Transmissive vs. Retro-Reflective Optical Sensors: Key Differences

This article compares transmissive optical sensors and retro-reflective optical sensors, highlighting the differences between the two.

What is an Optical Sensor?

Introduction:

Optical sensing refers to the method of detecting light rays.
An optical sensor is a device that converts light rays into an electrical signal, similar to a photoresistor.
Typically, an optical sensing system includes a transmitter (e.g., an LED as a light source) and a receiver (e.g., a photodiode or phototransistor as an optical detector).

optical sensing using optical sensor

Optical sensors are categorized into transmissive and reflective (or retro-reflective) types.

transmissive optical sensor

The figure above illustrates a transmissive optical sensor.
It consists of a pair of components: a light emitter and a light receiver. These components are positioned facing each other across a small gap, all within a single module.
The sensor is activated when an object either interrupts or reflects the light beam.
It’s also known as a “through-beam sensor.”

Popular Transmissive Optical Sensors:

Vishay TCZT8020: Incorporates an infrared LED as the light source and a phototransistor as the detector.
Omron EE-SX

Retro-reflective optical sensor

The image depicts a retro-reflective optical sensor.
It comprises a light emitter and a light detector placed adjacent to each other, facing in the same general direction.
When both the transmitting and receiving components are housed within a single module, it’s referred to as a retro-reflective sensor. It can be triggered in one of two ways:
- 1. An object passing in front of the light beam reflects the light back to the detector. This requires the object to be naturally reflective or have a reflective patch applied.
- 1. A stationary reflector can be mounted opposite the light emitter. In this scenario, a detector positioned beside the light emitter is triggered when an object interrupts the reflected light beam.