identify 6 different complexities of visual performance
- light perception (threshold of vision)
- discrimination (ability to distinguish an object from its background)
- form vision and recognition (ability to identify letters and words)
- resolution (ability to see detail)
- localisation (locating object that is situated to one side of another object)
- higher tasks (visual system stimulating other responses eg. motor)
describe the point spread function (3)
the image of a point object formed by the eye is not a point -> actually a spread over an area
- graph of illuminance across this image is known as point spread function
- limits resolution in foveal and peripheral vision
identify optical (4) and anatomical (2) factors that are responsible for the point spread function
optical:
- uncorrected refractive errors
- diffraction
- aberrations
- light scattering (from occular media)
anatomical
- retinal
- neural
outline the retinal location and structure (1+2)
- retina lies against pigmented choroid (which provides nutrient supply and black out)
- outermost layer of retina is pigment epithelium (light absorption and scattering reduction)
- retina transparent with the exception of pigment epithelium
what is the ratio of ganglion cells to photoreceptor cells (1)
2.5
1 cone : 2.5 ganglion
what limits the acuity of foveal vision (1)
what type of light are they sensitive to?
- acuity limited only by the density of the cone mosiac
- cones in central fovea express pigments that are sensitive to green and red light
outline receptor theory for resolution (3+1+2)
- in fovea, cone cells have diameter of 1.5 microns
- seperated by an edge to edge space of 0.5 microns
- > minimum effective seperation between cone to cone is about 2 microns
- if the eye focuses on two adjacent point sources of light but falls on two consecutive cones, they will be percieved as only a single source
- only percieved as two seperate images if an unstimulated cone lies in between
- > min speration between images at retina is 4 microns
outline wave theory (3)
- image of a point formed by eye cannot be a point -> but is a spread over a finite area due to diffraction of light at the pupil boundaries
- at circular aperture (ie. pupil) this image formation takes form as a central bright disc surrounded by fainter rings
- central bright disc is called Airy disc (84% of light)
outline rayleigh criterion (2)
- rayleigh criterion is the criterion for the minimum resolvable detail
- > when the first diffraction minimum of Airy disc of one source point coincides with the maximum of the Airy disc of the other object point
outline grating resolution and acuity (3)
describe 2 methods (2+1)
- visual acuity of subject is sometimes measured using gratings as targets
- visual acuity is inversely related to minimum angle of resolution (MAR)
- grating cycle is the seperation of a pair of repeated points
methods:
- foucault grating (square wave grating)
- consists of series of black and white stripes
- luminance changes abruptly at edges - sinusoidal grating
- luminance changes sinusoidally across the grating
why does the wave theory (rayleigh criterion) not work perfectly in reality? (3)
- according to wave theory, diffraction at pupil margins gives minimum separable visual angle as an inverse function of pupil diameter
- > this relationship only true if optical system is aberration free
- in real model, the eye’s aberration will reduce its performance at larger pupil diameters
- best line width acuity is reached when pupil diameter is 3mm -> resolution becomes poorer beyond 3mm due to aberration
what are the requirements of a sharp retinal image (2)
when is the image blurred? (2)
- after refraction by the eye, the image forming pencils are to be:
1. astigmatism free (homocentric)
2. image vergence (L’) is equal to dioptric length of the eye (K’) - if there are no defects in optical image, blurring will result if:
1. the image does not lie on retina
2. in absence of astigmatism, the retinal image of a point object will be a circular patch of light called a blur circle
what does the focusing error in peripheral vision rely on (2)
- focusing error in the periphery depends on the eccentricity (off-axis angle)
- eccentric acuity limited by diffracton, refractive blur and the retinal limit
outline the retinal limits to peripheral vision (2)
- cone concentration decreases away from fovea -> visual acuity decreases in periphery
- in periphery, several photoreceptors are connected to a single ganglion cell
- > sensitivity is increased but acuity is reduced
explain how reduced acuity in the periphery can be an advantage (1)
retinal image in the periphery is highly aberrated and this could be distracting if detected with good acuity
(ie. better focus)
outline retinal light scattering (2)
how is it reduced (1)
- a portion of light incident on retina is scattered either out the eye or within the eye
- light scattered within the eye will result in reducing contrast of retinal image
- pigmented choroid absorbs light scattering
