Human Colour Vision Dr Huw Owens © Huw Owens - University of Manchester : 18/09/2018

The human visual system Theories of colour vision Colour Deficiencies Introduction The human visual system Theories of colour vision Colour Deficiencies Eye and brain Optical illusions © Huw Owens - University of Manchester : 18/09/2018

Girl, you had me, once you kissed me My love for you is not iffy I always want you with me I'll play Bobby and you’ll play Whitney © Huw Owens - University of Manchester : 18/09/2018

Intensity The intensity of a light source depends on the number of quanta it emits per unit of time. A range of physical units, known as radiometric units, has been developed to measure intensity. However, the effectiveness of light quanta depends on their wavelength properties (as the human visual system is more sensitive to some wavelengths than others. So in the context of vision, light intensity is usually specified in photometric units that take into account human sensitivity. Light Source Luminance (cd per m2) Photons (per m2 per sr per second) Photons per receptor Paper in starlight 0.0003 1013 0.01 Paper in moonlight 0.2 1015 1 Room Light 316 1018 1000 Paper in sunlight 40000 1020 100000 © Huw Owens - University of Manchester : 18/09/2018

What is colour? Colour exists only in the brain - it is a sensation. Vision is a sense; the other senses are touch, hearing, smell and taste Can an oven feel pain? We can separate the physical cause of the pain that we feel as being the heat of the oven - we need to try to do the same thing with colour! © Huw Owens - University of Manchester : 18/09/2018

Where is colour? © Huw Owens - University of Manchester : 18/09/2018

Some properties of the human visual system The visual system responds to contrast rather than intensity The colours that we perceive are not totally correlated with the physics of the world The visual system provides you with an interpretation of the world - it is not like a photograph of the world The purpose of (colour) vision is to allow you to consistently be able to recognise objects seen from various angles, distances and viewed under different lights © Huw Owens - University of Manchester : 18/09/2018

Indoors - light has 100 units of light at each wavelength Why is black black? Reflectance = 1% Reflectance = 99% Indoors - light has 100 units of light at each wavelength Light reflected = 1% of 100 = 1 unit Light reflected = 99% of 100 = 99 units Outside - light has 10000 units of light at each wavelength Light reflected = 1% of 10000 = 100 units Light reflected = 99% of 10000 = 9900 units © Huw Owens - University of Manchester : 18/09/2018

The human eye Retinal area 5*5cm – thickness: 0.4mm 1.413 Fovea 1.5mm 1.009 Vitreous humour is a viscous gel that fills the large posterior chamber of the eye, maintaining its shape and holdiong the retina against the inner wall. The watery aqueous humour is pumped into the eye continuously, enetering the eye near the attachment of the lens (ciliary processes) and leaving near the margins of the iris (canal of Schlemm). Fluid is replenished every 45 minutes) 1.336 1.376 © Huw Owens - University of Manchester : 18/09/2018

The eye and the camera © Huw Owens - University of Manchester : 18/09/2018

Pigment Iodopsin (Wald, 1949) Rods and Cones Rods 100 million per eye Pigment Rhodopsin (also known as visual purple) Responsible for twilight vision (Scotopic) Cones 5 million per eye Pigment Iodopsin (Wald, 1949) Responsible for daylight vision (Photopic) © Huw Owens - University of Manchester : 18/09/2018

Sensitivity of rods and cones © Huw Owens - University of Manchester : 18/09/2018

L,M,S cone sensitivities © Huw Owens - University of Manchester : 18/09/2018

Spatial distribution of cones on the retina © Huw Owens - University of Manchester : 18/09/2018

Theories of Colour Vision Young-Helmholtz-Maxwell Trichromatic theory Red, Green and Blue sensitive cones Signals processed by the brain Hering Opponent Theory Opposite pairs of sensations Redness v Greeness Yellowness v Blueness Lightness v Darkness © Huw Owens - University of Manchester : 18/09/2018

Image at the retina is real and inverted Visual Physiology Image at the retina is real and inverted Pupil small (1.0mm) in bright light, large (6.6mm) in low light Photoreceptor types – rods and cones Only cones in the fovea Photoreceptor signals travel to Lateral Geniculate Nucleus (LGN) via the optic nerve and then on to Visual Cortex Some pre-processing at the retina and LGN © Huw Owens - University of Manchester : 18/09/2018

Modern Theory of Retinal Processing © Huw Owens - University of Manchester : 18/09/2018

Defective Colour Vision Defective colour vision is carried on the X chromosome Females (XX) have two copies of the X chromosome. Males (XY) have one copy of the X chromosome. Female XX* (where X* represents the defect) is a carrier. Dichromats – three types: protanopes (missing Long-wavelength photopigment), deuteranopes (missing Middle-wavelength photopigment), tritanopes (missing Short-wavelength photopigment) © Huw Owens - University of Manchester : 18/09/2018

What is it like to be a dichromat? Dichromats will confuse all stimuli that lie along what are known as confusion lines. There can be, for a given type of dichromat, many such lines all emanating from the appropriate copunctal point. Of course, the drawing of the diagrams above does not suggest that dichromats have normal chromaticity spaces - quite the contrary. The confusion lines are plotted in the chromaticity space of the normal observer. The determination of these confusion lines is another diagnostic tool that could be used for colour defectives. Dichromatic wavelength discrimination is also much poorer than for normal subjects (Hsia & Graham 1997). The dichromat requires a just discriminable wavelength difference that is often ten or more times as great as the corresponding Dl for a trichromat. Wavelength discrimination curves for the protanope and deuteranope have a single minimum around 500nm and Dl increases rapidly on both sides of this region. A monochromat is a person who can match the spectrum with any chosen wavelength provided he is allowed to adjust luminances, Two types exist - rod monochromats and cone monochromats. Such achromats are extremely rare. normal deuteranope (red green) tritanope (blue yellow) © Huw Owens - University of Manchester : 18/09/2018

Ishihara confusion plates. Colour Vision Tests Ishihara confusion plates. Farnsworth-Munsell 100 hue discrimination test. Holmgren Wool Test – varicoloured skeins (length of thread or yarn wound in a long loose coil) of wool are spread out before the subject who is required to select those resembling three larger skeins, a red, a green and a rose. Nagel Charts – Cards with small circular coloured spots arranged in a ring. Subject required to say which cards have spots of a single hue and which have more than one hue. Colour Matching (Anomaloscope) © Huw Owens - University of Manchester : 18/09/2018

What is it like to be a dichromat? Dichromats will confuse all stimuli that lie along what are known as confusion lines. There can be, for a given type of dichromat, many such lines all emanating from the appropriate copunctal point. Of course, the drawing of the diagrams above does not suggest that dichromats have normal chromaticity spaces - quite the contrary. The confusion lines are plotted in the chromaticity space of the normal observer. The determination of these confusion lines is another diagnostic tool that could be used for colour defectives. Dichromatic wavelength discrimination is also much poorer than for normal subjects (Hsia & Graham 1997). The dichromat requires a just discriminable wavelength difference that is often ten or more times as great as the corresponding Dl for a trichromat. Wavelength discrimination curves for the protanope and deuteranope have a single minimum around 500nm and Dl increases rapidly on both sides of this region. A monochromat is a person who can match the spectrum with any chosen wavelength provided he is allowed to adjust luminances, Two types exist - rod monochromats and cone monochromats. Such achromats are extremely rare. normal deuteranope (red green) © Huw Owens - University of Manchester : 18/09/2018

Genetic Basis of Inherited (Congenital) Colour Blindness Male X Y The gene for colour blindness is carried on the X chromosome. Female X A colour-blind man cannot pass the X chromosome to his son. Dichromats will confuse all stimuli that lie along what are known as confusion lines. There can be, for a given type of dichromat, many such lines all emanating from the appropriate copunctal point. Of course, the drawing of the diagrams above does not suggest that dichromats have normal chromaticity spaces - quite the contrary. The confusion lines are plotted in the chromaticity space of the normal observer. The determination of these confusion lines is another diagnostic tool that could be used for colour defectives. Dichromatic wavelength discrimination is also much poorer than for normal subjects (Hsia & Graham 1997). The dichromat requires a just discriminable wavelength difference that is often ten or more times as great as the corresponding Dl for a trichromat. Wavelength discrimination curves for the protanope and deuteranope have a single minimum around 500nm and Dl increases rapidly on both sides of this region. A monochromat is a person who can match the spectrum with any chosen wavelength provided he is allowed to adjust luminances, Two types exist - rod monochromats and cone monochromats. Such achromats are extremely rare. Females can be carriers of colour blindness. If there are men on the mother’s side who are colour blind then there is a chance that the child could inherit the gene. If a woman is colour blind her son is almost certain to be so. © Huw Owens - University of Manchester : 18/09/2018

Ishihara Test http://tjshome.com/selftest.php Dichromats will confuse all stimuli that lie along what are known as confusion lines. There can be, for a given type of dichromat, many such lines all emanating from the appropriate copunctal point. Of course, the drawing of the diagrams above does not suggest that dichromats have normal chromaticity spaces - quite the contrary. The confusion lines are plotted in the chromaticity space of the normal observer. The determination of these confusion lines is another diagnostic tool that could be used for colour defectives. Dichromatic wavelength discrimination is also much poorer than for normal subjects (Hsia & Graham 1997). The dichromat requires a just discriminable wavelength difference that is often ten or more times as great as the corresponding Dl for a trichromat. Wavelength discrimination curves for the protanope and deuteranope have a single minimum around 500nm and Dl increases rapidly on both sides of this region. A monochromat is a person who can match the spectrum with any chosen wavelength provided he is allowed to adjust luminances, Two types exist - rod monochromats and cone monochromats. Such achromats are extremely rare. http://tjshome.com/selftest.php © Huw Owens - University of Manchester : 18/09/2018

UMIST online colour vision test Dichromats will confuse all stimuli that lie along what are known as confusion lines. There can be, for a given type of dichromat, many such lines all emanating from the appropriate copunctal point. Of course, the drawing of the diagrams above does not suggest that dichromats have normal chromaticity spaces - quite the contrary. The confusion lines are plotted in the chromaticity space of the normal observer. The determination of these confusion lines is another diagnostic tool that could be used for colour defectives. Dichromatic wavelength discrimination is also much poorer than for normal subjects (Hsia & Graham 1997). The dichromat requires a just discriminable wavelength difference that is often ten or more times as great as the corresponding Dl for a trichromat. Wavelength discrimination curves for the protanope and deuteranope have a single minimum around 500nm and Dl increases rapidly on both sides of this region. A monochromat is a person who can match the spectrum with any chosen wavelength provided he is allowed to adjust luminances, Two types exist - rod monochromats and cone monochromats. Such achromats are extremely rare. Which disk appears the best match to the one in the centre? http://www2.umist.ac.uk/optometry/UES/COLOUR0.HTM © Huw Owens - University of Manchester : 18/09/2018

Anomalous Trichromats Anomalous trichromats have three cone classes but one of the cones is normally shifted to a different peak wavelength sensitivity. Protanomalous Deuteranomalous Tritanomalous The design of the anomaloscope relies upon the fact that people with normal colour vision have two classes of colour detectors - the red and the green (the blue or S cones are not affected by this test). Normal observers can easily adjust the amount of red and green light in the additive mixture so that it provides a match on one half of the view to the monochromatic yellow target. The device is constructed such that the proportion of red-green light required for the match is 0.5 R/(R+G). The range of matches of normal observers is approximately 0.43 - 0.57 R/(R+G) for males and a little smaller for females (Neitz & Neitz 1998). Although this may seem a surprisingly large variance the R/(R+G) ratios for anomalous trichromats are approximately 0.18 and 0.85 for deuteranomolous and protanomalous observers respectively. It has been suggested that most of the variation in normal observers can be accounted for by variation in the spectral positioning of the L- and M-cone pigments. © Huw Owens - University of Manchester : 18/09/2018

Anomalous Trichromats 1 1 0.9 0.9 0.8 0.8 0.7 0.7 0.6 0.6 sensitivity sensitivity 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 There are two main classes of anomalous trichromats. These observers have three cone classes but use very different amounts of red and green light in amomaloscopic matches. A protanomalous observer’s matches will appear redder than the yellow target stimulus to a normal observer. A protanomalous observer is less sensitive to red because the normal L cone centred at 563 nm is replaced by an anomalous cone pigment shifted to lower wavelengths. In the diagram, the normal cone pigment is shown with a dashed line. Typically, the anomalous L cone has a maximum sensitivity at 553 nm. A deuteranomalous observer’s matches will appear greener than the yellow target stimulus to a normal observer. A deuteranomalous observer is less sensitive to green because the normal M cone centred at 532 nm is replaced by an anomalous cone pigment shifted to higher wavelengths. In the diagram, the normal cone pigment is shown with a dashed line. Typically, the anomalous M cone has a maximum sensitivity at 555 nm. 0.1 400 450 500 550 600 650 700 400 450 500 550 600 650 700 wavelength wavelength deuteranomalous protanomalous Green sensitive Red sensitive © Huw Owens - University of Manchester : 18/09/2018

Farnsworth-Munsell 100 hue test The design of the anomaloscope relies upon the fact that people with normal colour vision have two classes of colour detectors - the red and the green (the blue or S cones are not affected by this test). Normal observers can easily adjust the amount of red and green light in the additive mixture so that it provides a match on one half of the view to the monochromatic yellow target. The device is constructed such that the proportion of red-green light required for the match is 0.5 R/(R+G). The range of matches of normal observers is approximately 0.43 - 0.57 R/(R+G) for males and a little smaller for females (Neitz & Neitz 1998). Although this may seem a surprisingly large variance the R/(R+G) ratios for anomalous trichromats are approximately 0.18 and 0.85 for deuteranomolous and protanomalous observers respectively. It has been suggested that most of the variation in normal observers can be accounted for by variation in the spectral positioning of the L- and M-cone pigments. The 100-hue test is also a test of colour discrimination © Huw Owens - University of Manchester : 18/09/2018

Over thirty visual areas have been identified in old world monkeys. Eye and brain The purpose of the eye is to transduce a particular subset of electromagnetic radiation into neural signals Neural signals from the retina travel along the optic nerve to the Lateral Geniculate Nucleus (LGN) and then to the occipital lobe. Over thirty visual areas have been identified in old world monkeys. The retinal image is inverted and colour, motion and other spatial aspects of the image are analysed. (e.g. determining the level of Illumination by comparing the background and foreground) © Huw Owens - University of Manchester : 18/09/2018

Related and Unrelated colours It is difficult to judge colours in isolation (when there is no background to make the comparison with). These are called unrelated colours. What colour is the rectangle in the middle of the next slide? © Huw Owens - University of Manchester : 18/09/2018

© Huw Owens - University of Manchester : 18/09/2018

© Huw Owens - University of Manchester : 18/09/2018

© Huw Owens - University of Manchester : 18/09/2018

© Huw Owens - University of Manchester : 18/09/2018

Successive Contrast Stare at the fixation spot on the next slide for 30 seconds and then look at the blank screen. A faint after-image in the complementary colour is seen. This phenomenon is called successive contrast. © Huw Owens - University of Manchester : 18/09/2018

© Huw Owens - University of Manchester : 18/09/2018

© Huw Owens - University of Manchester : 18/09/2018

Simultaneous Contrast When the area around or next to the central coloured coloured area is also in the field of view we may see the phenomenon of simultaneous contrast. This may be due to contrast in lightness, chroma or hue but chroma appears to be least important. © Huw Owens - University of Manchester : 18/09/2018

Simultaneous contrast - lightness A mid grey when view against a black background appears lighter than the same grey viewed against a white background. The same applies to a mid green viewed against light and dark greens of the same hue (colour). The brain exaggerates these differences. This is sometimes known as the “law of juxtaposition”. © Huw Owens - University of Manchester : 18/09/2018

An example of simultaneous contrast © Huw Owens - University of Manchester : 18/09/2018

An example of simultaneous contrast - hue A purple against a blue background looks redder than a purple against a red background. © Huw Owens - University of Manchester : 18/09/2018

Chromatic aberration The lens of the eye cannot focus red light at the same distance as violet or blue light (at the same time). If green is focussed on the retina then red falls beyond the focal plane and blue falls before the focal plane. As a result some design containing red next to blue appear to shimmer. © Huw Owens - University of Manchester : 18/09/2018

Red-Blue design © Huw Owens - University of Manchester : 18/09/2018

Optical Illusions Many optical illusions are due to the brain expecting perspective to be involved in a scene. Others are due to dark being expected to be further away (receding) while light is expected to be in the foreground. Some famous illusions are due to the eye not being able to focus on two places at once. © Huw Owens - University of Manchester : 18/09/2018

The Ponzo or railway lines illusion © Huw Owens - University of Manchester : 18/09/2018

Tile illusion © Huw Owens - University of Manchester : 18/09/2018

Colour Appearance and Cognition Adelman, MIT © Huw Owens - University of Manchester : 18/09/2018

Escher’s Waterfall © Huw Owens - University of Manchester : 18/09/2018