vision science

the larger the n,

where is the angle ray?

size of angle?

closer to the Normal

small

refraction definition

bending of light (snells law)

diffraction definition

spreading at edges , limits resolution ( airy disc )

if you increase the wavelength, what happens to the refractive index, n?

n gets smaller, angle gets larger

less refraction with longer wavelengths, closer to surface than normal

small gap (aperture) = __more/less___ diffraction

longwave length = __more/less___ diffraction

small gap is MORE diffraction

long wavelength = more diffraction , because long wavelengths travel further than short

double slit experiment is showing ______ and ______

forms a circular aperture named?

diameter of airy disk?

interference and diffraction

bright/dark, bright/dark pattern

airy disc

2.44 radians

pupil size and diffraction relationship

the smaller the pupil size, the more diffraction

= images bigger than actual size

rayleigh criterion?

distance between 2 points before they look like 1

2 points will be resolvable if the first minimum of the diffraction pattern of one source overlaps the central maximum of the diffraction pattern of the second source.

i/wavelength^4

diffraction limit = 1.22 radians (pupil size matters)

mie vs rayleigh scattering

rayleigh scattering = small particles

is highly dependent on the wavelength = blue sky. shorter wavelength/ more scattering

mie = large particles

usually created white glare bc it is not wavelength dependant

solar radiation and perception of white

sun=blackbody

brain has perception of white, doest need a perfect even spectum , our cones even out/balance

same with articicial light (LED or fluorescents )

luminance , perceived brightness

absolute threshold, minimal light eye can detect = 10^-6

photons/second

seeing very bright very quickly and dark adaptation is slow

measured in log bc range is huge of luminance

photon absorption

1. random

2. principle of univariance: rods are colorblind. the photoreceptor responds same , no matter the wavelength

3. photopigment response:

when light hits photoreceptor, photopigment absorbs it. The energy of photon changes photopigment shape (isomerizes) = electrical signals

"photon hit me"

4. wavelength information is lost after photon is absorbed by photoreceptor

luminious efficiency function

refers to relative sensitivity of eye to light across wavelengths

linear scale

Cone= Vλ

rod: V'λ

efficiency: referring to how likely a photon will be absorbed.

takes perception into account

"V = vision in daylight (555 nm)."

"V' = vision at night (507 nm)."

proportion of light that remains after each structure it passes through

absorbs short wavelengths

Cornea: absorbs some blue & UV light → less short-wavelength light passes deeper.

Lens: absorbs strongly below 400 nm (UV), and increasingly absorbs blue with age.

Macula (yellow pigment): filters out some blue light before it hits cones.

Vitreous: absorbs UV and short wavelengths further.

-light that reaches cones has almost no UV in it and less blue

older lenses absorb more blue light , starts to look yellow = cataract

brewster angle

angle of incidence at which P-polarized light is totally transmitted

E= h * v

E = h * c/λ

h= 6.626x10^-34 J/s

corneal crosslinking , therapy with laser

strengthens cornea

thermal damage : proteins denatured in eye by tissue heating

photochemical damage: UV damages proteins and DNA molecules and pigments in RPE , damage accumulates

thermoacoustic damage: intense pulse of light produces vibrations

photoablation

UV Laser to remove material

photochemical interaction: UV damage to DNA /pigments

thermal interaction

increase in temperature in proteins

heat generation by absorption of light / photocoagulation/photothermal shrinkage of stromal collagen

plasma-induced

plasma ionization

corneal refractive surgery using lasers

photodisruption

shock-wave generation

tissue is split by mechnical forces , shock-wave and cavitation effects propagating into tissues

cones and rods sensitivity

cones: "photopic"

need bright light, color vision

violet is 100x less sensitive than peak

blue is 10x less sensitive than peak , but 10x more sensitive than violet

rods: at night "scotopic"

more sensitive, no color

by 100x than cone, depending on wavelength

mesopic is a mix of both

light as a wave explains what 4 things?

refraction, diffraction, interference, and polarization

light as a photon explains what?

how photoreceptors detect light

laser classes and safety

Class 1:

Always safe, even with long exposure.

Example: barcode scanners.

Class 1M:

Safe unless optics (like binoculars) are used to focus the beam.

Class 2:

Visible light (400-700 nm).

Power < 1 mW.

Safe due to blink/aversion reflex (

maximum permissible exposures

MPEs refer to the highest intensity of light exposure that is considered safe

1/10th the intensity of light that would produce damage in 50% the cases

- depends on wavelength, power (watts), exposure time (s) area of retina exposed (mm^2) , pupil size (mm^2) , and eye motion (deg/sec)

-how much energy in joules was delivered,

-does the WL used produce thermal or chemical damage first?

-if thermal , exposure long enough some heat dissipated?

effects of time, wavelength, and spot size on damage

lower MPE = bad

Longer time = more dangerous → MPE drops.

Visible light (400-700 nm) = worst for retina.

Smaller spot = more dangerous, bigger spot = safer.

How bright the light looks is NOT the same to how dangerous it is

X-axis = Wavelength (color of light).

400 nm = blue, 550 nm = green-yellow, 800 nm = near infrared.

Y-axis = Luminance threshold for damage (in Trolands).

Trolands = photometry unit (retinal illumination).

Log scale (each step = 10×).

Curves (30s vs 300s exposure):

Longer exposure = higher risk.

In the blue, damage builds up over time (photochemical).

In the red, damage is about the same regardless of exposure time.

Key Point:

Light at 800 nm (infrared) looks very dim to your eyes, but can still be just as damaging as visible green light at 550 nm.

In fact, it can look 10,000× dimmer while still causing the same damage.

550 nm (green) looks bright AND can damage.

800 nm (infrared) looks dim BUT can damage just as much.

Blue light causes slow, cumulative damage (photochemical).

longer the flash, the lower the power must be to avoid damage

longer exposure = lighely photochemical damage, can accumulate over time

brief exposure = thermal damage

Maximum Permissible Exposure

X-axis (bottom): Log exposure time (seconds). Longer exposure → more risk.

Y-axis (left): Log weighted retinal irradiance (W/cm²). Higher light intensity → more risk.

Black line = MPE (Maximum Permissible Exposure):

The safety limit for retinal light exposure.

Defined as 1/10th of the intensity that would cause damage in 50% of cases.

Anything below the line = safe, above = potential damage.

Yellow zones: Fundus cameras + focusing modes (clinical devices).

These operate below the MPE line, meaning they're safe for routine use.

Red boxes: Documented cases of light-induced retinal damage.

These are above the MPE line

MPE line = safety threshold (10× below damage level).

Fundus cameras & focusing systems fall within safe limits.

Light damage has been documented when exposure is above the MPE.

Both irradiance (brightness) and exposure time matter → even moderate light can be damaging if exposure is long enough

radiometry vs photometry

radiometry:

measurement and specification of power of a source of EM radiation

(Physical measures of light)

photometry : measurement and specification of a light source in terms of ability to produce a visual sensation

physical measures of light +eyes sensitivity, V(y)

conversions from radiometry to photometry

ΦV​=683×ΦE​(λ)×V(λ)

Φᵥ = luminous flux (lumens) → brightness to human eye.

Φₑ(λ) = radiant power (watts) at that wavelength.

683 lm/W = scaling factor at 555 nm (where eye is most sensitive).

V(λ) = luminous efficiency function → eye's sensitivity at that wavelength.

Same wattage ≠ same brightness.

Green light looks much brighter than red at the same physical power, because the eye is more sensitive near 555 nm.

That's why green laser pointers look far brighter than red ones, even if they have the same power (mW), brightness perceived = photometry = lumens

radiant and luminous flux

total emitted light from a point source is described as radiant or luminous flux

"total energy"

radiometry and photometry units

radiant flux:

radiant intensity:

irradiance

radiance

radiant flux:

radiometry: watt (power)

photometry: lumen (luminous flux)

radiant intensity:

radiometry: watt/w (J)

photometry: 1 lu/w (luminous intensity)

irradiance

radiometry: watt/m^2 (H)

photometry: illuminance (1 lux or 1lu/m^2)

radiance

radiometry: watt/w/m^2 (N)

photometry: 1cd/m^2 (luminance)

radiant and luminous intensity

totsl emitted light from a point source in a PARTICULAR DIRECTION is described as radiant or luminous INTENSITY

flux/unit solid angle, or steradian

solid angle

r^2 = 1 steradian

irradiance and illuminance

how much light falls onto surface unit area

watts/m^2

lumens/m^2 or Lux

retinal illuminance: describes amt of light falling on a patch of retina

radiance and luminance

light from direction from extended source (brightness from computer)

light is focued byy the _____ and _____ onto the ____

___ has a fixed focus____ can change shape

light is focued byy the cornea and lens onto the retina

cornea has fixed focus, changes vergance of light, accounts for 70% of focus

lens can change shape

middle layer of eye parts and function of parts

ciliary body(containing ciliary muscle): attaches to lens through small ligaments called zonules, controls fluid/pressure of aqueous humour

choroid: mainly blood vessels

iris: 2 muscles that constrict and filate the pupil . (sphincter = constriction) (dilator pupillae =dilation)

images are detected by photoreceptors in the ____

where do nerve fibers go across the back of eye ? this forms the...

retina (image forming system)

optic disc, nerves go through down to form optic nerve

fovea

region of retina that has high acuity

images fall on fovea

visual axis: from cornea to fovea . 5 degrees above the optic axis, crosses the optic axis through back of lens at the NODAL point

has high amt of cone photoreceptors

optic nerve:

carries all info out of the eye

axons of the ganglia cells (leaving the eye)

optic disc: part of fundus where the bundle of ganglion cells exits the eye

no photoreceptors = blindspot

fluorescein angiography

method to look at vasculature of patients retinal blood pressure , maybe diabetes

(no blood vessels where fovea is)

subdivisions of the macula

medial to lateral

fovea centralis,

foveola

perifovea

parafovea

foveola (inner)

fovea centralis

paraforvea

perifovea

capillary free zone

center of fovea

no cell bodies in center

straight to photoreceptors

retina

200-400 microns thick

5 cells: photoreceptors, bipolar cells, ganglion cells, horizontal and amacrine cells

photoreceptors: receive photons, converge light energy to chemical signals for rest of nervous system TO bipolar cells (electric synpase) TO

ganglion cells that project to rest of brain (midget and parasol)

-takes 35 miliseconds

rods> cones. ratios depend where in retina. No rod in fovea, no cones in peripheral

pigment epithelium

first layer closest to choroid (interior)

nutrient support,absord light

Cones:

function:

retinal distribution:

number

acuity

color vision

photopigments

temporal properties

Phototropic vision

Highest

6 million

lower sensitivity

high acuity

colorvision

S-,M-, L- opsins

fast temporal

rods

function:

retinal distribution:

number

acuity

color vision

photopigments

temporal properties

scotopic vision

high density in the perphery

100million

higher sensitivity

low acuity

rhodopsin

slow temporal property

retinitis pigmentosa:

begins with rods and goes to cones

cant make out objects at night

rods- peripheral . loss of peripheral vision = tunnel vision

-can only see color in center

cant see colors , low acuity

inner vs outer segments of rods and cones

outer segment: membranes that house pigment molecules (opsins)

inner: mitochondria, ribosomes, membranes to assemble opsins

synaptic convergence in rod and cone pathways

high level of synaptic convergence in rod pathway

(low in cone)

convergence increases sensitivity at expense of acuity

phototransduction in rods

light - electrical signals

visual pigment

rhodopsin (opsin + chromophore: 11-cis retinal Vitamin A) embedded in outer segment membranes

1.

photons --> activated rhodopsin --> metarhodopsin II --> transducing --> phosphodiesterase --> closing of ion channels --> membrane hyperpolarization

(strong signal amplification)

2. 11-cis retinal transforms to all-trans retinal and cannot absorb photons (bleaching)

bleaching

Rhodopsin is perfected bleached

1. bleached pigments need to be regenerated for renewed light signaling (20-30 minutes)

2. complex and slow biochemical process involving pigment epithelium

first drop: cones take over

plateau: rods still adjusting

second drop: rod gain function

vitamin a deficiency: no rhodopsin made . all lines flat near top

perceptual consequences of photopigment regeneration + retinal distribution and wavelength sensitivity of rods and cones

Diameter of light and threshold luminance

smaller diameter= more focused into cones/fovea = less sensitive

but the bigger diameter , reaches into peripheral / rods = more sensitive

wavelegnth of test stimulus

short wavelength / 400 more sensitivity of rods

long wavelength / 700

no sensitivity of rods

rods = 500

cones = 550

retinal circuits

How is the refractive index (n) of a transparent medium defined?

n equals the speed of light in a vacuum divided by the speed of light in that medium (n = c/v), so n is always 1 or greater.

When light passes into a medium with a higher refractive index, which way does the refracted ray bend and what happens to the angle of refraction?

It bends toward the normal, and the angle of refraction is smaller than the angle of incidence.

Which end of the visible spectrum does the eye refract most strongly, and what aberration does this produce?

Short wavelength (blue) light is refracted most, so blue focuses in front of red. The result is longitudinal chromatic aberration of roughly 2 diopters across the visible spectrum.

What is the Airy disc?

The central bright spot of the diffraction pattern formed by a circular aperture, surrounded by concentric dark and bright rings.

What two wave phenomena does the double slit experiment demonstrate, and what pattern appears on the screen?

Diffraction at each slit and interference between the two emerging wavefronts, producing alternating bright and dark fringes (maxima and minima).

What pupil diameter gives the best retinal image quality in a normal eye, and why?

About 2 to 3 mm. Below that, diffraction increasingly dominates, and above it, higher order aberrations dominate.

Why does a pinhole improve acuity in uncorrected refractive error, yet worsen vision if the aperture is made too small?

The pinhole narrows the blur circle produced by defocus, but below about 1 mm diffraction spreads the image faster than the defocus blur is reduced.

State the Rayleigh criterion for resolving two point sources.

Two point sources are just resolved when the center of the Airy disc of one falls on the first dark ring (first minimum) of the other.

Which particle size produces Rayleigh scattering, and how does it depend on wavelength?

Particles much smaller than the wavelength of light. Scattering intensity is proportional to 1 divided by wavelength to the fourth power, so short (blue) wavelengths scatter far more.

Which particle size produces Mie scattering, and what does the scattered light look like?

Particles about the same size as or larger than the wavelength of light. Scattering is only weakly wavelength dependent, so the scattered light appears white or gray and is thrown mostly forward.

What determines whether a light source is perceived as white?

Balanced excitation of the three cone types, not a flat or even spectrum. The visual system normalizes across the spectrum, so spiky sources such as LEDs and fluorescent tubes still look white.

The spectral emission of the sun is approximated by what kind of radiator?

A blackbody radiator at roughly 5800 K.

Why is luminance expressed on a logarithmic scale?

Because the eye works across roughly 14 log units of luminance, from about 10^-6 cd/m2 at absolute threshold to about 10^9 cd/m2 at the sun's surface, a range too wide for a linear axis.

Which is faster, light adaptation or dark adaptation, and how long does each take?

Light adaptation is fast, essentially complete within seconds to a couple of minutes. Dark adaptation is slow, needing about 30 to 40 minutes to reach full rod sensitivity.

About how many absorbed photons does a dark adapted observer need in order to see a flash?

Roughly 5 to 14 absorbed photons, spread across separate rods, out of about 50 to 150 photons arriving at the cornea.

State the principle of univariance.

Once a photoreceptor absorbs a photon, its response varies only in magnitude and never in kind, so the cell cannot signal which wavelength was absorbed. Wavelength information is lost at the moment of absorption.

What governs whether a photon reaching a photoreceptor is actually absorbed?

Absorption is a probabilistic event. Each photon has only a probability of being caught, set by the pigment's absorption spectrum at that wavelength.

What chemical change does an absorbed photon produce in a visual pigment?

It isomerizes 11-cis retinal to all-trans retinal, which changes the shape of the opsin and starts the phototransduction cascade.

What does a luminous efficiency function describe?

The relative sensitivity of the visual system to light of equal radiant power, plotted as a function of wavelength.

What is the Purkinje shift?

The shift of peak spectral sensitivity from 555 nm toward 507 nm as vision moves from photopic (cone) to scotopic (rod) conditions.

Why does dim red illumination preserve dark adaptation?

Rods are essentially unresponsive to wavelengths longer than about 640 nm, so red light stimulates cones without bleaching rhodopsin.

How many photons must a single rod absorb to generate a detectable response, and roughly how many absorbed photons are needed for a human to see a flash?

One absorbed photon can excite a rod; conscious detection requires only about 5 to 14 absorbed photons spread across a group of rods.

What is the term for the proportion of incident light that remains after passing through a structure?

Transmittance, the ratio of transmitted radiant flux to incident radiant flux.

How is the overall transmittance of light through several ocular structures in series calculated?

Multiply the individual transmittances together; the transmittances are a product, not a sum.

What is Brewster's angle?

The angle of incidence at which p-polarized light is entirely transmitted with no reflection, so the reflected beam is completely s-polarized.

What equation gives Brewster's angle for light going from medium n1 into medium n2?

tan(theta B) = n2 / n1

Why are polarized sunglasses built to transmit only vertically polarized light?

Glare reflected from horizontal surfaces such as water, snow, and roads is predominantly horizontally polarized, so a vertical transmission axis blocks it.

How does photon energy vary with wavelength?

Inversely: the shorter the wavelength, the higher the energy per photon.

Approximately how much energy does one photon of 500 nm light carry?

About 4 x 10^-19 joules.

Which light damage mechanism causes solar retinopathy after eclipse viewing?

Photochemical damage to the retinal pigment epithelium and photoreceptors, with only a minor thermal contribution.

Which wavelength band carries the greatest photochemical risk to the retina, known as the blue light hazard?

Roughly 400 to 500 nm, with peak hazard near 440 nm.

What is photoablation as a laser-tissue interaction?

High energy ultraviolet photons directly break molecular bonds so tissue fragments are ejected, removing material with almost no heat spread to surrounding tissue.

What is a thermal laser-tissue interaction, and what is its main ophthalmic use?

Absorbed light is converted to heat that coagulates and denatures tissue proteins; it is the basis of retinal photocoagulation.

What is plasma-induced ablation?

An ultrashort, very high irradiance pulse causes optical breakdown, forming an ionized plasma that removes tissue precisely and stays confined to the focal volume.

What is photodisruption?

Plasma formation that generates a shock wave and cavitation bubbles, splitting tissue by mechanical force rather than by heat.

Which two ophthalmic laser procedures are the classic examples of photodisruption?

Nd:YAG posterior capsulotomy for posterior capsular opacification and Nd:YAG laser peripheral iridotomy for angle closure.

How does photodisruption differ from plasma-induced ablation, given that both create a plasma?

Photodisruption uses higher pulse energy, so the plasma generates shock waves and cavitation that tear tissue mechanically; plasma-induced ablation stays confined to the plasma volume with no significant mechanical rupture.

What does treating light as photons, rather than as a wave, explain about vision?

It explains how photoreceptors detect light: absorption is quantal, so a single photon is either absorbed by one photopigment molecule or not absorbed at all.

What happens inside a photopigment molecule when it absorbs a photon?

The chromophore 11-cis retinal isomerizes to all-trans retinal, activating the opsin and starting the phototransduction cascade.

What is the principle of univariance?

A photoreceptor's response encodes only how many photons it absorbed, not their wavelength, so any single receptor type is color blind.