What is an Illuminance Sensor? | How does it work?


An illuminance light sensor senses the presence of light. Photodiodes and photoresistors are two types of devices used to sense light. They are responsible for the ambient screen brightness in your smartphone. When the photosensor in your phone detects high intensity it increases your phone’s brightness automatically. This is one of the many applications of the illuminance sensor. They are used in the shipping industry for security, and in agriculture and setting up solar panels.

How do they work?

Photoresistor based photosensors are cheaper than the photodiode ones. A photoresistor is a light-dependent resistor. The resistance of this resistor changes with respect to the intensity of light that falls into it. They are not as precise as photodiodes in light detection. Therefore, photoresistors are used in applications that involve comparing light levels or simple light on/off scenarios. The resistance of the photoresistor is inversely proportional to the amount of light. So, the resistance of the resistor decrease with an increase in light intensity.

Unlike photoresistors, photodiodes are more accurate. Photodiodes are diodes that generate a voltage when illuminated with light. When light falls into the photodiode’s PN-junction, it generates a voltage. This voltage is proportional to the intensity of the light, the higher the intensity of the incident light, the higher will be the current. This relation is used to find the intensity of light. This is the same principle applied for a solar panel. A solar panel is a big photodiode that generates electricity.

The light intensity at the sensor side is measured in lux, whereas the intensity at the source is measured in lumens. Lux is the ability to perceive the light from the source, as dispersion causes very little light to reach the detector. The lower the lux rating of a sensor, the better the detection.


Photoresistors are cheaper, and they are handy while installing automated light on/off systems at home. They are also used in streetlights to automate the on/off function as well. Some cameras use a photoresistor as the light meter to measure the available light at the camera.

The ambient brightness feature in a smartphone is the best example of a photodiode application. They are used in smoke detectors, cd and DVD player that demands very high precision detection. They also find applications in the medical field as well. In dry areas, farmers use this technology to find places with suitable light for farming. And, photodiodes help in finding the spot with maximum light intensity for setting up solar panels as well.


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