Task 3 Flashcards
What colours do we perceive?
-Unrelated/Related colour
- Unrelated color: color you can receive without any additional color (red, blue, green)
- Related color: when you have a grey room, you cannot receive brown because they are related to each other
- Achromatic / Chromatic colours
- Selective Transmission
• Selective reflection (some wavelengths more reflected more than others)
- chromatic colors/hues (blue, green, red)
- achromatic colors like white, grey, black when light is reflected equally across spectrum
• the color of things that are transparent, such as liquids, plastics, glasses is created by selective transmission (only some wavelength pass through the objects or substance)
Colour Mixing
- Subtractive
- Additive
• When mixing paints, parts of the spectrum are absorbed and reflected by one colour and other parts are reflected and absorbed by the other colour those wavelengths that are reflected from both of them (“mixture”) are highlighted
Subtractive colour mixture = the creation of colours that occurs when paints of different colours are mixed together, both paints still absorb the same wavelengths they absorbed so only absorbed wavelength they both have in common middle wavelength in common so green
Additive colour mixture = the creation of colours that occurs when lights of different colours are superimposed
–> colours occur when we mix colours associated with which wavelengths are reflected into the eye. Mixing lights causes more wavelengths to be reflected (each light adds wavelengths to the mixture); mixing paints causes fewer wavelengths to be reflected (each paint subtracts wavelengths from the mixture).
Perceptial Dimension of colour
- Spectral /NS
- Hues
- Satuation
- Value
- Color solid
- Spectral colors: colors that appear in the visible spectrum (only 6-7 colors)
- No spectral colors: colors that do not appear in the spectrum because they are mixed colors (magenta Is mixed with blue and red)
- Three perceptual dimensions of color:
- Hues: dominant wavelength experience of chromatic color (green, red, yellow, blue) or combinations (length of wave)
- Saturation: determined by amount of white that has been added to a particular hue e.g. create more colors by changing intensity of light to make colors brighter, dimmer or by adding white to change color´s saturation (adding white to deep red makes it become pink so less saturated (desaturated from red)
- Value: light to dark dimension of color
- Color solid: systematically arrangement of colors within a three- dimensional color space
Trichomatic Theory of Color Vision:
- Color vision depends on ratio of activity in three different receptor mechanisms with different spectral sensitivities (red, green, blue); perception of color determined by pattern of activity in different kinds of receptors
- light of particular wavelength stimulates each receptor mechanisms to different degrees –>activity results in the perception of color, each wavelength is represented in the nervous system by its own pattern of activity in the three receptor mechanisms
TT
- Cone Pigments
- Population Coding
• Three different cone pigments differ in the photopigment they carry
o Short- wavelength pigment: max. absorption 419 nm “blue cone”
o Middle- wavelength pigment: max absorption 531 nm “green cone”
o Long- wavelength pigment: max. absorption 558 nm “red cone”
• All visual pigments made up of large protein component called opsin and small light sensitive component called retinal differences in structure of long opsin part of the pigments responsible for the three different absorption spectra
• Population Coding: multiple cones get input and together that gets coded into a color
Cone Responding
-Metamerism
• Relationship between responses of three kinds of receptors and perception of color
• Different firing patterns to different colors
• Blue signaled by large response in the S Receptor Smaller response in the M receptor and even smaller in the L receptor
• White: equally large in all receptors, yellow in small response in S receptor and large, equal responses in M and L receptors
• Helps to predict which colors should result when combining light of different colors
• Blue light high activity in the S receptors, yellow light high activity in M and L receptors combining all three receptors equally: white
• Metamerism: lights in two fields are physically different (contain different wavelengths) but perceptually identical (look the same), two physically different stimuli are perceptually identical
o Metamers: two lights have different wavelengths distributions but are perceptually the same color
o Result in the same pattern in three cone receptors e.g. proportion of 620 nm red light (large response in L receptors), 530 nm green light (large response in M receptor) adjusted to mixture matches the color of 580 nm light is yellow (M and L receptors)the two-mixed wavelength create same pattern of activity in the cone receptors as single 580 nm light
Vision with one Receptor Cone
-Principle of Univariance
Light can also be described as small packets of energy called photons with light energy. Visual pigment molecule isomerizes when molecule absorbs one photon of light activates molecule that triggers the process that activates visual receptor and leads to seeing light
If intensity of light e.g. 480 nm light isomerizes 100 visual pigment molecules and 600 nm light isomerizes 50 molecules one is twice as many and should cause larger response in receptor resulting in brighter light perception
BUT if both 600 nm and 480 nm are same isomerized the lights will appear identical
Principle of univariance = Once a photon of light is absorbed by a visual pigment molecule, the identity of the light’s wavelength is lost. This means that the receptor does not know the wavelength of the light that is absorbed, only the total amount of light it has absorbed.
expresses the fact that a single cone in isolation cannot determine colour! Two or more adjacent cones having different response curves are needed to distinguish colour from intensity
Vision with Two Receptor Types
- Adding second pigment can distinguish between wavelength independent of light intensity
- One pigment absorbs more 600 nm light than 480 nm light so intensity that causes pigment 1 to generate the same response to the wavelength causes pigment 2 to generate much larger response to the 600 nm light (480 nm larger response in p1 than smaller in p2 and 600 nm smaller response in p1 than in p2) ->response by ratio information, both pigment together: two different wavelength detections
Colour in Cortex
- Single Opponent neurons
- Double opponent neurons
- found in intertemporal cortex
o Single-opponent neurons =M+L neurons, neurons that increase firing to medium wavelengths presented in the centre of the receptive field and decrease firing to long wavelengths presented in the surround (or vice versa) LGN, Retina, Visual Cortex see one colour
–>important for perceiving the colour whithin regions
o Double-opponent neurons = Neurons that have receptive fields with side-by-side regions, respond best to medium WL in which stimulation of one part of the receptive field causes an excitatory response to wavelengths in one area of the spectrum and an inhibitory response to wavelengths in another area of the spectrum, and stimulation of an adjacent part of the receptive field causes the opposite response Visual Cortex, more complicated information perceiving boundaries, see two colour can see the edge between the yellow and the black like a contrast
Colour Constancy
- Colour constancy = we perceive the colour of objects as being relatively constant even under changing illumination
- Interaction between illumination (sunlight, lightbulbs) and reflection properties of an object (green
Colour Constancy
-Thungsten
WL distribution; bulb contains more energy at long WL, (sunlight contains equal amounts of energy at all WL (white light characteristic))
sweater reflects longer WL when illuminated by tungsten light than when illuminated by sunlight
although the WL composition differs is color constancy
Different Mechanisms how the eye´s sensitivity is affected by color of the illumination
• Colour perception can be changed by chromatic adaption (=prolonged exposure to chromatic colours) e.g. adaption to red light selectively bleaches your long WL cone pigmentdecreases sensitivity to red light see the reds and oranges viewed with left, adapted eye as less saturated and bright than those with the right eye
creates partial colour constancy (= the perception of the object is shifted after adaption but not as much as when there was no adaption) the eye can adjust its sensitivity to different wavelengths to keep colour perception approximately constant as illumination changes e.g. room with tungsten yellowish light, eyes adapt to the long WL rich light, decreases sensitivity to long WL long WL light reflected from object less effect than before adaption, so only small effect on your perception of colour or different colours in different seasons e.g. lush in summer with a lot of green and arid in winter with more yellowsdecrease perception of yellow in the arid scene
Effect of Surroundings
• An object’s perceived colour is also affected by the object’s surrounding e.g. green area perceived to be slightly more yellow under tungsten light than in daylight (colour constancy works less if object is in isolation but works best if object is surrounded with many different colours)
Lightness Constancy
• seeing whites, greys, blacks as staying about the same shade under different illuminations
• depends on
1) illumination (total amount of light that is striking the objects surface and
2) reflectance (proportion of light that objects reflects into eyes) e.g. object looks
black if 5% light reflected, grey 10 to 70% of light, white 80-95% reflectance
Intensity Relationship and Ratio Principle
- illumination is the same over whole object, lightness is determined by ratio of reflectance of object on reflectance of surrounding objects e.g. ratio of black square surrounding white squares is 9/90=0,1 under low illuminations 900/9000=0,1 under high illuminations
- as long as this ratio remains the same, perceived lightness will remain the same
