Determining Letter Heights Explained for Students (Easy Guide)
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Original Question
Determining Letter Heights for Distance Reading Objective: To understand the concept of visual acuity and learn how to determine the size of text for signs. Description: Visual Acuity is the ability to discriminate fine detail, and it depends on the accommodation of the eye. Visual acuity is measured in terms of the reciprocal of the visual angle (VA) and is measured in minutes of arc. The VA can be calculated with the following formula: VA (minutes) = (3438 X H)/D Where: H is the height of the object being read D is the distance from the object NOTE: The greater the visual acuity the smaller the size of detail that can be distinguished; this formula holds for small visual angles. For signs it is often recommended that characters should subtend a visual angle of at least 25 minutes of arc at the intended viewing distance. Characters should be in sans-serif fonts, with character width to height ratios of 0.6:1 to 1:1. “Normal” acuity is taken as 1.0 minutes (20/20 vision, ability to resolve 1 minute of arc at 20 feet). If a person can see at 20 feet what a normal person can see at 10 feet, their vision is 20/10. On the other hand, if a person can only see at 20 feet what a normal person can see at 40 feet, then their vision is 20/40. These scores are known as “Snellen” scores. They were developed by Hermann Snellen in the 1860s. We can use acuity to determine heights of letters for distance reading (e.g. on signs). The formula is: Ws = 1.45 x 0.00001 x S x d Hl = Ws/R Where: Ws = stroke width S = denominator of Snellen Acuity score d = reading distance Hl = letter height R = stroke width-to-height ratio of the font expressed as a decimal (for 1:5, it would be 0.20) It’s best to design signs for people with at best, 20/40 vision. Goals: 1. Students will define visual acuity and how to measure it. 2. Students will calculate the height of letters for signs given stroke width-to-height ratios, distances, and acuity scores. 3. Students will be able to determine if size of letters is large enough to be read at a necessary distance given a person’s visual acuity. Equipment: measuring tape, calculators Exercise: 1. Go outside and look for 4 signs. Insert a picture of each sign in this document. Use these 4 signs Speed Limit Sign, Gas Station Price Sign, Exit Sign, Restaurant Menu Signboard. 2. For each sign, determine the distance at which you would need to read the sign in order to make a decision. You will need to support this decision. 3. Measure the stroke width-to-height ratio for the font and calculate the visual angle (analyze the letters in caps). 4. Using the above formulas, determine the height that the letters so that they can be read at the above determined distance. 5. Conclude whether the letters are large enough. What are the consequences for the user? What do you notice about the type of font, the contrast of the sign, and the readability of it? Do the visual angle calculations correspond to the desired font height if you want at least 25 minutes of arc? Please fill in your data in the table and provide your calculations beneath it: Table 1. Visual Acuity Data from Liberty University Signage Sign Distance needed to read it Stroke width-to-height ratio of font Desired font height Font Height Is font large enough? VA (min) Additional Question: What are the adverse effects from a sign having lettering that is too small?
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