FREAk feature descriptor sampling pattern : distance measurement unit

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I've been looking into the FREAK descriptor's code lately. The very first part of its algorithm constructs the sampling pattern that'll be used in choosing the pixels that'll serve to calculate the descriptor of the point of interest.
The sampling pattern is based on concentric circles. The rays of the smallest and the biggest circles were define like so:
const float smallR = 2.0/24.0;
const float bigR = 2.0/3.0;
What is the unit of this measurement, I mean 2.0/3.0 refers to 2 thirds of a pixel, of a centimeter (I know cm is not relevant in this context at all)... If it's 2 thirds of a pixel, how do we measure this in an image?