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Ch. 1 - Electronic Imaging Theory and the SPOT Camera
The SPOT Cooled Color Digital Camera
User’s Guide to the SPOT Cooled Color Digital Camera, 6/9/98
7
The SPOT Cooled Color Digital Camera
In order to provide you with an understanding of what happens when you take a picture,
the following sections highlight some of the basic operating principles of the SPOT
camera.
Three Pictures
The SPOT camera uses the single CCD, three pass method. To take a picture (also
known as capturing an image), the SPOT camera takes a red picture, a green picture, and
a blue picture. A color filter, controlled by the software that accompanies your camera,
changes from red to green to blue for each exposure. Thus, SPOT can use one CCD chip
to compute precise red, green, and blue values for each pixel in the final image.
Pixel Digitization
Following each exposure, SPOT reads the CCD chip and digitizes each pixel as it comes
off of the CCD chip, and before it is sent through the cable into the computer. This
technique minimizes noise and maintains the lack of distortion that is inherent with CCD
chip geometry.
A Cooled CCD Chip
The CCD chip in the SPOT camera is cooled in order to reduce thermal noise. The effect
of thermal noise on a digital image is similar to the "snow" that you see on your TV when
tuned in to a station with a weak signal. If you could reduce the snow (i.e., the thermal
noise), the weak station (or the dim image) would be much clearer. Cooling the CCD
chip cuts the amount of thermal noise in half for every 5
°
C drop in temperature. The
SPOT camera uses a Peltier thermoelectric cooler to keep the chip cooled to 37
°
C below
ambient room temperature. Thus, if the room temperature is 25
°
C, the CCD chip is
12
°
C. This makes the SPOT camera an excellent option for dim images.
!
Note: In order to keep the CCD chip from frosting over, a desiccant packet is added
to the air space surrounding the chip before the camera is sealed. Over time, the
desiccant can become saturated with moisture and lose its effectiveness. To change
a desiccant packet, refer to Appendix __.
Gain
Normally an auto exposure routine in a digital camera sets the exposure time so that the
brightest pixel on the CCD chip is completely “filled” with electrons, or is “full well”
(see the Digital Photography section). This simple technique works well for bright
images, but, as an image gets dimmer, exposure times can get uncomfortably long.