QImaging optiMOS Signal Timing

This document describes the QImaging optiMOS signal timing, based on the 'expose out' signal setting and the exposure duration. The signal timing is shown with acquisition controlled by an external trigger input signal.

The camera maximum frame rate depends on the expose out signal setting in a way that may not be immediately obvious. The camera is fastest with expose out set to 'first row', other settings result in a slower maximum frame rate.

Contents:

Rolling shutter

The QImaging optiMOS has a rolling shutter. This means that camera rows are exposed at different times. On each image exposure, the first pixel row is exposed first, followed sequentially by other rows, until the last row of the sensor is reached. This exposure sequencing requires approximately 10ms (approximately 10us per row) to cover the entire sensor, independent of the exposure interval.

This rolling shutter design has some implications for imaging:

The readout time for the full sensor is 10ms, independent of exposure time and independent of binning. The readout time for a fraction of the sensor is approximately linear in the number of rows, so if the camera is set to use an ROI that covers half the rows, the readout time will be approximately 5ms.

The time at which each row is exposed is shifted in time approximately 10us from its neighboring rows. That is, each row starts exposing and ends exposing 10us after the preceding row starts exposing and ends exposing. Consider a 25ms exposure, which is much longer than the 10ms readout time. If we call the time at which the exposure starts t=0, then we have the timing:

The values given in the above example are approximate, the QImaging optiMOS documentation describes the precise row-to-row timing and the overall readout timing.

I/O signals

The QImaging optiMOS I/O signals used for camera timing are:

Trigger in
The trigger input signal to the camera. The camera is set to perform one exposure (acquire one image) on each external trigger input pulse.
Expose out
The expose active output signal from the camera. The camera signals that it is exposing an image. This signal is designed to control a light source or a shutter. The behavior of the expose out signal depends on a camera setting, described following.
Trigger ready
The trigger ready signal from the camera.
Readout
The readout active signal from the camera.

Expose out: first row

This section describes the behavior of the signal timing when the expose out signal is set to 'first row'.

The 'expose out: first row' setting means that the expose out signal is asserted when the first row is exposing. Other rows expose later, with the last row exposing 10ms after the first row exposes. The expose out signal is asserted for the exposure duration.

Expose out: first row, 25ms exposure

For a 25ms exposure, the signal timing is:

The signal timing in this document is shown with oscilloscope measurements. The signal vertical display is set to 5V/division, and the horizontal display is set to 5ms/division. The oscilloscope setup is the same for all images.

Notice the behavior of the signals:

Trigger in
The external trigger in (oscilloscope channel 4) is a 500us pulse every 100ms. The external trigger in starts the image exposure.
Expose out
The camera asserts the expose out signal when the image exposure begins, immediately on the rising edge of the external trigger in signal. The expose out signal remains asserted during the exposure of the first row, for 25ms. The camera deasserts expose out after the first row has finished exposing. Not shown is that the other camera sensor rows continue exposing until they are read out, the last sensor row continues exposing for approximately 10ms.
Trigger ready
The camera deasserts the trigger ready signal as the image exposure begins. The camera asserts the trigger ready signal as the first row finishes exposing and is read out. The camera could begin a new acquisition, while the other camera rows are continuing to expose and be read out.
Readout
The camera asserts the readout signal as it begins reading out the first row. The readout duration is 10ms, because this is a full-sensor image. The readout finishes approximately 35ms after the start of acquisition (10ms from start of exposure until the last row exposure begins, plus 25ms exposure duration).

Expose out: first row, 5ms exposure

For a 5ms exposure, the signal timing is:

The signal behavior is the same as expose out first row, 25ms exposure. Only the exposure duration has been reduced, so the expose out signal duration is 5ms instead of 25ms, and the readout finishes after 15ms instead of after 35ms.

Expose out: any row

This section describes the behavior of the signal timing when the expose out signal is set to 'any row'.

The 'expose out: any row' setting means that the expose out signal is asserted when any row is exposing. The first row is exposed first, with the last row exposing 10ms later. The expose out signal is asserted for the exposure duration plus 10ms.

When the expose out setting is 'any row', the camera is not ready to begin another exposure until after exposure readout completes. This lowers the maximum frame rate as compared to the expose out setting 'first row'.

Expose out: any row, 25ms exposure

For a 25ms exposure, the signal timing is:

Notice the behavior of the signals, as compared to 'expose out: first row':

Expose out
The camera asserts the expose out signal when the image exposure begins, immediately on the rising edge of the external trigger in signal. The expose out signal remains asserted as long as any row is being exposed. The last row exposure begins 10ms after the exposure starts, and continues for 25ms, a total of 35ms.
Trigger ready
The camera deasserts the trigger ready signal as the image exposure begins. The camera asserts the trigger ready signal after the expose out signal is deasserted, which is also when the readout ends. The trigger ready signal is not asserted until 10ms after the trigger ready signal is asserted in the case of 'expose out: first row'.

Notice that the maximum camera frame rate is lower with 'expose out: any row' than with 'expose out: first row'. In 'expose out: any row', the camera is not ready to begin a new exposure (trigger ready asserted) until after all rows have been exposed.

Expose out: any row, 5ms exposure

For a 5ms exposure, the signal timing is:

The signal behavior is the same as expose out any row, 25ms exposure. Only the exposure duration has been reduced, so the expose out signal duration is 15ms instead of 35ms, and the readout finishes after 15ms instead of after 35ms.

Expose out: all rows

The expose out: all rows setting is a 'synthetic' setting: the exposure duration for each row is approximately 10ms longer than the exposure setting value, and the camera asserts the 'expose out' signal while all rows are exposing. The 'expose out' signal is intended to control a light source or shutter.

The QImaging optiMOS camera has a rolling shutter. The first sensor row is read out before the last sensor row is read out, so to achieve an equal exposure duration on all rows, the first sensor row begins exposing before the last sensor row begin exposing. The readout time is approximately 10ms, so the exposure start time shift from first sensor row to last sensor row approximately 10ms. This means that after approximately 10ms, all rows are exposing. When the camera begins sensor readout, the camera reads the first row first, so that row is no longer exposing.

In the 'expose out: all rows' setting, the camera exposes for 10ms more than the exposure setting. 10ms after the exposure begins, the camera asserts the 'expose out' signal for the exposure setting duration. This allows an external light source or shutter to expose the entire camera sensor for the exposure interval. Then the camera deasserts the 'expose out' signal and begins readout. The rows continues exposing until they are read out, with the last row reading out 10ms later.

Expose out: all rows, 25ms exposure

For a 25ms exposure, the signal timing is:

Notice the behavior of the signals, as compared to 'expose out: first row':

Expose out
The camera asserts the expose out signal 10ms after the rising edge of the external trigger in signal, when all rows have started exposing. The expose out signal remains asserted for the 25ms exposure duration.
Trigger ready
The camera deasserts the trigger ready signal as the image exposure begins. The camera asserts the trigger ready signal after the expose out signal is deasserted, which is 10ms before the readout ends.

Expose out: all rows, 5ms exposure

For a 5ms exposure, the signal timing is:

The signal behavior is the same as expose out all rows, 25ms exposure. Only the exposure duration has been reduced, so the expose out signal duration is 5ms instead of 25ms, and the readout finishes after 25ms instead of after 45ms.

IGOR Pro: QImagingOptiMOSTrigger.ipf

The IGOR Pro SIDX sample script QImagingOptiMOSTrigger.ipf shows how to control the QImaging optiMOS expose out mode. The function CameraSetUp controls the imaging parameters.

Copyright Bruxton Corporation