Your sight test explained

There's more to a routine eye examination than you might expect.

Here's a brief guide to some of the procedures that we'll carry out during your visit:


Visual fields test (the 'flashing lights' test)

Certain medical conditions such as glaucoma, stroke, brain tumours or other neurological problems, can cause losses in your central and peripheral vision. In many cases, however, the back of the eye can still look healthy and the patient may not be aware of any deficit.

Visual field screening is therefore used to alert us of any possible problems.

During this test you'll sit in front of a small concave dome with an illuminated target in the centre. The eye that is not being tested is covered. The device presents small dots of light at various places on the dome and you'll click a button whenever a light is seen. A computer then automatically maps your visual field which is assessed by our optometrist.

We use a Humphrey field analyser which is a computerised device made by Carl Zeiss. It's widely regarded as the "gold standard" in fields testing, and is the instrument of choice in most hospital eye departments.



Autorefractor (the 'balloon' test)

An autorefractor is used to give us an "objective" idea of your spectacle prescription. It's also used to check if your contact lenses are fully correcting your vision.

When you look into the device, you'll see a picture of a road with a balloon at the end. The picture will move in and out of focus - this is a technique used to stop your eyes over-compensating for any refractive error. Your prescription is then estimated using a beam of infrared light which is tracked as it passes through your eye.

This can be thought of as a modern version of 'retinoscopy', a technique where the optometrist used to repeatedly swipe a bright light across your eye while adjusting lenses in a trial frame. By observing the reflections, the optometrist could roughly calculate your prescription.


Subjective refraction



The autorefractor test provides a starting point for what's called a "subjective" refraction. Using an instrument called a phoropter, different lenses will be presented to you while looking at a chart and you'll be asked "which is better?".

This feedback refines the objective prescription to one which will provide you with the clearest vision.







Tonometry ('puff of air' test)


Tonometry is performed to measure your intraocular pressure (IOP). It is an important test in the evaluation of patients at risk from glaucoma. If the pressure in the eye is elevated, damage can be done to the sensitive nerve fibres at the back of your eye, leading to permanent vision loss.

We principally use "non contact tonometry", which most people refer to as the "puff of air test". Here, a very quick puff of air is used to applanate (flatten) a small part of the cornea. Pressures are estimated by detecting the strength of the air required to do this. Despite how that might sound, there's absolutely no discomfort involved!




If your pressures appear to be high, we'll check them again using "Perkins" or "contact tonometry". This is the method used in hospitals and offers a greater degree of accuracy. It involves contact with your eyes, so the optometrist will use anaesthetic drops which include fluoroscein, a dye which fluoresces under UV light. We'll usually do this on a different day or time, as pressures can vary throughout the day and a separate appointment allows us to establish an average figure.

If your pressures are still high at this stage, we may refer you to the Hospital Eye Service for further examination. In most cases, treatment is by way of eye drops (applied daily) which reduce the pressure and accordingly the risk of any vision loss.



Pachymetry (used in cases of suspect pathology only)

Pachymetry is a simple procedure to measure the thickness of your cornea (the clear window at the front of the eye). A probe called a pachymeter is gently placed on the cornea and the thickness is assessed using ultrasound waves.

Pachymetry can be useful in cases of suspect glaucoma or high intraocular pressures, because corneal thickness has the potential to influence eye pressure readings.  If a patient has a thick cornea, the pressure reading may be artificially increased. Conversely, if the patient has a thin cornea, the pressure may measure artificially lower

Pachymetry can also help monitor extended-wear contact lens patients, giving us warning of steepening or warpage of the cornea that may occur from long-term wear. It can also be useful in assessing a patient's suitability for refractive (laser) sugery.



Slit Lamp Examination

A slit lamp is an illuminated microsope used routinely to examine the health of your eyes. The exterior of the eye is first examined, then another lens (called a "Volk" lens) is added so the interior can be studied.

In the past, optometrists had to use a hand-held device called an ophthalmoscope to examine the back of the eyes, however this involved quite close proximity to the patient. The modern Volk method feels much more relaxed and non-invasive.

We use a state of the art Zeiss slit lamp, which is also capable of photo and video capture.

Please note that, for anyone over the age of 60, we normally use drops to widen the pupil so we can obtain a better view of the back of your eyes. This will affect your ability to read or drive for a few hours afterwards.



Retinal Photography


During your eye examination, we'll take a picture of the back of your eyes using a state of the art Nidek retinal imaging system. This allows us to assess in more detail any changes to your eyes on subsequent visits. Previously, optometrists had to rely solely on written notes but this method has obvious limitations- you can't describe the back of the eye in words in the same detail as a photograph. Retinal photography has particular benefits in assessing patients with pathology- we can monitor conditions more effectively, and give the hospital or GP more meaningful data should we need to refer you.




Optical Coherence Tomography

We can also perform a more in-depth scan of the retina which is outwith the scope of a standard NHS eye examination.
For more details, click here.



Ishihara ('colour blindness') test   (used where signs/symptoms indicate)


The Ishihara test is used to detect colour deficiencies. It was named after its designer, Dr. Shinobu Ishihara, who was a professor at the University of Tokyo


The test consists of a number of coloured plates, each of which contains a circle of dots. With "vanishing plates" the dots form a number or shape clearly visible to those with normal colour vision, but are invisible, or difficult to see, to those with a red-green colour vision defect.

You'll also be shown "hidden digit plates", where only individuals with a colour defect can recognise the figure, and "transformation plates" where individuals with a colour vision defect will see a different figure from individuals with normal colour vision.


 Finally, if you have colour vision deficit,  "diagnostic plates" will be used to determine the type and it's severity.



Corneal Topography (for contact lens wearers or those with corneal pathology)

For new contact lens fits, we use a device called a topographer to create a detailed 3D map of the cornea to assess your suitability for lens wear, and to create bespoke designs where necessary

The device is also used to assess and monitor corneal pathologies such as kerataconus.





For information about common eye problems, download our guide