Time of Flight Optical Sensors

Time of flight optical sensors determine displacement and distance by measuring the time it takes light to travel from the instrument to a target and back. These position sensors use either lasers or LEDs as light sources. Light emitting diodes (LEDs) are less powerful than lasers and, therefore, have a shorter effective distance. Because LEDs are relatively weak sources of light, however, they do not have safety classifications and can be used in different applications. The lasers in time of flight optical sensors are designated as safety class I, II or III devices. 

Selecting time of flight optical sensors requires an understanding of measurement methods. Some methods analyze a single trip or a pulse of light. Others calculate the measured range over a number of trips by using a continuous wave. For very long-range distance measurements (up to many miles), time of flight laser rangefinders with pulsed laser beams are used. The laser emits very brief, very intense pulses of light. The amount of time the pulse takes to reach the target and return is measured and converted to a distance measurement. Depending upon the application’s requirements, the scan field for time of flight optical sensors can use a single point of light or a line.

Time of flight optical sensors use either visible or invisible light beams. Sometimes, a visible beam can make the sensors easy to mount and aim at a target. Occasionally, however, a visible beam may be undesirable. For instance, in a factory or processing center where a number of people working, a visible beam may be distracting and lead to accidents. In such applications, time of flight optical sensors with invisible light beams are desirable or preferable.

Product specifications for time of flight optical sensors include measuring range and accuracy. Range resolution and sampling rate are also important. Resolution is the smallest change in distance that a position sensor can detect. Outputs for time of flight optical sensors are designated as analog voltage, current or frequency; digital transistor-transistor logic (TTL), serial or parallel signals; or even a switch or an alarm.

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