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Sensors, Pixels and Image Sizes

Image Sensors—Introduction

A CCD is like a threedecker sandwich. The bottom layer contains the photosites. Above them is a layer of colored filters that determines which color each site records. Finally, the top layer contains microlenses that gather light. Courtesy of Fujifilm.
Click to explore how exposure determines how light or dark an image is.
Digital cameras have roots going back almost 200 years. Beginning with the very first camera all have been basically black boxes with a lens to focus the image, an aperture that determines how bright the light is, and a shutter that determines how long the light enters. The big difference between traditional film cameras and digital cameras is how they capture the image. Instead of film, digital cameras use a solid-state device called an image sensor. In some digital cameras the image sensor is a charge-coupled device (CCD), while in others it's a CMOS sensor. Both types can give very good results. On the surface of these fingernail-sized silicon chips are millions of photosensitive diodes, called photosites, each of which captures a single pixel in the photograph to be.

An image sensor sits against a background enlargement of its square pixels, each capable of capturing one pixel in the final image. Courtesy of IBM.

When you take a picture, the camera's shutter opens briefly and each photosite on the image sensor records the brightness of the light that falls on it by accumulating photons. The more light that hits a photosite, the more photons it records. Photosites capturing light from highlights in the scene will have many photons. Those capturing light from shadows will have few.

After the shutter closes to end the exposure, the photons from each photosite are counted and converted into a digital number. This numbers represents the color of a single pixel and it, along with pixels captured by all of the other photosites on the sensor are used to reconstruct the image by setting the color and brightness of matching pixels on the screen or printed page.

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