Limits of Vision: Image Formation Flashcards
How is light perceived from the eyes?
The eye is a “simple” optical instrument, formed by just two positive lenses, the cornea and the crystalline lens. Together, they combine to project images of a scene onto the photoreceptor layer of the retina. Before the light reaches the lens, it travels through the pupil, which controls retinal illumination and limits the rays of light entering the eye. The pupil varies in diameter from less than 2 mm in bright light (‘photopic’) to more than 8 mm in the dark (‘scotopic’). After the light is refracted by the lens, it enters the posterior chamber, which is filled with the (hopefully) transparent vitreous humor, and (hopefully) converges on the retina. At the retina, light is transduced into electrical signals by photoreceptors, then passed on to ganglion cells for encoding and transmission. The signals are then further processed throughout the visual cortex and beyond.
Why do we get an airy pattern?
Due to diffraction - all waves diffract when passing through aperture, the positive interference and negative interference leads to that concentric circle appearance. Such that even a point light source such as a laser produces an airy pattern.
How does diffraction limit angular resolution?
What is modulation transfer function (MTF)?
Thus what does it indicate?
The Modulation Transfer Function (MTF) expresses how much contrast in the original input signal is successfully transferred through the optics of the eye, as a function of spatial frequency.
(i.e. how well the eye preserves detail/contrast)
Thus it indicates the overall quality of the image produced. (I.e. it is the sum of diffraction + abberrations + scatter).
What is the Rayleigh Criterion?
The Rayleigh criterion is where the distance between airy disks is equal to the radius of the airy disk. So essentially you can still resolve the two points however the disks are touching.
Describe the relationship between pupil size and diffraction upon minimum angle of resolution?
As pupil size increases, we expereince less diffraction and as a result a smaller minimum angle of resolution (i.e. acuity increases (based ONLY on diffraction).
Describe the relationship between MTF and pupil size, and how this affects minimum angle of resolution.
As pupil size increases, modulation transfer function (represented as a fucntion of spatial frequency) decreases and as a result the minimum angle of resolution increases (i.e. acuity worsens).
[This is because diffraction isn’t the only thing that affects image quality - abberrations and scatter also affect image quality]
What is the wavefront aberration and why does it occur in the eye?
The cornea is not a perfectly curved surface and as a result we get a distortion of the wavefront. The difference between the ideal wavefront and wavefront produced by a (naturally) irregular cornea is the wavefront aberration.
What are lower order abberrations of the eye and what are they known as?
Lower order aberrations [LOAs] are also known as refractive errors. They include: myopia, hyperopia and astigmatism.
What are higher order abberrations of the eye?
Higher order aberrations [HOAs] are more complex irregularities, with unfamiliar names such as coma, spherical aberration and trefoil. These types of aberrations can produce vision errors such as difficulty seeing at night, glare, halos, blurring, starburst patterns or double vision (diplopia)
What is aberrometry?
The measurement of W ( wavefront aberration)
What is the classic method of measuring wavefront abberration?
Hartmann Shack method
What does this symbol Ø represent?
Pupil size
What is the effect of a Coma (commatic) aberration and why does it occur?
Off axis points of light become commet shaped.
This is because different parts of the lens vary in magnifciation (i.e. magnifying power of the centre of the lens will be different to the periphery) and as a result a series of asymetrical circle of different sizes are created.
[Across an image this can look like spiral shaped warping]
[Coma abberrations can be postiven or negative]
What is the effect of spherical distortion abberration why does it occur?
- Light rays striking the lens off-centre are refracted more/less than those striking close to the centre.
- Causes blurriness at the edges of an image.
- “Positive” spherical aberration means peripheral rays are bent too much. “Negative” spherical aberration means peripheral rays are not bent enough.
What are Zernike polynomials?
A fancy name for the graphical representation of all types of abberrations in the eye.
What is the ideal pupil size for best image formation/ acuity?
What is intraocular light scatter and what is it’s affect?
Local irregularities of the refractive index within the ocular media, leading to straylight (inbound light being spread out at large angles over the retina) - A.K.A anything in the eye that causes scatter.
The effect of scatter is reduced contrast and the appearance of a ‘veiling glare’.
What are the causes of intraocular light scatter in healthy eyes?
What are pathological causes of increased intraocular scatter?
When is the effect increased intraocular light scatter particularly noticeable for a px?
Night driving
Strongest impact in mesopic and scotopic conditions, and in the presence of a strong light source away from fixation
What does neural sampling refer to and how can this limit image formation?
Neural sampling refers to ganglion receptive field size which increases in the periphery.
Most ganglion cells pool across multiple photoreceptors, with more peripheral ganglia having increasingly large receptive fields- this decreases image quality in the periphery.
