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Chapter 5 Operation 63
Frame Transfer
The MicroMAX fully supports frame transfer readout. Operation in this mode is very
similar to the operation of video rate cameras. Half of the CCD is exposed continuously,
raising the exposure duty cycle to nearly 100%. The other half of the CCD is masked to
prevent exposure, and it is here that the image is “stored” until it can be read out.
Figure 22
shows the readout of a m
asked version of our sample 4 × 6 CCD. The shading represents
the masked area (masking is on the array).
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 22. 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 can be determined by dividing the time needed to shift all
rows from the imaging area by the exposure time. See the equation below.
(3)
CCD Array 1 MHz Readout Time
MicroMAX:512BFT
EEV CCD57-10 512 x 512
0.35 sec. for full frame
Table 7. Approximate Readout Time for the Frame-Transfer CCD Array