SZH coaxial illuminator LED mode: Reevaluating the Jigginess (part 4)

[xiphmont]

The end result of more playing around with the plastic LED collimaters: they're not going to work well. Dang.

The microscope wants a small diameter parallel light beam, and the plastic collimaters just won't do that. Even the tight spots overfocus and overmagnify, and there's no real room for adjustment.

Noq2's approach works because of:

a) brute force: 60W of LED light is the equivalent of around 350W of halogen light

b) a large die LED (7mm diameter!): that's more like a small COB

In his case it's a 'close enough' approximation to infinity focus because of the huge die and tons of output. There's enough nearly parallel light in there along with all the rest to make it work. Most of the light that makes it into the optical path (which is likely only 5-10%, the beamsplitter doesn't combine much) is just lighting up the inside of the microscope body, but the light that does make it all the way through is diffuse and even and nice. Brute force works!

Me, I want as much of the light to be usable as I can get.

So I'm probably back to using lenses. The stock Olympus setup uses two air-gapped elements with a total focal distance of 11-12mm placed on either side of a halogen bulb. The bulb filament intentionally sits just in front of the focal point to defocus the image slightly. (EDIT: Actually, I'm wrong here; it's just past the focal point in order to produce a focused image of the bulb filament at the point of the iris in the microscope body).

Olympus uses some really nice glass. The first condenser element is a partial shortpass to filter out some of the infrared. The second weaker element nearly touches the first and finishes the collimation job. Two 45 degree front-surface (!) mirrors direct the light into the beam combiner body.

The biggest constraint on the collimater design is the diameter of the optical path through the scope, which is a little under 15mm at multiple points. Opposing that, we want to collect as much light as possible from the LED into the condenser, which means putting the lenses as close to the emitter as possible, and so choosing the shortest practical focal length. Focal length trades off against beam width; the shorter the focal length, the more the 'image' of the LED die is magnified and the wider the final collimated light beam.

The original Olympus optical design expands the image of the halogen bulb filament into a beam of approximately 15mm diameter, matching the optical path. The Cree XP-L LEDs I'm using have a die almost the same major dimension as the bulb filament.

A more powerful LED with a bigger die probably isn't useful if we're going to use a single-stage collimator. And an XP-L is easily the highest-flux LED I can get with a die this small.

The XHP70.2 might put out 5x as much light, but if that's over 5x as much area, it's not a net gain given the constraints (I'm going to test it anyway, but I don't have high hopes). The big die of the XHP70.2 isn't a problem for noq2 because he's not using any optics that magnify its apparent size. He could usefully apply a 15mm COB.

This also means we're not going to improve on the original Olympus lens choices without going to a more complex beam reduction design that probably won't fit. (EDIT: Actually, I can probably fit a Keplerian design in there)

I'm going to try it Olympus's way. Given how dang nice those lenses are, I'm totally yoinking them. And since I'm messing so much with the physical layout, I want some adjustment ability. Which means at this point-- I'm most of the way back to my original design. Oh well. At least much of it is actually built and this has become an iterative process. A little at a time rather than one fell swooooooop.

So... The next step is making some new lens tubes.

And mounting the Olympus lenses in them (using nice, reversible, not messy O-rings).

This gives me the lenses at the proper separation in a durable, flexible package. Now I have to make a tube mount that fits inside the lighting enclosure.