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Ch. 1 - Electronic Imaging Theory and the SPOT Camera
Digital Photography
User’s Guide to the SPOT Cooled Color Digital Camera, 6/9/98
5
Introduction
This chapter is a brief overview of the operational theory behind the SPOT Cooled Color
Digital Camera. The next two sections, Digital Photography and Color Images provide a
short introduction to electronic imaging theory, followed by a description of how this
theory applies to the SPOT camera. The final section highlights some of the unique
features of the SPOT camera.
Digital Photography
Modern electronic imaging is based on the
charged coupled device (CCD)
. All of
today’s digital cameras have a CCD chip or chips. In digital cameras, the CCD chip/
sensor replaces the film of traditional cameras as the means by which the camera records
the image. The surface of the CCD chip is composed of light-sensitive cells arranged in a
checkerboard pattern. Each cell of the checkerboard is known as a picture element, or
more commonly, a
pixel
. The following is a simplified description of what happens when
you take a picture with a digital camera:
1.
The camera’s optical system forms images on the “checkerboard” of pixels.
2.
The CCD is exposed to the image for a period of time.
During this period, each photo-sensitive cell receives photons of light, converts the
photons to electrons, and then stores the electrons in the cell. The process by which
each cell accumulates electrons can be compared to a well filling with water. As
more light hits a cell, the electron level in the well rises. The more electrons that are
in the cell, the more voltage it will have when read out by the digital camera.
3.
Following the exposure, a digital camera does three things:
a.
It measures the voltage of each cell.
b.
It converts the voltage to a binary number.
c.
It transmits this number down a cable to your computer.
4.
The computer reconstructs the image by assigning a brightness value to each pixel in
the final image. Each brightness value is proportional to the voltage of the
corresponding cell on the CCD chip.