One might ask himself, how is it the Resorce Centre for the Blind identifies students who need low vision aid or Braille study?
Every new school year (beginning in September) the Resource Centre for the Blind in collaboration with nurses attached to the Ministry of Health sets out into the primary schools of Grenada Cariacou and Petite Martinique to ‘Vision Screen’ students of the Grade six, the class where students are being prepared to sit the Secondary School ‘Common Entrance’ exams, as well as the first grade students that have just entered the school system as previous studies have shown that eye diseases and concerns tend to develop mainly during this age range (3-4 and 11-12)
This is an observational exam that results only in our recommendation that students see a licensed optermologist who can then give a professional opinion as to the student’s visual state and what their visual acuity really is. After this is done, the Resource Centre for the Blind then provides services such as large print photo copies, Braille lessons and teacher guidance as is required.
The main tool used during our observational study is: The Snellen Chart.
Description
The traditional Snellen chart is printed with eleven lines of block letters. The first line consists of one very large letter, which may be one of several letters, for example E, H, N, or A. Subsequent rows have increasing numbers of letters that decrease in size. A patient taking the test covers one eye, and reads aloud the letters of each row, beginning at the top. The smallest row that can be read accurately indicates the patient's visual acuity in that eye.
The symbols on an acuity chart are formally known as "optotypes." In the case of the traditional Snellen chart, the optotypes have the appearance of block letters, and are intended to be seen and read as letters. They are not, however, letters from any ordinary typographer's font. They have a particular, simple geometry in which:
• the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter "C"
• the height and width of the optotype (letter) is five times the thickness of the line.
Only the nine letters C, D, E, F, L, O, P, T, Z are used in the traditional Snellen chart.
Wall-mounted Snellen charts are inexpensive and are sometimes used for rough assessment of vision, e.g. in a primary-care physician's office. Whenever acuity must be assessed carefully (as in an eye doctor's examination), or where there is a possibility that the examinee might attempt to deceive the examiner (as in a motor vehicle license office), equipment is used that can present the letters in a variety of randomized patterns in other countries.
According to BS 4274:1968 (British Standards Institution) "Specification for test charts for determining distance visual acuity" the minimum illumination for externally illuminated charts should be 480 lx, however this very important parameter is frequently ignored by physicians, making many test results invalid.
Snellen fraction/measurment
Visual acuity = Distance at which test is made / distance at which the smallest optotype identified subtends an angle of 5 arcminutes.
"20/20" (or "6/6") vision
Now, how is it EXACTLY that we go about actually useing this tool? Come back to find out next post :)
