# Laser power measurement

1. ## Laser power measurement

Hello everyone,
I need to measure the power (in Watt) from my laser beam (red - 630nm Wavelength - 10mW max power) as a part of my project.
I have to use phototransistor as it is a compulsory part of the lesson.
It seems that all the documents related to my problem is associated with the "photodiode" method.
Please excuse my English as I'm not a native speaker.

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2. ## Re: Laser power measurement

The problem of absolute power measurement is that it requires calibration.

Photo diodes have a relative low sensitivity variation and high linearity, it might work for a 10 mW laser without grey filters and achieve some 10 % power accuracy without calibration.

Phototransistors are almost useless as power sensors due to
1. large sensitivity variation
2. low saturation power
3. nonlinearity

If you want to use it though you need grey filters to reduce the laser power to a small fraction before exposing the photo transistor.

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3. ## Re: Laser power measurement

I guess I have to stick with this one since photodiode is not available in my country.
I have to make a characteristic graph between the Voltage (V) of the incident beam with the power measured (W).
I'm thinking out make a voltage divider circuit like this (Common collector), I guess measuring V0 according to the changes of the Voltage (V) will reveal something.
Any thoughts on that ?

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4. ## Re: Laser power measurement

I have seen another method that is less accurate, but it could work. It consists in a black heatsink attached to a diode with a metal package (i.e. TO-220). The diode is used as temperature sensor, and measuring ambient and heatsink temperatures will allow you to calculate the power of the laser, which is pointing to the heatsink generating a rise of temperature. You will have to know the total thermal resistance to achieve good results, which can be obtained with callibration.

It's another method, maybe it's useless to you since you must use a phototransistor, but maybe it's useful to somebody else.

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