Time of flight

Measuring a distance without contact means that a tape measure or a caliper cannot be used: a signal is needed that reaches the target and comes back. The time of flight method exploits exactly this: a signal is emitted (typically a LASER beam), the time it takes to return is measured and, knowing the propagation speed, the distance is calculated.

Since the speed of light is known and constant, ToF allows fast, accurate and repeatable measurements even over long distances, and it is insensitive to the nature of the surface in a wide range of cases.

How time of flight works

The sensor emits a light signal toward the target; the light is reflected and returns to the receiver. By measuring the round-trip time t and knowing the speed of light c, the distance is obtained as d = c × t / 2. Since light travels about 30 cm in one nanosecond, the measurement requires very precise timing electronics.

The visible red LASER beam makes alignment on the measuring point easier and keeps a small diameter even at a distance, ensuring a localized and repeatable measurement.

Pulsed ToF and phase measurement

There are two main ways of measuring the time of flight:

• Pulsed ToF: the delay of the reflected pulse is measured directly. It is suitable for long distances and for poorly reflective targets.
• Phase measurement (amplitude modulation): the phase of the emitted signal is compared with that of the received signal. It offers high accuracy (down to the millimeter) over medium distances and is typical of precision industrial distance sensors.

Many distance sensors combine the two techniques to cover a wide range while maintaining high accuracy and a high measuring rate.

Advantages of ToF measurement

• Non-contact measurement, even on moving, hot or hazardous objects.
• Wide measuring range: from a few centimeters to hundreds of meters (more with a reflector).
• High accuracy, down to the millimeter with phase measurement.
• Analog and digital outputs for integration into automation systems.

Applications and instruments

ToF is used to measure level in tanks and silos, position and displacement of overhead travelling cranes, cranes and machines, clearances and distances on the production line, and in safety and anti-collision systems. The industrial LASER distance sensors distributed by RODER cover these applications: models and technical data in the linked catalogue pages.

Related products

RODER instruments for this topic. Go to the catalogue pages for models, technical data and documentation.

Frequently asked questions