Linear Visible Spectrum
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Colour Wave Length Frequency Photon Energy
Violet 380 – 450 nm 668 – 789 THz 2.75 – 3.26 eV
Blue 450 – 495 nm 606 – 668 THz 2.50 – 2.75 eV
Green 495 – 570 nm 526 – 606 THz 2.17 – 2.50 eV
Yellow 570 – 590 nm 508 – 526 THz 2.10 – 2.17 eV
Orange 590 – 620 nm 484 – 508 THz 2.00 – 2.10 eV
Red 620 – 750 nm 400 – 484 THz 1.65 – 2.00 eV

Atmospheric opacity (opposite of transmittance) to various wavelengths of electromagnetic radiation, including visible light.

Spectroscopy is the study of objects based on the spectrum of colour they emit, absorb or reflect.

Colour Spectrum
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Approximation of spectral colours on a display, results in somewhat distorted chromaticity

Rendered Spectrum
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A rendering of the visible spectrum on a gray background produces non-spectral mixtures of pure spectrum with gray, which fit into the sRGB colour space.

For colour-accurate reproduction, a spectrum can be projected onto a uniform gray field. Consequent mixed colours can have all their R, G, & B coordinates non-negative, and can be reproduced without distortion. This accurately simulates looking at a spectrum on a gray background.

High-energy Visible Light

Here’s where biology becomes relevant, or maybe it’s the psychology, or perhaps it’s both. The eye is very similar to a camera, but the brain is not like a video recorder. The brain is not like a computer with fixed hardware or transistors and capacitors executing some sort of software code. The neurons of the brain are probably best described as wetware — a fusion of hardware and software (or perhaps something completely different). I don’t feel qualified to say much about this part of the process. Once the visual information leaves the eye, basic physics ends and neuro-cognition takes over.

Colour is determined first by frequency. Let’s start by determining what a typical person would see when looking at electromagnetic radiation of a single frequency. Physicists call this monochromatic light. (It literally means “single colour”, but the actual meaning is “single frequency”). Low frequency radiation is invisible. With an adequately bright source, starting somewhere around 400 THz (1 THz = 1012 Hz), most humans begin to perceive a dull red. As the frequency is increased, the perceived colour gradually changes from red to orange to yellow to green to blue to violet. The eye doesn’t perceive violet so well. It always seems to look dark compared to other sources at equal intensity. Somewhere between 700 THz and 800 THz the world goes dark again.

By defining a colour space, colours can be identified numerically by their coordinates. These physical or physiological quantifications of colour, however, do not fully explain the psycho-physical perception of colour appearance.

Colours of the Visible Light Spectrum

Colour Wave Length Interval Frequency Interval
Red 700 – 635 nm 430 – 480 THz
Orange 635 – 590 nm 480 – 510 THz
Yellow 590 – 560 nm 510 – 540 THz
Green 560 – 520 nm 540 – 580 THz
Cyan 520 – 490 nm 580 – 610 THz
Blue 490 – 450 nm 610 – 670 THz
Violet 450 – 400 nm 670 – 750 THz

Colour, Wave Length, Frequency and Energy of light

Colour (nm) (THz) (μm−1) (eV) (kJ mol−1)
Infrared > 1 000 < 300 < 1.00 < 1.24 < 120
Red 700 428 1.43 1.77 171
Orange 620 484 1.61 2.00 193
Yellow 580 517 1.72 2.14 206
Green 530 566 1.89 2.34 226
Blue 470 638 2.13 2.64 254
Violet 420 714 2.38 2.95 285
Near ultraviolet 300 1 000 3.33 4.15 400
Far ultraviolet < 200 > 1500 > 5.00 > 6.20 > 598

Electromagnetic radiation is characterised by its wavelength (or frequency) and its intensity. When the wavelength is within the visible spectrum (the range of wavelengths the human eye can see, approximately from 390 nm to 700 nm), is known as “visible light”.

Most light sources emit light at many different wave lengths. A source’s spectrum is a distribution giving its intensity at each wavelength, even though the spectrum of light reaching the eye from a given direction determines the colour sensation in that direction.

The intensity of a spectral colour, relative to the context in which it is viewed, may alter its perception considerably. For example, a low-intensity orange-yellow is brown, and a low-intensity yellow-green is olive-green.

If objects scatter all wavelengths with roughly equal strength, appear white. If they absorb all wavelengths, they appear black.

Colour in the Eye

Eye Cones
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Normalised typical human cone cell responses (S, M, and L types) to monochromatic spectral stimuli.

The ability of the human eye to distinguish colours is based upon the varying sensitivity of different cells in the retina to light of different wavelengths. Humans being trichromatic, the retina contains three types of colour receptor cells, or cones. One type, relatively distinct from the other two, is most responsive to light that we perceive as blue or blue-violet, with wavelengths around 450 nm. Cones of this type are sometimes called short-wavelength cones, S cones, or blue cones. The other two types are closely related genetically and chemically: middle-wavelength cones, M cones, or green cones. These cones are most sensitive to light perceived as green, with wavelengths around 540 nm, while the long-wavelength cones, L cones, or red cones, are most sensitive to light we perceive as greenish yellow, with wavelengths around 570  nm.

Behavioural and functional neuro-imaging experiments have demonstrated that colour experiences lead to changes in behavioural tasks and lead to increased activation of brain regions involved in colour perception, thus demonstrating their reality, and similarity to real colour perception, albeit evoked through a non-standard route. As many as half of all women are retinal tetrachromats (having four pigments in cone cells in the retina, compared to three in trichromats) and functional tetrachromacy (having the ability to make enhanced colour discriminations based on that retinal difference). This means that 50% of women in general are more objective to colour shade differences.

Behavioural and functional neuro-imaging experiments have demonstrated that these colour experiences lead to changes in behavioural tasks and lead to increased activation of brain regions involved in colour perception, thus demonstrating their reality, and similarity to real colour perceptions, albeit evoked through a non-standard route or type or synethesia.

Although Aristotle and other ancient scientists already researched the nature of light and colour vision, it was Newton that discovered that light was identified as the source of the colour sensation. In 1810 Goethe published his comprehensive Theory of Colours in which he ascribed physiological effects to colour that are now understood as psychological.

A colour reproduction system “tuned” to a human with normal colour vision may present inaccurate results to other observers. Shades of Soul proves to create a more reliant/relevant guideline by using Human Eye Colour being a constant factor. People’s colour perception may differ but by looking at eye colour that already matches the *Harmonising, *Contrast and *Intente Shades, (without realising the frequencies) it suggests the best colour options to each person.

The different colour response of different devices could be a challenge if not properly managed. For colour information stored and transferred in digital form, colour management techniques, such as those based on ICC profiles (International Colour Constance), can help to avoid distortions of the reproduced colours. Colour management does not circumvent the gamut limitations of particular output devices, but can assist in finding good mapping of input colours into the gamut that can be reproduced.

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