Introduction: Making a Green Laser Pointer a Little Bit Safer
I use a cheap, allegedly less than 5mW, green laser pointer to point things out in the night sky at public astronomy events. Unfortunately, recent NIST tests show that a lot of green laser pointers generate a lot of extra visible and infrared light. The infrared one can do something about.
I ordered a 9.5mm IR cut (not IR pass!) filter from an ebay seller. It was $7, shipped, for a pair. Direct from China, of course.
Step 1: Testing the Filter
Trust but verify. Once the filter came, I had to check to ensure that the filter was doing its job. Fortunately, these days, most of us have infrared sources around, namely TV remote controls, and infrared detectors, namely phone cameras with inadequate infrared block filters. I made sure that my Galaxy S2 phone's front camera clearly showed the infrared LED pulsing very brightly on my TV remote. Then I put the filter that came in the mail over the lens. And now the LED was almost invisible--just a dim, dim pulsing shade of white showed up. Wouldn't have seen it were it not pulsing. This isn't a quantitative measurement, but obviously it was blocking a lot of infrared.
I also made sure that it didn't matter which side up the filter was pointing. One side of the filter had a shiny coating. Both seemed equally good at blocking the TV remote's infrared LED. But just to be safe, I was going to use the shiny side on the laser pointer side (because one normally puts the shiny side of a filter on the side of the source).
Step 2: Glue Filter On
I put E-6000 glue around the output hole on the laser pointer and glued the filter on, shiny side towards the laser.
Once the glue set a little, I put a bead of glue around the edge. And then eventually I put duct tape around the filter, and to some degree on the front of it (except where it counts), to make sure the filter wouldn't come off too easily in my pocket.
Ideally, I'd have mounted the filter inside, but I couldn't get the tip off the laser (and hurt myself with the pliers while trying).
Step 3: Final Testing
Now it's time to test how much infrared is getting through. Of course, the super-bright green light coming out of the laser was going to swamp the infrared. So I used an infrared-pass filter. I.e., the dark part of a piece of exposed film negative, folded in half. I verified that this passed infrared by shining my TV remote control through it. It was a little dimmer than without the filter, I feel, but it was plenty bright. I then shone the laser through it. I made sure I wasn't pointing the laser at the phone's sensor, as that would damage the sensor. While the laser is very directional, I would expect that even without pointing the laser at the sensor, there should be a lit-up infrared circle on the underside of the exposed film if a lot of infrared is passing through. (Certainly the visible part of the laser really lights up whatever it goes on.) And there was very little showing up.
Ideally, I should have done a control test before sticking the filter on. But I didn't think of that.
No doubt, the laser IS STILL DANGEROUS! But it is a little bit less so. And that matters at star parties where there are children.