68 I-PentaMAX System Manual Version 3.A
Software averaging can improve the S/N ratio by as much as the square root of the
number of scans. Unfortunately, with a high number of scans, i.e., above 100, camera 1/f
noise may reduce the actual S/N ratio to slightly below this theoretical value. Also, if the
light source used is photon-flicker rather than photon shot-noise limited, this theoretical
signal improvement cannot be fully realized. Again, background subtraction from the
raw data is necessary.
This technique is also useful in high light level experiments, where the camera is again
photon shot-noise limited. Summing multiple pixels in software corresponds to
collecting more photons, and results in a better S/N ratio in the measurement.
Frame Transfer Readout
The I-PentaMAX 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 26 shows the readout of a masked version of our sample 4 × 6 CCD. The shading
represents the masked area (masking is on the array).
1
2
3
4
A1 A2 A3
B1 B2 B3
C1 C2 C3
D1 D2 D3
A4 A5 A6
B4 B5 B6
C4 C5 C6
D4 D5 D6
A1 A2
A3
B1 B2
B3
C1 C2
D1 D2
C3
A1 A2 A3
B1 B2 B3
C1 C2 C3
D1 D2 D3
A4 A5 A6
B4 B5 B6
C4 C5 C6
D4 D5 D6
A1
B1
C1
D1
D2
A2
B2
C2
Figure 26. 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
