20 Questions
What is the term used to describe the image spread of a point source on the retina?
Point spread function (PSF)
What are the potential optical limitations of the eye that affect spatial resolution, including diffraction?
Refraction and diffraction
Which factors can limit spatial resolution?
Retinal factors, optics of the eye, and neural pathway capacity
What is the term for the ability to resolve detail, as in a task of resolution acuity?
Spatial acuity
What is the term used to describe the image spread of a thin line object on the retina?
Line spread function
Where can Adler’s Physiology of the Eye be accessed?
Library
What primarily limits visual acuity at the fovea?
The density of cone photoreceptors
What is the approximate separation required for the resolution of two lines at the fovea?
0.5 arc mins
At what distance from the fovea do rods peak in density?
18° or 5mm
What is the range of cone density in the rod-free area (about 1 degree of angle)?
98000 to 324000 cones per mm$^2$
For optimal visual acuity at the fovea, the signal from a single cone must be processed through:
A single ganglion cell
What is the term used for the connection of more than one photoreceptor to a ganglion cell?
Spatial pooling of cone signals
At what pupil size is visual acuity at the fovea primarily limited by photoreceptor density?
2mm or more
What limits visual acuity in peripheral vision?
Spatial pooling of cone signals
What is the equation for the size of the Airy disc?
$r = 1.22 \times \frac{\lambda}{d}$
What pupil size is the eye diffraction limited at?
2mm
What criterion is followed to resolve two closely spaced lines?
Rayleigh criterion
What factor contributes to the spread of light in optical systems?
Aberrations
What graph represents the spread of light with respect to retinal distance?
Brightness versus retinal distance
What impacts the quality of the retinal image in the eye's media?
Light scatter and absorption
Study Notes
Optics of the Eye: Understanding Light Spread and Factors Influencing Image Quality
- The spread can be represented on a graph of brightness versus retinal distance, with factors like diffraction, aberrations, light scatter, absorption, and focus affecting the spread function.
- Diffraction, caused by light bending around an edge or through an aperture, results in a bright central area (Airy disc) and concentric rings in the diffraction image of a point source.
- The size of the Airy disc is given by the equation: r = 1.22 * λ/ d, where r = angular radius, λ = wavelength of light, and d = pupil diameter.
- Aberrations in optical systems contribute to the spread of light, especially for larger pupils, degrading optical image quality.
- Measuring the line spread function for different pupil sizes shows that as the pupil size decreases, ocular aberrations also decrease, and the eye becomes diffraction limited at a pupil size of about 2mm.
- Light scatter and absorption in the eye's media can impact the quality of the retinal image, with light scatter worsening in the aging eye and shorter wavelengths being absorbed more.
- The minimum visible acuity is achieved when the light increment (retinal illuminance) reaches a threshold, increasing in contrast as the size of the line spread function on the retina increases.
- Minimum resolvable acuity, or resolution, is achieved when the visual system can distinguish ΔI/I, and two closely spaced lines can be resolved if the separation between their respective line spread functions is sufficiently wide, following the Rayleigh criterion.
- Achieving the Rayleigh criterion equates to about 1 cone separation at the fovea.
- The text is sourced from "Adler’s Physiology of the Eye, 10th edition: Chapter 17; Visual Acuity, G Westheimer" and "Campbell and Gubisch 1966."
- The information is available on the website "foundationsofvision.stanford.edu" and "webvision.med.utah.edu."
- Dr. Sarah J Waugh is the author of the content and the source of the lecture material.
Test your understanding of the optics of the eye with this quiz. Explore the factors influencing image quality, including diffraction, aberrations, light scatter, and absorption. Dive into concepts like the Airy disc, line spread function, and visual acuity to enhance your knowledge of how light spreads in the eye and impacts the retinal image.
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