A light sensor, as its name suggests, is a mechanical or electronic apparatus that detects light. It is used to provide information on distance, shape, speed, dimensions, and even types of substances contained in a wide array of objects. Within industries ranging from architecture to offset printing, light sensors are essential tools used for their high level of precision. Often, they are also employed in day-to-day use by the layperson.

For example, every time a motion sensitive porch light or burglar alarm is activated, a light sensor has just been used. The way these products work is fairly straightforward. A light sensor inside the porch light or burglar alarm detects variance in its field of light sensitivity. Once the predetermined variance threshold has been reached, an electrical impulse is sent to activate the bulb or alarm.

In the manufacturing industry, a simple LED coupled with a light detector, referred to as an "eye," can count the number of products made. The eye detects a marking on the product itself which is called a "target." Each time the target passes the eye, the particular light signature of the target registers on the eye, counting each target as one product. Due to the high speed and precise nature of light, these devices can count at a much higher rate than a human worker.

In web-based operations, such as printing and film extrusion, light sensors are used to stop a machine in the event of a break in the roll of paper or film. As a result, less down time is required to get the machinery up and running again. When the roll, or "web," breaks, a light sensor positioned close to the web senses the difference of light, tripping an automatic shutoff switch within the machine.

Light sensors are also employed in space exploration. They are used to make topographical maps of planetary surfaces, as well as various other celestial bodies. When detecting the qualities of light coming from a given star, planet, or comet, a light sensor is used to gather the information required to resolve the inquiry.

There have been many recent advancements in the realm of light sensor technology. Vast areas of the microcosmic and macrocosmic worlds heretofore left unknown are now open for exploration. There are many new technologies being explored in the field which may come to fruition in the coming years.