Reflective photocells with LED or LASER and photodiode. Anaolog output (device)
LED or LASER microchips (NIR or VIS) with a near photodiodes or phototransistors dice, are able to light a target and receive the reflective signal back and place an electrical signal out.
Very small dimension of the device, embedded electronics to check and drive the device.
Long array or modular transmissive optical barriers with analog or digital output (arrays)
Long array with SMT/SMD devices or microchips (photodiodes, phototransistors) for high resolution and/or high speed optical barriers
Infrared light or visible
Electronics to drive and signal conditioning