QUESTION: What makes laser light travel in a straight beam and can it be dispersed over a larger surface? I've been reading up on laser light therapy for hair regrowth and seen helmets built with lasers for that purpose costing $500 or so. I was thinking there must be some cheaper approach like dispersing the light from a few red dollar store laser pointers. Thoughts?
ANSWER: You can totally disperse light, and it doesn't travel in a perfectly straight beam. It might seem that way to us on the macroscopic level...but even then the beam can be expanded with optics. The difficulty is the power and wavelength, as well as the theraputic value of such treatments (not proven). Going beyond a certain wavelength and especially beyond a certain power level requires a huge increase in the cost over simple solid state laser pointers. There's really not a way around the cost barrier you're talking about unless you go to a larger manufacturing scale.
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QUESTION: How can the dot of a laser pointef be expanded to say a 3" diameter circle? I have talked to people who cut hair that claim it is somewhat promising techology for something so simple. Red is the wavelength I've read works but why would the power output need to be great? Why not longer term exposure to lower power light? I just assume it needs to be laser light so it is sure to penetrate the scalp.
You would use a simple beam expander for the task of expanding the laser light. However, first you have to ask yourself two things: 1) does laser light exposure actually stimulate hair growth and 2) since no one knows an exact mechanism by which it does so, what intensity do you need? If you expand the beam, you lower the intensity. A 3" diameter beam will have an intensity of 1.8 then thousandths of the intensity at a 1mm diameter beam, just because of how much you spread it out. Factor in optical absorption in the beam expander (which is just a series of lenses) and you're down to about a flat 10,000th. There, a 1 mW laser pointer goes down to the equivalent intensity of lasers in the hundred nanowatt range. That's ridiculous, you'd get more red light of your scalp by simply going outside. You definitely need enough light that it's far brighter than sunlight, which is about 700W/m^2 at the Earth's surface. That's bright. Ever tried to use a laser pointer outside in the sunlight? You can't even see it, despite the pure color. Trust me, the intensity of such light is simply far too low to do anything, even before you expand the beam and reduce the laser intensity by a factor of 10,000.