Lecture 1 eye and visual stimulus

Q: What are the main internal structures of the eye and their functions?

Aqueous humour → clear fluid in anterior eye

Lens → transparent structure that changes shape during accommodation

Vitreous → gel filling posterior eye

Choroid → vascular layer supplying nutrients

Retina → neural tissue containing rods and cones

Q: What are the main external structures of the eye and their functions?

Iris → muscular diaphragm controlling pupil size

Pupil → opening through which light enters the eye

Cornea → transparent refracting surface

Sclera → structural protection

Conjunctiva → covers sclera and inner eyelids

Limbus → junction between cornea and sclera

Cilia → eyelashes

Punctum lacrimale → tear drainage opening

Lacrimal caruncle → tissue in medial canthus

Q: Compare the anterior and posterior segments of the eye.

Anterior Segment

Contains aqueous humour

Includes:

Anterior chamber (cornea → iris)

Posterior chamber (iris → lens)

Posterior Segment

Contains vitreous humour

Also called the vitreous chamber

Q: Why are the fovea and macula important?

Fovea = highest visual acuity

Contains only cones

Central fovea is avascular

Macula surrounds the fovea

Appears yellow because of pigment

Q: Compare the visual axis and optical axis.

Visual axis → line from fixation point to fovea

Optical axis → line through optical centre of the eye

They are not the same.

Q: How can light be described?

Light can be described as:

A wave

A ray

A quantum (photon)

Each model explains different aspects of vision.

Q: What does each model of light explain?

Wave

Diffraction

Interference

Polarisation

Rays

Optical pathways

Photons

Photoreceptor interactions

Q: Compare wavelength and frequency.

Frequency = cycles per second (Hz)

Wavelength = distance between wave peaks

Higher frequency = shorter wavelength

Lower frequency = longer wavelength

Q: How are energy, frequency and wavelength related?

Q: What wavelengths make up visible light?

Approximately:

380-750 nm

Colours:

Violet ≈ 380 nm

Blue ≈ 460 nm

Green ≈ 520 nm

Yellow ≈ 580 nm

Orange ≈ 600 nm

Red ≈ 620 nm

Q: Compare radiometry and photometry.

Radiometry

Measures all electromagnetic radiation.

Photometry

Measures visible radiation weighted by human visual sensitivity.

Q: Compare luminous flux and radiant flux.

Luminous Flux (F):

Total visible light emitted

Unit = lumen (lm)

Radiant Flux (Fe):

Total electromagnetic energy emitted

Unit = watt (W)

Q: Compare luminous intensity and radiant intensity.

Luminous Intensity (I)

Luminous flux in a given direction

Unit = candela (cd)

Radiant Intensity (Ie)

Radiant power per steradian

Unit = W/sr

Q: Compare luminance and illuminance.

Luminance (L)

Light coming off a surface

Unit = cd/m²

Illuminance (E)

Light falling onto a surface

Unit = lux (lm/m²)

Remember:

Luminance = leaving

Illuminance = arriving

Q: Which photometric units must you know?

Q: What determines retinal illuminance?

Retinal illuminance depends on:

Stimulus luminance

Ocular transmittance

Pupil area

Q: What is a troland?

Q: Compare luminance, brightness and lightness.

Luminance

Physical measurement of light.

Brightness

Perceived amount of light.

Lightness

Perceived reflectance/grey level of a surface.

Equal luminance does not always produce equal brightness.

Q: Explain the eye and visual stimulus.

• Light enters through the pupil.

• Iris controls pupil size.

• Cornea and lens provide refraction.

• Lens accommodates to focus images.

• Retina contains rods and cones.

• Choroid supplies nutrients.

• Fovea contains only cones and provides highest visual acuity.

• Macula surrounds the fovea.

• Anterior segment contains aqueous humour.

• Posterior segment contains vitreous humour.

Light may be considered:

• A wave

• A ray

• A photon

• Frequency and wavelength are inversely related.

• Energy is directly proportional to frequency and inversely proportional to wavelength.

Visible light spans approximately 380-750 nm.

Radiometry measures all EM radiation.

Photometry measures visible light weighted by human sensitivity.

Key photometric quantities:

• Flux → lumen

• Intensity → candela

• Illuminance → lux

• Luminance → cd/m²

• Retinal illuminance → trolands

Retinal illuminance depends on:

Eret=L×aE_{ret}=L\times aEret​=L×a

where retinal illuminance equals luminance × pupil area.

Luminance is a physical measurement, brightness is a perception, and lightness is the perceived reflectance of a surface.