Human Eye & RGB Colour
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This diagram is about the perception of colour. It explores how our eyes respond to wavelengths of light corresponding with red, green and blue.
- As we look at the world on a sunny day a vast range of wavelengths of visible light enter our eyes. Each wavelength corresponds with the perception of a different spectral colour.
- When wavelengths of light corresponding with only red, green and blue enter the eye in different proportions, our eyes are still able to see a full gamut of colours.
- The colours we see when wavelengths corresponding with just red, green and blue light are called RGB colours.
- Because RGB colours are produced by mixing red, green and blue light in different proportions they are not spectral colours.
- The RGB colour model does not correspond directly with the light sensitivity of the three different types of cone cells in the human eye. Cone cells work together and cross-reference the information they receive to deduce colour.
- RGB colours include magenta which is produced by mixing red and blue in different proportions in the absence of green.
Description
Human Eye & RGB Colour
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About the diagram
About the diagram
- This diagram is about the perception of colour. It explores how our eyes respond to wavelengths of light corresponding with red, green and blue.
- As we look at the world on a sunny day a vast range of wavelengths of visible light enters our eyes. Each wavelength corresponds with the perception of a different spectral colour.
- When wavelengths of light corresponding with only red, green and blue enter the eye in different proportions, our eyes are still able to see a full gamut of colours.
- The colours we see when wavelengths corresponding with just red, green and blue light are called RGB colours.
- Because RGB colours are produced by mixing red, green and blue light in different proportions they are not spectral colours.
- The RGB colour model does not correspond directly with the light sensitivity of the three different types of cone cells in the human eye. Cone cells work together and cross-reference the information they receive to deduce colour.
- RGB colours include magenta which is produced by mixing red and blue in different proportions in the absence of green.
Some basic facts:
- Light is electromagnetic radiation (radiant energy), which, detached from its source, is transported by electromagnetic waves (or their quanta, photons) and propagates through space.
- Even if humans had never evolved, electromagnetic radiation would have been emitted by stars since the formation of the first galaxies over 13 billion years ago.
- The experience of colour is a feature of human vision that depends first of all on the construction of our eyes and then on the particular wavelength, frequency and amplitude of visible light that strikes the retina at the back of each eye at any particular moment.
- Light enters the eye through the cornea, through the pupil and then through the lens. The lens shape is changed for near focus and controlled by the ciliary muscles.
- Photons of light falling on the light-sensitive cells of the retina (cones and rods) are converted into electrical signals that are transmitted to the brain by the optic nerve and results in sight and vision.
Now a summary:
Things have colour because light has properties that are visible to the human eye. But what does this mean?
- The colour of objects depends firstly on the light source and the wavelengths it emits.
- The way any object appears to an observer depends on the material it is made from and what happens when light strikes (or is emitted from) its surface.
- Light striking an object may undergo absorption, dispersion, reflection, refraction, scattering or transmission.
- The appearance of an object to an observer also depends on the mental processes that lead to colour perception.
- Spectral colours are produced by a single wavelength of light or by a band of similar wavelengths.
- RGB colour is an additive colour model in which red, green and blue light is added together in various proportions to reproduce a wide range of other colours. The name of the model comes from the initials of the three additive primary colours – red, green, and blue.
Remember that:
When light strikes an object at least one of the following things happens:
- Absorption. When light strikes an opaque medium the wavelengths that are not reflected are absorbed and their energy is converted to heat.
- Dispersion. Chromatic dispersion refers to the way that light separates into its component wavelengths and the colours corresponding with each wavelength become visible.
- Reflection. The term reflection refers to a situation where light strikes the surface of an object and some wavelengths are obstructed and bounce off. If the surface is smooth, light is reflected away at the same angle as it hits the surface. The term reflection refers then to what happens to wavelengths of light that are neither absorbed (by an opaque medium) nor transmitted (through a transparent medium).
- Refraction. The term refraction refers to the way a light wave changes direction and speed as it travels from one medium to another.
- Scattering. Scattering takes place when light waves are reflected in random directions at the boundary between two media. Scattering can also take place when light strikes particles or other irregularities within a medium through which light propagates.
- Transmission. In optics, transmission refers to the passage of electromagnetic radiation through a medium.
Some key terms
A human observer is a person who engages in observation by watching things.
- In the presence of visible light, an observer perceives colour because the retina at the back of the human eye is sensitive to wavelengths of light that fall within the visible part of the electromagnetic spectrum.
- The visual experience of colour is associated with words such as red, blue, yellow, etc.
- The retina’s response to visible light can be fully described in terms of wavelength, frequency and brightness.
- Other properties of the world around us must be inferred from patterns of light.
- To be clear about the RGB colour model it is useful to remember first that:
- The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
- A spectral colour is a colour corresponding with a single wavelength of visible light, or with a narrow band of adjacent wavelengths.
- The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. However, because of the way the eye works, we can see many other colours which are produced by mixing colours from different areas of the spectrum. A particularly useful range of colours is produced by mixing red, green and blue light.
- RGB colour is an entirely different approach to producing and managing colour.
- RGB colour is an additive colour model in which red, green and blue light is combined in various proportions to reproduce a wide range of other colours. The name of the model comes from the initials of the three additive primary colours, red, green, and blue.
- Except for the three primary colours, RGB colours are not spectral colours because they are produced by combining colours from different areas of the visible spectrum.
- RGB colour provides the basis for a wide range of technologies used to reproduce digital colour.
- RGB colour provides the basis for reproducing colour in ways that are well aligned with human perception.
- When an observer has separate controls allowing them to adjust the intensity of overlapping red, green and blue coloured lights they are able to create a match for a very extensive range of colours.
- When looking at any modern display device such as a computer screen, mobile phone or projector we are looking at RGB colour.
- Magenta is an RGB colour for which there is no equivalent spectral colour.
Cone cells, or cones, are a type of neuron (nerve cell) in the retina of the human eye.
- Cone cells are cone-shaped whilst rod cells are rod-shaped.
- Cone cells are responsible for colour vision and function best in bright light, as opposed to rod cells, which work better in dim light.
- Cone cells are most concentrated towards the macula and densely packed in the fovea centralis, but reduce in number towards the periphery of the retina.
- There are believed to be around six million cone cells in the human retina.
Primary colours are a set of colours from which others can be produced by mixing (pigments, dyes etc.) or overlapping (coloured lights).
- The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to the electromagnetic spectrum that results in the perception of colour.
- A set of primary colours is a set of pigmented media or coloured lights that can be combined in varying amounts to produce a wide range of colour.
- This process of combining colours to produce other colours is used in applications intended to cause a human observer to experience a particular range of colours when represented by electronic displays and colour printing.
- Additive and subtractive models have been developed that predict how wavelengths of visible light, pigments and media interact.
- RGB colour is a technology used to reproduce colour in ways that match human perception.
- The primary colours used in colour-spaces such as CIELAB, NCS, Adobe RGB (1998) and sRGB are the result of an extensive investigation of the relationship between visible light and human colour vision.
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