Color vision

visible light to human

opsin+retinal within human photoreceptor can only absorb wavelength between 390nm -760nm

other animals can absorb other wavelengths → see wavelengths that humans cannot

What is the limiting factor for vision color?

Vision is possible because the opsins react to electromagnetic energy that enters the eye

how can electromagnetic energy enter the eye?

by illumination and reflectance

illumination

• illuminants produce electromagnetic radiation

• monochromatic light source: one single wavelength is produced:

  • laser

  • light emitting diodes (LEDs)

  • sodium lamp (589nm)

• polychromatic light source: multiple wavelengths are produced

  • the sun

  • light bulbs

  • candlelight

reflection, transmission and absorption

When electromagnetic energy hits an object, it can either be absorbed, transmitted or reflected.

• Wavelengths absorbed by the material pigments do not enter the eye

  • Color vision is NOT determined by the absorbed wavelengths.

• Only wavelengths that are either transmitted or reflected enter the eye to cause vision

  • Color vision IS determined only by transmitted or reflected wavelengths.

• Absorption and reflection are vary between 0% to 100%.

• Incident light that is not absorbed will be reflected, transmitted or scattered.

what determine color?

Color is determined by the electromagnetic wavelengths

1. entering the eye and 

2. causing photoreceptor reaction

additive color

color perception caused by wavelengths added together by illuminants that produce electromagnetic radiation of wavelength

subtractive color

Color perception caused by wavelengths being subtracted out by materials that do NOT produce electromagenetic radiation

→ they absorb wavelengths instead and those wavelengths are subtracted out → does not enter the eye

refraction

Concept: 

Shorter wavelengths refract (bend) more 

Longer wavelengths refract (bend) less

when passing into a medium of different refractive index.

This is how a prism splits white light:

a mixture of all sorts of wavelengths between 400-760nm 

into its component colors of the rainbow.

This is why red is always at the top of rainbows (bc of longest wavelength - bend the least)

And blue at the bottom. (shortest wavelength - bend the most)

what happen when light pass thru a refracting instrument?

• any lens system is a 2 prism stacked upside down → can be seen as a refracting instrument

When polychromatic light pass through the eye (or any other lens):

1. Long wavelengths converge farther behind the lens than short wavelengths.

2. Only one wavelength can be brought into focus at any point.

3. All other out-of-focus wavelengths cause blur.

chromatic aberration

the blurring caused by all the wavelengths that does not focus on the retina

Chromatic blur from chromatic aberration is inherent to every single eye. 

There is no way around it. 

the range and intensities of wavelengths entering the eye and causing photoreceptor reaction depends on what?

• the type of illumination

• materials in the environment

• refraction in the anterior segment

• opsin sensitivity

distinction between electromagnetic wavelength and color

• Color is not synonymous w electromagnetic wavelength

• Electromagnetic wavelengths are physically quantifiable 

• Colors are subjective, immaterial, perceptual constructs of our minds

• There is no such thing as color in the world (bc every species/individuals perceive colors differently)

Color vision

• Color vision: ability to make discriminations based solely on spectral content (the chromatic info at different wavelengths)

color attributes

hue: the color — red, orange, yellow

saturation (chroma): describes the amt of colordness vs whiteness (đậm hay lợt)

brightess (value): the relative lightness or darkness of the color (tối hay sáng)

Munsell’s color tree

Problems with the Munsell Tree:

Only describes perception

No relationship to the physical causative agent (electromagnetic radiation)

No way to know how to produce or reproduce a color

• all vision perceptions starts w photoreceptors

• color vision starts w cone activity

What determines the relationship between electromagnetic radiation and subjective brightness of color?

The relationship between:

1. The physical electromagnetic radiation, and 

2. The subjective brightness of color / sensitivity

is given by the luminosity function.

The luminosity function describes spectral sensitivity

of human observers

Does wavelength determine color perception?

Wavelength does not determine color perception.

It is the relative activity of each cone type that determines color.

What affect cone response?

• Cone response is both dependent on wavelength and intensity.

• peak sensitivity of opsin at around 550nm (the most abundant wavelength on earth)

  • in human, 550nm is limey-green

  • also absorb similar wavelength but lesser extents

    

the relationship between light absorption and phototransduction

proportional (tỉ lệ thuận)

1. More light absorbed → more signal.

2. Less light absorbed → less signal.

→ opsins absorption and visual signaling is wavelength dependent.

How does light intensity affect opsins absorption and visual signaling?

1. Less light (e.g. evening / nighttime): less light to absorb → less signal.

2. More light (e.g. noontime): more light to absorb → more signal.

problem w being dependent on both wavelength and intensity

given: photoreceptor activity is 100% at 550nm = 100 photons of a 550nm light are absorbed by opsin

given: photoreceptor activity is 50% for 620nm and 480nm = only 50 photons is absorbed for every 100 photons of light

→ difficult to determine if the activity is from 620nm or 480nm

so if we have 200 photons (increase light intensity), we will see 100 photons absorbed.

→ difficult to determine how many photons are absorbed by each wavelength

• could be absorbed by 550nm, 480nm, or 620nm.

therefore, activity from one type of photoreceptor alone cannot distinguish wavelength from intensity differences!! 

can we distinguish light wavelength and intensity differences based on one type of photoreceptor activity?

no because we simply cannot tell how many photons are absorbed by which wavelength

the principle of univariance

uni-: one  ; -variance: variable

• we cannot tell intensity differences from wavelength differences at night when ONLY rod are active because:

  • rods activity is excited by the amount of photon it absorbs only, so:

    • A short-wavelength (blue) light that causes the receptor to absorb 10 photons will produce the exact same response as a long-wavelength (red) light that also causes the receptor to absorb 10 photons.

👉 Same number of absorbed photons = same response, even if the wavelengths are different. So we cannot tell the wavelength of the light that excite rod cell based on its activity .

dichromatic vision

di-: two , -chromatic: color = two-cone vision

Given 2 cones with different peak sensitivity, at 450nm and 620nm.

1. 100 photons at 480nm will cause:

• 50 photons to be absorbed and 50 units of activity in the 550nm preferring cone

• 90 photons to be absorbed and 90 units of activity in the 450nm preferring cone

2. 100 photons at 620nm will cause:

• 50 photons to be absorbed and 50 units of activity in the 550nm preferring cone

• 0 photons to be absorbed and 0 units of activity in the 450nm preferring cone

therefore, 

We can now disentangle wavelength confusions by looking at the activity of the second cone type. 

Dichromatic vision with intensity light

Given: 200 photons at 480nm:

• 100 photons to be absorbed and 100 units of activity in the 550nm cone 

• 180 photons to be absorbed and 180 units of activity in the 450nm (preferring cone)

For 100 photons at 550nm:

• 100 photons to be absorbed and 100 units of activity in the 550nm preferring cone 

• 0 photons to be absorbed and 0 units of activity in the 450nm (preferring cone)

Therefore, we can also disentangle wavelength + intensity confusions

by looking at the activity of the second cone type.

Limitation of dichromatic vision

Given: 100 photons at 550nm:

• 100 photons to be absorbed and 100 units of activity in the long wavelength cone 

• 0 photons to be absorbed and 0 units of activity in the short wavelength cone

200 photons at 620nm:

• 100 photons to be absorbed and 100 units of activity in the long wavelength cone 

• 0 photons to be absorbed and 0 units of activity in the short wavelength cone

We still cannot distinguish between 550nm and 620nm. 

→ is only helpful when the light has a wavelength that excite both cones

what is the useful range of wavelength for the dichromatic system?

450nm-550nm

wavelengths outside of that range is useless

• animals have difficulty distinguishing between non-nutriotious leaves and nutritious fruit.

the trichromatic system

• evolvement of short wavelength opsin → solution to the dichromatic limitations

• explains why M and L-opsins are genetically, biochemically and functionally very similar while S-opsin is very different

types of opsins

L : long wavelength (peak absorbtion around 550nm)

M : medium wavelength (peak absorbtion around 480nm)

S : short wavelength (peak absorbtion around 420nm)

color vision from the trichromatic system

• the color perceived by the eye is the combination of photoreceptor activity from the 3 cone types

• all different combinations of wavelengths that initiate the same 3 photoreceptor activity will cause the same perception of color (see attachment)

color metamers

Color metamers are different spectral distributions that cause identical color perception.

metameric stimuli

• wavelengths or a mixtue of wavelengths

• appear matched in color

• cannot be distinguished from each other

ex: white color can be from red blue green wavelengths entering the eye, or a multitude of wavelengths from sunlight

genetic deficiencies that affect color vision

• color vision is enabled by 3 genes encoding for the opsins in 3 cones

• defect of these genes will cause vision loss

• gene for S cone is autosomal

• gene for M and L are sex-linked in the X chromosome

→ males (XY) are more prone to color vision defect 

types of inherited color vision deficiencies

• monochromacy: 

  • rare

  • requires 2 genes to be disrupt

  • grayscale vision

• dichromacy:

  • one gene disrupted

  • protanopia: no L-cones (1% males)

  • deuteranopia: no M-cone (1% males)

  • tritanopia: no S-cones (rare)

• anomalous trichromacy

  • impaired color vision

  • 3 cone types present

  • more common than dichromacy

  • protanomaly: 2 M-like cones

  • deuteranomaly: 2 L-like cones

  • tritanomaly: reduced S-cone function 

what does vision allow organisms to do?

The purpose of vision is to enable organisms to 

1. Observe the environment

2. Make appropriate decisions

3. Behave successfully

4. Obtain behavioral feedback through more observations

opsin selection pressure

most abundant wavelength on earth’s surface is about 500-600nm

• animals develop opsins sensitivity to 550nm to maximize daily life efficiency (easy detection, more info about environmental conditions)

why is the most abundant wavelengths at the earth’s surface between 500-600nm?

1. the radiaiotn of spectrum of sunlight

2. reflectance from natural objects on earth (mainly vegetation)

facts

• trichromats: animals benefit from 3 cone type color vision

• priamtes have trichromacy

• majority of diurnal mammals are dichromats (only 2 cone - reduced colot vision)

• monochromats (1 cone type - grayscale vision) is RARE, found in a few nocturnal and marine species

• nocturnals use only rod vision bc only short wavelengths penetrate water

color matching

• combinations of light that appear to be the same color for most people

1. Mathematical formulas derived from empirical observations

2. That explain why certain combinations of wavelengths

3. Cause the same L-, M-S-cone activations 

4. And look the same.

why is color matching useful?

1. Calculate various combinations of wavelengths that will look identical

2. Devise alternative combinations of wavelengths to reproduce the same color in different systems

e.g. yellow light bulb vs yellow on the TV vs yellow on print all appear yellow but are caused by very different combinationsofwavelengths 

clinical application of matching

• nagel anomaloscope — test for anomalous color vision

primary colors

• since we only depend on cone activity, we don’t need all the wavelengths

• most colors derived form wavelength of visible spectrum can be reproduced by adding different amounts of primary colors

• primary colors = 630nm, 532nm, 465nm

• primary colors do not correspond to the peak wavelengths of the L/M/S cones

• primary colors are selected to be primary based on the perceived “pureness” of the hue

  • they mostly stimulate one cone type each

  • mixing them in different amounts can make all other colors we can see

Tristimulus values and color space

• The space of all possible colors that can be displayed

  by mixing RGB defined a color space

• RGB color space — see attachment

characteristics of the CIE 1931 XYZ color space

• the boundary of the color space are:

  • The limits of human hue perception

  • Elicited by monochromatic wavelengths

  • bounded by the white outline

• white color is called equal energy white —- cordinates [0.33,0.33]

color matching

the process where the observers adjust the RBG until it matches the target color

polychromatic colors

• all colors on the line connecting 2 points (colors) on the triangle are produced by mixing those colors

• all colors inside the triangle are produced by mixing all 3 RBG colors

how does the CIE 131 XYZ color space better than the Munsell color tree?

by relating perception to wavelength

how is the dominant wavelength of any color measured?

by :

1. plotting the color coordiantes

2. connecting a straight line from equal energy white thru the color coordinates

the dominant wavelength is given by the intersection of the line and the boundary of the color space

how to calculate saturation?

measure the distance between:

• the white coordinate and the color

• white and the dominant wavelength

• divide 1 by 2

  • 0 = desaturated (grayscale - no color)

  • 1 = saturated, pure color

color gamut

• the range of reproducible colors

• technology cannot display all the colors that humans can perceive

  • i.e. RGB subspace

• manufacturers define their own subspace

• common gamuts are in attachment

MacAdam Ellipses

• he found that some colors are more easily discriminated than others

• The MacAdam Ellipses are regions in the color space perceived to be the same color as the center.

• colors within an ellipse are indistinguishable — metamers

• 

Color discrimination in humans are:

1. Best in blue hues

2. Medium with red hues

3. Worst with green hues 

the amount of ellipses show how many different colors we can distinguish at each region

confusion lines and color blindness

• The CIE 1931 xy color space also indicates colors that appear the same to colorblind people.

• Confusion lines are lines connecting colors that will be mixed-up by a color-blind person.

  • All colors between two confusion lines appear indistinguishable.

  • Co-punctal points are where the confusion lines intersect.

For deutans, the intersection is beyond visibility.

non-spectral colors

these colors do not have a dominant wavelength, can be written as the negative of the complement of the dominant wavelength

color perception organization

organized in three channels :

• Red-green axis

• Blue-yellow axis

• Brightness (not considered further on this lecture) 

color opponency

• it is not possible to perceive reddish green or blueish yellow 

• if the given color is red, and a certain amount of green is added to red, when the amount of these 2 colors are balanced, the red and green will completely dissapear

chromatic valence functions

reveal color opponency in human vision and can be measured with with hue cancellation.

For example, pure red is obtained by cancelling the yellow in 700nm light with blue.