Multiple exposures in one frame¶

Multiple exposures in one frame

The multiple exposure feature accumulates several exposures (sub-exposures) into a single frame.
At the end of each exposure, the pixel values are transferred to a pixel buffer, and the sensor starts again from zero.
The readout of the frame starts after the last exposure period has finished.
Once the required number of exposures is completed, the resulting output frame is the sum of all exposures for each pixel.
As you can see in the text below, this feature can work in two modes - either with exposure time defined by the XiApi parameter or controlled by the duration of the trigger pulse.
In both modes, every trigger pulse starts one sub-exposure. The frame is read out and delivered after the last trigger of the burst.

The number of exposures can be defined using the XiApi parameter XI_PRM_EXPOSURE_BURST_COUNT.

According to Sony, the maximum number of exposures per frame is 4095.
However, please note that there is a gap (FOT) after each exposure, during which the next exposure cannot start. Depending on the sensor and bit depth, it is ~50 - 160 µs.

Camera models supported¶

Some of the XIMEA cameras support multiple exposures captured in a single frame.
The models supporting this feature are based on the Sony Pregius sensors mentioned in the tables below.

Sony Pregius (IMX25x)¶

Present in the following camera families:

xiC camera family
xiX camera family

Sensor model	Camera model	Chroma	Interface
Sony IMX252	MC031MG-SY	Monochrome	USB3
Sony IMX252	MC031CG-SY	Color	USB3
Sony IMX252	MX031MG-SY-X2G2	Monochrome	PCIe Gen2 x2
Sony IMX252	MX031CG-SY-X2G2	Color	PCIe Gen2 x2
Sony IMX250	MC050MG-SY	Monochrome	USB3
Sony IMX250	MC050CG-SY	Color	USB3
Sony IMX250	MX050MG-SY-X2G2	Monochrome	PCIe Gen2 x2
Sony IMX250	MX050CG-SY-X2G2	Color	PCIe Gen2 x2
Sony IMX255	MC089MG-SY	Monochrome	USB3
Sony IMX255	MC089CG-SY	Color	USB3
Sony IMX255	MX089MG-SY-X2G2	Monochrome	PCIe Gen2 x2
Sony IMX255	MX089CG-SY-X2G2	Color	PCIe Gen2 x2
Sony IMX253	MC124MG-SY	Monochrome	USB3
Sony IMX253	MC124CG-SY	Color	USB3
Sony IMX253	MX124MG-SY-X2G2	Monochrome	PCIe Gen2 x2
Sony IMX253	MX124CG-SY-X2G2	Color	PCIe Gen2 x2

Sony Pregius S (IMX53x, IMX54x)¶

On Pregius S sensors the feature uses the Sony "Multi exposure Master Fast trigger" mode. It is controlled by the same XiApi parameters as on the IMX25x cameras.

Sensor model	Camera model	Chroma	Interface
Sony IMX540	MX245MG-SY-X2G2	Monochrome	PCIe Gen2 x2
Sony IMX540	MX245CG-SY-X2G2	Color	PCIe Gen2 x2
Sony IMX530	MX245MG-SY-X2G2-HDR	Monochrome	PCIe Gen2 x2
Sony IMX530	MX245CG-SY-X2G2-HDR	Color	PCIe Gen2 x2
Sony IMX541	MX203MG-SY-X2G2	Monochrome	PCIe Gen2 x2
Sony IMX541	MX203CG-SY-X2G2	Color	PCIe Gen2 x2
Sony IMX531	MX203MG-SY-X2G2-HDR	Monochrome	PCIe Gen2 x2
Sony IMX531	MX203CG-SY-X2G2-HDR	Color	PCIe Gen2 x2
Sony IMX542	MX161MG-SY-X2G2	Monochrome	PCIe Gen2 x2
Sony IMX542	MX161CG-SY-X2G2	Color	PCIe Gen2 x2
Sony IMX532	MX161MG-SY-X2G2-HDR	Monochrome	PCIe Gen2 x2
Sony IMX532	MX161CG-SY-X2G2-HDR	Color	PCIe Gen2 x2
Sony IMX530	MC245MG-SY-HDR	Monochrome	USB3

The camera models above include all their connector variants (e.g. -FL, -FV, -FF, -TC, -UB).

Exposure is defined by the XiApi parameter "XI_PRM_EXPOSURE"¶

In this mode, the trigger defines the start of each exposure, but the length of the exposure is defined by the XI_PRM_EXPOSURE parameter. Set exposure length using XI_PRM_EXPOSURE parameter and set XI_PRM_TRG_SELECTOR to XI_TRG_SEL_EXPOSURE_START.
The trigger can be a software trigger or a hardware trigger on a GPI. With a burst count of 5, the camera needs 5 triggers to deliver one frame.

C++:

// Set exposure
xiSetParamInt(xiH, XI_PRM_EXPOSURE, 1000);
// Set trigger source (software trigger or e.g. XI_TRG_EDGE_RISING for a GPI)
xiSetParamInt(xiH, XI_PRM_TRG_SOURCE, XI_TRG_SOFTWARE);
// Set trigger selector
xiSetParamInt(xiH, XI_PRM_TRG_SELECTOR, XI_TRG_SEL_EXPOSURE_START);
// Set the number of times of exposure in one frame
xiSetParamInt(xiH, XI_PRM_EXPOSURE_BURST_COUNT, 5);

XI_IMG image = {};
image.size = sizeof(XI_IMG);
xiStartAcquisition(xiH);
// Each trigger starts one sub-exposure
for (int i = 0; i < 5; i++)
    xiSetParamInt(xiH, XI_PRM_TRG_SOFTWARE, 1);
// The frame is delivered after the last trigger
xiGetImage(xiH, 5000, &image);

Python:

from ximea import xiapi

cam = xiapi.Camera()
cam.open_device()
img = xiapi.Image()

# Set exposure
cam.set_exposure(1000)
# Set trigger source (software trigger or e.g. 'XI_TRG_EDGE_RISING' for a GPI)
cam.set_trigger_source('XI_TRG_SOFTWARE')
# Set trigger selector
cam.set_trigger_selector('XI_TRG_SEL_EXPOSURE_START')
# Set the number of times of exposure in one frame
cam.set_exposure_burst_count(5)

cam.start_acquisition()
# Each trigger starts one sub-exposure
for i in range(5):
    cam.set_trigger_software(1)
# The frame is delivered after the last trigger
cam.get_image(img, timeout=5000)
print(list(img.exposure_sub_times_us))

cam.stop_acquisition()
cam.close_device()

Exposure defined by the length of the trigger pulse¶

In this mode, both the start of the exposure as well as the length of the exposure is defined by the trigger pulse. Set XI_PRM_TRG_SELECTOR to XI_TRG_SEL_EXPOSURE_ACTIVE. The exposure length will be defined by the trigger pulse length.
This mode requires a hardware trigger on a GPI.

C++:

// Set GPI1 as trigger input
xiSetParamInt(xiH, XI_PRM_GPI_SELECTOR, XI_GPI_PORT1);
xiSetParamInt(xiH, XI_PRM_GPI_MODE, XI_GPI_TRIGGER);
xiSetParamInt(xiH, XI_PRM_TRG_SOURCE, XI_TRG_EDGE_RISING);
// Set trigger selector
xiSetParamInt(xiH, XI_PRM_TRG_SELECTOR, XI_TRG_SEL_EXPOSURE_ACTIVE);
// Set the number of times of exposure in one frame
xiSetParamInt(xiH, XI_PRM_EXPOSURE_BURST_COUNT, 5);

Python:

# Set GPI1 as trigger input
cam.set_gpi_selector('XI_GPI_PORT1')
cam.set_gpi_mode('XI_GPI_TRIGGER')
cam.set_trigger_source('XI_TRG_EDGE_RISING')
# Set trigger selector
cam.set_trigger_selector('XI_TRG_SEL_EXPOSURE_ACTIVE')
# Set the number of times of exposure in one frame
cam.set_exposure_burst_count(5)

Notes¶

Note 1 (FOT): In both of the above modes, there is a short period (FOT) after each exposure during which time the next exposure cannot start. The length of this period depends on the sensor:

Sony Pregius (IMX25x): FOT = 11 * line_period
Sony Pregius S (IMX53x, IMX54x): FOT = 8 * line_period

The line_period depends on various other parameters (e.g. bit depth, ROI, interface), see Line Period in the Camera performance calculator.

Example for MC245MG-SY-HDR at full resolution (Multi exposure Master Fast trigger mode):

Bit depth	Line period	FOT	Max. trigger frequency
8 bit	13.454 µs	~108 µs	~9.07 kHz
12 bit	20.181 µs	~161 µs	~6.09 kHz

The maximum trigger frequency is calculated as 1 / (FOT + T_TGED + T_OFFSET + T_EXP_MIN), where T_TGED = 0.1 µs, T_OFFSET = 2.46 µs and the minimum exposure T_EXP_MIN = 0.05 µs. Longer exposures reduce the maximum trigger frequency accordingly.

Note 2: None of the above two modes support the start of exposure during readout (trigger overlap feature).

Note 3: Limitations of the IMX sensors in multi-exposure mode lead to an increase in noise in comparison to a single exposure frame.

Note 4: The multi exposure mode does not work ideally when ultra short exposure times are used (<15us).

Note 5 (trigger required): When XI_PRM_EXPOSURE_BURST_COUNT is bigger than 1, the camera works only with a trigger (XI_TRG_SEL_EXPOSURE_START or XI_TRG_SEL_EXPOSURE_ACTIVE). If no trigger source is set (free run), the camera does not deliver any images. If fewer triggers than the burst count arrive, no image is delivered (xiGetImage returns timeout).

Note 6 (image header): The exposure times of the sub-exposures are available in the image header:

C++: XI_IMG.exposure_sub_times_us[5]
Python: img.exposure_sub_times_us
The array holds the first 5 sub-exposures, unused entries are 0. The field exposure_time_us contains the total exposure time of the frame (exposure * burst count).

Applications¶

A typical application is tracking a light spot in the dark, where the resulting image captures the trajectory of the point over time.

Another application is measurement with modulated (pulsed) illumination, similar to lock-in detection. The camera exposes only during selected phases of the light modulation, synchronized by the trigger signal, and accumulates many short sub-exposures into one frame. This increases the signal of weak, fast-modulated light while the amount of data and the frame rate stay low. Comparing frames taken at different phases (e.g. light on / light off) removes the background.