The Human Eye and RGB Colour

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To find out more about the diagram above . . . . read on!

The Human Eye and RGB Colour

Look carefully at the diagram at the top of the page. Now check out the following questions (and answers)!

  1. What are spectral colours?
  2. What are RGB colours?
  3. What is a cone cell?
  4. Does the peak response of cone cells in the human eye correspond directly with RGB?
  5. Why do objects appear to be different colours to an observer?

About the Diagram

Introducing the diagram! Read back and forward between the image at the top of the page and the explanation below!

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.

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 muscle.
  • 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 interpreted as 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.

Follow the blue links for definitions . . . . or check the summaries of key terms below!

Some Key Terms

Move to the next level! Check out the following terms.

Electromagnetic spectrum

The electromagnetic spectrum includes electromagnetic waves of all possible wavelengths of electromagnetic radiation, ranging from low energy radio waves through visible light to high energy gamma rays.

Rainbow

rainbow is an optical phenomenon produced by illuminated droplets of water. Rainbows are caused by reflectionrefraction and dispersion of light in individual droplets and results in the appearance of an arc of spectral colours.

  • Rainbows can be produced by meteorological phenomena, waterfalls, lawn sprinklers and other things that create a fine mist of water.
  • A rainbow is formed from millions of individual raindrops each of which reflects and refracts a tiny image of the sun towards the observer.
  • It is the dispersion of light as refraction takes place that produces the rainbow colours seen by an observer.
  • If the sun is behind an observer then the rainbow will appear in front of them.

https://en.wikipedia.org/wiki/Rainbow

Visible light

Visible light is a form of electromagnetic radiation.

  • Visible light is the range of wavelengths of electromagnetic radiation that are perceived as colour by a human observer.
  • The range of wavelengths that produce visible light is a very small part of the electromagnetic spectrum.
  • Other forms of electromagnetic radiation include radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays.
  • Visible light is perceived by a human observer as all the spectral colours between red and violet plus all the other colours that result from mixing them together in different proportions.
  • The complete range of colours that can be perceived by a human observer is called the visible spectrum.

Visible spectrum

The visible part of the electromagnetic spectrum is called the visible spectrum.

  • The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
  • As light travels through the air it is invisible to our eyes.
  • Human beings don’t see wavelengths of light, but they do see the spectral colours that correspond with each wavelength and colours produced when different wavelengths are combined.
  • The visible spectrum includes all the spectral colours between red and violet and each is produced by a single wavelength.
  • The visible spectrum is often divided into named colours, though any division of this kind is somewhat arbitrary.
  • Traditional colours referred to in English include red, orange, yellow, green, blue, and violet.

Wavelength

Wavelength is a measurement from any point on the path of a wave to the same point on its next oscillation. The measurement is made parallel to the centre-line of the wave.

More Information

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Slides

Slides available for download from lightcolourvision.org share common specifications:
  • Titles: All slides has titles.
  • Backgrounds: Black, framed with a violet gradient.
  • Size: 1686 x 1124 pixels (3:2 aspect ratio).
  • Slides are available in three file formats: SVG, PNG, AI.

Slides (black backgrounds) are optimized for viewing on screen or with a projector.

Diagrams (white/transparent backgrounds) are optimized for printing on A4 pages in portrait format.

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Diagrams

Diagrams available for download from lightcolourvision.org share common specifications:
  • Titles: No titles.
  • Backgrounsd SVG: White.
  • Backgrounsd PNG: Transparent.
  • Backgrounds AI: Fully editable.
  • Size: 1686 pixels wide. So all diagrams reproduce at the same scale when inserted into Word documents etc.
  • Labels: Calibri 24pt Italic.
  • Diagrams are available in three file formats: SVG, PNG, AI.

Diagrams (white/transparent backgrounds) are optimized for printing on A4 pages in portrait format.

Slides (black backgrounds) are optimized for viewing on screen or using a projector.

FILE TYPES - SVG

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Download the image at the top of this page as an SVG file:
  • All images available for download at lightcolourvision.org are 1686px wide.
  • Text on SVG images with black backgrounds has been converted from fonts to outlines.
  • Text on SVG images with white (or transparent) backgrounds use Calibri 24pt Italic.

Did you know:

  • SVG (.svg) stands for Scalable Vector Graphic.
  • SVG files scale up or down without loss of definition because they are like line drawings.
  • SVG files are much smaller in size than most other file formats (e.g JPEG or PNG).
  • You can insert SVG files directly into MS Word 365 or PowerPoint 365.
  • You can view SVG files by dragging the file into a browser tab.
  • You can edit SVG files using programs such as Adobe Illustrator.

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Download the image at the top of this page as a PNG file:
  • All images available for download at lightcolourvision.org are 1686px wide.
  • Because all images are a standard width they reproduce consistently on an A4 page.

Did you know:

  • PNG (.png) stands for Portable Network Graphics.
  • PNG files are raster files as are JPG’s (.jpeg) files, so they behave like photos.
  • Our PNG’s are 32bit files.
  • You can insert or paste PNG’s directly into MS Word 365 or PowerPoint 365.
  • You can view PNG files by dragging them into a browser tab.
  • You can edit PNG files using programs such as Adobe Photoshop.

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  • All images available for download at lightcolourvision.org are 1686px wide.

Did you know:

  • AI (.ai) files downloaded from lightcolourvision.org can be re-edited using Adobe Illustrator.
  • Adobe Illustrator can save and export AI files to other formats including PDF (.pdf), PNG (.png), JPG (.jpeg) and SVG(.svg.

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