Measuring CW laser power without fooling yourself

What does a laser power meter actually measure?

It measures the total optical power of a continuous beam that reaches the sensor, expressed in watts. That is not brightness at a point and not the intensity of a spot. If part of the beam misses the sensor, the meter cannot count it.

The SANWA LP10 uses a silicon photodiode sensor with a Ø9 mm area and a diffusion sheet, on a dockable stick probe. It measures from 0.01 µW to 40.39 mW and shows the result on a 4039-count digital display with a 41-segment bar graph. Its listed measured object is a CW laser only: a laser that emits steadily. A pulsed laser is outside what the LP10 is specified for.

Why does wavelength matter?

A photodiode does not respond equally to every colour. The same optical power at two different wavelengths produces two different electrical signals, so the meter’s response is wavelength-dependent. To turn a signal into watts, the meter is calibrated at one wavelength, and readings at any other wavelength have to be corrected.

The LP10 gives direct readings at 633 nm, the helium-neon line. For other wavelengths in its 400–1100 nm range, a spectral-sensitivity conversion table is supplied in the case cover. The routine is simple: take the reading, find your laser’s wavelength in the table, and apply the correction the table gives.

Be careful with the accuracy figure. SANWA states ±5% at the calibrated wavelength. SANWA does not publish the accuracy after a table conversion, so do not assume the corrected reading carries the same ±5%. If you work at one wavelength other than 633 nm every day, SANWA lists a special order, through an authorized agent, for factory customization to any wavelength within 400–1100 nm.

How do I choose the right range?

The LP10 has four ranges: 40 µW, 400 µW, 4 mW and 40 mW. Pick the range that holds your expected power with the least headroom. A reading near the top of a range uses most of the 4039-count display, while a small reading on a large range is mostly the bottom few counts, with poor resolution to match. Across the 40 mW range, 4039 counts spread over 40.39 mW means steps of about 0.01 mW, so a 0.5 mW beam would show as only about fifty counts.

If the display overloads, move to the next range up. If the beam is above 40 mW, it is beyond what the LP10 measures, and you need a different instrument rather than a guess.

What is the difference between laser power and illuminance?

Laser power is radiometric: energy per second in the beam, in watts, regardless of how the eye sees it. Illuminance is photometric: light falling on a surface, per unit area, weighted by the sensitivity of the human eye, in lux. They answer different questions and are not interchangeable.

The SANWA LX20 is the illuminance instrument. It covers 0.1 lx to 403.9 klx in four ranges, and its silicon photodiode approximates CIE relative luminous efficiency, which is the standard description of the eye’s response. Because of that eye weighting, a lux meter would read an infrared beam as almost nothing even though the beam carries real power. Use the LX20 for room lighting and display brightness, and the LP10 for the power in a beam.

What habits stop you fooling yourself?

Confirm the actual wavelength of the source rather than trusting a label, and note which correction you applied. Make sure the full beam falls inside the sensor window and that the probe is held steady, since movement changes the reading. The LP10’s MIN/MAX hold helps you see drift over time. Silicon sensors also respond to room light, so check the reading with the laser blocked and note any baseline.

Finally, record the range, the wavelength and any conversion used alongside the number. A power reading without them is hard to reproduce later. A power meter does not make a laser safe to look at, so follow your site’s laser safety rules.