dav said:
i need help with some ir stuff thats confusing to me.
i dont know if this is the right place to post in
or if there already is a thread on this somewhere.
#1 what is ir and how can you test if there is any
#2 is the ir consentrated around the beam? does it spread everywhere the instant
the laser is turned on? does it reflect of objects (walls)
#3 if a laser has ir filter do i still need laser goggles?
any help would be appreciated.
Hi Day,
#1. They are longer wavelengths just above the visible spectrum beginning around 700nm or so and going to 1600nm or maybe more I'm not sure where exactly the upper boundary is. You can test best by having a meter with a thermal head and then mount or hold the beam steady onto the head, record 532nm power, then put an R72 IR passing camera filter in the path, the green will disappear and what is showing on the meter will be pure IR. Of course you want to make sure you're still pointing it directly at the thermal head as you can't tell visually since it'll be invisible wavelengths you're measuring.
I also use an Edmund multi-coated IR filter that removes about 99% of IR to cross check, but I've found that the R72 IR passing filter works better, especially if you're not positive about the quality of the IR filter. Those cheap green IR filters (yes I even sell those too) don't do any where near a perfect job and some are just terrible.
#2. It's not Just around the beam so to speak, it exists in the 532nm part of the beam as well. It's not true that the IR spreads out like an LED or flashlight. The reason being is that these lasers use an IR laser diode that emits coherent light and being coherent means in part that does not spread out like normal light from an LED or flashlight would. Since the lasers lenses and coating are designed for the 532nm, the IR wavelength will be a little bit "out of focus" . This will result in a higher divergence; but it's not as much as one would expect. I've done tests with an IR sensitive card (you can get them from edmund, but not that cheap), they will light up when IR hits them. The tests I've done at ~30 feet distance shows that in the pen pointers I've tested, the divergence of the IR portion is about 1.5x that of the 532nm, ie. 532nm spot at 30ft/ 10 meters was around 1.1 cm (1.1mrad) while the IR was ~1.7cm (1.7mrad).
#3. IR or no IR, it only takes a little over 5mw right in the eye to cause permanent eye damage, so yes.
On your last question about how much an IR filter will reduce the output, any added surface is going to reduce all light 532nm and any other by ~3%, since a filter has two surfaces, the front and the back, it would be at least 6%. As far as amount beyond that it dends on how much IR was being emitted to begin with; for instance if a 100mW 532nmpointer (as measured on a thermal head) was recording recording 35mW after an R72 IR passing filter was put it the beam path, then you'll know that the IR portion was actually ~37mW (remember the filter is reducing the IR by ~6% as well) and from that you would know that the 532nm portion was 100-37 = 63mw.
It get much more complicated with a meter that uses a silicon sensor as they are linear across the spectrum and are not accurate at all if the beam contains any IR mixed with the 532nm. This is because a silicon sensor is way more sensitive to IR, so when set to measure 532nm, 1 or 2mw of IR mixed in can increase the meters readout by 30mw or more.
So you can see that's not a question that can be answered easily, but it easy to measure if you have a thermal head and an R72 camera filter.
