Spectral Instruments Series 800, User Manual, Page: 39 / 70

Spectral Instruments
Pt # 2500- 39
The flat field is also sensitive to illumination angle of the incoming light. Since
collimated light shows up many optical defects, such as dust, in a way that wide
angle illumination does not, it is important that master flats and target images are
both obtained with very similar (if not identical) illumination beams.
4.3.4 Understanding The Scaling Effects
The example above derived the scaling number from the brightest area in the
master flat image. After correction, the values of pixels in the target image are
essentially unchanged in the area of the ROI from which the mean was
determined. Any number you might have used would work for scaling the target
image to preserve significance in the pixel values provided it is large enough. It
does not say that the magnitude of the pixel values is now “correct”.
Provided that the master flat image is exposed so that it is about half of the full
range of the ADC, a scalor of half the maximum ADC value retains the
theoretical maximum pixel dynamic range.
Clearly, the value of the scalor affects the pixel values of the resulting images.
For inter-comparison among a number of different sets of images or sequences of
images a constant scalor for the entire set is essential - as is a constant setup so
the same master flat field image reflects the spatial attenuation and the constant
scalor assumption is valid.
To determine the scale factor to correct images to an absolute scale requires
introduction of known objects into the application “field”. Most measurements
are relative. You look for changes in intensity within an image or from image to
image. Few measurements require absolute measurement scales.