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3.2 Acquisition modes
3.2.1 Free-Run
Also known as continuous acquisition. In this mode the sensor delivers a constant stream of image data at the maximum speed available
by the current bandwidth, without any external trigger. Each image exposure is sequentially started automatically when possible.
For all sensors the exposure of the next frame overlaps with the data readout of the previous frame.
This Overlap mode gives the highest number of frames per second (
FPS ).
Figure 67: acquisition mode - free run
The frame rate in free run mode depends inversely on the frame time. In general the frame time roughly equals to the readout time or to
the exposure time, depending on which one of the two is larger. This means that when exposure time is larger than the readout time, the
frame rate gradually decreases with increasing exposure time (frame_rate
∼
1/t_exp).
This is however not true for the IMX455, IMX461 and IMX461 sensors where the frame time is always a multiple of the readout time and
its value equals n*t_readout where n is the integer part of division of exposure time by readout time (n = t_exp // t_readout).
In this mode the timing depends on the Exposure Time and Data Readout Time. In situation when the exposure time is comparable or longer
than readout time, the exposure active signal might have constant active level during acquisition. This might be caused also by different
propagation delay for rising and falling edge of opto isolated outputs, see
Optically isolated digital outputs . Polarity inversion might help to
make visible the separated exposure pulses. Some camera models support limiting of FPS. When set the camera will limit the frame rate
so it does not exceed the set value. Please see:
Frame_Rate_Control . This is also applicable in case of triggered acquisition.
3.2.2 Trigger controlled acquisition/exposure
Unlike in the free-run, each image exposure can also be triggered with an input trigger signal. In this mode, the sensor waits in stage until
the trigger signal arrives. Only then, the exposure of first frame is started, which is followed by the data readout. XIMEA cameras supports
several triggered modes along with single image exposure after one trigger. The trigger signal can be either edge sensitive or level sensitive.
In the case of “level sensitive”, it can be used to control length of exposure or acquisition itself. Generally trigger sources can be divided
into two groups:
Software trigger
The trigger signal can be sent to the sensor using a software command. In this case, common system related latencies and jitter apply.
Hardware trigger
A hardware trigger can be sent to the sensor using the digital input described in Optically isolated digital inputss . Triggering by hardware
is usually used to reduce latencies and jitter in applications that require the most accurate timing. In this case rising edge of input signal
is suggested as the delay of opto coupler is smaller as well as introduced jitter. Triggering by hardware is usually used to reduce latencies
and jitter in applications that require the most accurate timing.
XIMEA Technical Manual, Version: v240108 65