Snellen Latter Size Calculation- The easiest way to calculate Snellen optotype size.

What is the formula for Snellen Latter size calculation?

• In Snellen chart, optotypes or letters are constructed in a square so the height and width of Snellen optotypes or letters are same.
• The formula to identify the size (Height & Width) of Snellen optotype is:

Here,

• W = Height or Width of Snellen Optotypes
• D = Denominator of Snellen fraction in Meter

You might be thinking of Where I find the number “0.00145”

• The actual formula for Snellen Latter size calculation is:

• Tanθ is the angle formed by the optotype which is 5min arc and constant of each letter.
• Bigger the size of the letter or optotype bigger the viewing distance that keeps angle at nodal point of eye constant.
• So, tanθ is constant that is 5 min arc or 0.00145 radian.

That’s how I modified the formula in simple way by replacing the tanθ by its numerical value 0.00145.

So, the formula to identify the size of Snellen optotypes is:

W = (D) X 0.00145

Example for Snellen Latter size calculation:

What is the size of a 20/60 letter on a standard 20-ft Snellen chart?

• We know,
  • W = (D) X 0.00145

Here,

•             W= ??
•            D = 60ft or 18.288m

• So,         
  • W = 18.288 X 0.00145
  • W =  0.0265 meter
  • w  = 2.65 cm (0.0265×100) When we multiply meter (m) by 100 we get value in centimetre (cm).
  • w = 26.5 mm (0.0265×10) When we multiply centimeter (cm) by 10 we get the value in millimeter (mm).

• Using this formula we can find:
  • Size of 6/60 is 87 mm,
  • Size of 6/36 is 53 mm, 
  • Size of 6/24 is 35 mm,
  • Size of 6/18 is 27 mm,
  • Size of 6/12 is 18 mm,
  • Size of 6/9 is 13 mm,
  • Size of 6/6 is 9mm.

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