3: Eye and Colour Vision
- Page ID
- 128453
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- 3.1: Anatomy and function
- Light enters at the cornea, then traverses the aqueous humour, passes through the pupil, which limits the power entering the eye and reduces aberration. Light then traverses the crystalline lens and vitreous humour and finally is focussed on the retina. The highly curved cornea gives most refracting power to the eye, sometimes adjusted by spectacles or contact lenses. Muscles change the shape of the crystalline lens to adjust the focal length. This is called accommodation.
- 3.2: Retina, rods and cones
- Usually, light is focussed to form a real, inverted image on the retina. In the retina, light is absorbed by photoreceptor cells, which, via retinal neural cells, produce binary electrical pulses (action potentials) in the optic nerve. Rod cells respond to all wavelengths and so give maximal sensitivity. Cone cells of three types, maximally sensitive to red, green and blue, provide colour vision, to which we return later.
- 3.3: Resolving angle and time
- Performance of the eye, assessed in physical terms. (This and next page.) The ‘pixel spacing’ of photoreceptors and the focal length determine a theoretical geometrical limit for angular resolution: ~0.05, which the eye’s performance sometimes approaches. In bright light with very small pupil, the eye can reach the limit imposed by diffraction (discussed later). Stereo vision contributes to depth perception for close objects. Motion looks smooth at ~ 25 frames per second.
- 3.4: Sensitivity
- Physical performance of the eye, continued. The eye captures ~10% of photons passing through the pupil. Little light is lost by reflection on axis at the cornea but scattering and absorption in the lens and humours lose about half. Photoreceptors capture ~20% of photons in retina. The variation in pupil area contributes to a large dynamic range: from minimal detectable to damage intensity is about 120 dB (very roughly the same as the ear).
- 3.5: Compromises
- We examine necessary compromises among different aspects of performance of the eye. Aberration due to geometry reduces spatial resolution. Aberration but can be reduced by reducing the pupil size, though at the cost of reduced sensitivity. Increasing integration time increases sensitivity but reduces time resolution. Increasing the ratio of focal length to pixel size would increase spatial resolution, but at the cost of reducing sensitivity.
- 3.6: Colour vision
- Three types of cone cell photoreceptors in the retina are called red, green and blue (RGB) for (approximately) the colours of spectral wavelengths to which they respond maximally. Their combined response largely determines perceived colour. Additive mixing of RGB light or RGB pixels can, to our visual system, approximately simulate most visible spectral colours. For example, simultaneous stimulation of collocated red and green cones gives yellow.
- 3.7: Colour Mixing
- To our three-colour RGB vision, spectral colours can be simulated by subtracting bands from white light using just three filters (e.g. paints, ink or pigments). The subtractive primaries (CYM: cyan, yellow magenta) are the complementary colours to the additive primaries (R,G,B). A cyan filter or dye absorbs red light, yellow absorbs blue, magenta stops green. C+Y+M absorbs R, G and B, which in principle yields black (B). C+Y=G, C+M=B, Y+M=R.