FastVision FASTCAMERA Series, Руководство пользователя, Страница: 17 / 32

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by programming the clocks per line register (see serial command
0x04 below). By slowing down the line rate, you can dial the shutter
speed (at the cost of maximum frame rate), to any time above 132 clocks. So to
get a shutter speed of 3.0 microseconds you would set the clocks per line to 200
clocks. The highest full frame readout rate would be, or 192 full frames per
second.
8.2 Pixel Gain and Offset
The offset and gain of each pixel can be calibrated by setting the pixel gain and offset
arrays in the DDR memory of the camera. It is a good idea to do this as the large number
of converters, need to be calibrated for best results.
Note : Only the pixels in the selected ROI are processed (see serial commands 0x02 and
0x03).
The offset table is set by taking several images with no light input, averaging them and
uploading them to the offset (or dark field) table. These values are subtracted from the
sensor pixel values (clipped so less than zero values are zero), before applying the gain
table values.
The gain table values are obtained by taking a defocused image of a uniform object, with
enough light to get to 75% of saturation at most on the brightest point in the image.
Using the resulting image gain values are computed (in the simplest case the pixel
values and the ratio with 75% of full scale (191 (8 bit) or 768 (10 bits)) determines the
gain.)
Note: It is assumed that you are trying for a uniformly lit field of view (i.e. a Flat Field). It is
important that you have as nearly as is practical uniform field of light, as large or small
gain values can introduce significant artifacts in your images.
The gain table contains 8 bit values, which are formatted unsigned 1.7 format, that is one
binary digit followed by 7 fractional digits. This makes the range of gains 1.992 down to
0.00390625.
The equation for the pixel gain and offset is:
CP[i,j] = Saturate((2*Gain[i,j]/256)* Clip (P[i,j] – Offset[i,j]))
CP = Corrected Pixel
P = Raw sensor Pixel
Clip = 0 if P[i,j]-Offset[i,j] is negative, otherwise its P[i,j]-Offset[i,j].
Saturate = FullScale if its input is greater than or equal to FullScale, otherwise it is it’s
input value.
FullScale = 255 for 8 bit pixels 1023 for 10 bit pixels.
The camera has 120 MB (or more) of memory which can be used to store images, and
do averaging. Each mode of operation is explained in the following sections. If the
memory option is enabled, then sensor data goes to memory instead down stream.
8.3 Memory Modes (Mem)
8.3.1 FIFO memory mode
In this mode memory is used as a first in first out memory (FIFO). The memory
fills with images, until it is full, and the stops filling. At any time the user may