Optalert uses pulses of invisible infra-red (IR) light from an LED positioned below and in front of the left eye, housed in a specially designed glasses frame. These light pulses are very brief (each lasting less than 100 microseconds), are repeated 500 times per second, and are directed in a "cone of invisible light" at the wearer’s left eyelid.
The total amount of invisible light reflected back from the eye and eyelid is detected by a phototransistor mounted beside the LED. This phototransistor also detects the level of environmental IR light immediately before each light pulse, and subtracts this from the combined level during the pulse, thereby removing the unwanted effect of environmental light.
A microprocessor housed in the arm of the glasses controls the timing and other characteristics of the invisible light pulses, and digitises the analogue output from the sensors. The power supply and serial output from the glasses are provided via a lightweight cable connected to a USB port, which is mounted alongside the vehicle driver’s seat. This connects to the processing unit permanently installed in the vehicle. This unit provides a variety of different analyses of the recorded signals, including generation of the driver’s Johns Drowsiness Scale (JDS) score and production of visual and audible warnings when predefined scores are reached.
