Machining a "perfect" Hemispherical Mirror

I am trying to create a simple part that acts as a reflector for light striking it, for a somewhat special purpose. When dealing with mirrors, the focal-point of a perfect hemisphere would be the radius of the sphere. That is the shape which fits my needs for reflection. (The outside part shape is basically a cylinder with hemisphere in one of the ends.)

Two questions then arise:

Which metal of these two metals, when polished to a mirror finish reflects light better, without re-oxidizing from strictly air-contact? Brass alloy, or aluminum alloy? I would probably take the extra step if needed and apply a silver coating with one of the spray kits available. But only if necessary after polishing.

Will any milling bits provide a perfect hemisphere when cutting, or will this need to be done on a CNC lathe? (The part is roughly no larger than 55mm diameter/30mm length. The very tip of the hemisphere at the cylinder inside center is not as crucial as the other 80-0 degrees of the inside cut down the sides.) I do expect to have to polish the final part by hand to a mirror finish, just need to get close enough with the machining-phase.

Any suggestions or does anyone around here have the equipment to do this part? I can produce a CAD file of what would be needed, if needed.

Thanks for any help here.

You could do it with a CNC mill with a ball endmill; a CNC lathe is less likely to be able to bore that much of the hemisphere in one pass because the boring bar has a minimum radius.

I don't think any manual machine could do it to your satisfaction.

Some telescopes use spherical mirrors, but not a whole hemisphere. If you do not need a full hemisphere, you may be able to buy a telescope mirror for a Maksutov type telescope that would work. Otherwise, you may be able to use some kind of telescope building techniques and materials to make what you want.

The focal point location depends upon where the source object is. You did not say where you intended to put the source. The center of a sphere is the focal point only when the source is also at the center. You can get a fine mirror finish out of aluminum with a diamond bit. It will oxidize though and it is typical to coat it with something like SiO or SiO2. "Perfect" is a relative term when it comes to optics. Hand polishing is an art and requires the proper tools to measure the shape frequently during the polishing. So now I'm curious, what are you making?

Telescope mirrors are coated in aluminum, but then over-coated with a quartz coating to prevent oxidization.

I had a mirror sounds just like what you're looking for. Little over 2" diameter, spherical. about a 3" focal length. Got it at a surplus place online some time back. Don't remember where, tho. Run a search for surplus optics.

I've ground telescope mirrors from borosilicate. Not something you want to get into unless you're planning to build a telescope.

Hey HyperSpeed , I'd do it on my CNC Mill before my lathe with a radius adapter but have both options. You can email me if you have a CAD file (STP, ......or native Solidworks or Pro-E).

Scott

Yes, I completely understand "perfect" means a lot in optics, I just meant a fairly high-quality surface and hemisphere shape when made. (Say as good as medium-grade ground glass surface would be, which usually sells for $25-40 for a ground glass lens of same relative size [Edmund Optics glass for example]). Just a reflector that looks somewhat professional when finished--not with machining marks inside the hemisphere that will be hard to correct and polish by hand, or if chucked in a lathe at low speed to polish. It was more generally meant, in that, it reflects a quality image of the source, the size of the source, back onto the source. The mirror base is to be mounted very near the light source plane, so that the light emitted re-draws an image back onto the source LED itself. I figured, a hemisphere, minus a plane thickness equivalent to how much the source "protrudes" forward into the mirror, would place the same size image back onto the source. What someone else said about not a complete hemisphere, well, that's why I actually said the middle/front 20 degrees doesn't need to be so finely tuned--it won't be there. The cylinder will be ground down, while testing output and exit aperture beam angle, until I find the opening aperture size that I need for the optics being used. I'm simply capturing excess light that spills to the sides, and using that light to re-enter the main image beam which strikes the focusing lens that's out in front of the assembly to collimate the LED image. It needs to be a cylinder exterior shape, to start off, because I am going to use the object to mount some very small electronics to, inside the device I am building, and this seemed like the easiest shape to work off of for my needs and probably for a CNC, too. I would just call it a light recycling-aperture after said and done. An LED is used as the source, and LEDs are great because they are essentially a 1-dimensional square emitting plane the light comes from. This helps me position the reflector with epoxy around outside edges at the correct focal distance from the source emitting plane, so the extra recycled light lands back onto the LED, and lands on it in-focus (at the focal point), producing the same size image as the LED emitting area itself. I developed and tested a few prototypes from coated meniscus lenses over 6 years ago and it indeed works. More light comes out the aperture as a brighter, more intense projected image after passing through a convex lens. Just have new needs for a new project in this situation.

Thanks SCP, I will contact you.

If anyone is confused by my description (I wrote it in a hurry), let me know and I could post a diagram showing how the design actually works, and also showing why it works so well.

A drawing and a clearer explanation of what you are trying to accomplish would be useful. If you are trying to make a very uniform light projector with a well defined angular cut-off cheaply, there's a very simple solution.....

Bob

I diagram would be very helpful since you start out talking about having the LED at the center of the hemisphere and the image being form back on the LED, but then you later talk about things being collimated. A hemisphere, literally one half of a full sphere, is a terrible optic and won't collimate anything. It sounds like you just want a light bucket that send most of the light out one end. There are lots of way to do that without resorting to bouncing light back to the source and hoping it eventually goes where you want it. It sounds like what you really want is an integrating sphere.

JMZawodny said:

I diagram would be very helpful since you start out talking about having the LED at the center of the hemisphere and the image being form back on the LED, but then you later talk about things being collimated. A hemisphere, literally one half of a full sphere, is a terrible optic and won't collimate anything. It sounds like you just want a light bucket that send most of the light out one end. There are lots of way to do that without resorting to bouncing light back to the source and hoping it eventually goes where you want it. It sounds like what you really want is an integrating sphere.

Click to expand...

About 15 years ago we had a contract to design a uniform light source for radiometric scene illumination. We coupled light sources into an integrating sphere and coupled the integrating sphere to a high order Compound Parabolic Concentrator to control the illuminated spot size a fixed distance. We were able to deliver up to 70% of the optical power into a well defined 1 M circle of illumination with a sharp cut-off with a irradiance uniformity better than 5%.

Both the integrating sphere and CPC can be made by electroless nickel plating on a machined plastic plugs The diamond machining the plugs is not inexpensive, but replicate the parts is not very expensive. Overcoating the integrating sphere with a diffuse specular reflector, and depositing a high reflectivity overcoat on the CPC is required to get high efficiency and uniformity.

Bob