VISN2131 Exam 2010-Solution

VISN2131-Exam 2010-Solution


2a)



OR, you can add them using Graphical addition:





Using Prentice’s rule the prism power due to the cyl:

To calculate the prism power due to the cyl:

4b)


The total prism power due to the sphere and the cyl:

5a) The peak CS value obtained with Foucault gratings is (4/) times larger than the peak CS

value obtained using sinusoidal gratings. This is due to the fact that the fundamental

harmonic of a square wave, which is a sine wave of the same frequency, has an amplitude

that is (4/) times larger than the amplitude of the square wave. This ratio corresponds to

0.1 log unit. The CSF measured using a sine wave grating peaks around 4~5 cycles/ deg and

falls down at lower frequencies. But if the CSF is measured with square wave gratings then at

low frequencies the higher harmonics make a subject more sensitive when the grating is

less than the for peak CS and the CSF tends to flatten at such low frequencies.

b) Spherical aberration arises due to the fact that paraxial rays go to one focus and marginal

rays go to a different focus. It can be expressed in dioptres as the difference in the power at

the periphery to the power close to the axis. It can be described in terms of Longitudinal

Spherical Aberration (LSA) or Transverse Spherical Aberration (TSA). LSA is given in terms of

the dioptric interval between the paraxial and marginal focal planes. TSA is given in terms

of the spread of the marginal rays at the paraxial focal plane.

Spherical aberrations can be minimized by reducing the pupil size. However, this will limit the

amount of light throughput. It can also be minimized by flattening the surface near the

periphery. In the eye the cornea is naturally flatter near the periphery! Further, the

crystalline lens refracts the light more strongly near the centre by virtue of the graded


distribution of the refractive index. This has the effect of reducing the spherical aberration.

The human eye is also less sensitive to the marginal rays and more sensitive to the paraxial

rays (the cause for this is associated with the shape of the cones: Stiles-Crawford effect). This

reduces the effect of spherical aberration in human vision.

5d) Purkinje III is formed by the convex anterior surface of the crystalline lens and IV by the concave

posterior surface. Therefore, they move in opposite directions when the eye moves. The anterior

surface of the lens is flatter and rougher than the posterior surface of the lens. Therefore, compared

to IV, III will appear larger, poorly defined and will move faster when the eye moves.

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VISN2131 Exam 2010-Solution