Eye Testing

Testing Vision, Investigations, Symptoms and Signs

The most widely used test is the Snellen chart: a range of letters, starting large, and finishing small, designed to measure the ability to see one minute of arc (1/60 degree). A single minute arc equates to one inch over one hundred yards, for instance, or one pixel of Snellen’s 25 pixel optotype at 20/20 vision.

The ability to see these letters, and distinguish the pixels from each other is a reliable way to measure the patient’s ability to complete daily activities which require the use of sight. This test measures visual acuity. Visual acuity = Distance at which test is made / distance at which the smallest letter read subtends an angle of 5 arcminutes.

The measurement from the Snellen chart is given as a fraction comparing the patient’s eyesight to ‘standard’ eye sight: ie, a score of 20/40 indicated that the patient can see at twenty feet what a normal person could see at forty. It is said that normal vision is actually slightly better than 20/20, more like 20/16. In some countries, like Australia, eyesight is no longer measured in feet: it is now measured in metres. 6/6 is standard, worse than 60/6 (200/20) is considered legally blind, assuming it cannot be corrected with glasses. Hawks have 20/2 vision. 20/40 is considered the acuity required to drive.

Although the measurement of visual acuity from Snellen does give a decent indication of ability to perform standard tasks, it fails to take into consideration a variety of other factors, and hence 20/20 vision may not be good. Examples of factors which contribute to vision impairment are colour blindness (generally measured with green and red dots creating a number), and the inability to track motion.

The Snellen chart has also been criticised on the fact that to read the top line, you only have to read one letter; compared to reading multiple at the lowest line. Thus factors such as isolation, cluttered space, the ability to distinguish between characters, not just pixels, and what not are not taken into consideration.

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The near vision test takes into account what font size of standard Times New Roman, can a person read at a distance of 40 cm (14 inches). N8-N10 is what most books and newspapers are. Occasionally, near vision is measured at both 40cm and 100cm, in which case the subject receives two scores, ie. N8 & N14. This is relevant for certain professions such as piloting aircraft. N10 is considered normal. Light conditions can affect the results of this test, as can tiredness.

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Common symptoms of cataract include:

• a painless blurring of vision;
• glare, or light sensitivity;
• poor night vision;
• double vision in one eye;
• needing brighter light to read;
• fading or yellowing of colours.
• occassionally, a cataract can be painful, but it is rare.

The amount and pattern of cloudiness within the lens can vary. If the cloudiness is not near the centre of the lens, you may not be aware that a cataract is present.

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To diagnose cataracts, visual acuity is measured, a pupil-dilation test is completed, and if necessary, a slit-lamp test is also conducted. In the pupil dilation test, the pupil is dilated using eye drops, then examined with an ophthalmoscope. In the slip test, a lamp which only allows a slit of light through, allows the doctor to determine the exact location of the cataract.

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The Snellen Chart

Further reading: (melb uni, God bless them, have a decent site... worth checking out) :
http://en.wikipedia.org/wiki/Arc_minute
http://en.wikipedia.org/wiki/Visual_acuity
http://en.wikipedia.org/wiki/Image:Snellen06.png
http://en.wikipedia.org/wiki/Snellen_chart
http://www.lowvisiononline.unimelb.edu.au/Screening/acuityNear.htm
http://www.medem.com/medlb/article_detaillb.cfm?article_ID=ZZZSXEVUF4C&sub_cat=0
http://eye.upmc.com/Cataracts/Diagnosis.htm