CVI Mascot - VIC

What is CVI?

Documents

Visual Acuity

Reduced visual acuity can be caused by the eye or brain conditions, or both. With CVI we are looking at visual acuity in relation to the brain.

This is the letter E But how do we know it is the letter E?This is the letter E But how do we know it is the letter E?

We know it is the letter E because of these two spaces. With this letter E the two spaces are both white rectangles.We know it is the letter E because of these two spaces. With this letter E the two spaces are both white rectangles.

Without the two spaces, the E would look like this. It is only when you can see the spaces that separate the block to create its unique shape, that you can recognise the letter.Without the two spaces, the E would look like this. It is only when you can see the spaces that separate the block to create its unique shape, that you can recognise the letter.

As the spaces become narrower, correctly identifying the letter becomes harder.As the spaces become narrower, correctly identifying the letter becomes harder.

For someone with reduced visual acuity, it may become impossible.For someone with reduced visual acuity, it may become impossible.

Visual acuity if often thought of in terms of the smallest thing you can see, for example a marble, a peanut, a grain of rice. These are not measurements, they are observations.

To be useful, visual acuity needs to be measured.

The measurement of visual acuity is the measurement of the width of the line that allows you to separate things, to let you see them clearly.

Visual acuity is a measure of the thinnest black / white line gap and line thickness that can be seen apart from each other.

As things look smaller the further away they are, measures of visual acuity have to take this into account.

This is done in two different ways.

1. The Two Number Way

  • 6/6 is normal vision.
  • the first number 6 is the distance from which the test was done
  • the second number 6 is the distance the image tested would look the same size as the 6/6 image, therefore:
  • 6/60 means that at 6 meters from the test chart, the smallest letters, numbers or pictures that can be seen are those that someone with typical vision could see at 60 meters
  • note in some countries the first number of the test is 20, for example 20/20 vision. This is the same test, just measured in feet (imperial) rather than meters (metric).

2. The Angle Way

This is a complex calculation preferred by some optometrists, ophthalmologists and vision scientists, because the result is more precise, and the scale is decimal. Visual acuity when measured this way is recorded as LogMAR.

This diagram shows how visual acuity is measured using the angleThis diagram shows how visual acuity is measured using the angle

Normal (6/6 or 20/20) vision equates to 0.0LogMAR (see conversion table below).

There are two types of visual acuity

1. Threshold visual acuity
Meaning a measure of the smallest black / white detail that can be seen. This is tested with each eye in turn.

2. Functional Visual Acuity
As used on this website, means the measurement of the smallest detail (not blob or mark) that can be seen comfortably at the highest speed possible with both eyes open in normal lighting conditions, even at the end of the day when the person is getting tired.

As clarity of vision is lost with reducing acuity in these LogMAR charts (below), look at what has happened to the clear lines that defined the letters.

The lines of separation are rarely nice neat straight white lines, with thick black borders as with the letter E.The lines of separation are rarely nice neat straight white lines, with thick black borders as with the letter E.

Remember our ladybirds?

Clarity of vision, that is to see the ladybirds clearly, can only be achieved if you can see the separate parts. With the bottom picture there is not enough information to confidentially recognise the 'red blobs' as ladybirds.

Normal visual acuity

degraded visual acuity

This is the large ladybird. We can see that the head and body are separate. We can see the black spots separately from the red shell. We can see the legs as separate from the green background. It is our ability to see these things separately that helps us give meaning to the image.

Our visual acuity is the measurement of the minimum thickness of the line that separates things, and the thickness of the things themselves, allowing clarity of vision. With this measurement you will be able to understand:

  • the minimum size things and their characteristic detail have to be, and how far apart these need to be, to be identified
  • how things look from the perspective of the person with low vision

To assess visual acuity, we are looking for a line thickness, and identical gap thickness. As with our letter E (above) if the space between the lines is narrower than is visible, then what is on either side merges to become one, meaning the clarity is lost, and the letter cannot be identified.

It maybe that a professional has already assessed visual acuity.

Many teachers of children with low vision use a functional visual acuity assessment kit called'Puppetface' for young children.

Measurements - What do they mean?

Visual Acuity Measurements Conversion TableVisual Acuity Measurements Conversion Table

Above is a conversion chart. 0.00 (highlighted) is considered 'normal' visual acuity, also called 6/6 or 20/20 vision. Numbers less than this mean that vision is better than 'normal'. Numbers bigger than this means that vision is less than normal.

The first two columns, Imperial and Metric, are the same measurements, just using feet or metres as the measure. The first number 20 (Feet) or 6 (Meters) is the distance you are standing from the assessment letters. If the test is conducted in a smaller room without a 6m distance available, then the letters on the testing chart are made proportionately smaller to allow for the closer distance, to ensure a consistent accurate result. The distance from the visual target at which the test is done is key to measuring visual acuity.

6/19 vision would mean that the smallest detail the person with that level of vision sees at 6m, can be seen by a person with normal (6/6) vision at 19m. This can be a useful way of understanding how someone with reduced acuity sees things - finding out how different something looks when viewed from just over three times the distance.

Another way is to look progressively further to the side of the required larger target until you can just make it out. This is quite like what the target looks like for people for whom this is the smallest target they can make out when looking straight at it from the same distance. If you master this method, by knowing how far to the side you need to look to just make out the target, you can then always look this far to the side of anything you are showing the person with reduced visual acuity, to check it is just visible (visual acuity), and how much less to the side you need to look to check if it is easily visible (functional visual acuity). This method works because our clarity of vision is less to the side than in the centre and it gets lower the further away from the centre we are concentrating on what we are looking at.

Line Width

The line width needed to see nearby relates to the minimum thickness a line has to be, at the person's 'arms-length', approximately which of course gets greater the older the child.

Connor's visual acuity is 1.5 LogMAR. From our table we can see that the metric equivalent is 6/190. That means that Connor's mother would need to stand 190m away from whatever Connor was looking at from 6m (or more than thirty times further away), to achieve a comparable acuity simulation for herself.

Arms-Length - Between 15 and 30cm depending on size.

Typically, it is the distance we:

  • hold a pen to write
  • hold a book to read
  • hold a toy to play with
  • have our computer screen from our eyes
  • Note - music usually has to be further away (and therefore made bigger) to make room for the instrument.

Connor's visual acuity of 1.5 LogMAR can be converted to a line width of 2.78mm. At arms length, anything with a width below 2.78mm (just over 1/4cm) is not visible to Connor. This includes the spaces. In the diagram below both the lines and spaces are of about thickness Connor can see (depending on the size of your screen).

If the lines were kept the same width, but the spaces reduced to 0.5mm (not to scale), as below:

Then this (below) is what Connor would see due to his 6/190 visual acuity.

The spaces are too small to be visible to Connor. Connor could no longer see the lines. Connor saw a big blob of black!

For Connor - everything that is presented to him at arms-length needs to be at least 2.78mm to be visible. This includes the detail - if it is a picture, all the details in the picture need to be at least 2.78mm wide.

For all Connor's favourite picture books his parents have enlarged the details (like eyes and other facial features), so that Connor can see these details to be wider than 3mm and separated by more than 3mm, to make sure he can see these details.

Understanding visual acuity in terms of a minimum line width gives people a critical guide for how big everything has to be for the person to just see it. Not just the book, but the text within the book. Not just the pictures in the book, but the detail in the pictures in the book. Not just the toy but the detail on the toy. Everything.

To see how different levels of visual acuity affect what visual information is available to us, the following picture has each been edited to demonstrate the different levels of reduced visual acuity.

Image with normal acuityImage with normal acuity

Image as it might be seen by someone with 6/19 visual acuity Image as it might be seen by someone with 6/19 visual acuity

Image as it might be seen by someone with 6/38 visual acuity Image as it might be seen by someone with 6/38 visual acuity

Image as it might be seen by someone with 6/95 visual acuity Image as it might be seen by someone with 6/95 visual acuity

These images (made with a beta version of SightSim software), give an approximate indication of how someone with the measured levels of vision shown sees when compared with someone with typical vision

Further examples of images that have been edited to show different reduced levels of visual acuity can be found in Simulated Images section.

The reason the images become harder to see clearly and make sense of is because the detail - the visual information that enables us to see things separately - is too small and lost.

One important element to see from these pictures is that variety of the primary colours helps one to see the elements as separate as well.

Assessing visual acuity does not aim to provide the person with the minimum size things have to be to be strained to be seen. In the same way a spectacle prescription should allow comfortable vision, when assessing functional vision the aim is to make things easy and comfortable to be seen all the time.

Some Effects of Reduced Visual Acuity

Once you have the measurement of visual acuity, look at the things the person uses and consider how much of the visual information they can see to make meaningful sense:

Does dolly have a mouth and nose?Does dolly have a mouth and nose?

Does the hat still have a cat?Does the hat still have a cat?

Does the butterfly still look like a butterfly?

Does teddy have eyes?Does teddy have eyes?

Does dinner look tasty?Does dinner look tasty?

HELP SUPPORT US

Your generous donations will be put to immediate use in supporting our charity...

Donate Here

About Us

At CVI Scotland we are devoted to helping people understand cerebral visual impairments, and together working towards developing the understanding of this complex condition.