Table of Contents
Advertisement
www.asdi.com
B.3
Gain and Offset
B.4
Light Source
ASD.Document 600540 Rev. C
Gain and offset are determined during optimization by an optimization
3
algorithm in the RS
application. You can observe this process by depressing
the OP button on the top right-hand toolbar. The gain, offset and integration
time will then appear in the lower left hand corner. These parameters are set
by the optimization routine and cannot be set manually.
The Gain Value is the inverse of actual Gain. A decrease in Gain Value will
educe a higher Gain. The maximum Gain Value is 16. Under the low light
levels that create such a gain, small changes in Offset will translate into large
changes in dynamic range.
Offset causes a downward shift (from Y=32,768 DN) in a SWIR spectrum
such that the max value of the spectrum and the minimum value of the
spectrum (Dark Current - off screen) are equidistant from the line y=32,768.
Under ideal conditions, where there's enough light, there will be about 32,768
DN of signal immediately after optimization, +/- 1,000. This means that
optimization "creates" about 16,000 DN of dark current and about 16,000 DN
of dynamic range. But this dynamic range will disappear with either a slight
increase in signal (if the signal started out low to begin with) or a slight
increase in dark current (increases with ambient temperature).
When using ASD's ProLamp illumination source, the less intense the lamp
setting (knob on side of light source), the more diffuse (homogeneous) it will
be. An average spot size of reasonable homogeneity is about 10 cm at a 50 cm
lamp distance. Two lamps improve this geometry significantly, especially for
large targets. If lighting conditions are so poor that you are continually
re-optimizing, try optimizing with a brighter source:
•
For outdoor optimization, tilt the Spectralon® panel directly towards the
sun during optimization.
•
For enclosed illumination of targets requiring contact measurements, we
recommend ASD's High Intensity Reflectance probe.
•
Under low light conditions, consider using a reference panel < 99-100%
white and then post-process out the difference between the white and gray
reference in ViewSpec.
84
Appendix B Reference Information
FieldSpec® 3 User Manual
Advertisement
Table of Contents
