User`s manual

Spyder3 GigE Vision SG-14 Monochrome Cameras User’s Manual 55
Teledyne DALSA 03-032-20123-01
Appendix A: Clear Dark Current
Gate Dark Current Clear
Image sensors accumulate dark current while they wait for a trigger signal. If the readout is not triggered
in a reasonable amount of time, then this dark current accumulation may increase to an excessive
amount. The result of this happening w ill be that the first row, and possibly additional rows (frames), of
the image will be corrupt.
The sensor used in this camera contains two sources of dark current that will accumulate with time: 1) in
the photo sensitive area, and 2) in the gates used to clock-out the charge.
The gate dark current can account for approximately 20% of the total dark current present. While the
exposure control has direct control over the amount of dark current in the photo sensitive area, it has no
control over the charge accumulated in the gates. Even with exposure control on, at low line rates, this
gate charge can cause the camera to saturate.
Using the Set Readout Mode (srm) command, the camera user can control the camera's behavior in
order to minimize the dark current artifact.
The modes of operation selected by the srm command are: Auto, On, or Off.
Note: When auto clear is ON in the HS, dual line mode, the leading TDI stage is transferred separately
and stored in the camera memory. TDI summing is recreated by adding stored data to the trailing TDI
stage in the camera processing chain. Due to digital summing, dark noise will increase by √2. A digital
add will not increase dominant shot noise.
Auto Mode (srm 0)
Note: Teledyne DALSA recommends Auto mode for most users. In this mode the camera will
automatically start and stop dark current clear, depending on line rate.
Stop Max line rateDC Clear th
HFLF
Watchdog th
100Hz
DC Clear ON
DC Clear OFF
Freq decreasing
Freq increasing
Figure 27: Gate Dark Current Clear in the Auto Mode.
To avoid corrupted lines due to jitter in External Trigger, the dark current clear switchover is controlled
by hysteresis thresholds. Thresholds (LF and HF) are set to higher frequencies, 30-45% of the maximum
line rate, so switchover will be transparent in an image.
However, if the external trigger frequency jumps back and forth over both thresholds in three consecutive
lines, then a corrupted line will occur.
Threshold frequencies for each model are outlined in the tables below.
Table 12. Auto readout mode thresholds
HS Mode
Transition Frequencies (Hz)
Model
LF
HF
Maximum Line Rate
SG-14-01K40-00-R
9818
11230
36000
SG-14-01K80-00-R
18149
20747
68000