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Mention of lighting
 
What is light?
Light is a form of energy composed of electromagnetic waves that spread through space at the speed of 300.000 km per second, all similar, except for the frequencies, to radio waves.

An electromagnetic radiation is composed of a magnetic field variable in accordance with sine curve law.
  The characteristic dimensions are the maximum value (or efficiency value) of the intensity of the field and the light wavelength ( ) which in turn determines the colour of the light.
In time terms, the wavelength finds equivalence in the time period (T), the time consumed between two successive maximum values of homogenous mark. The correlation between the time period T and the wavelength is obvious, given that the propagation speed c of electromagnetic waves through space is known and constant.
   
Luminous flux
 

Luminous flux measures the light potency useful in terms of vision.
Logically it should also be measured in watts, but the unit used is the lumen (symbol lm). One lumen corresponds to 1/683 W of yellow light, corresponding to radiation with a wavelength of 555nm.

Although precise mathematical relationships exist between lumen and watt, it is important not to confuse the potency absorbed by electric lightbulbs with the flow emitted; in fact, only a minimal part of the potency absorbed is transformed in visible light radiation.
For simplicity, but without respect for the reality of physical law, light flow is defined as "the quantity of light emitted by a lightbulb"; thus said, it is not a question of quantity of energy (joule) but of potency (watts translated into lumen).

Furthermore it is to be emphasised that it is the potency transmitted that is weakened when it meets or crosses means of dissipation.
Thus, the luminous flux emitted by a lighting appliance is less than that emitted by lightbulb it contains due to the losses caused by absorption by the reflector and/or diffusor (concept of appliance output).
   
Light intensity
  Light intensity is a dimension that measures the light potency of rays emitted from the source in a given direction.
In order to understand the concept, with reference to Figure 3, one can imagine a spherical source, for example an opaline globe, that diffuses light uniformly in the entire solid angle of 12.56 sterad (a three-dimensional angle measured in steradiants). If the total light flow is, for example, 1000 lumen, in every direction one achieves a light intensity of 1000:12.56 candles (symbol cd), that is around 79.57 cd.

Light intensity (symbol l) can in this way be defined as the infinitesimal relationship between light flow and the angle at which it is emitted.
Light intensity is measured in candles (that is in lm/sterad). Sometimes, and not without much confusion, this dimension is attributed to a lightbulb imagining it as a uniformly emitted light source: under this hypothesis one candle is equivalent to 12.56 lumen.
The concept of light intensity is the basis of the entire calculation of illuminotechnics, based on defined physical elements (a point per point calculation).

Whilst the flow (minimal weakening apart) is a characteristic typical of light sources (lightbulbs), intensity is typical of lighting appliances; for a precise calculation of illumination it is necessary to know the light intensity emitted by a lighting appliance on at least two significant vertical right-angles (photometric indicator).
 


IIllumination
  IIllumination is mathematically defined as the relationship between the flow incident to a surface and the surface itself measured in m2 (lumen/m2=lux). Only the incident flow should be considered, that is as it hits the surface on the perpendicular. If the light ray is not perpendicular to the surface the relationship is multiplied by the cosine of the angle of incidence with respect to the vertical.

This first rough concept becomes exact only in the case of uniform parallel flow (for example sunlight with respect to a surface of modest dimensions). In general situations light is diffused by a surface much smaller than that illuminated and the rays diverge for this reason. It follows that illumination E decreases in line with distance squared.

The units of measure (light flow, light intensity, illumination) are well chosen so that the illumination of a point is given by the relationship between light intensity of the incidental ray and the distance squared.
For up to 1 m from the source, illumination is numerically equal to light intensity of the incident ray. For example along a ray with an intensity of 100 cd one will find, respectively:

- up to 1 m 100 lx;
- up to 2 m 25 lx;
- up to 4 m 6,25 lx etc.
   
Luminance
  Taking a further look, with the human eye, the effect of light flow, light intensity and illumination can be evaluated better if the concept of luminance is introduced.

The luminance of a body from which the eye receives light is defined as the relationship between light intensity (candela) of the ray that hits the retina and the emitting surface (m2). Luminance (measurement symbol L) is measured in nit (symbol nt, dimension cd/m2).
   
Colour
  Every wavelength comprised in the range of visible electromagnetic radiations corresponds to the perception of colour by the human eye, variable from red (700nm) to blue-violet (435nm), passing through yellow (570nm).

"Pure" colours, that is formed by a perfect radiation sine curve, are only theoretical; in reality the eye, with respect to electromagnetic waves, behaves like a multi-band antenna in such a way that the colours are perceived as additional syntheses to monochrome radiations.
Thus, for example, white does not exist as a primary colour, but is obtained by a suitable dose of red, green and yellow.

The colour of sunlight is the same white at which artificial light is aimed.
If a light source is not able to produce determined lightwaves in determined doses, the colours of the objects illuminated are subject to distorsion.
   
Tonality
  The artificial light produced by electric lightbulbs is not perfectly white; it can tend towards red (warm light) or blue (cold light) if, respectively, there is an abundance of red or blue radiations.
The method most used to define tonality is that of degrees Kelvin.
Tonality is normalized by the CIE in three groups according to the following symbols:

- W up to 3300 K indicate warm tones;
- I from 3300 to 5300 K indicate daylight;
- C over 5300 K indicate cold light.

In the range of tonalities comprised between 3000 and 6000 K colours are perceived correctly thus the choices are subjective or influenced by special usage (for example the aspect of flesh improves if illuminated by light with warm tones; whilst the aspect of fish is better highlighted by cold tonalities).
   
Chromatic yield
  The chromatic yield of a lightbulb expresses its capacity to reproduce the natural colours of the objects illuminated.
The colour of such objects depends on the light reflected: thus, an object appears red because it absorbs all other radiations and reflects only combinations of monochrome lights that in synthesis are red.

Without going into complex detail it is fairly evident that an object is not able to reflect a radiation it does not receive. Therefore, chromatic yield is perfect only if artificial light produces the same radiations as sunlight and in the same doses. This optimal condition clearly cannot be reached. The level of imperfection is conventionally measured by a numeric index preceded by the symbol Ra:

the number 100 indicates the perfection (100% colour yield) and 0 indicates absolute imperfection (monochromatic vision).
Light and environment
Our modern society requires an increasing quantity of electricity for industry, ... ...
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