22.4 CAMERAS

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Cameras use available light from a scene.

The light passes through a lens that focuses the beams on a plane inside the camera. The focal distance of the lens can be moved toward/away from the plane in the camera as the scene is moved towards/away.

An iris may also be used to mechanically reduce the amount of light when the intensity is too high.

The plane inside the camera that the light is focussed on can read the light a number of ways, but basically the camera scans the plane in a raster pattern.

An electron gun video camera is shown below. - The tube works like a standard CRT, the electron beam is generated by heating a cathode to eject electrons, and applying a potential between the anode and cathode to accelerate the electrons off of the cathode. The focussing/deflecting coils can focus the beam using a similar potential change, or deflect the beam using a differential potential. The significant effect occurs at the front of the tube. The beam is scanned over the front. Where the beam is incident it will cause electrons to jump between the plates proportional to the light intensity at that point. The scanning occurs in a raster pattern, scanning many lines left to right, top to bottom. The pattern is repeated some number of times a second - the typical refresh rate is on the order of 30Hz

Charge Coupled Device (CCD) - This is a newer solid state video capture technique. An array of cells are laid out on a semiconductor chip. A grid like array of conductors and insulators is used to move a collection of charge through the device. As the charge moves, it sweeps across the picture. As photons strike the semiconductor they knock an electron out of orbit, creating a negative and positive charge. The positive charges are then accumulated to determine light intensity. The mechanism for a single scan line is seen below.

Color video cameras simply use colored filters to screen light before it strikes a pixel. For an RGB scan, each color is scanned 3 times.