Deterministic Triggering for Time Sensitive Applications – Software or Hardware Triggers?

Capturing an image at exactly the right moment is what makes it valuable. Timing is the key whether it is an image of a car violating a red light or a photo of items to be analyzed on assembly lines.

The ability of a camera to trigger deterministically is critical to capturing an event at the appropriate time, which is within microseconds. Since the triggering reliability, delay, and repeatability will be different for each imaging solution, it is crucial for users to select an appropriate camera that will meet their timing requirements (Figure 1).

Various imaging solutions will operate with differing triggering reliability, repeatability, and delay, so it is important to choose a camera that will fulfill your timing requirements.

Figure 1. Various imaging solutions will operate with differing triggering reliability, repeatability, and delay, so it is important to choose a camera that will fulfill your timing requirements.

Hardware and Software Triggers

An input trigger is used to instruct cameras to capture images. This trigger is typically initiated by one of two sources - hardware trigger and software trigger. The hardware trigger is normally in the form of an electrical signal or impulse.

The software trigger comes from a system or computer controlling the camera. The end result will be the same for both of these triggers, but there is a drastic difference in terms of their implementation.

While sending a software trigger, the message is transmitted through a combination of networks, buses, drivers, and the CPU of the camera, where it is interpreted as a signal to trigger the shutter of the camera.

There are various factors that influence the time taken by the signal to reach and instruct the camera to trigger, including system implementation, the robustness of the drivers, network congestion, and the overall processing speeds of the camera. All of these factors contribute to the margin of timing error between the trigger and the initiation of exposure by introducing a degree of variance to the trigger delay time.

A key advantage of the hardware triggers is their ability to connect to the camera directly, generally through a purpose-built interface, bypassing the network and infrastructure that support the imaging system. This leads to a much shorter trigger delay and quicker camera response.

The transmission of an electrical signal in place of a software-encoded message allows bypassing layered transmission delays between the camera and the host, drastically decreasing the trigger delay variance and ascertaining deterministic triggering.

Deterministic Triggering in Time Sensitive Applications

Deterministic triggering is important to significantly improve the efficacy of the solution in time-sensitive applications. This is the reason for the tendency of solution designers to stray from DSLRs and point and shoot, as they can have long software trigger delays with a large variance.

These cameras are designed for consumer level imaging and involve little engineering effort to building powerful drivers, APIs and SDKs, to deal with software triggers. Also, the variation of these software-based tools between different camera models causes integration problems when new models are launched.

The software trigger delay in some DSLR camera models is up to 250 ms. If these cameras are employed in a red light enforcement system, and a vehicle crosses a red light at a speed of 80 km/h or 50 mph, the vehicle would have crossed 5.5 m before taking an image.

This is one of the several reasons why industrial solution designers approach industrial camera manufacturers with relevant technical know-how to develop application-specific imaging tools.

This information has been sourced, reviewed and adapted from materials provided by Lumenera Corporation.

For more information on this source, please visit Lumenera Corporation.

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