In an earlier article Decoding Snellen Chart In 4 Mins https://www.quickguide.org/post/decoding-snellen-s-chart-in-4-mins we have touched upon the science behind Snellen Chart for measuring distance visual acuity. It may be important here to now focus on the science behind measuring near visual acuity. In many countries, we largely represent patient's near visual acuity in terms of N notation chart -N6, N8, N10, etc. It is time to look into the science behind such numbers, how they are derived and what this means for the patient.
In 1951, FW Law published Standardization of Reading Types in the British Journal of Ophthalmology. The aim was to standardize near acuity test reading charts that was found wanting unlike the distance acuity Snellen charts that was well established by then. He found the Jaeger notation that was widely practiced for recording near acuity as highly obsolete and without standardization of the reading fonts. He proposed, that the Times New Roman font ( a name derived from the famous The Times paper) as a standard font for reading charts. This would ensure standardization of style, length, and height of the fonts across the reading chart. Thus the N-notation represents the record of what the patient reads as per the font size (point size).
Different fonts differ in height, despite the same point. For example, on your computer/laptop if you select the Arial with 10 point, it will be of a different height than that of Times New Roman with the same point. Therefore Times New Roman was selected as the standard.
The N notation reading charts could be used in various distances from the eye. Thus any reference to near visual acuity in terms of N notation should be referenced with regard to the distance the patient is holding the chart from eye. Therefore a patient reading N6 at 40 cm from eye would not be the same as a patient reading N6 at a distance of 30 cm from the eye. If the intention is to evaluate two different multifocals in terms of reading speed or near visual acuity, then the distance from the eye should be standardized. However, the near visual acuity and at what distance the patient reads will depend on the add power of the multifocal IOL as well as the patient's reading habit.
A patient reading N6 at a distance of 40 cm from the eye would mean, that the letters will have to subtend at 5 min of arc at the nodal point of eye at that distance from the eye to chart ( to understand min of arc and nodal point of eye please refer to Decoding Snellen Chart in 4 mins https://www.quickguide.org/post/decoding-snellen-s-chart-in-4-mins )
Now let us look into the maths. 1 point of the font size is 1/72 of an inch (an inch equals 25.4 mm). Therefore the height of 1 point of Times New Roman letter is .35 mm. This is illegible and impossible to read unless you hold a magnifying glass over it :-) .
A patient reading N6 at 40 cms would mean the font size are 6 times multiplied by .35 mm or 2.1 mm of height and the letters are subtending at 5 min of arc at the nodal point of eye at that distance. If a patient reads N10, what that means is that the height of the Times New Roman letters has to be 3.5 mm ( 10 times multiplied by .35 mm) to help subtend the letters at 5 min of arc at the nodal point of eye when the patient is holding the book at the same distance (40 cm).
Frank Law therefore requested the following standardization in the N notation
Times New Roman font be adopted
That the spacing shall be standard
The points of 5, 6, 8, 10, 12, 14, 18, 24, 36 and 48 be used.