A CCD (charge-coupled device) has a grid of sensors that react to light.

A television tube or other CRT (cathode ray tube) detects electrons on its screen.

This photometer measured brightness by comparing shadows

A sensor is a device that measures a physical quantity and converts it into a 'signal' which can be read by an observer or by an instrument. For example, a mercury thermometer converts the measured temperature into the expansion and contraction of a liquid which can be read on a calibrated glass tube. Video cameras and a digital cameras have an image sensor.

There are many different types of sensors. Some are used in everyday objects, and some are separate.