Maximum frame rate possible in asynchronous (external trigger) mode

Last Revision Date: 6/22/2015

This article describes the relationship between frame rate and shutter time in external trigger mode and how to calculate the maximum frame rate when externally triggering a camera.


Historically, the maximum triggered frame rate has been limited by the fact that the camera was unable to overlap image transfer with the trigger input. The theoretical maximum triggered frame rate when using Trigger_Mode_0 depends on the shutter time (in seconds) and the maximum frame rate of the camera in free-running mode (in Hz). For example, a 640x480 Dragonfly that is capable of acquiring images at 30Hz in normal “free-running” mode cannot acquire images at 30Hz when in asynchronous “triggered” mode. This relationship can be calculated as follows:

Max_Frame_Rate_Trigger = 1 / ( Shutter + ( 1 / Max_Frame_Rate_Free_Running ) )

For example:

Shutter = 0.0020s 
Max_Frame_Rate_Free_Running = 30Hz 
Max_Frame_Rate_Triggered = 1 / ( 0.0020 + ( 1 / 30 ) ) = 28.30Hz


Newer versions of firmware1 (1.1.x.x and later) overcome the limitations described above, providing the ability to overlap integration and transmission to achieve full frame rate triggering2. Trigger_Mode_14 is our unique trigger mode that is very similar to Trigger_Mode_0, but allows for triggering at faster frame rates.

Using Trigger_Mode_14, the trigger may be overlapped with the readout of the image, similar to continuous shot (free-running) mode. If the trigger arrives after readout is complete, it will start as quickly as the imaging area can be cleared. If the trigger arrives while the image is still being read out of the sensor, the start of exposure will be delayed until the next opportunity to clear the imaging area without injecting noise into the output image. The end of exposure cannot occur before the end of the previous image readout. Therefore, exposure start may be delayed to ensure this, which means priority is given to maintaining the proper exposure time instead of to the trigger start.

This mode works well for users who want to drive exposure start with an external event. However, users who need a precise exposure start should use Trigger_Mode_0.

For more information regarding the trigger modes supported by our cameras, please refer to the Digital Camera Register Reference.

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