Cameras have always relied on light-sensitive surfaces in order to capture images for posterity. However, the nature of that reflecting surface has changed over the last few decades. While photographic film used a chemical process to capture an image, an image sensor can perform the same process by electronic means. An image sensor is a device that converts an optical image formed by an imaging lens into electronic signals, according to the book "Image Sensors and Signal Processing for Digital Cameras."
About Diodes
Electric image sensors can detect light with the aid of diodes. You can also see diodes in all manner of electric circuits. In fact, diodes make up one of the building blocks of modern electronics. Diodes mostly consist of a material called a semiconductor, which conducts electricity only when certain conditions are met; for instance, when you shine a light on it. A diode actually consists of two joined semiconductors that are doped with impurities so they can behave in a certain desired way.
How Diodes Work
The doped semiconductors, when joined together, form what is called a p-n junction. Through this p-n junction (p-n stands for positive-negative), electric current can flow one way, but not the other. This makes diodes extremely important in electronic circuits, where the direction of the current flow is vital for electronic devices to work properly. Over the years, engineers have found a multitude of uses for diodes as they have experimented with different semiconducting materials and doping methods.
Diodes and Light
Over time, people found that certain materials, when used for semiconductors in a diode, emitted light when a current passed through them. This phenomenon is called "radiative transition" and became the basis for light-emitting diodes, or LEDs. Nowadays, LEDs are used for everything from digital watches to automobile headlights and as backlights for television screens. It turns out the opposite can be achieved: diodes that absorb light and turn it into electric current, instead of emitting light whenever electric current passes through them.
Photodiodes
These types of diodes are called photodiodes, and make up the main components in image sensors. A typical image sensor in a digital camera contains a vast number of microscopic photodiodes. When each photodiode gets hit with light, it accumulates electrons on its cathode (the negative side of the p-n junction). The camera then measures the accumulated charge on each of the photodiodes. The overall pattern of electrical charges reflects the amount of light and shade in a given image. When capturing color images, each diode is covered with either a red, blue or green filter that lets the camera measure a single primary color per diode.
Types of Image Sensors
Of all image sensors, the most commonly used type is the charged-coupled device, or CCD, according to the book "Video Production Handbook." CCDs represent a more mature, rugged technology that results in small image sensors that deliver great results. Another type of image sensor is the complementary metal-oxide semiconductor, or CMOS. Although traditionally being susceptible to noise, CMOS technology has begun to catch up with CCDs. CMOS sensors also tend to consume up to 100 times less energy than CCDs, and are also cheaper to produce.
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