Vision Flashcards
What is the range of wavelengths of visible light
What does this correspond to
~390 (violet) to 700 (far red) nm
The spectrum of sunlight
What are the 4 basic variables analysed by the visual system
Intensity (1) and Wavelength(2) and their variation in space (3) and time (4)
What is light from a light source quantified as
As illuminance (lux)
How is light reflected from objects quantified
Luminance (cd/m^2)
From visual threshold to saturation, by how much can light intensity in the environment vary
What about for reflectance of natural objects
By a factor of ~10^10
Reflectance it natural objects varies nobly ~20 fold and is independent of illuminance
What is the most commonly used measure of light stimulus strength
Relative intensity or contrast
ΔI / I
Where I= mean background
And ΔI= increment in intensity
Briefly give an overview of how an image is formed in the eye
An inverted image is focussed by the cornea and lens on to the retina, with an aperture controlled by the pupil
How are size and distance in the outside world expressed in the retina
In angular terms
As size and distance are expressed in angular terms on the retina, what is 1 degree equivalent to
1 degree = 60 arc minutes
This is about the width of a thumbnail at arm’s length or 300μm on the retina
What is 1 radian in degrees
~57 degrees
What is diffraction
The spreading of waves around objects
Such as light spreading out as it passes through an aperture
What is the point spread function
Express this as an equation
Because of diffraction, even with perfect lenses, the image of a point source is a blurred circle
The angular diameter of point spread function is:
d~ λ/D
Where D= diameter of lens (or aperture if limited by diaphragm or pupil);
λ= wavelength
d~λ/D
What does this mean for the diffraction limit in relation to the lens size
The smaller the lens/ aperture, the larger the diffraction limit
True or false
Lens aberrations degrade the image
True
Other than diffraction, the image can, in practise, be further degraded by optical imprecisions including spherical and chromatic aberration and glare
What is spherical aberration
For a spherical surface, rays towards the edge are more strongly refracted
What is chromatic aberration
Different colours are focussed at different depths
The human eye is well focussed for green but poorly for blue light
Describe how glare can affect image formation
Small particles in the optical media scatter light in all directions, reducing contrast of the image
When does the point spread function approach the diffraction limit
At small pupil diameters when spherical and chromatic aberration is modest
What happens to the point spread function when the pupil dilates
Off axis rays contribute to image formation and aberrations become more significant, broadening the point spread function despite the reduced effects of diffraction
Do eyes have refractive defects
Some eyes may have refractive defects such as ametropic
What does emmetropic mean
Can focus sharply on an object at infinity
If an eye cannot do this it is ametropic
What is myopia and what is its incidence
Short sight
20% of population
What is hypermetropia
How common is it
Long sight
30% of population
What does myopia predispose
Retinal detachment, degeneration and glaucoma