Why does a pinhole improve vision?
Because refractive error from beams of light that is not refracted correctly onto the retina is eliminated.
What is the equation for Rayleigh’s Criteria?
A(radians) = 1.22 λ /d
How do you convert radians to degrees?
x (180/π)
Optics of the eye spread, point of light forms ________ distribution on the retina
Gaussian (Normal)
Define minimum resolvable distance
To be able to resolve two stars, images on the retina have to be a certain distance apart
Limits of Visual Acuity
- Undersampling and aliasing
2. Need enough receptors to fully reconstruct Fourier wave form
What are the four types of Visual Acuity? and what do they measure
- Localisation - asking the patient to detect differences in location of features.
- Recognition - Ability to recognise letters or shapes e.g. Snellen Chart, LogMAR chart
- Resolution Acuity - ability to discriminate against two or more stimulus. e.g. 30” to 1’ checkerboards, falling C
- Detection - ability to determine if the stimulus is present or not. e.g. VFT and children
Grating Acuity
One cycle of grating = two detail widths of Snellen width
High Spatial Frequency vs Luminance Contrast
- At high spatial frequencies the loss of contrast in the retinal image is greater that at low frequencies reducing recognition.
- Contrast sensitivity reaches a maximum at 4-6 cycles per degree
What do you do if the patient can see the biggest letter on the chart?
- Move the chart closer till they can see it and record new test distance
- Move to counting fingers
- Hand movement
- Light perception
- No light percerption
