Q I got a negative (-) power value on my clamp on power meter. Please confirm if the arrow label (→) is oriented in the same direction as the flow of power from the power supply to the. The power meter may then temporarily display a negative reading, even though the laser output itself has not changed. In other words, the laser is usually not the problem; the measurement conditions are. The basic process is straightforward: turn the meter on, set it to the correct wavelength, clean your connectors, plug in, and read the. 1. 1 Safety 1 General Information The PM100A Handheld Optical Power Meter is designed to measure the optical power of laser light or other monochromatic or near monochromatic light sources and the energy of pulsed light sources.