How to choose the energy level and threshold

September 19, 2012

OphirBlog

When measuring energy with a thermal head there are two parameters that can be adjusted: the energy scale and the trigger threshold. Although it might seem complicated at first, choosing the right energy scale and threshold can be accomplished by following these simple guidelines.

First, we must choose the energy scale. This is easy. Choose the lowest scale that is still higher than your laser energy.

Now we need to set the threshold. A high threshold is important so noise or vibration doesn’t trigger an energy event. On the other hand, if the trigger threshold is too high, we might cause the meter to reject actual pulses as noise or degrade the accuracy of the measurement. For this reason, we suggest the following guidelines:

For mid-level readings, between 10% and 85% of full scale, it is advised to stick with the default, medium. If the environment is so noisy that you get false readings from noise on the medium setting, then set the threshold to high.
For readings >85% of full scale but that do not exceed 110%, then set the threshold to high and the meter will read even if you exceed 100%.
For readings that are less than 10% of full scale, go to the next lower scale. If you cannot do this because some of the readings are over 11% or you are already on the lowest range, then set the threshold to low.

What if you are on the lowest energy scale and your energy is so small that it might not even trigger on low threshold? There are three possible solutions for measuring such a low signal:

Measure the average power of a series of pulses. As long as you know the repetition rate for your laser, the average energy per pulse is equal to power / rep rate.
Again, have the laser fire a train of pulses, but this time measure the total energy as if it were one long pulse. If you know how many pulses were fired, the average pulse energy is the total energy / the number of pulses.
Switch to a sensor that is more sensitive than the one you have and that is more suited to the low energies you are measuring.

At Ophir, we believe it is best to give the user as much control and flexibility as possible. We could have made everything simpler with one energy scale and a medium trigger threshold. This would be simpler but less helpful. Instead, our power meters have defaults geared to the average laser. If your laser isn’t quite run-of-the-mill though, no problem! Just switch the range and/or threshold to best suit your needs.

For more information see this FAQ or comment below with a question.

You might also like to read: How to Use Your Power Meter at Any Wavelength