Latency

Latency is the time it takes for an image or data packet to travel from a camera to the computer that processes it. In machine vision systems, knowing this delay is important because it affects how quickly the system can react—whether that’s inspecting a fast-moving product on a conveyor or triggering an action based on what the camera sees.

The total delay consists of four main components: processing delay, queuing delay, transmission delay, and propagation delay, each contributing to the time it takes for data to reach the host system.

Processing Delay and Queuing Delay have no fixed equations; instead, they depend on factors such as the camera’s internal processing speed, image preprocessing operations, trigger settings, buffer sizes, and the level of network traffic or congestion. 

Processing Delay

The time it takes for the camera or processing system to analyse or prepare a packet of data.

Example: Preprocessing steps, such as vertical binning or triggering, can add a few microseconds to a few milliseconds.

Queueing Delay

The time data waits in a buffer or queue before being transmitted. This depends on network traffic and the camera’s internal buffering. It can vary significantly if multiple cameras share the same network.

Transmission Delay

The time to push all the bits of a packet onto the network cable.
Equation:

• L = packet size (bits)
• R = network bandwidth (bits per second, bps)

So, for a 1 MB image over a 1 Gbps Ethernet link:

Propagation Delay

The time for the signal to physically travel from the camera to the host system.
Equation:

• d = cable length (meters)
• s = propagation speed (~  m/s for typical Ethernet cable)

Example: For a 100 m cable:

Note: Propagation delay is usually much smaller than transmission delay.

Total Latency

The total nodal latency for a single packet can be estimated as:
 

Scenario

You are using a JAI GigE camera to inspect products on a high-speed conveyor. Each product must be imaged and analyzed within 10 milliseconds to trigger a sorting mechanism.

1. Capture: The camera exposes the sensor for 2 ms to get the image.
2. Processing delay: The JAI camera performs onboard operations (ROI cropping, debayering) for ~0.3 ms.
3. Queueing delay: If another image is already being sent, the new image may briefly wait in the camera’s buffer, ~0–0.1 ms.
4. Transmission delay: The 2 MB image is sent over 1 Gbps Ethernet, ~16 ms
5. Propagation delay: The signal travels over a 10 m cable, negligible ~0.05 µs.

In the above scenario, the system would not meet the 10 ms inspection window on a standard 1 GbE network. Upgrading to a higher bandwidth network card would significantly reduce transmission delay, and thereby lower the total latency so that the system can meet the 10 ms inspection window.