You might need to talk to a chemist at the paint supplier.

Isn't 940nm in the infra red range? (An thus the colour that we can see/perceive is not necessarily indicative of absorbtion for that wavelength.)

The paint chemist may confirm that a particular component of the paint is absorbent in that wavelength range and advise you of other coatings with suitable concentrations.

Yes, 940nm is infrared wavelenght. I know that mate black paint will do a good job, but I prefer to find a chemical way to get that non-reflective surface finish. If I didn't find anything simple, then I will consider to paint the parts. The optical housing finish has to resist to -40°C to +80°C because the unit is outside in an enclosure.

Again, there is a lot of prior art out there with this that you can tap into. Telescope making is one, but the same technology is employed in optics for cameras.

I would be researching that tact first before reinventing the wheel.

Even NASA sponsors technological assistance to businesses requesting help and at no charge. I don't remember the name of the program, but it would be worth the effort to pursue.

Powder coat it.

Texture is probably more critical than color at infrared. I developed diffuse reflective sensors for flushing toilets. A dark wool tweed was always the worst. There is also a problem with any material that has a brushed finish in that the arc of the brush stroke causes a bit of a spherical reflector which can be a serious problem.

You might also explore the moth eye black concept.

A common practice would be to cut a fine inside thread. This creates a series of light baffles that trap most of the stray light.

Yes, this is what I thought ... I did grooves equivalent to 40 threads/inches on all surfaces that I wanted to protect and I still got some reflections. But this treatment help to low the reflection level.

I don't know what your application is, but I'm guessing your optical system is forming an image - otherwise the stray light might not be a problem. Does your system have a field stop, and have you checked that it is properly sized and positioned?

The system has no field stop because I need all the light.

You have a pretty big NA here, and your'e right - no place for a field stop. I think that you are also right that a surface treatment is the best and possibly only solution. If the optical black anodize (plus threads) doesn't work I'd probably go for a dimensional material like black fabric as some of the earlier posts have suggested.

Maybe the thread cut needs an angle of 10 degrees rather than say 60 degrees.

What if you cut a thousand grooves per inch with deep grooves. Razor blades packed together make a good black surface.

Can send you Bamboo Woven Mat or Grass woven Mat.Please state size you need.

Or do try Black matt paper which you should get at your local artist's materials shop.

Acktar.com produce adhesive black film and vacuum applied coatings for optical instruments used in space telescopes. Could be expensive though.

I used them when I was with a well known optical company in Germany.

Yes, Acktar produce good stuff but it's highly expensive ...

I am looking for a affordable surface finish for 500 - 1000 optical pieces/year.

Something like a special type of anodization. Painting the parts will be my last choice.

Would you believe soot deposit from a candle flame will give you an almost perfect black, but the deposit is fragile, It would be ok in a sealed box where it would not be disturbed.

The anodising will always reflect but, if you need the anodising on other surfaces it is easy to cover the critical surface using a matt black spray can. If it's internal the durability is not an issue

There are good suggestions in previous posts, in our dark enclosures & cameras we use matt black paint, you have to consider fluorescence as well as reflection although that may not be a problem at your wavelength. Interpon do a 'very matt black' powder or you can get photographic matt black stove enamel.

If the shape allows, fitting a sleeve is a good option. Matt paper works well as mentioned earlier. We often machine a black plastic mask from 25% glass filled Noryl, the filler makes the surface dull & we leave a deliberately rough machined surface.

I just tried blasting the inside surface of an anodised part, that leaves a fine matt gray finish.

Good idea but I did that prior to anodize and it helps to reduce the reflections but not as good as mate black paint. Actually I sent samples to a metal surface finish compagny name Pioneer and they claim that they can do '' Optical Black Anodize''. This is a special way to anodize the aluminum and this treatment supposed to be equivalent to mate black paint. I will test it next week.

I was thinking that by blasting after anodising you could keep the cosmetic finish but have a matt blasted finish where reflection is a problem.

I'd be very interested to hear how you get on with the optical black anodising.

Ah OK, blasting after anodizing ... light blasting to keep the black ! ... I will try.

I will keep you in touch of the ''optical black anodize'' (also mane as Martin Marietta Black) testing of next week.

Our small blaster is very light, only about 1 ft³ inside, it's made for dental work.

Hi Nigh,

I just test the '' Optical Black Anodize'' also called Martin Marietta Black patent number #4,111,762 available on USPTO.

The test with 940nm wavelenght is very good. As good as mate black paint. This coating reflect only 0.5% of the light source. As per my tests, regular black anodize reflect more than 15x more than the optical black anodize.

This is the stuff I looking for ...

Pops7

Thanks for the info, I wonder if I can find someone in the UK to supply that finish. I guess the reflectance at other wavelengths would be different, most of our work is in the visible or UV, IR is our enemy & is detected as noise.

There is at least one anodiser compagny offering ''Optical Black Ano'' in UK because I find one last week but I can't remember the name ... try: cylex-uk.co.uk/anodisers

The anodiser that I get optical black finish is : pioneer metal finishing

If you go to the compagny website, you will find the reflectivity as a function of wavelenght curves.

Hi there Nigh, I was wondering if you had managed to find a UK based supplier to apply that finish at all? And if so you might be able to put me in touch!

Kind Regards,

Hayden

We ended up using an Interpon powder finish, "Interpon 610 - AN200E black - very matt".

You might try a green colour since it absorbs light very well and was designed by mother nature. I have no personal experience but have read many times that a green background absorbs light and heat to a higher degree than some other colours. Start with a forest green in a matt finish.

I don't think green absorbs more than other colours, it's just that our eyes are more receptive to wavelengths at around 540nm so it is a very visible colour (unless you are colour blind).

Roll a tube out of silica carbide wet/dry sand paper. Also, dust spray it with flat black from a can. 120 through 500 grit works well.

Glad you found something to do the job...(re: post 28).

And, certainly it is understood that you don't have a "NASA-budget" with which to work.

Nonetheless, couldn't help but notice that nobody had yet referred-to the CR4 blog pertaining to the "Blackest Black", based on THIS article (the video is pretty cool).

(CR4 blog) _{Los Bravos "66}

Try black flocking. It is often used in jewelry boxes and gives a great non-reflective surface that absorbs stray light. You should be able to find it at craft supply stores.

Scribe a series of parallel troughs on the surface, say 0.3mm to 0.5mm deep, 0.1mm apart so you end up with a cross-sectional sawtooth profile surface. Not economic, but it will give a result you want.

That can work, on circular parts we have a groove like a very fine, shallow screw thread cut to break up the surface. Not as good as a matt surface but still effective.