Microfiber Footed

Microfiber Footed
how do I know if my laser is really 50 milliwatts?

So I bought this "50" mw laser from amazon.com. It shipped from microfiber products online. It was only 80 bucks. I need a way to determine the mw of the laser without getting an expensive detector. Basically, I can see the beam at night and sometimes in a fairly-lit room, but only when it's near me. If I walk like 10 feet away, I can only see the beam when its VERY dark.
Thanks!

First of all, measure its power consumption: measure the voltage of the battery (use an AVO on volts range) and the current flowing (use an AVO on mA range, wired in series with the batteries: current should flow into + terminal, out of - terminal) and calculate the product. Remember V * A = W, so V * mA = mW. If there is less than 50mW coming out of the batteries, then there's no way in hell that the thing can be putting out 50mW.

To measure the output power, you will need some way of absorbing the laser's energy and converting it to heat. Half-fill an aluminium drinks can with water, add thermal insulation and a thermometer, and shine the laser through the aperture into the water. The aluminium should keep bouncing the light around until the water has absorbed it all. Note the rate at which its temperature rises.

Dump out the water and refill with the same amount of fresh water at the same temperature as you originally started. Set up everything exactly as before but this time, connect a resistor to a variable-voltage power supply and immerse this in the water (note you will need to cover the lead wires and solder joints with some waterprof sealant). Adjust the PSU so that the resistor is dissipating 50mW (remember watts = volts * amps and amps = volts / ohms) and note the rate of temperature rise this time.

If the temperature rose quicker with the laser than with your home-made 50mW immersion heater, then its output power is greater than 50mW.

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