Light barrier
Photoelectric timing device that triggers the exact time as the vehicle passes – standard for timing special stages.
The light barrier (also photocell) is a photoelectric timing device used to time special and regularity stages. A beam of light is stretched across the road; when the vehicle interrupts the beam, the light barrier triggers the timing at that very moment. This captures the passage to the hundredth of a second.
Triggering is usually done by the front of the vehicle or the bumper at the height of the beam. Light barriers are often set up secretly, so that crews drive precisely not only at the visible measuring point. Many devices have a short dead time after triggering to prevent double measurements.
An alternative to the light barrier is a pressure tube laid across the road, which triggers the time by air pressure – it reacts to the wheels rather than the front of the car. A stage with light-barrier-controlled timing is also called a light-barrier test.
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Related terms
Light barrier test
A regularity test in which timing is controlled by photoelectric light barriers. Timing starts at the first barrier and ends at the second, triggered by the front of the car rather than the wheels.
Pneumatic tube test
A regularity test whose timing is triggered by driving over pneumatic tubes – similar to the light barrier test, but via the wheels.
Pressostato
A pressure-sensitive tube laid across the road that, in Italian regolarità events, detects the front wheels passing over it and triggers the timing. Alternative: a photocell.