I was reading the flashlight thread and started wondering about using lasers as actual flashlights instead of a pointer or rangefinder. The big problem with using lasers is that everything is red or green or whatever. What would happen if I used 3 lasers, red, green, and blue, and focused them on the same spot? Would I have effectively a white light?
On the same subject, could I de-collamate the beams to illuminate a larger area?
Thanks
Testy
Yes, you can use three lasers to create effective white light. This is one way they made white light holograms. Be aware, however, that with three sources like that, you won’t get quite all the visual color space within the spectral locus. Of course, your LED monitor does essentially the same thing.
It doesn’t even have to be three separate lasers – you can use an appropriately-coated end mirror on a Krypton Ion laser and get all the colors you need – that’s how Laserium generated its different colors.
As for your econd question, not only can you de-collimate a laser, it inevitably happens on its own. Lasers aren’t truly collimated, in the sense of “all the rays being perfectly parallel to each other” or even "the envelope containing x% of the light being perfectly parallel to the axis. Even in a perfect TEM[sub]00[/sub] beam – the best, most perfectly-collimated you can get – the boundaries of the region of x% of the maximum intensity describe hyperboloids of revolution around the optical axis. The beam is only considered “collimated” over a distance called the “Rayleigh range”, whose length is dictated by the wavelength and the minimum beam size. The problem is keeping the beam effectively collimated as long as possible. But in many applications the beam spreads out over a pretty large area. The smaller the focal spot, the larger the area.
Laser beams in different oscillatory modes, or a combination of modes, spread out even more rapidly, as does well-collimated non-lasere light, such as a searchlight beam.
Cal
Thank you for this. I was just wondering whether there would be any advantages to making a laser-flashlight using 3 lasers of the appropriate colors and a movable lens of some sort to de-collimate the beams to illuminate a larger area. There are some fairly powerful lasers that can be battery operated and it might be interesting to try.
OTOH, spreading the beams to illuminate a larger area might negate the advantages of using a laser in the first place. AFAIK, laser light is nothing special except for its ability to put most of the light on a very small target.
Thanks
Testy
I don’t see any advantage using a laser over using an ordinary light source. Lasers are inherently inefficient in terms of light out/energy in. They also carry warnings about damage to peope’s eyes, and use of powerful lasers is restricted (and use of powerful handheld lasers is likely to get even more restricted, as they go up in power. Some places already have laws against their use). Besides, they freak people out. At one of my jobs a selling point of our devices was that they didn’t use lasers. Our competitor’s devices used lasers, and had to carry warning labels.
Besides, LEDs have become incredibly powerful, and have lifetimes in excess of 100,000 hours. Easier and cheaper to use LEDs than lasers.
Cal
Good points. When I do the math, it seems that I’d need a heck of a laser to illuminate a 1 meter circle. 200 mW on a 1mm dot is very intense. The same power spread over a 1 meter circle, not so much. Oh well, another interesting idea that didn’t work. Thank you for your help on this.
Regards
Testy
Another advantage of LEDs and other conventional sources over the coherent output of lasers – no speckle.
There are some cases where you actually want speckle, but for most applications it’s annoying and distracting.
http://www.perimed-instruments.com/support/theory/laser-speckle-theory
Cal
Thanks for the links. Yeah, the speckle can get to be a pain after a while. The LEDs are probably the best bet for right now and God knows there’s plenty of LED flashlights available.
Thanks again for the help
Testy