Vision. The eye. Image formation. Eye defects & corrective lenses. Visual acuity. Colour vision. Lecture 3.5

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Nigel Atkins
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1 Lecture 3.5 Vision The eye Image formation Eye defects & corrective lenses Visual acuity Colour vision

2 Vision Perception of light--- eye-brain system Physiology-- describes how the image is processed by eye-brain combination Perceptual Psychology high level processing of information e.g. Hollow mask illusion Physics --- image formation on the retina

3 eye Vision Iris Muscular diaphragm Controls size of pupil Colour of the eye

$Camera provides main refractive power of the eye 1.$ $376) Aqueous humour (refractive index =1.$

4 Vision Physics ---image formation on the retina iris lens pupil Visual axis Optic axis retina fovea macula cornea iris Ciliary muscle Vitreous humour Aqueous humour Optic nerve Cornea (outer transparent aperture structure, Camera provides main refractive power of the eye 1.376) Aqueous humour (refractive index =1.336) Pupil (variable opening in iris) Lens (refractive index =1.437) Vitreous humour (refractive index =1.336) Retina (back surface: photo-receptors) CCD array

5 Vision Physics ---image formation on the retina Lens-cornea system focuses light onto the retina (rear surface of the eye) Lens contributes only about 20-25% of the refractive power of the eye Shape of the lens is altered by the ciliary muscles Iris regulates the amount of light entering the eye by controlling the diameter of the pupil Fovea -- most sensitive part of retina slightly off centre

6 Vision Retina Retina consists of millions of photo-receptors called rods (1.3x10 8 ) and cones (7x10 6 ) Converts light energy into electrical energy These structures send electrical impulses to the brain via the optic nerve rods & cones chemically adjust their sensitivity depending on light levels --takes about 15 minutes getting used to the dark

7 Vision + iris lens pupil cornea iris Ciliary muscle Visual axis Optic axis Vitreous humor retina fovea macula Optic disk Optic nerve At the optic disk there are no photo-receptors Blind spot on retina at optic disk Not noticable because each eye compensates by seeing what the other doesn't. +

8 Vision Accommodation = s' s f Lens to retina distance (image distance s ) is fixed Focal length (and hence the power) of the lens must vary to ensure objects at various distances can be brought to a focus at the retina process called Accommodation Normal eye is capable of focusing on objects over a range from infinity (far point) (Ciliary muscle relaxed) to the near point of 25 cm. Ciliary muscle tense 25cm 2.3cm Near point increases with age

9 Example Calculate the refractive powers of the eye required to view an object clearly (a) at far point and (b) at near point = = s' s f Strength( diopters( D)) Image distance is constant 2.3 cm =0.023 m (approximate diameter of eyeball) (a) At the far point ( infinity) object distance s is very large, therefore 1 0 s 1 1 s' f + 0 = = Strength( diopters) Strength =1/s = 1/f = 1/(0.023m) = 43.5 diopters Strength = 43.5 diopters

10 Example (b) At the near point the object distance =25cm (0.25m) = = Strength s' s f 25cm 2.3cm 1 1 Strength = + = diopters Eye, when viewing object anywhere between near point and infinity adjusts its strength from 47.5 D to 43.5 D Lens of the eye provides this required change in strength Ciliary muscle controls shape of the lens Normal accommodation ability 8.5%

11 Eye defects Farsightedness (hyperopia) Object at normal near point: Eye lens cannot be made sufficiently converging to bring object to focus on the retina: lens to weak (focal length to long) Near point 25cm Converging eyeglass lens required to ensure focus at retina Near point eyeglass

12 Eye defects Near-sightedness (myopia) Eye lens cannot relax sufficiently to focus a very distant object on the retina, lens to strong (focal length to short) Diverging eyeglass lens required to ensure focus at retina

13 Example A woman with an eye defect has a far point of 80 cm. Name the defect and calculate the focal length of a suitable corrective lens. Myopia Object at 80 cm Remember far point of normal eye is at infinity When she views object far away, lens must produce a virtual image of the object at her far point = s s' f 80 = f = - 80 cm f

14 Eye defects Astigmatism Results from an asymmetry in the cornea (irregular curvatures) Cornea: different curvature in different directions. e.g. different foci for rays that propagate in two perpendicular planes Test chart Astigmatism can be corrected using cylindrical lenses, correctly orientated

15 Astigmatism Eye defects cornea retina light lens Rays in horizontal plane not in focus light cylindrical lens corrects astigmatism

16 Presbyopia Eye defects Loss of accommodation (presbyopia) Inability for eye to accommodate increases with age. Lens becomes larger & less flexible and the ciliary muscles are less able to distort the lens Eventually affects nearly everyone. difficulty in seeing near and distant objects clearly Correction bifocal or varifocal lens

17 Visual acuity Rows of photo-receptor cells on retina 2µm Shows two images just resolved Images of two adjacent dots must fall on two Non-adjacent retinal cells Ideally there should be a few unexcited cells separating the ones on which the images appear--- otherwise the images will not be clearly resolved

18 Visual acuity ability of the eye to see fine detail Also known as spatial resolving power Visual acuity is mainly limited by: 1. photoreceptor density on the retina 2. Diffraction effects, θ W = eyeball width =2.3 cm. y * y * d Eye can resolve points with an minimum angular separation of 3 x 10-4 radians = 1.0 minute of arc Calculate Minimum Spatial separation y at a distance of 6 m y θ = y = θd d Visual acuity Angular separation θ = ( -4 ) d = 6m y = 6 m 3 10 =1.8 mm y d

19 Example Determine the size of the image on the retina of (a) a dot 1.8 x10-2 cm in diameter at a distance 60 cm from the eye and (b) a person of height 1.7m at a distance of 10m from the eye. M s ' = s M = h ' h (a) h /h = -s /s s = 2.3 cm h = h(-s /s) = -1.8 x10-2 cm x (2.3cm/60cm) h = x10-4 cm = - 6.9µm (b) h = h(-s /s) = -1.7m x (2.3cm/10m) = cm

20 Visual acuity (resolution) Diffraction effects Associated with the wave nature of light Diffraction is the spreading of light as it passes through an aperature or passes by the edge of an object. Diffraction effects are dependent on the size of the aperature or object greatest when the dimensions approach that of the wavelength of the light Clear circular spot the size of circular aperture produced on screen Clear circular spot the Size of circular aperture produced on screen Small aperature: Diffraction effect Fuzzy circular spot, much larger than aperture, produced on screen

21 Visual acuity (resolution) Diffraction effects Recall: For circular aperture Minimum angle of resolution Rayleigh criterion 1.22λ θ min = d Pupil has, on average, 3 mm diameter. Assume λ=500 nm θ min 1.22λ nm = = = rad d 3mm Diffraction effects (2x10-4 rad) < resolving power of eye (3x10-4 rad) Other less important effects that limit visual acuity are illumination, contrast, location of image on retina. 20/20 vision Normal sighted person Someone with 20/20 or 6/6 vision (visual acuity) is just able to decipher a object (letter) that subtends a angle of 1 minute of arc at the eye. 20/20 not perfect: young adults 20/16>>>20/12 Hawk 20/2 Large pupil, & small photo-receptors θ = min 1.22λ d

22 Vision Threshold Low light levels Vision threshold Very low Described in terms of single photons Photons incident on the cornea needed to perceive a flash of light. approximately half of photons reach the retina approx 5 are absorbed Vision: - Signal processing Occurs in neural network of retina Important aspects of image selected and transmitted to brain Image movement on retina important for vision achieved by small rapid eye movements

Colour Vision Isaac Newton (1643 1727) demonstrated that

passing through a prism is separated into its constituent

$Reason: refractive index of the glass in the prism varies$

23 Colour Vision Isaac Newton ( ) demonstrated that white light is composed of several colours White light passing through a prism is separated into its constituent colours. Reason: refractive index of the glass in the prism varies with wavelength (colour) This variation of refractive index with wavelength is called Dispersion Wavelength: 400 nm nm

24 Colour Vision Objects have a particular colour because of their reflection/absorption characteristics. Example; an apple appears green because all wavelengths except green are absorbed; green is reflected. White light White light White light White light White light Various levels of absorption will result in an object having more complex shades of colour.

Colour Vision Visible spectrum Infrared Wavelength Ultra violet Colour vision is related to the wavelength of light Light sensitive cells in the retina:

25 Colour Vision Visible spectrum Infrared Wavelength Ultra violet Colour vision is related to the wavelength of light Light sensitive cells in the retina: Rods and Cones Rods: Highly sensitive Low light level vision Peripheral vision little colour response Cones: Less sensitive Colour vision concentrated in fovea

26 Colour Vision 3 kinds of cones: each one sensitive to a different array of wavelengths ref. Dowling 1987 Absorption Sensitivies Colour blindness results from absence of one or more types of cones The sensitivities of the three cones overlap and the perceived colour is due to the relative response of the three cones.

Electromagnetic Waves Visible waves Wavelength: 400 nm - 700 nm Frequency: 10 15 Hz Characteristics The

27 Electromagnetic Waves Visible waves Wavelength: 400 nm nm Frequency: Hz Characteristics The only waves the eye can see -What color does the eye see best? e.g. Blue text on red background is hard to read.

Electromagnetic Waves Primary and secondary colours The eye responds to red, green and blue colour (RGB) these are the primary colours. Mixing the primary colours gives rise to.

28 Electromagnetic Waves Primary and secondary colours The eye responds to red, green and blue colour (RGB) these are the primary colours. Mixing the primary colours gives rise to. secondary colours. e.g. monochromatic green+red light appears yellow just like monochromatic yellow light. Application: Each pixel in a colour LCD made of red, green, blue subpixels.

Colour Vision Colour matching in restorative work Spectral distribution of illumination important; natural light, artificial light may be different Spectral distribution Sun Relative

29 Colour Vision Colour matching in restorative work Spectral distribution of illumination important; natural light, artificial light may be different Spectral distribution Sun Relative Intensity Light from Filament lamp Wavelength (nm) Colour matching should be carried out in natural light or artificial light with spectral distribution close to that of natural light

30 Colour Vision Colour constancy concerns the constant perception of an object s colour under different illumination conditions. Example: green apple appears green in white sunlight of midday. It also appears green in sunset illumination which is mainly red. A range of illumination wavelengths is required. Colour constancy does not happen with single wavelengths

PHYSICS. Chapter 35 Lecture FOR SCIENTISTS AND ENGINEERS A STRATEGIC APPROACH 4/E RANDALL D. KNIGHT

PHYSICS. Chapter 35 Lecture FOR SCIENTISTS AND ENGINEERS A STRATEGIC APPROACH 4/E RANDALL D. KNIGHT PHYSICS FOR SCIENTISTS AND ENGINEERS A STRATEGIC APPROACH 4/E Chapter 35 Lecture RANDALL D. KNIGHT Chapter 35 Optical Instruments IN THIS CHAPTER, you will learn about some common optical instruments and