Optical and Light Sensors

Optical sensors detect light and convert it into an electrical signal. They rely chiefly on the photoelectric and photoconductive effects, in which photons striking a material free charge carriers or change its conductivity. Because light covers a range from infrared through visible to ultraviolet, optical sensors are designed with a spectral response matched to the wavelengths of interest, whether for measuring ambient brightness, detecting a beam, or reading data from an optical medium.

Several detector types are in common use. The light-dependent resistor (LDR), or photoresistor, lowers its resistance as illumination increases and is simple and inexpensive but slow. Photodiodes generate a current proportional to incident light and respond quickly, making them suitable for communication and precise measurement. Phototransistors add internal amplification for greater sensitivity, and avalanche photodiodes provide internal gain for very low light levels. Photocells based on the photovoltaic effect generate a voltage directly when illuminated.

Optical sensors underpin many applications: automatic lighting and camera exposure control, optical encoders for position and speed, barcode and disc reading, infrared remote-control receivers, and laser-based measurement such as laser Doppler velocimetry. They are frequently paired with a light source, such as an LED or laser diode, to form an emitter–detector pair that senses reflection, transmission or interruption of a beam. The choice of detector depends on the required speed, sensitivity and the wavelength of the light involved.

Frequently asked questions

What is the difference between a photodiode and an LDR?

A photodiode produces a current proportional to light and responds quickly, while an LDR simply changes resistance with illumination and reacts much more slowly.

Why use an avalanche photodiode?

It provides internal gain through avalanche multiplication, making it useful for detecting very low light levels where an ordinary photodiode would be too insensitive.

What does the spectral response of an optical sensor mean?

It describes how strongly the sensor responds at different wavelengths, so a sensor is chosen to match the infrared, visible or ultraviolet light it must detect.