Overview of colour vision Flashcards
Describe light as electromagnetic radiation
- Light as a wave and equation:
wavelength = speed of light / frequenc
What are photons
Radiation is emitted from a source in small packets of energy = photons
what speed do photons travel at?
speed of light
how do photons vibrate?
Photons vibrate at a frequency that increases with their energy
what is frequency?
the number of cycles per second pass a given point
describe the electromagnetic spectrum
as you go right wavelength increase and as you go left frequency increases
high energy photons vibrate…
at high frequency; shorter wavelength
what is visible light on the EM spectrum
narrow band
how does light quality vary?
-along 2 dimensions
-intensity and wavelength or chromaticity
what is intensity?
-can be measured as energy (watts/unit area)
-relates to brightness in visible spectrum
what is wavelength/chromaticity?
relates to colour
how to measure light as radiant energy?
can be measured in watts/m^2
how to measure light as luminance?
candela/m^2
-scaled by interaction conventions according to spectral sensitivity of human eye
how to measure light as photon flux (quantal flux)?
- photons per unit area per unit time
- on the longer wavelength end
- stimulus intensity
Brightness is the same as luminance. TRUE OR FALSE
FALSE
what is hue?
Hue = colour appearance, which changes from blue to green to yellow to red as wavelength increases
what are spectral colours?
Spectral colours are those that can be elicited by single wavelength
what are non-spectral colours?
Many colours cannot be equated with specific wavelengths: purple, pink, brown… black, white (non-spectral)
what does colour depend on?
Colour depends on wavelength but cannot simply be equated with wavelength
what does colour vision entail?
Colour vision entails the ability to see colours and discriminate objects on the basis of colour
describe colour as surface spectral reflectance
-Objects appear coloured due to the wavelength composition of the light reflected from the surface
- The SSR is a fixed physical/chemical property of a surface
- SSR describes the proportion of light reflected at each wavelength of the visible spectrum
Is SSR the physical property that corresponds to colour?
Coloured objects do not reflect a single wavelength corresponding to a spectral colour
colour appearance depends…?
Colour appearance depends on the relative amount of different wavelength
Quality of the visible spectrum depends on …?
- an organisms spectral sensitivity
- e.g. humans have rather low sensitivity to wavelengths <400 nm
- Wavelength. Chromaticity or perceived hue, changes with wavelength
- signalled by comparison of cone signals
what does human colour vision depend on?
3 spectral classes of cone
what are the 3 spectral classes of cone
Blue, green and red; more correctly termed SW, MW and LW cones
Explain photoreceptor spectral sensitivity
- Summed photoreceptor signals determine luminous* sensitivity over visible spectrum
- determined by sums of photoreceptor signals across classes
- Brightness increases when total signal summed from photoreceptor increases
Vertebrates have a duplex retina. What are rods and cones for?
- rods for scotopic vision
- cones for photopic vision
The quality of light varies along two dimensions. State them.
- Intensity - other things being equal, luminance and perceived brightness increase with intensity
- Wavelength - chromaticity or perceived hue, changes with wavelength
Wavelength discrimination is a prerequisite for colour vision. Explain.
- The ability to discriminate changes in wavelength independently from changes in intensity
- need to show discrimination based purely on wavelength differences
- control for achromatic differences in intensity
Is wavelength discrimination possible with a single spectrally-tuned photoreceptor?
- No, e.g. a low intensity at 560 could produce the same response as a higher intensity 450
- In theory, any two stimuli in the spectral range could be adjusted in relative intensity to produce the same response
- cannot disentangle wavelength and intensity - a single photoreceptor is colour blind
Principle of univariance: a single cone is colour blind
- Phototransduction signal depends on rate of photon absorption (quantal catch)
- Probability of absorption depends on wavelength
Wavelength discrimination requires comparison of photoreceptor responses
- Two spectral classes:
- LW peak 560 nm
- SW peak 440 nm
- A decrease in intensity at 550 would decrease activity in both photoreceptors
- but a decrease in wavelength from 550 would decrease response of LW but increase response of SW responses
Photoreceptors spectral classes
mono-, di-, tri-, tetra-chromats
MONOCHROMATS:
animals with only a single spectral class of photoreceptors
DICHROMATS:
two spectral classes: SW plus MW or LW, most mammals; some humans with colour deficiency
TRICHROMATS:
three spectral classes: SW, MW, LW, old world primates (humans); some invertebrates - bees
TETRACHROMATS
4 spectral classes, most vertebrates (birds, reptiles, fish); some invertebrates - butterflies
