Ch 3 – Preparing to Take a Picture
Image Setup Options
User’s Guide to the SPOT Cooled Color Digital Camera, 6/9/98
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For example, an Auto-Gain Limit of 16 allows the camera to choose from the full range
of gain options. However, if you have found that gain settings of 8 or 16 are too noisy
for the type of work that you do, you can restrict the camera from selecting these options
by setting the Auto-Gain Limit to four. When the Auto-Gain Limit option is set to four,
the camera is limited to one of the lower three of the five gain options: 1, 2, or 4. The
following table illustrates the five levels of gain that correspond to the SPOT camera’s 24
bit RGB capabilities, as well as the relationship between gain, the fraction of full well
that the brightest pixel is filled to, and 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
The advantage of allowing the camera to use a higher gain level for dim images is that
the exposure time is reduced significantly. For each doubling of gain number, exposure
time is halved. Thus, an image with an exposure time of 5 minutes at full scale (a gain of
1) would take only 1.25 minutes at a gain of 4. Unfortunately, background noise
increases proportionally to the gain. Thus, the shortened exposure time would result in
an image with four times the amount of background noise. The following table illustrates
the relationship between gain, exposure time, and background noise:
Auto-Gain Limit
Possible Auto - Gain
Settings
Exposure Time
(dim images)
Background Noise
(dim images)
1
1
Longest
Lowest
2
1, 2
é
ê
4
1, 2, 4
é
ê
8
1, 2, 4, 8
é
ê
16
1, 2, 4, 8, 16
Shortest
Highest
To set the Auto-Gain Limit, follow these steps:
1.
Ensure that the camera is set to use the Auto-Exposure option.
2.
Click on the Auto-Gain Limit select list, and select the number that you want to limit
the gain to.
