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Basics of lightingIn the second oldest form of electric lighting – the incandescent lamp – an electric current passes
through a thin high-resistance wire, nowadays always of tungsten, to heat it to incandescence. Over
time, evaporation of tungsten atoms from the filament blackens the inside of the bulb and makes
the filament thinner until it eventually breaks at its thinnest point, ending the life of the lamp.
In a gas discharge lamp, an electric current passes through a gas between two electrodes at the
opposite ends of a closed glass tube. Collisions between free electrons and the gas atoms excite the
gas atoms into higher energy levels.
In a low-pressure sodium lamp, visible radiation is directly produced by the discharge of sodium.
High-pressure sodium lamps operate at much higher gas pressures, resulting in more inter-atom
interactions than with low-pressure lamps, leading to a broadening of the emitted radiation pattern.
The (compact) fluorescent lamp is basically a low-pressure mercury gas discharge lamp with the
inner surface of the discharge tube coated with a mixture of fluorescent compounds — called
phosphors — that convert the invisible ultraviolet radiation emitted by the mercury discharge into
visible radiation.
High-pressure mercury lamps contain mercury vapour confined in a quartz discharge tube (called:
burner) that operate at a pressure between 200 and 1500 kPa, at which pressure the discharge
process is found to emit a large proportion of its energy in the visible part of the spectrum.
Metal halide lamps have been developed from high-pressure mercury lamps by adding other metals
in the form of halide salt to the discharge.
A more recent development is the ceramic metal halide lamp that features a discharge tube made of
ceramic material instead of quartz glass.
The discharge of a solid-state lamp happens in a solid state material: orbit changing electrons cause
atoms to get ‘excited’ that subsequently fall back to their natural state thereby releasing its surplus
energy in the form of radiation.
Contrasting colours also have a mutual influence on each other. The general effect is that under the
influence of a surface of strongly saturated colour, other surfaces will take on a hue of the
complement of that colour.
As a general rule in interior situations, it can be taken that for satisfactory results the luminance
contrast ratio (= ratio between highest and lowest luminance) in the field of view should not be
larger than 3 and not smaller than 1/3.
Colour rendering of light sources - NIST
The attribute of colour rendering of light sources is often interpreted as indicating object colour
quality. However, colour rendering is actually defined as the “effect of an illuminant on the colour
appearance of objects by conscious or subconscious comparison with their colour appearance under
a reference illuminant”
, In the calculation of the CRI, the colour appearance of 14 reflective samples is simulated when
illuminated by a reference source and the test source. The reference source is a Planckian radiator or
a CIE Daylight source, matched to the correlated colour temperature (CCT) of the test source. After
accounting for chromatic adaptation, the difference in colour appearance ΔEi for each sample
between the test and reference light sources is computed in CIE 1964 W*U*V* uniform colour
space. The general colour rendering index (Ra ) is simply the average of Ri for the first eight samples,
all of which have low to moderate chromatic saturation. A perfect score of 100 represents no colour
differences in any of the eight samples under the test and reference sources.
Shortcomings:
- Uniform colour space used is outdated
- The assumption that complete chromatic adaptation to the chromaticity of the light source
takes place, fails at extreme CCTs.
- None of the reflective samples used in the calculation are highly saturated.
- Taking the average isn’t always good
- The definition is flawed
Correlation between colour quality metric predictions and visual
appreciation of light sources
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Reader week 3
Chapter 1. Light and radiation
1.1 Radiation
Two theories of electromagnetic radiation:
The first is Maxwell’s electromagnetic wave theory, suggesting that electromagnetic radiation
(including light) consists of ripples or waves propagated in fields of electric and magnetic force.
The other theory, Planck’s quantum theory, postulates that all forms of radiation consist of very
small portions of energy called “quanta”. Different types of electromagnetic radiation are thought to
consist of quanta of different energy content, the higher energies corresponding to the shorter
wavelengths in Maxwell’s theory.
1.2 Light amount and radiation energy
The energy of an incandescent body is distributed over different wavelengths according to its
temperature. The point of maximum emission shifts towards the blue end of the spectrum (shorter
wavelength) as the temperature increases. Bodies emitting electromagnetic radiation as a result of
heating are called thermal radiators.
Radiant energy is the energy of electromagnetic waves. The quantity of radiant energy may be
calculated by integrating radiant flux (or power) with respect to time and, like all forms of energy, its
SI quantity is Q and its unit is the joule.
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