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Point Grey Research Grasshopper Technical Reference
4.5.
Image Acquisition
4.5.1.
Camera Power
The Grasshopper allows the user to power-up or power-down components of the camera using
the CAMERA_POWER register 0x610. The exact components, e.g. image sensor, A/D converter,
other board electronics, will vary between camera models. By default, power is OFF both at
startup and reinitialization.
After writing 0x10000000 to register 0x610 to power-up the camera, the camera automatically
reports a value of 0x00000001 until power up is complete. The default power-up delay time is 100
ms, or two frame times. The maximum delay is 1 s. On GRAS-03K2 models, the default delay
time is 250 ms. The camera does not keep or transmit any images acquired during power-up,
regardless of isochronous or asynchronous transmission mode.
The auto-exposure algorithm does not run while the camera is powered down. It may therefore
take several (n) images to get a satisfactory image, where n is undefined.
4.5.2.
Shutter
The Grasshopper supports automatic, manual and one-push control of the CCD shutter time.
Refer to the General Specifications section for detailed information on supported shutter time
ranges. Shutter times are scaled by the divider of the basic frame rate. For example, dividing the
frame rate by two (e.g. 15 FPS to 7.5 FPS) causes the maximum shutter time to double (e.g.
66ms to 133ms).
Formulas for converting the fixed point (relative) shutter values reported by SHUTTER register
0x81C to floating point (absolute) values are not provided. Users wishing to work with real-world
values should refer to the Absolute Value CSR Registers section of the PGR IEEE-1394 Digital
Camera Register Reference.
The terms "integration" and "exposure" are often used interchangeably
with "shutter".
The time between the end of shutter for consecutive frames will always be constant. However, if
the shutter time is continually changing (e.g. shutter is in Auto mode being controlled by Auto
Exposure), the time between the beginning of consecutive integrations will change. If the shutter
time is constant, the time between integrations will also be constant.
The Grasshopper will continually expose and read image data off of the sensor under the
following conditions:
1. The camera is powered up (see Camera Power above); and
2. The camera is not in asynchronous trigger mode. When in async trigger mode, the
camera simply clears the sensor and does not read the data off the sensor.
It is important to note that the camera will continue exposing images even when isochronous data
transfer is disabled and images are not being streamed to the PC. The camera continues
Revised 26-Nov-10
Copyright (c) 2010 Point Grey Research Inc.
Camera Operations and Features
35
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