Teledyne Princeton Instruments Lansis, System Manual

Page: 61 / 101

Chapter 9 Full Frame Readout 61
EAR99 Technology Subject to Restrictions Contained on the Cover Page.
9.2.1.3.4 Open Before Trigger
Figure 9-15 illustrates the timing diagram for Readout Per Trigger mode combined with
Open Before Trigger shutter mode.
Figure 9-15: Full Frame Timing Diagram: Readout Per Trigger, Open Before Trigger
When Open Before Trigger is selected, the active shutter is partially synchronized with
the experiment.
Upon arrival of the first trigger pulse at the SYNC connector, the shutter:
• Remains open for the configured exposure period;
• Closes;
• The CCD is read out.
Once data readout is complete, the active shutter reopens and waits for the next frame.
Open Before Trigger is most useful when the time between the trigger pulse and the
desired incoming signal is not sufficiently long enough to allow the active shutter to
completely open.
Unfortunately, this mode exposes the CCD to ambient light while the shutter is open
between frames. If the ambient light is constant, triggers occur at regular intervals, and
continuous cleaning is enabled, then, depending on the specific cleaning pattern used,
most of the average background light can be removed, except for noise attributed to
jitter (refer to
Section 7.3.1, Clean Until Trigger , on page 41.) If a short cleaning pattern
is used, not all ambient light will be removed since only a portion of the CCD will be
cleaned on each pattern, and the result is a blurry residual.
Additionally, shot noise of the background cannot be subtracted which may be
significant. Consider applications using High gain (i.e., 1e
-
/ADU,) and ambient equals
10% of full scale (i.e., 6500 e
-
.) The shot noise is then approximately 80 ADU, which is
significantly greater than the camera’s read noise.
As with Normal shutter mode, accurate background subtraction may not be possible for
the first frame.
In addition to signal from ambient light, dark charge accumulates during the wait time.
Any variation in the external sync frequency also affects the amount of dark charge,
even if light is not falling on the CCD during this time.
** If Enabled
t
O
t
O
t
C
EXPOSE
READOUT
TRIGGER
SHUTTER
Open Before
Trigger
Wait For
Trigger
WFT
Clean Until
Trigger **
Pgm’d
Cleans
Clean
Until
Trigger **
ACQUIRE
t
R
t
exp
44
11
-0
174
_
0
0
3
3