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This page covers components which emit or detect light, or change their appearance under the control of an electrical input: how to recognise them and understand how thy might fail.

Summary

Of all our senses, sight is our richest source of information about the world around us. Naturally, then, this is often the best way in which electronic devices can communicate with us. This can be through simple indicator lights through to highly complex large format diplay screens, and can embody several different technologies. Conversly, our gadgets often need to detect light, whether just to detect when a beam has been interrupted, for example by closing a lid, or in order to image a scene as in a camera. Again, several principles are used across different devices.

In total, this amounts to a fair number of different types of component, but it's generally easy to identify them, to understand what they're supoosed to be doing, and whether they are indeed doing it.

Emitters of light

Filament bulbs

Filament bulbs consist of a very fine coiled tungsten wire which glos white hot when a current is passed through it. A glass envelope containing an inert gas protects it from very quickly burning out. Filament bulbs arevery inefficient and so can no longer be sold for domestic lighting.

Quartz halogen bulbs are similar but have a fused silica envelope which contais a smll quantity of a halogen gas. This combines with the tungsten as it gradually evaporates from the filament, and decomposes on the hot filament, depositing the tungsten back on it.As a result, the filment can be run significantly hotter without shortening its life, increasing its efficiency and producing a whiter light.Even so, the efficiency is not great.

LEDs

Discrete, COB, IR/UV

Photo-sensors

Light level, Transmissive, reflective, colour temerature

Displays

7 segment, dot-matrix

And now ...

... you might like to continue by reading about Passive components.