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How Light is Generated
To understand how lamps work and hence select the right light source for a particular application, we need to understand how light is generated
Light is a form of energy that can be released by an atom. It is made up of many small particle-like packets that have energy and momentum but no mass. These particles, called light photons, are the most basic units of light.
Atoms release light photons when their electrons become excited. Electrons are the negatively charged particles that move around an atom's nucleus (which has a net positive charge). An atom's electrons have different levels of energy, depending on several factors, including their speed and distance from the nucleus. Electrons of different energy levels occupy different orbits. Generally speaking, electrons with greater energy move in orbits farther away from the nucleus. When an atom gains or loses energy, the change is expressed by the movement of electrons. When something passes energy on to an atom, an electron may be temporarily boosted to a higher orbit (farther away from the nucleus). The electron only holds this position for a tiny fraction of a second; almost immediately, it is drawn back toward the nucleus, to its original orbit. As it returns to its original orbit, the electron releases the extra energy in the form of a photon, in some cases a light photon.
The wavelength of the emitted light (which determines its colour) depends on how much energy is released, which depends on the particular position of the electron. Consequently, different atoms will release different light photons. The colour of the light is determined by which atom is excited.
Light sources emit radiation across the electromagnetic spectrum. Some of the radiation is emitted in the form of visible light. See below.
The visible spectrum is a narrow band between 380 and 780 nanometres which can be detected by our eyes. The eye is able to discriminate between different wavelengths and the brain interprets this as colour. Violet and blue are situated at the shorter wavelength end of the spectrum, with reds occurring at longer wavelengths, and green/yellow at the centre. In order to develop energy-efficient lighting products, the most important goal of the lamp engineer is to concentrate as much energy as possible into this narrow visible band, while limiting the amount that is wasted in the invisible adjacent ultraviolet and infrared regions.
Natural Light
In order to understand artificial light sources, we need to consider natural light, daylight, or SKYLIGHT. (Source SLL Guide 2009).
Light from the sun is scattered by the atmosphere, and the distribution and amount of light received at ground level is dependent on atmospheric conditions. The International Commission on Illumination (CIE) categorise SKYLIGHT into a series of 15 sky distributions (BS ISO 15469:2004 Spatial distribution of daylight – CIE standard general )and give a formula that may be used for calculating the relative luminance distribution of the sky.  The following list details the types of sky distribution:
Description of luminance distribution
CIE Standard Overcast Sky, Steep luminance gradation towards zenith, azimuthal uniformity
Overcast, with steep luminance gradation and slight brightening towards the sun
Overcast, moderately graded with azimuthal uniformity
Overcast, moderately graded and slight brightening towards the sun
Sky of uniform luminance
Partly cloudy sky, no gradation, towards zenith, slight brightening towards the sun
Partly cloudy sky, no gradation, towards zenith, brighter circum solar region
Partly cloudy sky, no gradation towards zenith, distinct solar corona
Partly cloudy, with the obscured sun
Partly cloudy, with brighter circumsolar region
White-blue sky with distinct solar corona
CIE Standard Clear Sky, low luminance turbidity
CIE Standard Clear Sky, polluted atmosphere
Cloudless turbid sky with broad solar corona
White-blue turbid sky with broad solar corona CIE standard sky types
Skylight Distribution
The CIE standard helps with the distribution of daylight but it gives no information on the actual amount of daylight available at any particular time.  There are a number of meteorological stations that record the global and diffuse (not including light direct from the sun) horizontal plane illuminance values on an unobstructed site and this data can be used to predict daylight availability.  Whilst data is logged every five minutes or so at most measuring stations it is usually presented as a chart showing monthly averages of hourly values. The following chart shows typical data on daylight availability for the south of England, this will vary according to your location; BST corresponds to British Summer Time

Typical daylight availability chart
The colour of the light from the sun and sky depends not only on the on the colour of the light from the sun but also on the way that light is absorbed and scattered by the atmosphere.

The spectrum of daylight with a colour temperature of 6500k from CIE 15.2
The above chart shows the standarised spectrum of daylight from CIE 15.2 which gives formulae for the calculation of daylight spectra of different colour temperatures. In practice, as the sky condition is constantly changing it is difficult to give exact values of the colour of the sky. However, the following table lists approximate values of correlated colour temperature for various sky conditions.
Sky condition
Bright midday sun
Lightly overcast sky
Heavily overcast sky
Hazy sky
Deep blue clear sky
Light – achieving a balance
One of the skills of the lighting designer is achieving a balance between NATURAL light and ARTIFICIAL light. By capitalising on the natural light cast through windows, doors and skylights and then coupling this with a wide choice of artificial light sources within luminaires plus introducing  occupancy/presence controls, daylight sensors and other technologies, the lighting designer can achieve a fantastic lighting scheme which also makes the most of natural light available and hence saves energy as well.
Human Vision States
The eye consists of two key families of photo-receptors, which are sensitive to the wavelength and intensity of light. The entire surface of the retina is covered with Rod cells – so named because of their physical shape. Rod cells cannot distinguishcolour but are able to identify high and low brightness levels, and thus build up a picture of contrast of our external environment. This can be thought of as rather like a black and white photograph.
The centre of the retina, the fovea, is deficient in rod cells and instead populated by cone-shaped receptors. These are specifically present to introduce colour vision, and there are three types which are broadly receptive to red, green and blue wavelengths. Although we have limited colour vision beyond the centre of the retina, the brain remembers the colours of objects at the periphery of our vision and maintains a full colour image of our environment.
The two different kinds of cells each function within specific light level ranges. At high illumination levels, cone cells predominate and the eye sees in full colour. This situation is termed “Photopic Vision”. Photopic vision is by far the most important to the lighting designer, because it determines our sight during daytime and virtually all indoor illuminated environments. Fig. 6 – Photopic Vision
At very low lighting levels, e.g. starlight, there is no functionality of the cone cells. Only the rod cells are active and they build up a contrast image of our visual environment. No colour perception is possible and this state is called “Scotopic Vision”.Scotopic vision is less important and is rarely encountered. Even on a dark street there will be some degree of photopicvision. Fig. 7 – Scotopic Vision
At intermediate lighting levels the rod cells still predominate, but there remains partial functionality of the cone cells. Limitedcolour perception is possible in the brighter areas, and this condition is known as “Mesopic Vision”. It is a less well defined state existing between Photopic and Scotopic vision, whose precise nature is dependent on the actual illumination level. Fig. 8 – Mesopic Vision
The rod and cone cells each function more efficiently under certain wavelengths of light – further job of the lamp engineer to develop sources that radiate as much light as possible near the areas of peak sensitivity, in order to produce the maximum light for the minimum consumption of electrical energy. Figure 9 illustrates the Photopic and Scotopic spectral response curves of the human eye superimposed over the SPD of a high pressure sodium lamp. Since this radiates much of its energy near the peak photopic sensitivity of the eye, it has a very high luminous efficacy.
Fig. 9 – Relative Spectral Response of the Eye
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