Applied Biosystems SOLiD™ 4 System SETS Software User Guide ...

D
Appendix D Advanced Topic: Primary Analysis
Image metrics
Exposure metric is a real number that measures whether and by how
much an image is over- or under-exposed, relative to a particular
definition of ideal exposure. A value of zero indicates ideal
exposure. A value greater than zero indicates over-exposure, and one
less than zero indicates under-exposure. The scale is logarithmic
with base two; a value of 1 indicates that the image is exposed twice
as long as ideal.
The typical ideal exposure level of an image is one that maximizes
the dynamic range of the content, with some signal occupying the
lowest (darkest) range and some information extending to the
saturation (highest) point.
An overexposed image compresses too much of the content at
the top end of the range. Overexposure creates a non-linearity in
response and makes it impossible to differentiate between the
highest signals.
An underexposed image does not extend the content of the
image across the possible range. Underexposure creates a lowend
compression and fails to use the available range of the
system.
Note: For purposes of color-calling algorithms, slightly
overexposing the images yields better results.
178 Applied Biosystems SOLiD 4 System SETS Software User Guide