Understanding Latency in Eye Tracking

Question

I am having trouble understanding latency in the context of eye-tracking. I currently work with a 30Hz eye-tracker integrated into a head-mounted display for vision research.

The way I look at it, there is an overall delay in the time when the eye actually makes a movement to the time these coordinates are provided by the eye-tracking software. There are two components to this delay -> 1) The delay because of the frequency of imaging by the eye tracker (30fps -- 33.3ms) 2) The latency because of the actual algorithm that extracts data and provides coordinates.

Am I right in thinking that the total delay is the sum of 1) and 2) ?

I spoke with the company that makes the eye-tracker, and they said the latency in eye tracking is 60ms. Does that mean that my overall latency is 60ms + (1000/30Hz) ~ 93.3ms ?

Or does the 60ms figure somehow take into account the FPS of the eye camera?

Answer 1

Different vendors use different definitions of eye tracking latency so you have to ask your specific vendor what definition they use when they state 60 ms latency.

As an example Tobii, one of the largest eye tracking vendors, uses the following definitions:

Processing latency: Describes the time required by the eye tracker processor to perform image processing and eye gaze computations.

Total system latency: The duration from mid-point of the eye image exposure, to when a sample is available via the API on the client computer. This includes half of the image exposure time, plus image read-out and transfer time, processing time and time to transfer the data sample to a client computer.

Answer 2

I would assume that the number the company gave you 60ms is the total latency of everything. This is assuming that they are the ones providing the software and the camera.

If the cameras latency is not included then your calculation for the latency should be correct.