What happens if the light in the vehicle cab changes suddenly (going from daylight into a tunnel)?

Optalert is designed to compensate very rapidly for any change in environmental light intensity so it does not affect the measurement of drowsiness. Optalert does this by detecting the level of environmental IR light immediately before each light pulse (500 times per second) and subtracting this from the combined light level (pulse plus environment) during the pulse. This removes the unwanted effect of environmental light regardless of the source of this light, e.g. oncoming vehicle high beam lights, interior cabin light, etc.