![Diagnostic Instruments SPOT Скачать руководство пользователя страница 8](http://html1.mh-extra.com/html/diagnostic-instruments/spot/spot_user-manual_2492839008.webp)
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
8
With the Spot camera you can shorten exposure times by filling the brightest pixel on the
CCD to less than full well, and then using this lower level as
full scale
. The inverse
fraction of full well that the brightest pixel on the CCD is filled to is known as the
gain
.
For example, filling a pixel to half of full well cuts the exposure time in half. Since the
exposure time is cut in half, we call this a gain of 2.
The following table illustrates the relationship between gain, the fraction of full well that
the brightest pixel is filled to, and the effect of gain on exposure time:
Gain
Fraction of Full Well
Exposure Time (sample)
1
1
80 seconds
2
1/2
40 seconds
4
1/4
20 seconds
8
1/8
10 seconds
16
1/16
5 seconds
An 80 second exposure at a gain of one turns into a 5 second exposure at gain 16. This is
good in terms of the exposure time. Unfortunately, background noise is directly
proportional to gain. It is 16 times higher at a gain of 16 is than at a gain of one.
An Automated Exposure Process
Normally, for any given image, trial and error would have to be used to determine the
optimal combinations of gain and exposure time. In order to eliminate the need for trial
and error experimentation, the SPOT camera automatically determines this information
by sampling light levels before each exposure. Based on the light levels and the user-
determined Auto-Gain Limit, SPOT determines the optimal gain setting and exposure
time for the image.
!
Note: For more information on using the Auto-Gain Limit feature, refer to the
Chapter 3, “Preparing to Take a Picture.”