How to Measure Accurately Using a Thermopile Detector

This technical note provides information on how to set up an optical power meter and detector system in order to make accurate measurements when using a thermopile detector.

When choosing a thermopile detector, it is a safe practice to select a detector with a higher damage threshold specification than that of the actual laser beam power/energy density (J/cm2 or W/cm2).1 A good rule of thumb is to select a detector with a damage threshold that is at least twice the expected power density of the laser you are measuring. For maximum power measurement accuracy, select a detector that has its maximum average power (W) that is approximately twice the value you wish to measure. These guides will give some allowance for hot spots and unexpected fluctuations in the laser beam power.

If you are measuring powers close to the maximum average power of the thermopile, the displayed value may drift even though the power from the laser is staying constant. This occurs because the thermopile voltage is based on the temperature difference between sensors2. If the detector is unable to dissipate the heat efficiently, the readings will be influenced. Allowing the detector to thermally stabilize in the lab first will minimize measurement fluctuations

Recommended Steps

1. Connect the detector to the power meter. Turn the power meter on, and allow the power meter and detector system to warm up and thermally stabilize for 30 minutes or longer.

2. Select the right wavelength setting in the power meter setup screen for the laser or light source you are measuring. Refer to the Optical power meter user manual for the wavelength selection procedure.

3. When you are using the 1918-R, 1936-R, or 2936-R power meters, make certain that the analog filter is set to either 5 Hz or 0.5 Hz. A recommended digital filter setting is 100 or 1000. The procedure for setting the analog and digital filters can be found in the specific power meter manual.

4. Position the detector in the beam path with the light source off or blocked. Use the zero function in the power meter. Refer to the Optical power meter user manual for the zeroing procedure. Make certain that the readings are within the noise level of the detector being used.

5. For the best result, aim the laser beam at the center of the thermopile sensor. However, note that the response is almost independent of the input beam size and position. The sensor’s sensitivity will not vary more than +/- 2% of the measured power intensity, as a function of the position on the active area.

6. Place the detector normal to the input beam such that the beam is centered on the active area. Measurement values depend on the angle of incidence.

7. If you are working with a large spot size, high power lamp, such as a solar simulator, make certain that the beam diameter is within the active area of the detector. It is important that the light does not illuminate the metal housing, as it will affect heat dissipation and cause inaccurate measurement.

8. Begin taking measurements.