VISION EXPLAINED
In America, the Snellen Eye Chart is used which is based on standardized graded sized letters usually read at a distance of 20 feet.
Visual field = is the amount of distance seen left to right without moving the eyes from side to side.
20 / 20 = normal vision
A person with 20/20 can read the numbers in a telephone book and stock quotes on paper with the naked eye.
Normal Vision is the last 3 lines of an eye chart and extends from a vision of 20/10 to 20/20 and 20/30.
20/10 = better than normal vision!
The first set of numbers is the vision of the patient.
The second set of numbers is the vision of a normal person.
The numbers compare the two types of vision.
Patient (Compared to) Normal vision person
= P / N.
= P / N.
20/20 = normal
20/100 = Patient sees it at 20' / Normal person can see it from 100' away
20/10 = Patient can see it from 20' away / Normal person sees it at 10' away…the patient has better
than normal vision in this case!
Normal vision = 20/20
In order to get a driver's license = 20/40
In need of special educational accommodation/assistance = 20/80
The legal definition of blindness = 20/200
A person with 20/80 vision can read the headlines of newspapers…that is the type size of headlines, so that they can be seen from a further distance by most people, and also read by low vision people.
Visual Acuity = a measure of the spatial resolving power. It indicates the angular size of the smallest detail that can be resolved. (That can be seen clearly enough to identify correctly.)
The maximum visual acuity…considered to be 20/15 or 20/16.
O.D. = oculus dextrous (the right eye).
O.S. = oculus sinister (the left eye).
So…if a normal vision person sees an object at 600 feet, and the patient sees it only when at 5 feet, then if the normal person saw something at 2400 feet clearly, the patient would be able to see it at 20 feet. 5/600 = 20/2400.
Also…if a normal person can clearly see something at 225 feet, but the patient sees it only when at 10 feet, then if a normal person can see it at 450 feet away, the patient could see it at 20 feet away. Say you enlarge the thing the normal person saw at 225 feet so the normal person could now see it when at 450 feet…enlarge it 2x. The patient could see it after it was enlarged 2x when standing at 20 feet away because it was enlarged 2x. 10 x 2 = 20 and 225 x 2 = 450 10/225 = 20/450.
The same is true for vision of 5/600. If the patient has to be 5 feet from something in order to see it clearly, while the normal vision person can see it from 600 feet away. Then an item large enough for the patient to see at 20 feet, the normal person would be able to see from 2400 feet away…nearly ½ mile away. To figure this, take the patient vision of 5 feet compared to normal vision of 600 feet. Then enlarge it to where the patient will be able to see it at 20 feet…you will need to enlarge it 4x. Make the normal person's item 4x larger also so that they will be able to see the same sized item, and the normal person will now be able to see that 4x item from 2400 feet away. 5 x 4 = 20 and 600 x 4 = 2400 5/600 = 20/2400.
Synopsis / Recap:
The goal is to make something to where the patient can see it from 20 feet …
Remember how the comparison is written…
P / N (P=patient
/= compared
to N=normal vision person)
10/225……………..20 divided by 10 = 2 (If the patient can only see it when standing at 10 feet away, then this means you need to enlarge the item 2x so the patient can see it from 20 feet away).
The 225 is the distance a normal vision person could see the item. If we enlarge the item for the patient to be able to see it from 20 feet away, it will change the distance that the normal vision person will see it at also. If we enlarge it, the item can now be seen by the normal vision person when they stand at 450 feet because 225 x 2 = 450.
10/225 (x2) = 20/450
If a patient can now see the item at 20 feet away, yet the same item is normally seen from 450 feet away, that means the patient has far less than normal vision. The patient should have been able to see the item from 450 feet instead.
When visual acuity is below the largest optotype on the chart, either the chart is moved closer to the patient or the patient is moved closer to the chart until the patient can read it. Once the patient is able to read the chart, the letter size and test distance are noted. If the patient is unable to read the chart at any distance, he or she is tested as follows:
Name | Abbreviation | Definition |
Counting Fingers | CF | Ability to count fingers at a given distance. |
Hand Motion | HM | Ability to distinguish a hand if it is moving or not in front of the patient's face. |
Light Perception | LP | Ability to perceive any light. |
No Light Perception | NLP | Inability to see any light. Total blindness. |
Many humans have one eye that has superior visual acuity over the other. If a person cannot achieve a visual acuity of 20/200 (6/60) or above in the better eye, even with the best possible glasses, then that person is considered legally blind in the United States. A person with a visual field narrower than 20 degrees also meets the definition of legally blind.
A person's visual acuity is registered documenting the following: whether the test was for distant or near vision, the eye(s) evaluated and whether corrective lenses (i.e. spectacles or contact lenses) were used:
Distance from the chart
- D (distant) for the evaluation done at 20 feet (or 6 meters).
- N (near) for the evaluation done at 15.7 inches (or 40 cm).
- D (distant) for the evaluation done at 20 feet (or 6 meters).
Eye evaluated
- OD (Latin oculus dexter) for the right eye.
- OS (Latin oculus sinister) for the left eye.
- OU (Latin oculi uterque) for both eyes.
- OD (Latin oculus dexter) for the right eye.
Usage of spectacles during the test
- cc (Latin cum correctore) with correctors.
- sc: (Latin sine correctore) without correctors.
- cc (Latin cum correctore) with correctors.
- The abbreviation PH is followed by the visual acuity as measured with a pinhole occluder, which temporarily corrects for refractive errors such as myopia or astigmatism.
- So, distant visual acuity of 20/60 and 20/25 with pinhole in the right eye will be:
DscOD 20/60 PH 20/25 - Distant visual acuity of count fingers and 20/50 with pinhole in the left eye will be:
DscOS CF PH 20/50 - Near visual acuity of 20/25 with pinhole remaining at 20/25 in both eyes with spectacles will be:
NccOU 20/25 PH 20/25
"Dynamic visual acuity" defines the ability of the eye to visually discern fine detail in a moving object.
Another type of eye chart, can you read it:
No comments:
Post a Comment