SPECS 2D-CCD, Руководство пользователя

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Manual V2.docx | Status 10.12.2015 | Version 1
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M is the average pixel value of the corrected flat field frame
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IF is the flat field frame
These parameters are described more fully in the following sections.
6.4.1 Bias frame or dark frame IB
Flat fielding requires the acquisition of two calibration frames. First, a bias frame or a dark
frame should be taken. Bias clears the camera of any accumulated charge and reads out the
cleared CCD. The resulting image is a low signal value image. In this image, all of the pixels
have approximately the same value, which consists of the electronic offset of the system of
the inherent structure of the CCD.
Darkness clears the CCD of charge, allowing charge to accumulate for a specified amount of
time with the shutter closed and then reads out that charge (dark current). A dark frame
contains the standard bias component as well as the dark signal. The dark command is most
useful when taking long exposures with low light levels.
6.4.2 Flat field frame IF
The second calibration image, the flat field frame, measures the response of each pixel in
the CCD array to illumination and is used to correct for any variation in illumination over the
field of the array. The optical system most likely introduces some variation in the
illumination pattern over the field of the array.
The flat fielding process corrects for uneven illumination, if that illumination is a stable
characteristic of each object exposure. Thus, it is necessary to illuminate the CCD with a light
pattern that is as representative of the background illumination as possible. This illumination
should be bright enough, or the exposure time should be made long enough, so that the
CCD pixel’s signals are at least 25 percent full scale or preferably higher. For a camera
equipped with a 12 bit analog processing card, the level should be at least 1000 ADUs.
6.4.3 Non-calibrated object exposure IR
An exposure of the object of interest IR is acquired. The object frame needs to be corrected
for electronic offset by subtraction of the bias/dark frame from it (IR − IB). The flat field
frame must also be corrected for electronic offset by subtraction of the bias/dark frame
from it (IF − IB). It is then possible to ascertain the average pixel value of the bias/ dark
corrected flat field frame.
Full well capacity 6.5
Full well capacity defines the amount of charge an individual pixel can hold before
saturating. Saturation must be avoided in high performance CCD imaging because it