All photoelectric sensors use a beam of light to detect the presence or absence of a target.
Standard incandescent light was the source for the old electric eye. Visible red or infrared LEDs provide the light for photoelectric sensors today. Like any normal light source, the light emitted by the sensor spreads out and weakens over distance, limiting the sensor's useful range or the size of the target it can detect.
Many of those limitations are overcome when the LED light source is replaced with a laser. Laser light is coherent, monochromatic, and highly directional. Coherent light maintains a constant-phase relationship throughout the beam while its monochromatic properties make it behave mostly like a single wavelength of light.
Laser light also does not expand over long distances like ordinary light. The width of a 1-mm laser beamed from earth grows only to the diameter of a quarter by the time it reaches the moon. The tight beam is often used to detect small parts while its optical gain lets it easily span distances of 300 to 400 m (980 to 1,300 ft).
Lasers are categorized into four classes according to their potential for causing biological damage. The lasers used in photoelectric sensors are rated Class I or Class II placing them among the safest lasers available. Class I lasers pose no hazard. A person's normal aversion to bright light is expected to protect them from Class II lasers. A person would have to stare into a Class II beam for several minutes to risk eye damage.
Pepperl+Fuchs (am.pepperl-fuchs.com) provided information for this column.