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Changes in Exposure Value vs. Frame Height
The time (measured in frames) for a new exposure to fully take effect is dependent on the absolute value of the change in
exposure (in rows) as it relates to the difference of the frame height (FH) minus the last active video row (LAVR). An exposure
change that is greater than the frame height/last active row difference requires two frames before the new exposure takes full
effect. An exposure change that is less than or equal to the difference takes one frame to take full effect.
This may be expressed as:
If |dExposure| <= (FH - LAVR) then Frames to full effect = 1
If |dExposure| > (FH - LAVR) then Frames to full effect = 2
Example: For a video configuration of Frame Height (FH) = 525 and a last active video row position of 500 (490 rows with active
frame start at row 10), a change in exposure of 26 rows requires 2 frames to full effect.
Exposure vs. Frame Width (Number of Pixel Clocks per Line/Row)
The exposure is controlled by setting the number of rows (lines) to allow the pixels to acquire charge. Changing frame width,
measured in columns or Pixel Clocks per Row, has the greatest impact on exposure. Since the time to scan one row (T
the basis for defining integration time, the exposure or integration time as a function of row count is:
T
= RowCount * T
I
and is therefore directly proportional to the time to scan one row.
T
= PixClkPerRow * T
row
Exposure Control Method and Exposure/Data Clock Out Timeline
The exposure is controlled by an electronic shutter (see
always exposed to light in this method of exposure control (unlike the mechanical shutter method that shields the imager array
from incoming light when not exposing). Therefore the image array must have some way of adjusting the system so that only
the charge collected for the desired time of exposure is read out of the imager. The exposure control circuitry of the imager
performs this task by allowing the photo diode array to collect charge until the time the next vertical minus the exposure
(measured in rows) is reached. The unwanted charge collected to this point is then removed from the photodiodes and the
charge is allowed to build again over the desired exposure interval. The wanted charge is collected at the end of the line.
Vert
Charge on
Photo Diode
Since the charge is collected at the end of the frame, this charge data is not clocked out of the imager during this frame. The
data is clocked during the next frame interval. Therefore, the data read from the imager is delayed by one frame time:
IMAGETEAM™ 4X00 Series Integration Manual
row
PixClk
Electronic Shutter Exposure Method
Charge is dumped
(reset)
Expose
Read Out
Electronic Shutter Exposure
Exposure Time
(wanted charge)
Frame 1 Frame 2 Frame 3
1st
2nd
3rd
Frame 1 Frame 2
1st
2nd
Method, page 2-5). The pixel array is
) is
row
2 - 5
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