This is a laser imaging machine, which creates a 3D map of the optic disc. It takes a series of measurements of the “height” of hundreds of points on the optic disc and compares them to similar measurements taken from patients with and without glaucoma for comparison.
Many patients have optic discs which fall into a grey zone between normality and clearly glaucomatous. HRT can help to distinguish between these categories, but is by no means fool-proof. Various functions on the machine give quantitative measures suggesting likelihood of glaucoma (eg Frederick S Mikelberg (FSM) discrimant function, which I find quite useful). Other functions are being investigated all the time and will become available with future software updates for the machine.
HRT is also extremely helpful for monitoring progression in patients with glaucoma, particularly in the early stages of disease when no visual field defects are apparent.