P
AGE
22
A
PPENDIX
A. G
LOSSARY
4.2.1
PL-A650 and PL-A660 Series cameras return a still frame using the same rolling shutter
techniq
camera
e capture under ambient light by providing consistent exposures
throughout the frame. However, if used with synchronized flash, a slightly different
approach m st be employed. An exposure time of sufficient duration to integrate all
rows in the active region of interest must be used, with the flash firing between the time
of all rows being simultaneously exposed and prior to the start of data read-out.
reg
use
Wit
this
exposure time required for a still-image capture with an
time
camera operating at
gen
exp
pro
4.2.2
Mode
exposed simultaneously for a specified exposu
rolling shutter strategy of Video Mode. Exposure times can range from sub-milliseconds
about half a second. However, the build-up of dark current noise will be noticeable in
the output image for exposure times greater than 100 ms.
After the specified exposure time has elapsed, the image sensor is read out from top to
bottom at maximum speed. So, under ambient light conditions, the bottom rows of the
image sensor would continue to be exposed for the length of time required for read-out.
This results in the image being over-exposed towards the bottom of the image. Thus, it
is usually best to use Still Mode under controlled lighting conditions.
For Enhanced Control cameras (PL-A633 and PL-A634), one method to control the
lighting conditions is to use the built-in capability to synchronize a flash pulse during the
still frame capture under dark conditions. (In the PixeLINK Camera API, refer to API
functions
PimMegaReturnFrameAfterTrigger
and
PimMegaReturnStillFrame
.) Where dark
conditions are not possible and ambient light cannot be controlled, an electronically
triggered shutter can be used to cut off the ambient light at the end of the specified
exposure time. With the shutter closed, the read out from the image sensor can proceed
without any further influence from ambient light.
The PixeLINK Camera API functions also provide the ability to control a shutter,
synchronized with a still frame capture. Exposure times, flash pulse timing, and shutter
timing signals can be specified to 1/10 of a millisecond.
Using PL-A650 and PL-A660 Series Cameras in Still Mode
ue used for a video frame. These cameras therefore respond better than earlier
s to still imag
u
To determine the exposure and flash timing, consider the camera clock rate, size of the
ion of interest (columns and rows), and decimation. The simplest way to do so is to
PixeLINK Capture
or the
PL-A6xx Application
and set the appropriate subwindow size.
h the video preview playing, observe the measured frame rate. Take the inverse of
number to get the minimum
external flash. Use this number as a starting point to determine the proper exposure
and flash delay for your application. For example, with the
12 MHz, the full-resolution frame rate is 11.7 fps. The minimum still frame exposure is
then (1/11.7) = 85.5 ms. In our tests, we chose an exposure time of 120 ms to add a
erous buffer. We chose a flash delay of 90 ms as a starting point, and, after some
erimentation, we discovered that having the flash fire 110 ms after the trigger would
duce good results.
Using PL-A630 and PL-A640 Series Cameras in Still
When in Still Mode, the entire subwindow of a PL-A630 or PL-A640 Series camera is
re time, rather than using the row-by-row
to
PL-A630 to PL-A660 Series Cameras
PixeLINK
Megapixel FireWire Camera
System Guide
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