Mohammad Uhayd

Why may it be misleading to say that colour is the perceptual quality associated with the wavelength of light?

INTRODUCTION

Colour is an imperative function of the visual system; colour is the result of how the brain perceives visual information. An objects hue, saturation and brightness can be described using colour. Physics describes colour by its association with electromagnetic radiation, designating colour perceived through the retina with a range of wavelengths. Such electromagnetic radiation constitutes the visible light portion of the electromagnetic spectrum. (Color – The Physics Hypertextbook, 2020)

The key concept involved in the perception of colour is vision, a person can see in low light conditions however, one only sees colour when light reaches the critical intensity. The way in which the brain responds to visual stimuli must also be considered, even under identical conditions, the same object can appear to be different colours to different observers. Therefore, the understanding of the perception of colour depends on a multitude of factors such as vision, light, individual interpretation and an understanding of physics, biology and psychology. (Neuroscience 2nd edition, 2001)

 

An object is seen in colour when certain wavelengths of visible light are reflected off the object,  the reflected light enters the human eye first through the cornea, the outermost area of the eye, the  cornea bends light toward the pupil, which controls the amount of light travelling through the lens.

 

The light falls upon photoreceptors called cones in the retina; signals are sent along the optic nerve to the brain where it is processed by the occipital lobe. Photoreceptors are cells which are sensitive to light, for example cones, which are stimulated in bright environments. (Schwartz and Krantz, 2015.)

 

Research demonstrates that colour forms part of the perceptual quality which is associated with the wavelength of light. However, perceptual quality is beyond just the colours of objects and their associated wavelengths, rather the complexities of the visual system require an empirical approach. This essay aims to investigate what constitutes perceptual quality associated with the wavelength of light and why to say that colour is purely based on wavelength is misleading.

 

DISCUSSION

 

Human visual system

 

Extensive research and theories have been proposed to understand how colour is perceived, such as the theory proposed by physicist Thomas Young which was later developed by Herman Von Helmholtz. The theory closely relates to the widely accepted trichromatic theory. The proposed theory states that the colour of any light is determined by the output of the three cone systems within the retinae.  (Colour - The perception of colour, 2020), (The Early Theory That Explains How We Perceive Color, 2020)

Cones are imperative components of the visual system to be able to see colour, humans have approximately six million cones, each containing ‘photo pigments’ which are colour detecting molecules. Humans typically have three types of cones – red, green, and blue and each type of cone is sensitive to different wavelengths of light. To claim that cones only detect the three colours is misleading as visual experience demonstrates a plethora of colours in all tones and shades. In fact, humans can see up to seven million colours. All wavelengths of visible light together appear to be white, however, if two white circles were presented, a green and red filter applied to either circle, a green and red circle should be visible. This is because the filters only allow the certain wavelength to go through, however, if the two circles are merged the circle would appear to be yellow. But initially yellow was not visible, therefore if the wavelength of yellow is not present, how is the colour yellow visible to the eye? (How Do Humans Perceive Color -- Color Deficiencies, 2020)

In order to understand, an objective approach must be made by assessing the situation mathematically, the wavelength graphs in figure 1 demonstrate the wavelengths of light relative to sensitivity. Line A in figure 1, demonstrates the level of sensitivity for the colour green, when green is visible both red and green cones are responsive. The value for its sensitivity on the Y axis reads between values of 0- 100, with the units being nerve impulse per unit time, the visibility of green gives a value of 80 for cone green and a value of 60 for cone red. Additionally, Line B demonstrates the level of response when red light is visible, however as the red light is only detected by the red cone, there will only be a single value of 20. The red cone gives the sum of the detected values of 60 and 20, giving a total value of 80, this demonstrates the properties of neurones and photoreceptors to be additive.

Therefore, the visibility of red and green results in the brain receiving numerical values of 0,80 and 80, however there is another single wavelength that also gives these values of 0,80 and 80 and that is the wavelength of yellow. Remarkably the brain does not detect whether the values its receiving are wavelengths of yellow or wavelengths of green and red with no yellow at all. This phenomenon produces the colour yellow without a specific yellow wavelength, demonstrating why the initial statement is misleading as the figure below proves that the perceptual quality of colour is not only dependant on wavelengths of light but there is complex interplay within the visual system that affects perceptual quality of colour. Therefore, to say colour is the perceptual quality of wavelength is highly misleading as it can cause one to believe that wavelengths are coloured for example 450 nm light is blue, 520 nm light is green etc, which is incorrect as wavelengths are colourless. (The Perception Of Color, 2020)

 

FIGURE 1: Human colour receptors relative to sensitivity.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Simultaneous colour contrast

Another aspect affecting the perceptual quality of colour is the effect of background on the experience of colour, the background of an object can strongly influence an object’s perceived hue, saturation and brightness. ‘Simultaneous colour contrast’ is a concept describing the effects of background colour on inducing complementary hue into the object, for example a green background can cause the object to appear more red, similarly a blue background can cause the object to appear more yellow. The concept is most substantial when the background is much more saturated and radiant than the object.

 

 

 

 

FIGURE 2: Simultaneous colour contrast

Figure 2 demonstrates the simultaneous colour contrast, the yellow ring appears much redder on the green background than on the red background, exhibiting the induced hue effect. The effect of induced saturation is indicated as the pink ring appears more red in the green background than on the red. Finally, the effect of induced brightness is exhibited as the grey ring appears brighter on the black background than on the white background.  (What Is Simultaneous Contrast in Art?, 2020)

 

Assimilation effects

Furthermore, an additional theory called “assimilation effects”, is a phenomenon in which the perceived colour of a region shifts toward that of its neighbour. In figure 3, the spread of the white bars causes the blue background to appear lighter whereas the spread of the black bars causes the blue background to appear darker. (Assimilation: central and peripheral effects, 2020)

 

 

 

 

 

 

FIGURE 3: Assimilation effects

 

Chromatic adaptation

Chromatic adaptation is described as a dynamic mechanism of the human visual system to compensate for white point changes when viewing an object in different illuminations. The effects constitute the induction of the complementary colour, for example viewing a red field would make a subsequently viewed yellow or white colour to appear greener. Another effect is the reduction in saturation, a red field can cause a pink object to appear whiter, the effect of decreased brightness would cause a subsequently bright object to appear dimmer.

However, a limitation of chromatic adaptation is that highly saturated light can adjust an object’s appearance, suppose there is a red object in white light, the object would still appear to be the same colour in red light because its pigments reflect long wavelengths. However, if the object was illuminated with blue light then it would appear dark grey or even black, this is because the object absorbs short wavelengths and does not reflect them back to the eye, colour constancy can compensate only for moderate variations in white light.  (Chromatic Adaptation, 2020)

Colour constancy

Colour constancy refers to the tendency of objects to remain the same colour even under altering illumination, for instance a yellow lemon will appear yellow whether it’s in tungsten light or in sunlight. Under these situations, the lemon reflects light spectra to the eyes but instead remarkably it’s seen with its unchanging material properties. Colour constancy is not a property of objects but it is in fact a perceptual phenomenon that is a direct result of the mechanisms within the eyes and brain. Colour constancy is a prime example of perceptual constancy where the visual system takes a variable input and converts it into a stable perception. (Foster, 2020)

 

 

 

 

 

 

 

FIGURE 4: Rouen cathedral depictions by Monet under different light conditions.

However, many argue that colour constancy is not perfect, as although the visible colour is the same under different illuminations, the properties of the colour are substantially affected, such as saturation, hue and brightness. The changes in the colour of the illumination could be completely discounted as all objects would be visible as if lit by pure white light.  Claude Monet, a French artist took this idea to the extreme in his Rouen cathedral paintings in which he progressed a golden afternoon to a mauve evening through exaggerating change in colours.

 

 

CONCLUSION

 

In conclusion, it is misleading to say that colour is the perceptual quality associated with wavelength of light. Although it is generally believed that wavelength is the key concept involved in experiencing colour, there are many situations as described in the discussion demonstrating that there is a complex interplay of factors besides wavelength that forms the experience of perceived colour. The idea of colour not being a property  of wavelengths was asserted by physicist Isaac Newton In his statement “The rays to speak properly are not coloured , in them there is nothing else than a certain power and disposition of this or that colour…so colours in the object are nothing but a disposition to reflect this or that sort of rays more copiously than the rest”(Optiks,1704). Newtons theory is that the colours visible in response to different wavelengths are not contained in the light themselves but that rays “stir up a sensation of that colour”, The nature of colour has intrigued philosophers and scientists for many years, history speaks of the Greek philosopher Parmenides around 500 BC who argued that colour is merely a name; a thousand years later the physicist Galileo said that colour “only resides in consciousness”. These ideas reinforce the argument that to argue wavelengths being the perceptual quality associated with wavelength is deceiving as there are many properties that affect colour besides wavelength as well as the well-known fact that wavelengths do not contain any colour but simply according to Newton “stir up a sensation of colour”. (Study.com,2020)

 

 

BIBLIOGRAPHY

 

  1. The Physics Hypertextbook. 2020. Color – The Physics Hypertextbook. [online] Available at: <https://physics.info/color/> [Accessed 10 April 2020].

  2. 2001. Neuroscience 2Nd Edition. Sinauer Associates, Inc.

  3. Schwartz, B. and Krantz, J., 2015. Sensation & Perception.

  4. Encyclopedia Britannica. 2020. Colour - The Perception Of Colour. [online] Available at: <https://www.britannica.com/science/color/The-perception-of-colour> [Accessed 12 April 2020].

  5. Verywell Mind. 2020. The Early Theory That Explains How We Perceive Color. [online] Available at: <https://www.verywellmind.com/what-is-the-trichromatic-theory-of-color-vision-2795831> [Accessed 11 April 2020].

  6. ZME Science. 2020. How Do Humans Perceive Color -- Color Deficiencies. [online] Available at: <https://www.zmescience.com/medicine/color-defficiency/> [Accessed 14 April 2020].

  7. Light-measurement.com. 2020. The Perception Of Color. [online] Available at: <https://light-measurement.com/perception-of-color/> [Accessed 12 April 2020].

  8. LiveAbout. 2020. What Is Simultaneous Contrast In Art?. [online] Available at: <https://www.liveabout.com/definition-of-simultaneous-contrast-2577729> [Accessed 12 April 2020].

  9. Cogprints.org. 2020. Assimilation: Central And Peripheral Effects. [online] Available at: <http://cogprints.org/659/1/bressan5.html> [Accessed 13 April 2020].

  10. Www2.cmp.uea.ac.uk. 2020. Chromatic Adaptation. [online] Available at: <http://www2.cmp.uea.ac.uk/Research/compvis/ChromaticAdaptation/ChromaticAdaptation.htm> [Accessed 13 April 2020].

  11. Foster, D., 2020. Color Constancy.

  12. Study.com. 2020. [online] Available at: <https://study.com/academy/lesson/history-of-color-theory.html> [Accessed 12 April 2020].

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