Week 6

Question 1

Stiles Crawford Effect of the first kind

Answer 1

• Light entering the eye at different pupil heights is perceived to be of different luminous intensities
• Light passing through the periphery of the pupil appears to be dimmer compared to equal intensity of light passing though the centre of the pupil which appears to be brighter
• It is explained by the shape and size of the cone photoreceptors which are roughly orientated towards the centre of the pupil
• Because of the small size and coned shape the cone photoreceptors act as dielectric waveguides
• Due to the narrow confinement of light within cone photoreceptors, the light entering the cones propagate in one direction and funnel in a predictable way

Question 2

Stiles Crawford Effect of the second kind

Answer 2

• The colour of a monochromatic light (one wavelength) entering the eye near the edge of the pupil is perceived to be of different colour compared to the same wavelength of light passing though the centre of the pupil, regardless of the overall intensities of the two lights

Question 3

Lateral inhibition

Answer 3

• Occurs when an excited neuron supresses the response of its neighbouring neurons to optimise its response
• It increases the contrast and sharpness in visual response and also colour perception

Question 4

Snellen acuity charts

Answer 4

• Clinically visual acuity is specified as the visual angle subtended by the smallest number which can just be resolved
• Documented as test distance/distance at which a person with normal vision could read the letter
• Targets are constructed on 5x5 grids

Question 5

LogMAR charts

Answer 5

• There are 5 letters on each line, each drawn in a 5x4 grid
• Letters are chosen from “Sloan fonts” which give approximately equal recognition to each letter
• Letters are in logarithmic progression. As a result each line is the same proportional change from the previous line
• Both letter and line spacing are proportional to the letter size
• Every letter reads contributes to the visual acuity score

Question 6

problems with Snellen charts

Answer 6

• There are 5 letters on each line, each drawn in a 5x4 grid
• Letters are chosen from “Sloan fonts” which give approximately equal recognition to each letter
• Letters are in logarithmic progression. As a result each line is the same proportional change from the previous line
• Both letter and line spacing are proportional to the letter size
• Every letter reads contributes to the visual acuity score