Measuring the power of scanning lasers such as barcode scanners presents a problem. A bar code laser beam scans back and forth at a very high frequency so an ordinary photodiode power meter will not measure the power in the beam but rather the average power impinging on it, i.e. the power times the fraction of time the beam is on the detector. Therefore, when exposed to a scanned beam, the reading will be much lower than the actual power in the beam. For example, if a scanning laser delivers 2mW to a photodiode sensor and the beam is on the sensor only 1% of the time, the instrument will read only 0.02 mW.
Ophir offers a special photodiode sensor, the BC20, to measure the power of scanned light beams as well as other short duration light sources. It has a peak and hold circuit that measures the power coming from the sensor and stores the peak power from the start of the measurement period to the end of the measurement period. The highest value measured during the measurement period is held and displayed on the meter. At the end of the predetermined measurement period, the circuit is reset to zero and the measurement starts again. The BC20 can also measure continuous beams as well.
The photodiode of the BC20 is fast enough to measure and hold the peak of a signal lasting for at least 13us. For a scanned beam this works out to a scan speed of about 30,000 inches per second giving a 13us dwell time on the 1cm detector surface. For measuring pulses, the same thing applies: the pulse width has to be at least 13us.
The BC20 has two measurement period modes, “continuous” and “hold”. In continuous mode, the circuit is reset 3 times per second and in the hold mode once every 5 seconds.
The graph below shows the reading of the BC20 for a beam of slowly varying intensity (top graph) and a scanning or pulsing beam (bottom graph).
You might also like to read: