Lighting: how artificial lighting occurs and the characteristics of the devices.
ELEMENTS OF LIGHTING: LIGHTING OF ARTIFICIAL BUILDINGS . Lighting is the study of methods and techniques for the artificial lighting of interior and exterior environments. Its main objective is to determine the quantity of light, or luminous flux, that must be emitted by lamps, which are inserted into devices that allow their operation, called lighting devices.
Main photometric quantities
The fundamental physical quantities of photometry are luminous flux, luminous intensity, illuminance and luminance.
Luminous flux
Luminous flux is the quantity of light emitted by a source in the unit of time.
The unit of measurement is the lumen (lm). Luminous flux measures the light emitted by lamps and is always indicated in the technical data sheets provided by manufacturers.
Luminous intensity
Luminous intensity l is the amount of light emitted by a point source in a given direction. The unit of measurement is the candela (cd).
Illuminance
Illuminance E is the amount of light that reaches its surface and is inversely proportional to the distance between the light source and the surface. It is measured in lux (lx) defined as the flux emitted by the point source that falls on a surface of 1 m2.
Luminance
Luminance L is the luminous intensity emitted by the surface of an object referred to a surface and is given by the ratio between the luminous intensity emitted by the object in a direction and the area of a surface perpendicular to that direction. It is measured (in cd/m2) with instruments called luminance meters and the phenomena of intensity of visual sensation and glare depend on it.
Lamps
Lamps are light sources that emit light energy intended for the artificial lighting of internal and external environments.
The most commonly used light sources can be divided into the following types:
• Incandescent lamps, those in which the light energy is produced by a very thin metal wire kept in a suitable environment and made incandescent by the passage of an electric current;
• Gas discharge lamps, the radiant energy is not produced by heating a filament, as in incandescent lamps, but by the radiation emitted during the discharge that is generated between two electrodes immersed in a gas or metal vapours and subjected to a potential difference;
• Light-emitting diode (LED) emitters, are light emitters that use solar energy in a similar way to photovoltaic systems to accumulate electricity in a battery that powers the emitters, the switching on of which is regulated by a twilight sensor; •
Optical fibres, made up of small tubes connected at one end to a light generator that exits from the other end through a terminal.
Features of the lamps
The main features of the lamps are:
- Absorbed electrical power, expressed in watts;
• Operating electrical voltage, expressed in volts;
• Luminous flux emitted, expressed in lumens;
• Efficiency, expressed in lumens/watt;
• Colour rendering index Ra, expressed in relation to the yield of sunlight;
• Colour temperature, expressed in kelvin.
Energy consumption of lamps
The electrical energy consumption of a lamp, expressed in watt hours (kWh), is given by the product of the power absorbed by the lamp, expressed in watts, by the duration of the ignition period.
Efficiency and performance
The energy efficiency of a lamp is measured by its efficiency and is given by the ratio between the luminous flux and the absorbed power. The greater the efficiency of a lamp, the greater the quantity of light it produces for the same consumption. Luminous efficiency is measured in lm/W (lumens per absorbed watt).
Lighting systems
The lighting of an environment can be:
- Direct, if the luminous flux is directed downwards;
• Semi-direct, if the luminous flux is mainly directed downwards;
• Diffused, if the part of the luminous flux directed downwards is substantially equal to that of the flux directed towards the ceiling;
• Indirect, if the luminous flux is directed towards the ceiling.
Light Hue
Every light source, natural or artificial, emits radiation of different wavelengths that give rise to a specific spectral distribution and a color of different hue.
Daylight contains all the colors of the light spectrum in an almost uniform measure, while artificial light sources have very different spectral distributions depending on the type of lamp; it is therefore important to know the spectral distribution of lamps in order to design the lighting of an environment.
Correlated color temperature
The determination of the correlated color temperature Tcp of a light source is done experimentally, by heating a “black body” (i.e. a material that has the property of completely absorbing radiation, without re-emitting or transmitting it) until it assumes the same color as the source under examination.
The two typical natural sources of light during the day and at night, which are the Sun and the flame of a candle, have the following temperatures correlated by color:
- Sun: Tc= 6000 K with illuminance > 100000 lx;
- Candle flame: Tcp= 1900 K with illuminance < 5 lx.
Color rendering index
The degree of color alteration produced by a light source is expressed through the color rendering index Ra. The evaluation of the Ra index occurs experimentally and is expressed in relation to an index of absolute color rendering fidelity Irc conventionally assumed to be equal to 100 (sunlight).
Lighting fixtures
Lighting fixtures are the casing that contains the lamp and have the function of enabling its operation and of modulating and appropriately directing the flow emitted by the lamp. The elements that constitute a lighting fixture are:
- The fairing is the support of the device, which has the function of protecting the lamps from external agents and dust;
- The electrical group consists of the lamp-lamp holder system and the related electrical connections.
- The optical group is made up of a set of elements that allow the luminous flux emitted by the lamp to be directed in the desired way.
Characteristics of lighting fixtures
The efficiency of the lighting system is a coefficient that takes into account the efficiency of the fixture, given by the ratio between the flow coming out of the fixture and the flow emitted by the lamp (?=φa/φc), the characteristics of the room and the lighting system.
Decay of the lighting system
After a certain period of operation, the illumination produced by each lighting fixture is reduced to a greater or lesser extent depending on various factors, including the ageing of the lamps, the soiling of the fixture, the reduction in the reflective capacity of its surfaces, etc. The decay is expressed through a coefficient called the maintenance factor Mf, the value of which normally varies from 0.55 to 0.80.
