About the HSB colour model and colour brightness
The HSB colour model is an additive colour model used to mix light. Subtractive colour models are used to mix pigments and inks.
- The only difference between the RGB and HSB colour models is the way colours are represented in terms of colour notation and dealt with in software and apps.
- Both the HSB and RGB colour model deal with how to mix red, green and blue light to produce other colours.
- HSB is popular because it provides an intuitive way to select and adjust colours when using applications such as Adobe Creative Cloud for design, photography or web development.
- The HSB colour model can be used to describe any colour on a TV, computer or phone.
In the HSB colour model:
- Hue refers to the perceived difference between one colour and another and accounts for colour names such as red, yellow, green or blue.
- Hue can be measured as a location on a colour wheel and expressed in degrees between 00 and 2590.
- Saturation refers to the perceived difference between one colour and another in terms of purity.
- Saturation is measured between a fully saturated colour (100%) and an unsaturated colour that appears dull and washed out until all colour disappears leaving only a monochromatic grey tone (0%).
- A fully saturated colour is produced by a single wavelength or a narrow band of wavelengths of light.
- On HSB colour wheels, saturation is usually shown to increase from the centre to the circumference.
- Brightness (colour brightness) refers to the difference between a hue that appears bold and vivid at maximum brightness (100%) and then appears progressively darker in tone until it appears black at minimum brightness(0%).
- Colour brightness is often apparent in the difference between the way a colour appears to an observer in well-lit conditions compared with its subdued appearance when in shadow or when poorly illuminated.
About the HSB colour model and saturation
The HSB colour model is an additive colour model used to mix light. Subtractive colour models are used to mix pigments and inks.
- The only difference between the RGB and HSB colour models is the way colours are represented in terms of colour notation and dealt with in software and apps.
- Both the HSB and RGB colour models deal with how to mix red, green and blue light to produce other colours.
- HSB is popular because it provides an intuitive way to select and adjust colours when using applications such as Adobe Creative Cloud for design, photography or web development.
- The HSB colour model can be used to describe any colour on a TV, computer or phone.
In the HSB colour model:
- Hue refers to the perceived difference between one colour and another and accounts for colour names such as red, yellow, green or blue.
- Hue can be measured as a location on a colour wheel and expressed in degrees between 00 and 2590.
- Saturation refers to the perceived difference between one colour and another in terms of purity.
- Saturation is measured between a fully saturated colour (100%) and an unsaturated colour that appears dull and washed out until all colour disappears leaving only a monochromatic grey tone (0%).
- A fully saturated colour is produced by a single wavelength or a narrow band of wavelengths of light.
- On HSB colour wheels, saturation is usually shown to increase from the centre to the circumference.
- Brightness (colour brightness) refers to the difference between a hue that appears bold and vivid at maximum brightness (100%) and then appears progressively darker in tone until it appears black at minimum brightness(0%).
- Colour brightness is often apparent in the difference between the way a colour appears to an observer in well-lit conditions compared with its subdued appearance when in shadow or when poorly illuminated.
Ganglion cells
Retinal ganglion cells are located near the boundary between the retina and the central chamber containing vitreous humour. They collect and process all the visual information gathered directly or indirectly from the forty-something types of rod, cone, bipolar, horizontal and amacrine cells and, once finished, transmit it via their axons towards higher visual centres within the brain.
The axons of ganglion cells form into the fibres of the optic nerve that synapse at the other end on the lateral geniculate nucleus. Axons take the form of long slender fibre-like projections of the cell body and typically conduct electrical impulses, often called action potentials, away from a neuron.
A single ganglion cell communicates with as few as five photoreceptors in the fovea at the centre of the macula. This produces images containing the maximum possible resolution of detail. At the extreme periphery of the retina, a single ganglion cell receives information from many thousands of photoreceptors.
Around twenty distinguishable functional types of ganglion cells resolve the information received from 120 million rods and cones into one million parallel streams of information about the world surveyed by a human observer in real-time throughout every day of their lives. They function to complete the construction of the foundations of visual experience by the retina, ordering the eyes response to light into the fundamental building blocks of vision. Ganglion cells do the groundwork that enables retinal encodings to ultimately converge into a unified representation of the visual world.
Ganglion cells not only deal with colour information streaming in from rod and cone cells but also with the deductions, inferences, anticipatory functions and modifications suggested by bipolar, amacrine and horizontal cells. Their challenge, therefore, is to enable all this data to converge and to assemble it into high fidelity, redundancy-free, compressed and coded form that can continue to be handled within the available bandwidth and so the data-carrying capacity of the optic nerve.
It is not hard to imagine the kind of challenges they must deal with:
- Information must feed into and support the distinct attributes of visual perception and be available to be resolved within the composition of our immediately present visual impressions whenever needed.
- Information must correspond with the outstanding discriminatory capacities that enable the visual system to operate a palette that can include millions of perceivable variations in colour.
- Information about the outside world must be able to be automatically cross-referenced, highly detailed, specifically relevant, spatial and temporally sequenced and available on demand.
- Information must be subjectively orientated in a way that it is locked at an impeccable level of accurate detail to even our most insane intentions as we leap from rock to rock across a swollen river or dive from an aircraft wearing only a wingsuit and negotiate the topography of a mountainous landscape speeding past at 260km per hour.
It is now known that efficient transmission of colour information is achieved by a transformation of the initial three trivariant colour mechanisms of rods and cones into one achromatic and two chromatic channels. But another processing stage has now been recognised that dynamically readjusts the eye’s trivariant responses to meet criteria of efficient colour information management and to provide us with all the necessary contextualising details as we survey the world around us. Discussion of opponent-processing is dealt with in the next article!
Horizontal cells
Horizontal cells are connected to rod and cone cells by synapses and are classed as laterally interconnecting neurons.
Horizontal cells help to integrate and regulate information received from photoreceptor cells, cleaning up and globally adjusting signals passing through bipolar cells towards the regions containing ganglion cells.
An important function of horizontal cells is enabling the eye to adjust to both bright and dim light conditions. They achieve this by providing feedback to rod and cone photoreceptors about the average level of illumination falling onto specific regions of the retina.
If a scene contains objects that are much brighter than others, then horizontal cells are believed to prevent signals representing the brightest objects from dazzling the retina and degrading the overall quality of information.
Caption
The Neuronal Organization of the Retina Richard H. Masland
https://www.cell.com/neuron/fulltext/S0896-6273(12)00883-5?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0896627312008835%3Fshowall%3Dtrue
Fovea centralis
The entire surface of the retina contains nerve cells, but there is a small portion with a diameter of approximately 0.25 mm at the centre of the macula called the fovea centralis where the concentration of cones is greatest. This region is the optimal location for the formation of image detail. The eyes constantly rotate in their sockets to focus images of objects of interest as precisely as possible at this location.
The term impact parameter refers to a scale used on a ray-tracing diagram to measure the point at which incident rays strike the surface of a raindrop. Rays are given a value between 0.0 and 1.0 depending upon their point of impact.
- For a primary rainbow, all the incident rays of interest strike a raindrop between its horizontal axis (0.0 on the impact parameter scale) and the upper-most point (1.0 on the impact parameter scale). In the second case, the ray grazes the surface at 900 to the normal and continues on its course without deviation.
- For a secondary rainbow, all the incident rays of interest strike a raindrop between its horizontal axis (0.0 on the impact parameter scale) and the lowest point (1.0 on the impact parameter scale). In the second case, the ray grazes the surface at 900 to the normal and continues on its course without deviation.
- An impact parameter is useful because it allows the relationship between equidistant incident rays, the angle at which they strike the surface and their angle of refraction to be plotted.
An idealised raindrop forms a geometrically perfect sphere. Although such a form is one in a million in real-life, simplified geometrical raindrops help to make sense of rainbows and reveal general rules governing why they appear.
The insights that can be gained from exploring the geometry of raindrops apply to every rainbow, whilst the rainbows we come across in everyday life demonstrate that each individual case is unique.
Don’t forget that the idea of light rays is also a way to simplify the behaviour of light:
- The idea that light is made up of rays is so commonplace when describing and explaining rainbows that it is easily taken for granted.
- The idea of light rays is useful when trying to model how light and raindrops produce the rainbow effects seen by an observer.
- Light rays don’t exist in the sense that the term accurately describes a physical property of light. More accurate descriptions use terms like photons or waves.
Basics of raindrop geometry
- A line drawing of a spherical raindrop is the starting point for exploring how raindrops produce rainbows.
- The easiest way to represent a raindrop is as a cross-section that cuts it in half through the middle.
- A dot or small circle can be used to mark the centre whilst the larger circle marks the circumference.
- Marking the centre makes it easy to add lines that show the radius and diameter.
- Marking the centre also makes it easy to add lines that are normal to the circumference.
- A normal (or the normal) refers to a line drawn perpendicular to and intersecting another line, plane or surface.
- A normal is used in a diagram to connect the centre with a point where a ray strikes the circumference.
- The diameter of a circle is a line that passes through its centre and is drawn from the circumference on one side to the other.
- The radius of a circle is a line from the centre to any point on the circumference.
- The horizontal axis of a raindrop is a line drawn through its centre and parallel to incident light. The vertical axis intersects the horizontal at 900 and also passes through the centre point.
- The angle at which incident light strikes the surface of a raindrop can be calculated by drawing a line that shows where an incident ray strikes a droplet and then drawing the normal. The angle of incidence is measured between them.
- The path of light as it strikes the surface and changes direction as it is refracted at the boundary between air and water can be calculated using the Law of Refraction (Snell’s law).
- When light is refracted as it enters a droplet it bends towards the normal.
- The law of reflection can be used to calculate the change of direction each time light reflects off the inside surface of the raindrop.
- When light exits a raindrop the angle of refraction is the same as when it entered but this time bends away from the normal.
There are many optical effects similar to rainbows.
- A fog bow is a similar phenomenon to a rainbow. As its name suggests, it is associated with fog rather than rain. Because of the very small size of water droplets that cause fog, a fog bow has only very weak colours.
- A dew bow can form where dewdrops reflect and disperse sunlight. Dew bows can sometimes be seen on fields in the early morning when the temperature drops below the dew point during the night, moisture in the air condenses, falls to the ground, and covers cobwebs.
- A moon bow is produced by moonlight rather than sunlight but appears for the same reasons. Moon bows are often too faint to excite the colour receptors (cone cells) of a human eye but can appear in photographs taken at night with a long exposure.
- A twinned rainbow is produced when two rain showers with different sized raindrops overlap one another. Each rainbow has red on the outside and violet on the inside. The two bows often intersect at one end.
- A reflection rainbow is produced when strong sunlight reflects off a large lake or the ocean before striking a curtain of rain. The conditions must be ideal if the reflecting water is to act as a mirror. A reflected rainbow appears to be similar to a primary bow but has a higher arc. Don’t get confused between a reflection rainbow that appears in the sky and a rainbow reflected in water.
- A glory is a circle of bright white light that appears around the anti-solar point.
- A halo is a circle of bright multicoloured light caused by ice crystals that appears around the Sun or the Moon.
- A monochrome rainbow only occurs when the Sun is on the horizon. When an observer sees a sunrise or sunset, light is travelling horizontally through the atmosphere for several hundred kilometres. In the process, atmospheric conditions cause all but the longest wavelengths to scatter so the Sun appears to be a diffuse orange/red oval. Because all other wavelengths are absent from a monochrome rainbow, the whole scene may appear to be tinged with a fire-like glow.
A typical atmospheric rainbow includes six bands of colour from red to violet but there are other bands of light present that don’t produce the experience of colour for human observers.
- It is useful to remember that:
- Each band of wavelengths within the electromagnetism spectrum (taken as a whole) is composed of photons that produce different kinds of light.
- Remember that light can be used to mean visible light but can also be used to refer to other areas of the electromagnetism spectrum invisible to the human eye.
- Each band of wavelengths represents a different form of radiant energy with distinct properties.
- The idea of bands of wavelengths is adopted for convenience sake and is a widely understood convention. The entire electromagnetic spectrum is, in practice, composed of a smooth and continuous range of wavelengths (frequencies, energies).
- Radio waves, at the end of the electromagnetic spectrum with the longest wavelengths and the least energy, can penetrate the Earth’s atmosphere and reach the ground but are invisible to human eyes.
- Microwaves have shorter wavelengths than radio waves, can penetrate the Earth’s atmosphere and reach the ground but are invisible to human eyes.
- Longer microwaves (waves with similar lengths to radio waves) pass through the Earth’s atmosphere more easily than the shorter wavelengths nearer the visible parts of spectrum.
- Infra-red is the band closest to visible light but has longer wavelengths. Infra-red radiation can penetrate Earth’s atmosphere but is absorbed by water and carbon dioxide. Infra-red light doesn’t register as a colour to the human eye.
- The human eye responds more strongly to some bands of visible light between red and violet than others.
- Ultra-violet light contains shorter wavelengths than visible light, can penetrate Earth’s atmosphere but is absorbed by ozone. Ultra-violet light doesn’t register as a colour to the human eye.
- Radio, microwaves, infra-red, ultra-violet are all types of non-ionizing radiation, meaning they don’t have enough energy to knock electrons off atoms. Some cause more damage to living cells than others.
- The Earth’s atmosphere is opaque to both X-rays or gamma-rays from the ionosphere downwards.
- X-rays and gamma-rays are both forms of ionising radiation. This means that they are able to remove electrons from atoms to create ions. Ionising radiation can damage living cells.
Remember that:
- All forms of electromagnetic radiation can be thought of in terms of waves and particles.
- All forms of light from radio waves to gamma-rays can be thought to propagate as streams of photons.
- The exact spread of colours seen in a rainbow depends on the complex of wavelengths emitted by the light source and which of those reach an observer.
