Quality of vision measurement
What is it for?
Vision can be thought of being made up of two elements – quantity and quality.
Vision quantity is the visual acuity (see earlier section), measured by reading letters on a standard vision chart and is recorded as 20/20, for example.
However, it is possible to have 20/20 vision, but still not be satisfied with your vision due to quality of vision symptoms, such as halos, glare, starbursts, double images, or reduced contrast sensitivity.
Therefore, it is important to measure the quality of vision in addition to the quantity of vision. A number of different devices are available to measure different aspects of quality of vision.
What actually happens?
You rest your chin on a padded support and stare straight into the examining instrument. The clinician sits in front of you and aligns the instrument by having you focus on a visual target.
Depending on the device, you may be asked to provide feedback to what you can see by clicking a button.
How does it feel?
You feel nothing as the instrument does not make contact with your eye.
How does it benefit you?
Measuring the quality of vision in addition to the quantity of vision provides your surgeon with a complete picture of what you see.
This information can then be correlated with the other scans and tests to make a confident and informed diagnosis, and consequently make an appropriate treatment recommendation. For example, these can be used to assess the visual significance of early cataract formation.
Examples of devices that measure the quality of vision include the following.
The HD Analyzer evaluates how light is scattered by the eye (known as the objective scatter index), providing a picture of what a point light source looks like for the patient. For example, this picture would be a small circle in a normal eye, whereas the picture becomes larger and more distorted in an eye with a cataract.
The C-quant also evaluates light scatter, known as straylight, and works using feedback from the patient when viewing different image patterns.
The light distortion analysis system is a method for measuring the size and shape of halos and starbursts, based on patient feedback when looking at a point light source.
The highest standard of care includes measurement of at least contrast sensitivity before and after treatment to ensure that visual quality has been maintained.
It is uncommon for laser refractive surgery to significantly reduce quality of vision, and such changes are manageable with further treatment in the majority of cases.
These devices are found at only a few refractive surgery clinics, but are integral to the management of patients who are not satisfied with their quality of vision.