Please explain the illuminance sensor.

The illuminance sensor refers to a light sensor which detects the brightness of various emission sources such as incandescent lamps, fluorescent lamps, and sunlight.

A light receiving device for the illuminance sensor is mainly requested of the following features:

Even if the type of light source is changed, the sensor's output value to the brightness should be reproduced almost equivalently.
Excellent linearity of sensor output in response to high/low illuminance.
High light receiving sensitivity. (Detectable even at low illuminance.)
Less mounting space. (The sensor itself is small and does not require extra accessories.)
Excellent functionality as a sensor.