Light traps / black coatings

Tom B. · Post by **Tom B.** » Sat Sep 21, 2002 3:59 pm

I recall Colin was asking a while ago about non-reflective shutters / black coatings and noticed this excellent post yesterday on sci.optics (credits to the author, Phil Hobbs). My shutter suggestion would be to use as the shutter element a thin front surface mirror which would divert light into one of these reflective light traps. I expect the surfaces would have to be VERY clean to totally eliminate back-scattered light.

<
Flat black is a last resort for baffles--you'd be amazed at how non-black it can be, especially near grazing incidence, and especially if it's in your detector's field of view. You're much better off with shiny black surfaces arranged so as to trap the front surface reflections.

IIRC, Breault first made his reputation by showing the Hubble team what a mistake it would be to rely on flat black--something like a factor of 10**5 worse than properly designed shiny black baffles. The prototype
of this approach is Wood's horn, a curved, tapered glass tube with lampblack on the outside. Nowadays you'd use black paint, such as Krylon Ultra-flat black spray paint (Sherwin-Williams, Solon, Ohio) instead of lampblack, which still gives ~10**-4 reflectance against
fused quartz per bounce, near normal incidence. Krylon is hardware-store paint, but is a reasonable choice for most purposes.

A black-paint surface in air isn't a highly effective absorber, because of the index mismatch between the binder/carbon black mixture and air. It's very much better for internal reflections, i.e. a glass-paint interface instead of air-paint.

The OSA Handbook of Optics (Vol II, I think) has a articles on black surfaces and baffles that are worth reading before you go too far down the flat black trail.

Cheers,
Phil Hobbs
>

Colin Kaminski · Post by **Colin Kaminski** » Sat Sep 21, 2002 6:47 pm

This is the way professional shutters are built. What geometry would a "light trap" take? Is there a simple shape or would one need to make a bunch of grazing reflections into a spiral.

It should be noted that the ultra flat black is much better than the flat black but since there is so little binder it does not work on the back of reflection holograms. I am told it is more than 95% carbon by weight. I did not think to ask if that was dried or wet. I assume it is dried.

Tom B. · Post by **Tom B.** » Sat Sep 21, 2002 9:28 pm

I don't think a fancy geometry is required unless you need the trap to work over a wide angle, for which I suspect the tapering spiral horn shape to be best. For a collimated beam, two parallel plates, shiny side inside, at an angle to the beam should work fine. Will try coating one side of a couple of microscope slides next time I have the spray paint out. Black glass should work, too - I have a couple of welding filters I can try.

Krylon ultra-flat black doesn't seem to be available in town, but I noticed a posting somewhere from someone claiming that Krylon camouflage black is even better, and I think I have seen this locally - at WalMart? I also saw some complaints about initial outgassing from Krylon UFB paint messing up telescope optics, but this is a worst case situation - a large painted area in close proximity to a mirror in a closed tube. So far, for general blackening I have had best results with Tremclad flat black rust paint. The fast-dry version seems to work just as well.

Tom B. · Post by **Tom B.** » Tue Oct 01, 2002 3:03 am

Black paint on glass did not work well as a reflective + absorptive light trap. There was a noticeable amount of diffuse reflection from the paint layer. The good news is that a pair of No. 10 welding filters (very dark glass plates) worked near-perfectly. Diffuse reflection from the first spot was almost invisible, and after 3-4 bounces, the beam was also invisible. For a 45 degree angle, glass reflectivity should be at most about 10%, so the transmission after 3 bounces would be .01 cubed = 0.0001%, or one part per million.

Tom B. · Post by **Tom B.** » Tue Oct 01, 2002 3:11 am

My silly mistake: 10% is 0.1, so transmission after 3 bounces would be 0.001.

Colin Kaminski · Post by **Colin Kaminski** » Tue Oct 01, 2002 8:49 am

If you were to leave the end open even this .001 could not bounce back.

It is a special order but shade 14 is available for an even lower transmision if needed. Make sure not to use the gold coated welding filters.

Colin Kaminski · Post by **Colin Kaminski** » Tue Oct 01, 2002 8:56 am

Tom,

How did you decide to bounce the light into the trap? I would expect a mirror is too heavy for a meter movement.

Tom B. · Post by **Tom B.** » Wed Oct 02, 2002 3:31 am

The shutter mirror (to reflect into the light trap) need not be thick, or made out of glass, and could conceivably be made from thin shiny black plastic. Fooling around the the welding glass plates at various angles, I found that the angle of acceptance could be made pretty wide. A V arrangement of the plates at a 15-30 degree angle seemed to work well. I confess though, that I was more fascinated by the idea of a perfect light absorber than the practical non-reflective shutter application.

Colin Kaminski · Post by **Colin Kaminski** » Wed Oct 02, 2002 6:41 pm

Thank you Tom you have been very helpful. If you work on the bounce let me know. I am a couple of months from building one.