Building My First RGB Projector (Opinions?)

[tvojmuc]

Very thanks for information, I will buy a new fog machine
And some info about safety? What is max power somehow “allowed” in room – 10-50mW is OK and safe enough?

[Mr.Coffee]

Is it possible to use a 405nm to "boost" the lower end of the blue spectrum? I've got a 473 in my projector, and I was thinking about adding a couple hundred mw of 405 to mix with it, though from your description it sounds like it would be a waste of time. Ahh to have a 445. (though at this point the red and green are priorities)

[buffo]

If you already have 473, adding 405 to it *will* increase your color palette. Granted, your beams won't look much different, but you'll see it in graphics and abstracts on the wall. It's hard to describe, but you get some very interesting purple-magenta colors when you have some 405 in there. It also makes your white look better.

Daedal has both 473 and 405 in his projector, and we watched several shows at FLEM ver 6 with his projector side-by-side with one that didn't have the 405. The differences were subtle, but they were noticeable. It made enough of a difference that I decided it would be a good idea to add some 405 to my projector as well. Unfortunately, that project has gone nowhere. (The 8X burner diode is still sitting on the shelf in the original package it shipped in.) Maybe this winter....

As for tvojmuc's question - a "safe" amount of power for indoors is hard to define. If you mean safe for your eyes, then you really need to keep the *irradiance* down below 100 mw per square centimeters. (Which is 10X the MPE, in line with ILDA's new guidelines.) Note that I said irradiance, not power. (There's a difference) And to calculate irradiance, you need to know the beam diameter of the laser, the divergence of the laser, the power of the laser, and the distance from the projector to the audience. (Or you can take some real-world measurements with a power meter and a properly-sized aperture.)

As a general rule of thumb though, if you're going to run your projector in your home (thus assuming that the room is no longer than about 25 feet or so), you don't need a lot of power. A couple hundred milliwatts will look *very* bright in a dark room with a little fog. Even 100 mw will look really cool if you have all the lights off. (See the pics above, and that's only 50 mw!) When in doubt, always make the room as dark as possible. Then, with a little fog, even very low power lasers will still look awesome.

I think anything over 500 mw is probably overkill for a home projector. (To make a 500 mw projector safe for your eyes at such a close viewing distance will require a negative 2.5 to negative 3 power lens in front of the projector in order to diverge the beam enough.) Admittedly, so long as you keep the beams out of your eyes, a 500 mw projector will even look pretty good with some light coming into the room from adjacent rooms or windows. (Again, assuming you have fog in the air.)

Adam

[tvojmuc]

Thanks for this answer. Now I know that I need fog -OK- no problem.
This weekend I had my first test with XY scanner position calibration but only with 1mW Red laser. I'm still waiting for correct laser.

Could you explain me what is »negative 2.5 to negative 3 power lens” in your answer?

[buffo]

A lens with a negative focal length (or a negative power) is a lens that EXPANDS the light entering it, rather than focusing it down to a point. Typically, these lenses are double-concave in shape - that is, they look like this: )(

Placing such a lens in front of a laser projector will increase the diameter of the beam. (It increases the divergence.) This spreads the power out over a larger area, which reduces the irradiance, making it safer for your eyes.

Adam

[tvojmuc]

Thank you for this info.
Do you have some picture or scheme, how is look like typical setup of lasers set-up in box with 3 laser diode? With lens, cooler, some prisms, lens or..

[buffo]

The most common way to do it is to use a pair of dichros. These are special mirrors that reflect one color while allowing other colors to pass through them. Typically the Green laser is last in the chain (furthest away from the galvos). Then you use a pass-green, reflect red dichro to mix in the light from your red laser. Finally, you use a pass-green&red, reflect blue dichro to mix in the light from your blue laser. The final layout looks something like this:


However, there is another way to do it. You can purchase a tricho and do the same thing with it. (Trichos are found inside higher-end digital cameras, and are designed to split up the incoming light into the red, green, and blue components to be sent to individual CCD's inside the camera.) A Trichro can be had for around $50 on E-bay, making it slightly less expensive than a set of dichros, but the trade-off is that you end up with a clumsy looking beam path that spreads the lasers out in a wedge pattern. Here's a picture of what a trichro setup looks like:


Adam