DIY diffuser for a laser monochromatic light.

[Flynth]

For a beam expander, a simple telescope works; maybe a microscope objective lens, nice short
focal length, to fan the beam out, and a plastic Fresnel panel to re-collimate it; once the beam is
expanded, it cannot be much hazard to your eyes (hey, most of a footwide beam will miss your eyeball).
Since the light is monochromatic, color-corrected lenses aren't required.

Would the fresnel lens not mess up coherence? It probably will.

The projection setup project requires the expanded beam to cover the area of biggest optical flat I have (6in) and be only slightly divergent at this size. Let's say to 8in at a distance of 10in. So the laser can be mounted 10in above with an 8in screen next to it. The beam travels down 10in bounces back off the optical flat and the item and returns to the projection screen.

So the second lens you suggested a fresnel one for needs to be quite large in diameter (4in)and be close in power to the first lens. Finding a large diameter lens of sufficient power is a challenge.

The inner layers of the light panel behind a dead LCD display include good white
translucent material (is there a dead display somewhere in the closet?).

Hmm, I do have an old broken big screen lcd TV around... But the bottle works too

 

[whitmore]

Would the fresnel lens not mess up coherence? It probably will.

The projection setup project requires the expanded beam to cover the area of biggest optical flat I have (6in) and be only slightly divergent at this size. Let's say to 8in at a distance of 10in. So the laser can be mounted 10in above with an 8in screen next to it. The beam travels down 10in bounces back off the optical flat and the item and returns to the projection screen.

So the second lens you suggested a fresnel one for needs to be quite large in diameter (4in)and be close in power to the first lens. Finding a large diameter lens of sufficient power is a challenge.

All the old transparency projectors have a nice 10" Fresnel lens, you just need to locate the small expander
lens at/near the focus to make a parallel output beam. I've also got a few full-page-size acetate F-lenses of lesser quality...

This toy has two useful 4 inch fresnel lenses...
Art projector
and it was inexpensive at the thrift store...

 

[Flynth]

All the old transparency projectors have a nice 10" Fresnel lens, you just need to locate the small expander
lens at/near the focus to make a parallel output beam. I've also got a few full-page-size acetate F-lenses of lesser quality...

This toy has two useful 4 inch fresnel lenses...
Art projector
and it was inexpensive at the thrift store...

I can't find the source now, but I read fresnel lenses are unsuitable for coherent light imaging because there is a lot of diffraction on the edges of the rings. The projection setup is essentially an interferometer the beam has to be coherent. Any extra diffuse light will make the image washed out and you'll see all of the lens non-continuities in the final picture. If anyone think this is in error please let me know, because a fresnel lens is indeed the easiest to obtain.

Coming back to standard lenses. BTW, the thread should probably be renamed, because we've been talking not about a diffusor(that's essentially solved with my improvised one and another 2 excellent solutions given), but about an interferometer setup. So the below is about the interferometer.

Let's say I use a relatively cheap 40x microscope objective with a about 4mm(0.16in) focal length and the laser emits a collimated 3mm(0.12in) beam. To have the output remain coherent and be 100mm(4in) wide the other lens has to be 133(~5in)mm away if it had a focal distance of 4mm. However such a large lens with so small focal distance is impossible.

This is exactly the same problem to resolve people face with telescopes. The solutions are the same. This means I can forget about this device being compact :-( I have a small hobby(refractor) telescope with the main lens of 75mm. So in theory I could use it as is (with a 3d printed laser adapter) to expand the beam to 75mm. But do I have space for a meter long tube to stick out from what was hopefully going to be a compact/portable device? Not quite.

I might jerry-rig something to see how it would work as it is intriguing, but to have a really nice compact setup I'd have to lap myself (or buy?) a 4in mirror for a Newton telescope. An actual reflecting telescope would be unsuitable because of the extra little mirror in front of the main mirror. It doesn't hurt anything when using it as a telescope, but it's shadow would be visible in the projected image.

 

[whitmore]

I can't find the source now, but I read fresnel lenses are unsuitable for coherent light imaging because there is a lot of diffraction on the edges of the rings. The projection setup is essentially an interferometer the beam has to be coherent. Any extra diffuse light will make the image washed out and you'll see all of the lens non-continuities in the final picture.

An interferometer works with monochromatic light, doesn't need to be coherent; only holography requires
long-range coherent light.

 

[Flynth]

An interferometer works with monochromatic light, doesn't need to be coherent; only holography requires
long-range coherent light.

I've never saw this particular way to measure surfaces by interferometry with a non coherent beam. I actually do have a fresnel lens I bought for another purpose. I'll send the laser through it and I'll shine it at the wall. What do you think I'll see? I need a nice flat circle of even Illumination (but a gausian distribution will do to).

Ill find out shortly.

 

[Flynth]

I wish I was wrong. Using a fresnel lens would make it much easier. Unfortunately... See picture below.
See those concentric bands shouldn't be there.

 

[Firstram]

Take a side trip and learn to grind lenses!

 

[whitmore]

The use of lenses (Fresnel or other) expands the beam; you also need uniformity of illumination, which
requires a diffuser (opal glass is traditional). For the Fresnel lens expander, one wants a diffuser after
the big lens contributes its concentric rings.

 

[Flynth]

The use of lenses (Fresnel or other) expands the beam; you also need uniformity of illumination, which
requires a diffuser (opal glass is traditional). For the Fresnel lens expander, one wants a diffuser after
the big lens contributes its concentric rings.

I'm not sure if you saw the part of my previous post where I talk about two different ways to use a laser with optical flats.

No 1. With diffuse, non coherent light - this requires such light. I'm currently using a large white plastic bottle as my diffuser. It works great. I have an old large TV to pull it's diffurer off so I might make a better looking diffuser from it. Effectiveness wise the bottle is very good.

No 2. With coherent light (what I call a projection setup). This requires a coherent, clean beam. No you can't diffuse it, because it will not work. However expanding the beam is a requirement if you want to see your entire object. You point the beam at the object with the optical flat on top and you watch the reflection of it on a screen.

If I was to use a lens for no1 it would be a very short focal length microscope objective, but it's not needed. The diffuser scatters a point beam sufficiently.

For no2 you need lenses as described before. A fresnel lens is sadly unsuitable for it.

 

[whitmore]

I think I get it.
Not just a light field required, for near-contact interference, but two beams combining
distant from two spaced-apart surfaces...
It has to be a phase reference because of parallax.
Big lens is one way, big mirror is probably better.