Cones and colour definicies

Single Cone Sensitivity

• Imagine you have only one type of cone in your eye.

• This cone is most sensitive to 550 nm light (which is green).

• It also responds to other wavelengths (like red), but less strongly.

If we had one cone that is sensitive to green only

• If 1000 photons of green light (550 nm) hit the cone:
  → Cone responds strongly = 100 response intensity

• Since the cone is most sensitive to green, you get a big response.

If we had one cone that is sensitive to green only if the cone perceives a red dress instead

• If 1000 photons of red light (590 nm) hit the cone:
  → Cone responds weakly = 50 response intensity

• The cone is less sensitive to red, so the response is weaker.

What if we increased the brightness of the red light

• If you increase the brightness of the red light (e.g., 2000 photons):
  → Cone response increases to 100 (same as dim green).

• Now dim green and bright red give the same response → confusing!

In this case we now have the same amount of activation coming from the brightred and dim green

Principle of univariance

• A photoreceptor can only signal the amount of light absorbed, not the wavelength. Once a photon is absorbed, the receptor responds the same way regardless of the light's wavelength.

Monochromat

• A monochromat has only one functioning cone type — or none at all.

• They need only one wavelength to match any perceived color.

How Monochromats Perceive Color

• Monochromats can’t tell the difference between colors.

• They can only tell the difference based on intensity (brightness).

• Example: A bright red and a dim green may look the same because the response intensity is similar.

• its kind of like our dark adapted vision where at night we cant see differences between colours, but we can see what is more intense. 

Types of Monochromacy

Green Cone Monochromats – Only green cones work.
Rod Monochromats – No functioning cones, only rods work → Total color blindness.

Cones in seals

• Some whales and seals lack S cones (blue) and L cones (red).

• They are likely green cone monochromats — they only have functional green cones.

What if we only had two cone types?

ex)

• Cone 1 – Most sensitive to 550 nm (green).

• Cone 2 – Most sensitive to shorter wavelengths (like blue).

• This allows the brain to compare the ratio of activation between the two cones.

What if we showed the person with 2 cones a green light?

• If green light (550 nm) hits the cones:

  • Cone 1 (green) responds strongly.

  • Cone 2 responds weakly (because it’s less sensitive to longer green wavelengths).

What if we showed the person with 2 cones a red light?

• Cone 1 responds moderately.

• Cone 2 responds weakly (since red is even farther from its peak sensitivity).

→ Cone 1 = 50
→ Cone 2 = 10
→ Ratio = 50:10 (or 5:1)

What if we showed the person with 2 cones a brighter red light?

If you double the intensity of red light (2000 photons):

• Cone 1 = 100

• Cone 2 = 20

• Ratio = 100:20 → Still 5:1

• Ratio stays the same even if brightness increases!

Why Ratio Solves the Problem of only having two cones

• The brain uses the ratio between Cone 1 and Cone 2’s activation to tell color apart.

• Even if brightness changes, the ratio remains stable — so the brain can detect a difference in color rather than just intensity.

person with 1 cone only vs 2 cones only

• With one cone → Can't tell the difference between dim green and bright red.

• With two cones → The ratio between cone responses solves the ambiguity and allows color vision!

Activation Problem with One Cone

• A single cone’s activation depends on:
  -The wavelength of the light (color)
  - The intensity of the light (brightness)

• This means that two different wavelengths (colors) can cause the same response if the intensity is adjusted — creating ambiguity.

Having two cones are sufficient for solving this problem.

Why Three Cones Improve our perception better

• Two cones give basic color vision, but some colors may still look similar.

• Three types of cones allow for finer color distinctions:
  Short (S) – sensitive to blue (~420 nm)
  Medium (M) – sensitive to green (~530 nm)
  Long (L) – sensitive to red (~560 nm)

Trichromats

normal colour humans

Dichromat

two functioning cone types

• they only need two wavelengths of light to match any perceived colour. (if they were to do the colour beam experiment)

3 types of dichromats

1. Protanopia

2. Deutranopia

3. Tritanopia

Are males or females more liekly to be a dichromat

males (its x-linked)

How do we know what colours look like for people who are colourblind?

we can test people who are unilateral dichromats.

Unilateral dichromat

trichomatic vision in one eye and dichromatic in other

Dichromatism - Protanopia

• Loss of detail for long wavelengths. 

• cant see red properly

• neutral point=492

dichromats have neutral points

• where colours start to look grey (because they are missing that wave) 

Deuteranopia

Missing medium wavelength

• cant see green properly

• neutral point: 498

Tritanopia

• missing short-wavelength pigment

• neutral point → 570

• most rare and higher neutral point

Methods for diagnosing colour definciency

1. Colour-matching Procedure

2. Colour vision test

Colour-matching Procedure

determine minimum number of wavelengths needed to match any other wavelength (beams example)

Colour vision test

1. Farnsworth panel d-15 test

2. Ishihara plates

Farnsworth panel d-15 test

start with one colour and task is to arrange them into rainbow order

Someone who is missing a long wavelength cone can still divide between warm and cool colours but the details between each individual color is lost - we no longer have the clear gradation between blue to yellow. 

Ishihara plates

They're used to diagnose red-green color blindness by checking whether a person can distinguish the numbers or shapes.

• first is normal, second is someone with red-green colour blindness.