A rainbow is an optical effect, a trick of the light, caused by the behaviour of light waves travelling through transparent water droplets towards an observer.
- Sunlight and raindrops are always present when a rainbow appears but without an observer, there is nothing, because eyes are needed to produce the visual experience.
- A rainbow isn’t an object in the sense that we understand physical things in the world around us. A rainbow is simply light caught up in raindrops.
- A rainbow has no fixed location. Where rainbows appear depends on where the observer is standing, the position of the Sun and where rain is falling.
- The exact paths of light through raindrops is so critical to the formation of rainbows that when two observers stand together their rainbows are produced by different sets of raindrops.
This DICTIONARY OF LIGHT, COLOUR & VISION contains a vocabulary of closely interrelated terms that underpin all the resources you will find here at lightcolourvision.org.
- Each term has its own page in the DICTIONARY and starts with a DEFINITION.
- Bullet points follow that provide both context and detail.
- Links embedded in the text throughout the site (highlighted in blue) take you directly to DICTIONARY entries.
- Shorter SUMMARIES of terms appear on DIAGRAM PAGES under the heading SOME KEY TERMS. These entries strip definitions back to basics and can be viewed without leaving the page.
Why a dictionary of light, colour & vision
- One of the practical objectives of this website is to make the connections between the topics of light, colour and vision accessible to students and researchers of all ages.
- Our DICTIONARY aims to avoid a problem faced by websites such as Wikipedia where articles are often composed by contributors with narrow specialisation and their own topic-specific vocabulary.
- The layout of the DICTIONARY also aims to avoid situations where a single unknown word or phrase makes it difficult, if not impossible, for our visitors to find the information they need (as explained below).
Terms, definitions and explanations
- All the terms we have selected for the DICTIONARY are widely used and are applied consistently across the topics of light, colour and vision.
- The aim is to avoid definitions and explanations with different meanings in different fields.
- As far as possible definitions contain no more than two short sentences.
- The explanations that follow each definition are arranged as short bullet points that avoid paragraphs of information completely.
- Each bullet makes a stand-alone point and is intended to deal with a single piece of information that we believe is likely to be important to our readership.
- The writing style across all terms aims to be clear, accessible and engaging.
- The idea is to enable our visitors to find and digest information quickly and to confirm facts one at a time.
- Because our readership and their concerns are diverse, bullet points sometimes provide different perspectives on a single term or topic.
Show me the DICTIONARY OF LIGHT, COLOUR & VISION
Show me the DIAGRAM PAGES
Accommodation
The distance between the retina (the detector) and the cornea (the refractor) is fixed in the human eyeball. The eye must be able to alter the focal length of the lens in order to accurately focus images of both nearby and far away objects on the retinal surface. This is achieved by small muscles that alter the shape of the lens. The distance of objects of interest to an observer varies from infinity to next to nothing but the image distance remains constant.
The ability of the eye to adjust its focal length is known as accommodation. The eye accommodates by assuming a lens shape that has a shorter focal length for nearby objects in which case the ciliary muscles squeeze the lens into a more convex shape. For distant objects, the ciliary muscles relax, and the lens adopts a flatter form with a longer focal length.
Additive colour is shorthand for the additive mixing of wavelengths of light to produce colour. The method involves mixing wavelengths corresponding with primary colours at varying intensities and projecting them onto a surface or screen. When seen by an observer, light enters and stimulates the eyes and, depending on the intensity of the signal on each channel, produces the visual impression of a predicted colour.
- Whilst additive colour mixing is the method used to combine wavelengths of light, subtractive colour mixing is the method used with dyes, inks and pigments.
- An additive approach to colour mixing is used in the case of the emission of light by light-emitting diodes (or similar light sources) embedded into the screens of mobile phones, computers and televisions etc.
- An additive approach to colour mixing is also used with digital projectors. In this case, sufficient light must be produced on each channel to form intense images when focused onto a screen across a room.
- RGB colour is one of the additive colour models that combine wavelengths of light corresponding with red, green and blue primary colours to produce other colours.
- Red, green and blue are called additive primary colours in an RGB colour model because they can be added together to produce other colours. Red green and blue are often described as being components of the resulting colour.
- Different colours are produced by varying the intensity of the component colours between fully off and fully on.
- When fully saturated red, green and blue primary colours are combined, they produce white.
- When any two fully saturated additive primaries are combined, they produce a secondary colour: yellow, cyan and magenta.
- Some RGB colour models can produce over 16 million colours by fine-tuning the intensity of each of the three primary colours.
- The additive RGB colour model cannot be used for mixing different colours of pigments, paints, inks, dyes or powders. To combine these colourants subtractive colour models are used.
About additive and subtractive colour models
There are two main types of colour models, additive and subtractive.
Additive Colour Models
- Additive colour models are used when blending light to produce colour.
- The primary colours for most additive models are red, green, and blue (RGB).
- When combined at full intensity, they produce white light.
- The additive RGB model (and HSB colour model) is central to display technologies such as computer screens, TVs and phone screens.
- The additive spectral colour model is particularly useful for developing an understanding of the relationship between wavelengths of light within the visible spectrum and corresponding colours.
- Additive models are based on the way human eyes perceive colour, with each colour being produced by a combination of different wavelengths. In contrast, a subtractive model is based on the way pigments reflect light.
Subtractive Colour Models
- Subtractive colour models are used when working with pigments, inks and dyes.
- The primary colours for most subtractive colour models are cyan, magenta, and yellow (CMY).
- When combined cyan, magenta, and yellow produce black.
- The subtractive CMY colour model and CMYK colour model are central to printing technologies.
- In practice, the CMY colours often can’t produce a perfect black when mixed due to impurities in the pigments or inks, so a fourth ‘Key’ component (represented as K) is often used in printing to produce a true black.
About additive colour & the RGB colour model
The RGB colour model used by TV, computer and phone screens involves additive colour mixing.
- The RGB colour model produces all the colours seen by an observer on TV, computer and phone screens by creating arrays of red, green and blue pixels (picture elements) in different proportions.
- Red, green and blue are called additive primary colours in an RGB colour model because just these three component colours alone can produce any conceivable colour if blended in the correct proportion.
- Different colours are produced by varying the brightness of the component colours between completely off and fully on.
- When fully saturated red, green and blue primary colours are mixed in equal amounts, they produce white.
- A fully saturated hue is produced by a single wavelength (or narrow band of wavelengths) of light.
- When any two fully saturated additive primary colours are mixed, they produce a secondary colour: yellow, cyan or magenta.
- Some implementations of RGB colour models can produce millions of colours by varying the brightness of each of the three primary colours.
- The additive RGB colour model cannot be used for mixing pigments such as paints, inks, dyes or powders.
- The RGB colour model does not define the exact hue of the three primary colours so the choice of wavelengths for each primary colour is important if it is to be used as part of a colour-managed workflow.
- The RGB colour model can be made device-independent by specifying a colour profile such as sRGB or Adobe RGB (1998) which ensures consistent results regardless of the device used to output an image.
About Adobe RGB, ProPhoto RGB & sRGB
The most common colour profiles in photography are sRGB, Adobe RGB (1998), and ProPhoto RGB.
- Adobe RGB, developed in 1998, consists of the same red green blue colours as sRGB but the colour space has a larger gamut.
- It was developed to communicate with standard CMYK multi-function and inkjet printers and is commonly used for printing on fine art papers.
- When the RGB colour model is used on a modern computer screen, the Adobe RGB (1998) colour space aims to reproduce roughly 50% of the range of colours that an observer is capable of seeing in ideal conditions.
- The Adobe RGB (1998) colour space was developed to improve on the gamut of colours that could be produced by the earlier sRGB colour space, primarily in the reproduction of cyan-green hues.
- sRGB stands for standard red green blue and has the smallest colour space.
- It was developed by HP and Microsoft in 1996 for use with monitors, printers, and the World Wide Web.
- It is the most commonly used colour profile today because of its consistent reproduction of colours across different platforms.
- ProPhoto RGB has the largest colour space with a gamut that covers a significant part of the perceptual colour space of the human eye.
- ProPhoto RGB is used in high-end photography and editing workflows to preserve a wider range of colours and maintain the quality of the original image during processing.
About amacrine cell functions
Amacrine cells are a type of neuron found in the retina, the light-sensitive tissue lining the back of the human eye. They play a critical role in the processing of visual signals before these signals are sent to the brain.
Amacrine cells are known to contribute to narrowly task-specific visual functions such as:
- Spatial Contrast Enhancement: Amacrine cells contribute to a process called lateral inhibition, which helps to enhance the contrast between light and dark areas in a visual scene, thereby improving our ability to see edges and borders.
- Temporal Contrast Enhancement: Amacrine cells play a role in detecting changes in light intensity over time, which helps us to perceive motion and changes in a visual scene.
- Direction Selectivity: Certain types of amacrine cells are involved in detecting the direction of moving objects. These are known as starburst amacrine cells.
- Centre-surround antagonism: Amacrine cells interact with both bipolar cells and retinal ganglion cells to contribute to the centre-surround antagonistic structure of ganglion cell receptive fields.
- Complex Visual Processing: Amacrine cells form connections with multiple types of retinal cells, including bipolar cells and ganglion cells. This allows them to participate in complex processing and integration of visual information.
- Inhibitory Signalling: Many amacrine cells arstae inhibitory interneurons, which means they can inhibit the activity of other neurons. This inhibitory function plays a role in shaping the output of retinal ganglion cells, which send visual information to the brain.
- Regulation of Circadian Rhythm: Some amacrine cells release a pigment called melanopsin and are involved in non-image-forming visual functions, such as the regulation of circadian rhythms and the pupillary light reflex.
- Neurotransmitter Release: Amacrine cells can release a variety of neurotransmitters, including GABA, glycine, dopamine, and others, allowing them to modulate the activity of various neural circuits in the retina.
Amacrine cells
Amacrine cells interact with bipolar cells and/or ganglion cells. They are a type of interneuron that monitor and augment the stream of data through bipolar cells and also control and refine the response of ganglion cells and their subtypes.
Amacrine cells are in a central but inaccessible region of the retinal circuitry. Most are without tale-like axons. Whilst they clearly have multiple connections to other neurons around them, their precise inputs and outputs are difficult to trace. They are driven by and send feedback to the bipolar cells but also synapse on ganglion cells, and with each other.
Amacrine cells are known to serve narrowly task-specific visual functions including:
- Efficient transmission of high-fidelity visual information with a good signal-to-noise ratio.
- Maintaining the circadian rhythm, so keeping our lives tuned to the cycles of day and night and helping to govern our lives throughout the year.
- Measuring the difference between the response of specific photoreceptors compared with surrounding cells (centre-surround antagonism) which enables edge detection and contrast enhancement.
- Object motion detection which provides an ability to distinguish between the true motion of an object across the field of view and the motion of our eyes.
Centre-surround antagonism refers to the way retinal neurons organize their receptive fields. The centre component is primed to measure the sum-total of signals received from a small number of cones directly connected to a bipolar cell. The surround component is primed to measure the sum of signals received from a much larger number of cones around the centre point. The two signals are then compared to find the degree to which they agree or disagree.