The visual field test: how it is done and how to read the result

You fix your gaze on a point at the centre of a white dome, with one eye covered. Every so often, somewhere, a little light appears: sometimes bright, sometimes so faint that you doubt you saw it. Each time you see it, you press a button. It looks like a test of reflexes, but it is in fact one of the most valuable maps an ophthalmologist can have: the picture of what your eye sees, point by point, even in the areas where you do not notice that you cannot see.

The visual field test, or perimetry, is used mainly to detect and monitor glaucoma, but also to recognise damage along the pathways that carry images to the brain. Here is how it is done, how the result is read and why it almost always has to be repeated.

In this article

1. Visual field: what the test is
2. How the visual field test is done
3. How to read the report
4. Why the visual field test is repeated
5. In glaucoma and beyond
6. Frequently asked questions
7. Sources

Visual field: what the test is

The visual field is all the space an eye sees while looking fixedly at a point: not only the centre, where we read and recognise faces, but also the sides, the top and the bottom. In everyday life the brain fills in the gaps and uses information from the other eye: that is why a blind area at the margins can go unnoticed for years.

Computerised perimetry measures, at many points, the faintest light that the eye can perceive. A review identifies it as the current standard for testing the visual field and an important diagnostic tool in glaucoma; the report makes it possible to recognise a normal field, identify a defect, follow its course and understand whether it is due to glaucoma or to a neuro-ophthalmological disease¹. On its own, however, it is not enough: the diagnosis comes from the combination of the examination, the measurement of pressure and the visual field test¹.

How the visual field test is done

One eye is tested at a time, with the other covered and with lenses suited to the distance of the dome. You rest your chin on a support, fix your gaze on a central target and press the button at every stimulus you see. The machine varies the position and intensity of the lights and repeats doubtful points, to estimate the sensitivity of each area.

It does not hurt and there is nothing to pass: it is normal not to see many of the lights, because some are designed to be at the limit of perception. What matters instead is attention and fixation. In the CIGTS study, which analysed the baseline tests of 607 people with glaucoma, the variability of the results was greater in less alert people, in older people and in the first eye tested².

There is also a learning effect: in the same study the results of the second test were slightly better than those of the first². The first visual field of a lifetime, therefore, has to be read with caution.

How to read the report

The report contains maps and numbers. The most common items are these.

Report item	What it indicates
Greyscale map	An overall picture: the dark areas are those where the eye sees less
Values in decibels (dB)	The sensitivity of each point: the higher the number, the fainter the lights perceived
Probability maps	The points that deviate from what is expected for age
Mean deviation (MD)	How far, on average, the visual field deviates from that of a healthy person of the same age
Pattern standard deviation (PSD)	How far the damage is concentrated in specific areas rather than spread out
Reliability indices	Responses given without a stimulus (false positives), strong stimuli not seen (false negatives), losses of fixation

The mean deviation is the index on which many calculations of the speed of worsening are based⁵: in the CIGTS study, repeated at short intervals, it gave very similar values from one test to the next (a correlation of 0.91)². The reliability indices say how far to trust that single test. A 2018 review points out, however, that these indices too have limits, and that factors such as the stability of fixation and cognitive decline can alter results that are considered reliable⁴.

In practice, most tests pass the reliability thresholds: in a glaucoma clinic, 8,931 tests out of 10,010, that is 89%, had a proportion of false positives of no more than 15%⁶.

Why the visual field test is repeated

Even in a stable eye the result fluctuates from one test to the next. A review of variability described this physiological fluctuation: it is wider in people with glaucoma or high pressure, grows with distance from the centre and in areas that are already damaged, and also depends on learning, reliability, pupil size and age³. Statistical programs help, but do not replace clinical judgement³.

That is why a single test that is worse than the previous one is not a verdict. In the Early Manifest Glaucoma Trial, for example, worsening of the visual field was confirmed only when the same points were worse on three consecutive tests⁷.

How many tests are needed? A group of experts calculated that, in a person with average variability, recognising an overall loss of 4 dB in two years requires three tests a year⁵. Current recommendations indicate six reliable tests in the first two years; a 2025 study of 535 patients showed that doing two tests per eye in the same visit reached six in about 1.4 years, against about two and a half years with one test per visit⁶. This is one of the reasons why the visual field, in glaucoma, is repeated more often than many people expect.

In glaucoma and beyond

In glaucoma the visual field is one of the pillars of check-ups, together with examination of the optic nerve and measurement of pressure¹; the optic nerve is today often studied with OCT as well. We discuss this in our articles on what glaucoma is and on high eye pressure.

But the test also looks beyond the eye. Some defects have a shape typical of damage to the visual pathways in the brain: this is the case of homonymous hemianopia, the loss of the same half of the visual field in both eyes, which a stroke can cause with a major disability⁹. The rehabilitation therapies available help to compensate for the defect, while those that claim to restore the visual field are controversial⁹.

The visual field also matters for driving. A review of homonymous defects found that some people with these defects drive safely in on-road tests, others do not; the extent of the visual field alone, measured without moving the eyes, does not predict driving ability well¹⁰. You will find the European rules in our article on vision and driving licences.

Frequently asked questions

Does the visual field test hurt?

No. You look at a lit dome and press a button when a light appears; there is no contact with the eye and no radiation. It does require attention, however, and tiredness can affect the result².

Why does the result change from one test to the next?

Because the sensitivity of the eye fluctuates naturally, especially in areas that are already damaged, and because experience, attention and age matter^2,3. That is why a worsening is considered real only if it is confirmed over several tests⁷.

How often does the visual field test need to be repeated?

It depends on the disease and the risk. In glaucoma, the recommendations reported by the studies indicate six reliable tests in the first two years, to measure how quickly the visual field is changing^5,6.

Can the visual field test be done at home, on a tablet?

There are perimeters on tablets, on the web or with headsets. A systematic review of 18 studies found promising but still uncertain results: the ability to detect glaucoma ranged from 54% in mild forms to 91% in moderate or severe ones, and all the accuracy studies had a high risk of bias in at least one respect⁸. For now they do not replace the test in the clinic.

The visual field is a test that is simple to do and less simple to read: an isolated result says little, a series of results over time says a great deal. If you are asked to repeat it several times, it is not a sign that something has gone wrong: it is the way the studies indicate for following, with precision, the health of the optic nerve.