Specifications
Chapter 6 Exposure and Readout 59
54
12
Charge accumulates in
unmasked cells during
exposure.
Accumulated charge in
exposed cells is quickly
transferred under mask.
Shifting continues until all masked
data has been shifted into serial
register and from there to the Output
Node.
A2
A1
B2
B1
C2
C1
D2
D1
A3 B3 C3 D3
Charges in serial register shift into
Output Node, emptying the register
so the next line can be transferred in.
A2
A1
B2
B1
C2
C1
D2
D1
A3 B3 C3 D3
A4 B4 C4 D4
A6 B6
C6
D6
A5 B5 C5 D5
A2
A1
B2
C2 D2
A3 B3 C3 D3
A4 B4 C4 D4
A6 B6
C6
D6
A5 B5 C5 D5
B1 C1 D1
A3 B3 C3 D3
A4 B4 C4 D4
B3
A4 B4 C4 D4
A6 B6
C6
D6
A5 B5 C5 D5
C3 D3
3
Charge from cells A1-D1 shifted
to serial register. Exposed cells
accumulate new charge.
6
All data from first exposure has been
shifted out. Second exposure continues.
Initial conditions are restored.
A2
A1
B2
B1
C2
C1
D2
D1
A3 B3 C3 D3
A3 B3 C3 D3
A4 B4 C4 D4
A4 B4 C4 D4
A6 B6
C6
D6
A5 B5 C5 D5
Figure 30. Frame Transfer readout
Only the exposed region collects charge. At the end of the exposure, the charge is
quickly shifted into the masked region. Since the shifting is accomplished in a short time,
i.e., a few milliseconds, the incident light causes only minimal “smearing” of the signal.
While the exposed region continues collecting data, the masked region is read out and
digitized. The percentage of smearing given by the equation below is simply the time
needed to shift all rows from the imaging area divided by the exposure time.
Nt
t
xi
exp
(4)
Digitization
During readout, an analog signal representing the charge of each pixel (or binned group
of pixels) is digitized. The number of bits per pixel is based on both the hardware and the
settings programmed into the camera through the software. The ST-133 or ST-133A can
contain up to two A/D converters with different readout rates selectable through
software.
Dual A/D Converters Option
There is provision in the controller for two complete analog channels including separate
A/D converters. Because the readout noise of CCD arrays increases with the readout
rate, it is sometimes necessary to trade off readout speed for high dynamic range.