why measure vision

• What is vision?


  Perception of differences in the external world from retinal stimulation by light.
• What can vision include?


  Light changes, color, shape, position, etc.
• What are some types of vision?

  Binocular, distance, near, intermediate, double, blurred, photopic, mesopic, scotopic, spatial, temporal, peripheral.
• Why is it important to measure vision?


  To quantify what people can and cannot see.
• What are some reasons for measuring vision?

  Changes in health and disease, occupational requirements, statutory registration.
• What historical interest is noted regarding test charts?

  Interest in the ability to see detail, such as double stars.
• Who asked Snellen to produce a measurement tool?

  Donders.
• What are “optotypes”?


  Special characters designed to test distance vision.
• What is the purpose of the Snellen chart?


  To test distance vision.
• What are the terms used synonymously with the Snellen chart?


  Letter chart, Snellen chart, test chart.
• What are “optotypes” in the context of vision testing?


  Letters, characters, or symbols used for testing vision.
• What is the summary of the role of optometrists?


  They measure vision and determine refractive error.
• What is noted about the Snellen chart in the summary?


  It is the first systematically designed letter chart to test distance vision.
• What is the reason that a thumbnail and a car can appear to be the same size at a distance?

  They subtend the same angle at your eye.
• How can you match the size of your thumb to an object in the distance?


  By moving your thumb until it matches the size of the object you are looking at.
• What is the significance of angles subtended by objects at our eyes?


  • They fundamentally affect image size.
  • Letters at different distances can produce the same image height on the retina if the angle is the same.
• What are the smaller units used to measure angles besides degrees?

  Minutes and seconds of arc.
• How many minutes are in one degree?


  60 minutes.
• How many seconds are in one minute?


  60 seconds.
• Convert 3.25 degrees into degrees, minutes, and seconds.


  3 degrees, 15 minutes, 0 seconds.
• Convert 5.67 degrees into degrees, minutes, and seconds.


  5 degrees, 40 minutes, 12 seconds.
• Why is trigonometry important in this context?


  • It helps to work out the size of test types, such as letters.
  • It is essential for calculating angles and distances.
• What is the height of the letter E if it subtends a 5' arc at a distance of 6m?


  0.008726 m or 8.73 mm.
• What is the small angle approximation for angles up to approximately 10 degrees?


  sin(θ) ≈ θ and tan(θ) ≈ θ when θ is in radians.
• How many radians are there in 360 degrees?


  2π radians.
• How do you convert radians to degrees?


  angle in degrees = angle in radians × (360/2π).
• How do you convert degrees to radians?


  angle in radians = angle in degrees × (2π/360).
• What is the conversion of 5' arc into radians?


  0.00145 radians.
• What is a geometric progression?


  • A sequence where each term after the first is found by multiplying the previous term by a fixed, non-zero number called the common ratio.
  • Example: 1, 2, 4, 8, 16, 32 (common ratio of 2).
• Why are logarithms important in vision?


  • Our eyes respond on a logarithmic scale to various stimulus attributes.
  • Useful for handling large ranges of values.
• What is the common logarithm of a number?


  It is the exponent to which the number 10 must be raised to obtain that number.
• How can a range of light levels be expressed in logarithmic units?


  By referring to it as log units, e.g., 8 log units for a range of 100,000,000 times.
• What is the purpose of the Snellen Chart?


  To measure visual acuity
• What are the key design features of a Snellen chart?


  • Letter size
  • Letter styles (test type font)
  • Number of letters per line
  • Range of letters used (legibility)
  • Progression of letter sizes
  • Letter spacing
  • Line spacing
• Who developed the British Standards for manufactured products?


  The British Standards Institute
• What does the number by each line on a Snellen chart represent?


  The letter size in meters
• What does a '12m' letter on a Snellen chart indicate?


  It indicates the distance at which the letter height subtends 5' arc
• How is the physical height of a '6m' letter calculated?


  By using the tangent function based on the subtended angle
• What is the physical height of a '6m' letter?


  $8.73 \text{ mm}$
• How is the height of an '18m' letter calculated?


  $26.18 \text{ mm}$