Visual Optics Flashcards
What are the two ways that light can be described, and how is each used to understand light relative to the eye?
- as a wave or as a photon particle
- wave theory describes the physical changes light undergoes as it moves through the eye
- particle (photon) theory explains the energy transfer that happens when light hits the out segments of the photoreceptors
What are the two principle factors of a light wave, and define each?
- amplitude (A): maximum value of the field generated by the propagating wave (determines the wave’s intensity)
- wavelength (lambda): distance between adjacent wave crests (determines wave’e location in the electromagnetic spectrum)
What is the range of wavelength in the human visual electromagnetic spectrum?
390 nm (deep blue) to 760 nm (deep red)
True of false: the visible portion of the electromagnetic spectrum occupies a small portion of the overall spectrum?
true
True of false: many nonmammalian species, and some mammals, possess ultraviolet vision, enabling them to detect light with a wavelength shorter than 380 nm?
true
True of false, certain fish living in backwater rivers have evolved infrared vision, giving them the ability to detect light with a wavelength greater than 760 nm?
true
What other species, other than backwater river fish, has a retina that responds to infrared light?
cat (significance of this is no known)
The amount of energy in a given photon is inversely proportional to what?
its wavelength
shorter the wavelength the more intense the light, why blue light possess more energy than red light
Give a clinical example of the particle nature of light.
fluorescein staining - fluorescein sodium molecules absorb photons of blue light and reemit photos with a lower energy content in the yellow-green spectrum (process known as fluorescence)
What are the two functions of the two-part photopigment molecule found in the photoreceptor outer segments?
Part 1 - opsin: determines the wavelength of light that the photopigment will absorb (this color vision)
Part 2 - visual chormophore: uses the energy of the photon to undergo isomerization, initiating conversion of light stimulus to electrical signal.
What is the name for the process by which light energy is converted to an electrical signal?
phototransduction
What is the difference between luminous intensity and luminance?
- luminous intensity describes the intensity of a light source (measured in candela)
- luminance describes a lights brightness reflected from a surface (measured in foot-Lamberts or candela/m2)
(these two are related by not necessarily proportional - think of a street light which is intense but not very bright, and a transilluminator which is bright but not very intense)
In general, which system (scotopic of photopic) is active at luminance >3 cd/m2?
photopic
In general, which system (scotopic of photopic) is active at luminance
scotopic
What is mesopic vision?
the range of luminance during which both scotopic and photopic systems are active
How is luminance measure?
using devices called photometers
What are the two different types of photometers, and how do they work?
- visual photometers: provide a subjective reading because the observer compares the illumination of the measured light with that of a standard light
- photoelectric photometers convert the measured light into an electric current displayed by the instrument
Why are photometry measurements important in ERG recordings?
- they are used to describe such variables as threshold, ambient light, and stimulus parameters
What are the two main factors that limit the visible electromagnetic spectrum to 390-760 nm?
- absorption spectrum of the opsin component of the visual photopigment
- transmission, reflection, and attenuation of the various wavelengths by the other ocular media
Transmission, reflection, and attenuation of the various wavelengths by the other ocular media depends on what?
wavelength of light and the angle of incidence
What happens to transmission of light through the rabbit cornea from 370-500 nm to 310 nm down to 290 nm?
The amount of transmission goes from 89% to 93% down to 50%, then down to 2% at 290 nm.
Beyond what wavelength does light not transmit through the aqueous humor?
1950 nm
Explain how the aqueous and lens essentially act as filters for light passing through the cornea.
a large range in wavelengths of light pass through the cornea, and the lens and aqueous filter out the high and low ends of the range to essentially filter out UV and IR light, which protects the retina.
Does transmittance decrease through the cornea with age?
no, at least not in the cornea of the human and golden hamster
What happens to light that strikes a surface at an oblique angle?
it is reflected back, not transmitted into the new medium
What determines the critical angle for reflection?
the difference in the indices of refraction between the two mediums
Where does most of the reflection of light coming into the eye take place and why?
most takes place at the cornea because of the large difference in refraction indices between the cornea and air
What prevents the ophthalmologist form examining the iridocorneal directly in canine patients?
internal reflection of outgoing light back into the eye
How does the goniolens allow visualization of the iridiocorneal angel?
they decrease the difference in refractive indices between the cornea and air, this increasing the critical angle and permitting rays emanating from the ICA to pass through the cornea
Absorption of UV radiation has been implicated in what diseases in dogs, cattle and humans
dogs - superficial keratitis
cattle - squamous cell carcinoma
humans - cataract and macular degeneration
What happens to light entering the eye that is not transmitted or reflected?
scattered or absorbed by pigment (can have clinical applications such as diode lasers)
What three changes occur to light as it strikes denser media?
1 - velocity is reduced
2 - wavelength shortens
3 - it is bent (unless striking
What law is used to describe the amount of refraction that occurs as light passes from one medium to the other, and what determines it?
- Snell’s law
- determined by the angle of incidence and the refractive indices of both media
Define lens.
an object that bends (refracts) light
What happens when a pencil of light rays strikes a concave lens?
divergence (negative vergence) of light
The vergence power of a lens is measured in what units?
diopters
What is the equation for measuring diopters?
D (diopters) = 1/f (focal length of the lens)
What is the focal length of a lens?
Distance between the center of the lens and the point at which parallel rays of light are bent into focus by the lens
The focal length of a lens is directly proportional to what?
The curvature radius of the lens
What type of image does a convex lens create?
a real image
What type of image does a concave lens create?
a virtual or aerial image (rays are traced back behind the lens)
True of false, the vergence powers of lenses in an optical system are additive?
true
What happens to the refractive power of a lens placed in a medium other than air?
it decreases
What happens to light as it passes from the tears, through the anterior portion of the cornea, through the posterior portion of the cornea, and into the aqueous humor?
Refractive power of the tears is approximately 43 diopters, and the anterior portion of the cornea gives an additional 5 diopters, however those 5 copters are lost as light passes through the posterior cornea into the aqueous humor, which has a nearly identical refractive index to the tear film (1.336)
What is the curvature of the cornea in large eyes compared to smaller eyes?
large eyes have a flatter, less curved cornea compared to a more spherical and curved cornea
What are the two factors that affect the refractive power of the cornea?
curvature and refractive index
Ture or false, the central and peripheral cornea have the same curvatures and refractive powers?
false
After the cornea, what is the next significant refractive structure through which light passes?
pupil
When the pupil dilates, what happens to the range of distances at which objects remain in focus?
it decrease significantly
What happens to the range of focus in an eye when the pupil diameter increases from 1mm to 4mm?
range of focus goes from 0.56 - 5 meters, down to 0.78 to 1.4 meters
(this phenomenon is amplified in species such as the dog which have little accommodative capability)
As a rule of thumb, constricting the pupil by 1/2 the increases visual resolution by what factor?
2
What happens to the significance of chromatic and pupillary aberrations as the pupil dilates?
they increase
In essence, what is the pupillary light reflex balancing, and in what group of animals is this particularly important?
- maximal retinal illumination and visual resolution (increasing the one comes at the expense of the other)
- deep diving animals
In humans and many non aquatic species, the refractive index of the lens is approximately what?
1.41
Is the refractive power of the lens in aquatic species higher or lower than that of the lens in non aquatic animals?
It is higher (as high as 1.66) which significantly increases the refractive power of the lens.
What is the difference in the shape of the lens between aquatic and nonaquatic species?
The lens is more spherical in fish and aquatic mammals, while it is more discoid in terrestrial species
What is the reason for the increased refractive power of the lens in aquatic species
need to make up for the decreased refractive index in a cornea that is adjacent to water on both sides
Why is there little refraction of light as it passes from the lens to the vitreous?
they have very similar refractive indices
How does the vitreous play an important role in refractive development of the eye?
vitreous elongation during development increases the axial length of the eye, increasing the refractive path of light (this can lead to myopia/nearsightedness)
*this is another method used in some fish to compensate for decreased corneal refractive power)
True of false: the refractive index remains constant through a structure (such as the lens, cornea, retina, etc)?
false ( these difference typically don’t alter the refractive power of the eye to a noticeable degree except in some species such as the goldfish eye, which has a refractive index of 1.35 in the lens cortex and 1.57 in the nucleus)
What is the stimulus for the accommodative response?
a blurred or defocused retinal image
What are the 4 ways the vertebrate eyes achieve accommodation?
- change corneal curvature
- change distance between cornea and retina
- change curvature and position of the lens
- having two or more separate optical pathways of different refractive powers
What method is used to most commonly measure accommodation?
infrared photoretinoscopy (uses infrared light from the fundus to measure dynamic changes in the refractive error)
Briefly describe the actions that lead to disaccommodation.
sympathetic innervation indues relaxation of the ciliary muscles, which in turn stretches the zonules, leading to zonular tension and pull on the lens capsule, which causes the lens to become more discoid, decrease in axial thickness and decreases the lens’s refractive power
Briefly describe the actions that lead to accommodation for focusing on near objects.
parasympathetic innervation indues contraction of the ciliary muscles, which in turn relaxes the zonules, leading to release of tension on the lens capsule, which liberates the inherent elasticity of the lens and causes the lens to become more spherical, increase in axial thickness and increases the lens’s refractive power
What is the proposed mechanism for increased thickness of the primate lens?
realignment of lens fiber-end segments into overlapped or interfaced configurations at the lens suture
(in disaccommodation, compression of the spring-like fibers causes the overlap of fibers and segments at the suture to become undone)
Primates younger than 5 years can accommodate up to how many diopters?
34D
primates older than 25 years can only accommodate an average of 5D
What are the proposed mechanisms of presbyopia?
- reduction in ciliary muscle contractility
- changes in the refractive index of the lens
- age-related changes in the relative position of the lens and ciliary body
- loss of lens capsule and lens fiber elasticity
What is the elasticity of the feline lens capsule compared to humans?
it’s only 5% as elastic as humans
How do cats accommodate?
via translation (anteroposterior movement of the lens)
Describe the age related change in accommodation in the chicken.
from 20D at hatching to less than 5D at 1 year of age
What allows for translation in the feline lens?
relative abundance of meridional (longitudinal) fibers in the ciliary body compared to the scarcity of circular fibers
Parasympathetic stimulation of the feline meridional ciliary muscle fibers leads to what?
forward displacement of the lens up to 0.66mm, which induces between 2D to 8D of accommodation
What is the reported accommodative power of the dog?
1-3D
What is the depth of field for nearby objects in the dog?
100-33 cm
What is the depth of filed for nearby objects in the cat?
50-12.5 cm
Forward lens movement in the raccoon induces accommodation to what degree?
up to 19D
Lens displacement in mammals results from what?
ciliary muscle contracture
Lens displacement in teleost fish results from what?
retractor lentis - specialized smooth muscle that pulls the lens backward to focus on distant objects
True of false: rodents posses accommodative ability?
false (their small pupil size and short axial globe length provide them with a significant depth of focus)
True of false: ungulates do not possess accommodative abilities?
true (while they do have ciliary muscle, they lack significant fiber-end taper which precludes any overlap of end segments to effect accommodation)
What is the accommodative power of the horse lens?
+ or - 1D in either direction
What are the three ciliary muscles of the chicken?
- anterior Crampton’s muscle
- intermediary Muller’s muscle
- posterior Brucke’s muscle
What are the combined mechanisms of lenticular accommodation in the chicken?
- mediated by the intermediary and posterior ciliary muscles
- parasympathetically innervated by postganglionic ciliary nerves
- large ciliary processes and a ring of columnar epithelial cells at the equitorial periphery of the lens (annular pad) which increase the diameter of the lens and it’s contact with the processes
- lenses are soft and maleable
- corneoscleral sulcus is larger (due to scleral ossicles) allowing for a greater range of movement
- these modifications allow for direct squeezing of the lens, which thickens the lens up to 0.2mm, steepening the curvature, and bulging the lens into the anterior and vitreous chambers
- leads to 15-19D of lenticular accommodation
In the chicken, corneal curvature is mediated by what muscles?
anterior ciliary muscle (Crampton’s muscle)
Where does Crampton’s muscle originate and insert?
originates in the sclera and inserts on the cornea
What does contraction of Crampton’s muscle do to the curvature of the cornea?
flattens the peripheral cornea and increases the curvature of the central cornea
How much accommodation does corneal accommodation account for in chickens?
8-9D
What is the combined lenticular and corneal accommodative power of the chick lens, and how does this correspond to human children?
maximum of 25D in young chicks compared to 15D in young children
True of false, avian and reptilian species have the ability to accommodate each eye independently?
true (up to 6 diopters difference)
Define anisometropia
unequal refractive power between the two eyes
What other group of animals possess similar lenticular and corneal accommodative mechanisms to chickens?
lizards
Define emmetropia
condition in which parallel light rays from a distant object are focused on the outer segments of the retinal photoreceptors when the eye is diaccommodated
Define ametropia
condition in which parallel light rays from a distant object are focused either behind or in front of the outer segments of the photoreceptors when the eye is diaccommodated
Define myopia
condition in which parallel light rays from a distant object are focused anterior to the outer segments of the photoreceptors when the eye is diaccommodated (nearsighted)
Define hyperopia
condition in which parallel light rays from a distant object are focused posterior to the outer segments of the photoreceptors when the eye is diaccommodated (farsighted)
What are the two assumptions made when applying the technique of retinoscopy?
1 - light emerging from the eye (emergent rays) follows the same optical path as light entering the eye
2 - the fundus reflex originates at the level of the photoreceptor outer segments
What ist he artifact of retinoscopy?
gap during retinoscopy between where we actually measure reflected light and where we should measure it (basically the thickness of the retina)
* we should be measuring int at the photoreceptors, but reflection likely originates from the internal limiting membrane*
The artifact of retinoscopy can account for how much of a refractive error in small eyes?
10D to 11D
Describe the path of emergent rays from a emmetropic vs myopic vs hypermetropic eye
- emmetropic emergent rays are parallel
- myopic emergent rays are convergent
- hypermetropic emergent rays are divergent
Most mammals are within how many diopters of emmetropia?
1
What are the 9 dog breeds that were found to be emmetropoic?
- english springer
- german shepherd
- golden retriever
- siberian husky
- shetland sheepdog
- labrador retriever
- border collie
- samoyed
- other terriers
What percentage of dogs are hypermetropic, and what is the average refractive error?
8% at +3.25D
What dog breeds were found to be hypermetropic?
- australian shepherd
- alaskan malamute
- bouvier des flanders
What percentage of dogs are myopic, and what was the refractive error?
25% at -6.25D
What dog breeds were found to be myopic?
- rottweiler
- Collie
- Miniature Schnauzer
- Toy Poodle
What causes myopia in the labrador retriever?
elongation of the vitreous chamber
What causes myopia in the Toy Poodle?
steeper, more powerful lens
How does visual deprivation lead to myopia?
causes axial elongation of the vitreous body and eye.
True of false: kittens less than 4 months of age are myopic with a mean refractive error of -2.45D
true
Ture of false: coat length is a significant factor in myopia of cats?
true
(DSH more likely to be myopic)
(DLH more likely to be emmetropic)
Other than axial lengthening of the globe, what are some other anatomic changes to the eye induced by visual deprivation?
- shallow anterior chamber
- thinner lens
- changes in corneal curvature
What is the recommended range of power for canine intraocular lens implants?
40-41.5D (difference is probably related to breed differences)
Why is a 41 diopter IOL needed in the canine eye after cataract surgery when it has been determined that the aphakic canine eye is 14-15.2D deficient?
because the IOL is placed in the capsular bag which is surround by aqueous humor which results in a reduction of its over refractive power due to the small difference in refractive indices between the IOL and aqueous humor.
What is the recommended IOL power in a cat, and why does this differ from dogs when the two species have essentially the same axial globe length?
- 52-53 diopters
- cats have deeper anterior chamber, which means they have a shorter distance from lens to retina, therefore need a stronger lens
(human have a shallow anterior chamber compared to dogs, thus typically have an IOL with power of 19-22 diopters)
Define astigmatism
unequal refraction of light along the different meridians of the eye
How can you measure for astigmatism?
refract the eye in both the vertical and horizontal meridians (a difference of 0.5D or more in the same eye is defined as astigmatism)
(can also use a keratometer)
What is direct, or with the rule astigmatism?
when light entering the vertical meridian is refracted more than light entering the horizontal meridian
What is inverse or against the rule astigmatism?
when light entering the vertical meridian is refracted less than light entering the horizontal meridian
How is astigmatism corrected?
with a cylindrical lens
What percentage of dogs were found to have astigmatism, and what breed has a higher prevalence than the average?
- 1%
- german shepherds at 3.3%
True or false: the horse is naturally astigmatic, with a horizontal mean corneal curvature of 21.6 diopters and a vertical mean corneal curvature of 22.9 diopters?
true
horse has mean degree of 1.3D direct astigmatism
What are some pathologic changers that can lead to astigmatism?
- corneal disease resulting in edema or scarring
- focal cataracts
- procedure that involve incising the cornea (typical resolve)
Ture of false: most cases of astigmatism in the animal kingdom are pathologic.
false (most are physiologic)
Describe lower-field myopia and what species possess this trait.
- in these species eyes are emmetropic along the horizontal and upper visual field, but myopic along the lower visual field
- several reptiles and amphibians
*this adaptation is considered a passive accommodation mechanism)
How is lower-field myopia an evolutionary advantage?
allows the animal to keep the ground in focus with relaxed accommodation while foraging for food and, at the same time, monitor the sky for predators
The head movement that was attributed to the ramp retina theory in horses is now attributed to what action?
raising and lowering of the head is though to occur when a horse is interchanging monocular and binocular visual fields
What are spherical aberrations, and what structure of the eye do they apply to?
- light rays passing through the periphery are bent more than rays passing through the center
- cornea and lens
How is the spherical aberration measured?
the difference between the focal points of the peripheral and central rays (spherical aberration) is measured in diopters
What are the anatomic adaptations in the cornea and lens that correct for spherical aberration?
- lens: higher refractive index of the nucleus
- cornea: periphery is flatter than the center (axial) cornea
What other internal eye structure plays a large role in controlling spherical aberrations?
- the pupil (constriction block transmission of peripheral light rays that have passed through the cornea from passing through the lens)
- miosis allows only rays that pass through the central cornea and lens to reach the retina
What are chromatic aberrations?
basically, when white light enters a lens, each wavelength contained within the light has a different refractive index, therefore is refracted to a different degree. So blue light and red light will be refracted differently, and their light rays will be focused on the retina differently
What are some anatomic adaptations to help correct for chromatic aberrations?
- multifocal lenses consisting of concentric rings of differing refractive properties (terrestrial species with these lenses that have a slit pupil take advantage of the multiple refractive zones even in a constricted pupil)
- human fovea has red and green cones, so does not perceive blue light
