Online alcohol test

Little, and that is intended. The horizontal gaze nystagmus (HGN) test, part of the standardised American battery, asks a trained officer to move a pen in front of the eyes and judge at what angle the tremor appears, with the 45° reference. Two things make it impossible to repeat here: the judgement is that of an expert person, and the angle presupposes knowing how far the eye is from the pen.

So this is not that test, it is something else that looks at the same eyes. Instead of the angle of onset it measures the quality of tracking: how much of the target's range the eye covers, how far it strays from it, how often the tracking breaks into jerks (which is the first clue in the original battery, "lack of smooth pursuit") and how much of the tremor falls in the frequency band typical of nystagmus.

The distance problem is solved by measuring it instead of assuming it: the distance between the pupils is known to within a few millimetres, and from how many pixels it takes up in the frame we work out how far you are from the screen. From there every point on the screen becomes a true angle, and the test tells you how many degrees you actually reached instead of pretending it was 45.

Alongside the two eye tests there is an inhibitory-control test (looking the opposite way to a flash) and a reaction-time test. They are there because they do not go through the eyes: the tap on the screen is measured without the delay the camera introduces.

None of these numbers has been validated against a breathalyser, and there are no "normal" values for a webcam test. The only comparison that makes sense is with yourself: record the reference when you are clear-headed and rested, and see how far you stray from it the other times.