The ampère (symbol: A, at times informally abbreviated to amp) is the International System basic unit for measuring the intensity of electric current. Being one of the fundamental units of the International System, all the other electromagnetic units derive from it. An ampere is the intensity of electric current which, if maintained in two parallel linear conductors of infinite length and negligible transversal section positioned at a metre of distance from each other in space, produces a force equal to 2 x 10 newtons per metre of length between them.
The ampere takes its name from the Frenchman André-Marie Ampère, one of the main physicists who studied electromagnetism.

Alternate Current

Alternate current (AC) is an electric current characterised by the alternation of positive and negative pulses. In Europe, electricity is distributed under the form of sinusoidal alternate current at a constant frequency of 50Hz.
Using alternate current derives from the fact that:

  • transporting high electric powers (above all over long distances) is very efficient if carried out at high voltages, which can be reached fairly easily using transformers;
  • the alternators are of simpler construction and have a higher yield than a dynamo;
  • in direct current benefit cannot be made of the advantages of a three-phase system.

Color rendering index (CRI)

The color rendering index (CRI) of a luminous source indicates how natural the colours of the objects illuminated by it appear.

Color temperature

This is a term used in illuminating engineering, photography and other disciplines connected with quantifying the tones of light. It is measured in kelvin. The choice of colour temperature often depends on personal taste, the customs of a country, the architecture of places and subjective perception.
Light bulbs with tonalities from 2700 kelvin (very warm light) to 6500 kelvin (very cold light) are available.
Halogen lamps are characterised by a brilliant light, very similar to incandescent light bulbs. Compact fluorescence is available in different light tones, from warm to cold. LED lights are available both in warm and cold tonalities, or coloured.

CRI (Color Rendering Index)

See: Colour rendering index.

Direct current

Direct current (DC) is characterised by a flow of current having constant intensity and direction over time. In direct current the electrons always flow in the same direction inside the circuit, therefore they always rotate/circulate in the same direction.
In many devices the acronym for denoting direct current is DC, or the symbol of a continuous line can be used (—) flanked by three shorter lines(---), while for alternate current the acronym AC or the symbol (~) are used. In a direct current system, differently from an alternate current system, it is very important to respect the direction of the current, namely the polarity. Batteries in fact have a positive and a negative pole, which must be correctly connected to the load. Many electronic circuits, if powered incorrectly, can develop a fault, in particular if they are not protected by an anti-inversion diode.


The diode is a passive non-linear electronic component with two terminals (bi-polar), with the ideal function of allowing the electric current to flow in one direction and blocking it completely in the other. This is done by placing metal contacts to stop the freedom of movement and direction of the charge carriers. More complex structures based on a different principle developed from the initial one over time, for example transit time diodes, and new devices with three terminals, such as the SCRs and triacs, which abandoned the name of “diode”.
There are different types of diode, among which LEDs. These emit visible light if polarised directly: they are usually used for signalling on control panels and as warning lights, or as transmitters for remote controls and optical fibres. High luminosity models have recently been developed that are suitable for lighting engineering, and there are numerous lighting applications on today’s market that use LEDs as an alternative source to the traditional incandescent light bulbs and fluorescent bulbs, with great advantages in terms of energy saving, duration and colour rendering. Their direct polarisation voltage varies according to the wavelength of the light they emit, and the more current passes through them, the more light they emit: in general they require a minimum current of 4 mA (threshold current) to emit light in perceptible quantities. The current varies according to the type of LED used. LED diodes normally require on average 15 mA to emit a good level of light. In the case of HL (high luminosity) LEDs, the current rises to values of around 20-25 mA. New LEDs, with extra high luminosity, can absorb currents of many amperes; these can be coupled mechanically with a heat dissipator.

Extra low voltage

Extra low voltage has voltage values that do not create the risk of electrocution.

Extra low voltage is defined as the electric voltage interval that is:

  • in alternate current: equal to or less than 50 volts (effective value)
  • in direct current: equal to or less than 120 volts not undulated

    Electric systems that use very low voltage, also called zero category, can be classified into these categories:

    • SELV (Safety Extra Low Voltage): power supply from an independent source (battery) or from a transformer with double safety insulation. There must be absolutely no earth connection (class III) and insulation from any other circuit must be guaranteed with double insulation or metal screen placed on the ground. For these two reasons it must be impossible to connect the plug of a SELV system to the outlet of any other system.
    • PELV (Protective Extra Low Voltage): very similar to the previous item but with one point of the circuit connected to the earth potential. Less safe than the previous system, it is necessary where earthing is indispensable for operational reasons.
    • FELV (Functional Extra Low Voltage): even though there are nominal voltages that fall into the extra low voltage definition, safety insulation from low voltage systems is not guaranteed with these systems. FELV systems are used when extra low voltage is required for operational reasons (e.g. servo systems) but direct contact by humans is not programmed. Contact must be blocked by suitable IP protection and insulation.

    The reference standard is CEI 64-8 part 4

    FELV (Functional Extra Low Voltage)

    see: extra low voltage


    The hertz (symbol Hz = 1/s) is the unit used by the International System to measure frequency. It takes its name from the German physicist Heinrich Rudolf Hertz, who brought important contributions to science in the electromagnetism field. A hertz means one per second: 50 Hz -> fifty per second, 100 Hz -> one hundred per second, and so on. The unit can be applied to any periodic event. As an example, it can be said that a watch ticks at 1 Hz.


    An acronym for light emitting diode; it is an optoelectronic device that uses the optical properties of some semi-conducting materials to produce photons with the spontaneous emission phenomenon, namely starting from recombination of the electron-hole pairs. The first LED was developed in 1962 by Nick Holonyak Jr.. LEDs can have these types of luminous emission:

    • Continuous: the LED emits light constantly.
    • Intermittent: the LED emits light at regular time intervals, obtained with astable circuits or with intermittent LEDs.

    Lumen (luminous flux)

    The lumen (symbol: lm) is the unit of measurement of the luminous flux.
    It is equivalent to the visible luminous flux emitted by an isotropic source with a luminous intensity of 1 candela in a solid angle of 1 steradian.
    The measure of the visible luminous flux is based on the sensitivity curve of the eye to luminous radiation, and is less than the issued radiant flux.
    Lux and lumen are two different measures of luminous flux. The lumen measures the “quantity of light” on a unitary sphere (centred on the source); the lux instead measures the area over which the luminous flux is spread. 1 lumen over an area of 1 m2 corresponds to 1 lux, while the same lumen concentrated into 1 cm2 corresponds to 10,000 lux.


    The lux (symbol lx) is the unit of measurement for illumination, accepted by the International System. A lux is equal to a lumen divided by a square metre. It is a unit of measurement for visible light, therefore it depends on the characteristics of the human eye and its sensitivity to luminous radiation.
    Some illuminance data follows, to give an idea of the value of a lux:

    • Sunlight on average varies between 32,000 lx (32 klx) and 100,000 lx (100 klx);
    • Moonlight is equal to approx.. 1 lx;
    • The light emitted by a luminous star is only 0.00005 lx (50 µlx);
    • Under the reflectors of television studios, there are approximately 1,000 lx (1 klx);
    • An office illuminated in compliance with the current European Uni En 12464 standard, must have 500 lx.


    The newton (symbol: N) is the measuring unit of force and it is part of the International System of Units derived units. The newton takes its name from Isaac Newton, as recognition of his work in classical mechanics. It was implemented by the Conférence générale des poids et mesures (General Conference on Weights and Measures) in 1960. It is defined as the amount of force needed to accelerate 1 kilogram of mass at the rate of 1 metre per second squared.

    Pcb (Printed Circuit Board)

    In electronics, the term printed circuit refers to a type of electric component used to produce modern electronic circuits, such as electronic circuit boards. These printed circuits are used as follows:
    1. Electrical connection between various electronic components, in order to create a true electric circuit;
    2. Mechanical casing for components and accessories (heat sinks, connectors, etc…) in order to create a system in which each component has a precise geometric position. In addition, the mechanical workability of the casing means that its edges can be shaped (by milling or shearing) so as to mechanically house the printed circuit in containers of even complex shape.

    According to the type of substratum and production process, the printed circuit can be either rigid, flexible or a mixture of both, with rigid parts connected to each other by flexible sections. According to the number of conductive layers in the printed circuit, the production process becomes progressively more complex and costly. On the basis of technological complexity, proportional to the number of layers, the versions can be:

    • "single sided" (one copper layer)
    • "double sided" (two copper layers)
    • "multi-layer" (in the majority of applications there are from 4 to 8 layers, but even 20 or more layers can be present)

    PELV (Protective Extra Low Voltage)

    see: extra low voltage

    Power supply

    The power supply is an electrical device that rectifies the electric input voltage at output, in order to supply electricity, adapting it to use by other electrical equipment, and if necessary modifying also the voltage and current levels, therefore power, at output by way of a transformer.

    RGB (Red Green Blue)

    This is the name of a colour model, the specifications of which were described in 1931 by the Commission internationale de l'éclairage (CIE). Differently from images in levels of grey, this color model is additive and is based on the three colours: red, green and blue, from which the model takes its name. They must not, however, be confused with the subtractive primary colours, yellow, cyan and magenta. Using filters or other techniques, an image can in fact be broken up into these basic colours which, when combined, give almost the whole spectrum of visible colours, with the exception of purples.
    The RGB as an additive model: combining the three colours at their maximum intensity gives white (all the light is reflected). The combination of colour pairs gives cyan, magenta and yellow.

    SELV (Safety Extra Low Voltage)

    see: extra low voltage


    The volt is the International System derived unit for measuring the electric potential and the electric potential difference.
    It gets its name from Alessandro Volta, who in 1800 invented the voltaic pile, the first electrochemical battery. In the 1880s, the International Electrical Commission, now the International Electrotechnical Commission (IEC), approved the volt (as the unit of measurement) for electromotive force.
    The symbol for volt is V.


    This is a derived unit of power in the International System, and its symbol is W.
    A watt is equivalent to 1 joule per second (1 J/s) and is equivalent, in electrical units, to one volt per ampere (1 V · A), or at 1 N · m/s (newton by metres per second). The watt takes its name from James Watt for his contribution in the development of the steam engine.

    Zero category

    see: extra low voltage