Overview of Colour Vision

Question 1

What is light also known as?

Answer 1

electromagnetic radiation

Question 2

What is Light: electromagnetic radiation?

Answer 2

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

Question 3

Where does visible light fall in the electromagnetic spectrum?

Answer 3

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.

Question 4

High energy photons vibrate at…

Answer 4

high frequency; shorter wavelength

Question 5

What affects the quality of light?

Answer 5

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

Question 6

What is the Effect of light intensity on visual function?

Answer 6

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

Question 7

How is light intensity measured?

Answer 7

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

Question 8

How does wavelength affect the quality of light?

Answer 8

Quality of the visible spectrum depends on an organism’s photoreceptor spectral sensitivity:
- E.g. humans have low sensitivity to wavelengths <400nm

Question 9

What does colour vision in humans depend on?

Answer 9

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

Question 10

What are the Sensitivity Functions of SW, MW and LW cones?

Answer 10

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.

Question 11

What is spectral sensitivity?

Answer 11

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

Question 12

What does the peak of each curve correspond to?

Answer 12

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.

Question 13

What is hue?

Answer 13

Hue is the term for colour appearance, which changes from blue → green → yellow → red as wavelength increases

Question 14

What are spectral and non-spectral colours?

Answer 14

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

Question 15

Is colour wavelength?

Answer 15

To some extent it is

Colour depends on wavelength, but cannot simply be equated with wavelength

Question 16

What is the Debate surrounding colour vision?

Answer 16

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.

Question 17

Why do objects appear coloured?

Answer 17

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

Question 18

What does colour appearance of objects depend on?

Answer 18

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.

Question 19

What is a pre-requisite for colour vision?

Answer 19

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

Question 20

Is wavelength discrimination possible with a single spectrally-tuned photoreceptor?

Answer 20

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)

Question 21

Explain the Principle of univariance.

Answer 21

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

Question 22

What are the Photoreceptor Spectral Classes?

Answer 22

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)