Overview of Colour Vision Flashcards
What is light also known as?
electromagnetic radiation
What is Light: electromagnetic radiation?
Radiation is emitted from a source in small ‘packets’ of energy called photons, which travel at the speed of light.
Photons vibrate at a frequency that increases with their energy.
Consequently, we can think of light as a wave, where speed of light, wavelength and frequency are related:
ƛ= c/f
Where does visible light fall in the electromagnetic spectrum?
A relatively narrow band electromagnetic spectrum, between roughly about 400-700nm in wavelength is the actual visible spectrum. Radiant energy falling within this band of the spectrum is perceivable by light to humans and other animals.
High energy photons vibrate at…
high frequency; shorter wavelength
What affects the quality of light?
The quality of light varies along two dimensions:
Intensity (brightness)
Wavelength
-related to colour sensations evoked by light absorbed by the retina; chromaticity, or perceived hue, changes with wavelength
What is the Effect of light intensity on visual function?
Lower light intensities = rods active = no colour vision, poor acuity
-Scotopic vision
Higher light intensities = cones active = good colour vision, good acuity
-Mesopic/photopic vision
How is light intensity measured?
Light as radiant energy
- Can be measured in Watts/m2
Light as luminance
- Candela/m2
- Scaled, by international conventions, according to spectral sensitivity of human eye
Light as photon flux (quantal flux)
- Photons per unit area per unit time
How does wavelength affect the quality of light?
Quality of the visible spectrum depends on an organism’s photoreceptor spectral sensitivity:
- E.g. humans have low sensitivity to wavelengths <400nm
What does colour vision in humans depend on?
Human colour vision depends on 3 spectral classes of cone.
These are often called blue, green and red cones, but more correctly termed short wave (SW), middle wave (MW) and long wave (LW) cones
What are the Sensitivity Functions of SW, MW and LW cones?
SW cone peak absorbance → 420nm
MW cone peak absorbance → 530nm
LW cone peak absorbance → 560nm
Summed photoreceptor signals determine luminous sensitivity over the visible spectrum.
What is spectral sensitivity?
The probability of absorbing a photon
Peak sensitivity is where the probability of absorbing a photon is highest. It will still absorb photons at wavelengths away from peak sensitivity, but less effectively
-therefore, you can think of the photoreceptors as ‘broadly tuned’; a peak wavelength sensitivity, but they will still respond to wavelengths around that peak wavelength sensitivity
What does the peak of each curve correspond to?
The peak of each curve corresponds to wavelength of maximum sensitivity, maximum photon absorbance, meaning probability of capturing a photon at this wavelength is maximal. Photons of other wavelengths are still captured, but with lower probability, shown by decreasing sensitivity with wavelengths increasingly distant from the peak.
What is hue?
Hue is the term for colour appearance, which changes from blue → green → yellow → red as wavelength increases
What are spectral and non-spectral colours?
Spectral colours are those that can be elicited by single wavelengths (blue, green, yellow, red)
Non-spectral colours are those that cannot be equated with specific wavelengths, such as purple, pink, brown, black, white
Is colour wavelength?
To some extent it is
Colour depends on wavelength, but cannot simply be equated with wavelength
What is the Debate surrounding colour vision?
Colour vision entails the ability to see colours and discriminate objects on the basis of colour.
One argument is that colour is entirely subjective, an internal representation that has no corresponding reality in the external world.
Others argue that colour IS a physical property of the world.
Why do objects appear coloured?
Surface Spectral Reflectance (SSR)
- objects appear coloured due to the wavelength composition of the light reflected from the surface
- the surface spectral reflectance is a fixed physical/chemical property of a surface
- different SSRs of different surfaces mean different objects absorb and reflect different ranges of the wavelengths in the incoming light, therefore have different colours
- SSR describes the proportion of light reflected at each wavelength of the visible spectrum
What does colour appearance of objects depend on?
Coloured objects do not reflect just a single wavelength corresponding to a spectral colour. Colour appearance depends on the relative amount of different wavelengths reflected.
Objects do not just reflect one or a narrow band of wavelengths; it is always a broad spectrum, and it is the relative wavelength distribution that determines colour appearance.
What is a pre-requisite for colour vision?
Wavelength discrimination
- you have to be able to discriminate changes in wavelength independently from changes in intensity (because intensity and wavelength are potentially confounded)
- need to show discrimination based purely on wavelength
- control for (achromatic) differences in intensity
Is wavelength discrimination possible with a single spectrally-tuned photoreceptor?
No, due to the broad spectral range.
A low intensity at 560nm could produce the same response as a higher intensity at 450nm.
- any two stimuli in the spectral range could be adjusted in relative intensity to produce the same response
- wavelength and intensity are confounded
A single photoreceptor is colour blind and cannot separate wavelength and intensity (principle of univariance)
If you only had one spectral class of photoreceptor, you would only see the world in terms of luminance changes (different shades)
Explain the Principle of univariance.
The fact that an infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make colour discriminations based on wavelength.
To discriminate wavelengths and see in colour, you need at least two different types of photoreceptors to be operating at the same time.
One individual photoreceptor type cannot differentiate between a change in wavelength and a change in intensity. Wavelength discrimination requires comparison of photoreceptor responses
What are the Photoreceptor Spectral Classes?
Monochromats
-animals with only a single spectral class of photoreceptor are termed monochromats
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 (including humans); bees
Tetrachromats
- four spectral classes
- most vertebrates (birds, reptiles, fish)
