Notions in Lighting
Light and radiation:

Light is an electromagnetic radiation, that the human eye senses as brightness. The human eye is sensitive to a small part of the whole radiation spectrum, a range between 380 and 780 nm, between the violet and red colours. Because of this, UV-rays (>720 nm) and infrared rays (<380 nm) are invisible..

Luminous flux (f):

The luminous flux φ, is the power of the radiant energy that a lamp or other light source gives, evaluated according to the relative value established internationally by the CIE for the human eye sensitivity, which is maximum for wavelenghts of 555 nm (green colour).The luminous flux is measured in lumens (lm).

The luminous efficiency of a lamp is the relationship between the emitted flux and the consumed power, varying among the different types of lamps.

Luminous intensity (I):Quantity of visible light that is emitted in unit time per unit solid angle.

The steradian is the unit of solid angle and there are 4 steradians about a

point enclosed by a spherical surface. It represents the intensity with which the light is projected on a determined direction. It’s distribution is represented by polar curves (explained later on) and can be modified with the help of reflectors and diffusers.

The unit of luminous intensity is the standard candle or candela (cd).

Light intensity (E):Light intensity is defined as the luminous flux that falls upon a surface divided by the area or this surface. It is the magnitude that shows the luminous level that there is upon a surface. Light intensity is measured in luxes (lx).

Luminance (L):The luminance shows the impresion of brightness with wich the human eye sensess an illuminated surface from a determined direction. It is the luminous intensity per square unit of a light source (direct) or of an illuminated surface (indirect).

The indirect luminance can be calculated from the light intensity (E) applying the following formula:

Where r is the reflection coeficient of the surface, wich can be found on tables from several technical publications. This formula can not be used for too bright surfaces.

The luminance is measured on candles by square meter (cd/m2).

Colour rendering index (IRC ó Ra):Depending on the working place and of the visual work to be done, the artificial light must provide an adequated colour rendering. The capability of a light source to render the colours is measured with the colour rendering index Ra.The concept of colour rendering of a light source is defined by the aspect of the illuminated objects colours compared to the aspect that they show under a reference light (natural light, or continuous spectrum light)..

The Ra value is determined by lighting a group of eight sample colours established by the DIN 6169 standard, with the reference light and with the

light to be analized, valuing from 0 to 100 the colour rendering of each sample. Doing the mean of the eight colour indexes, the Ra index is obtained. For Ra=100 the colours resulting with the light source are identical to the ones obtained with the reference light source.

Colour temperature:The colour temperature of a light source depends of the distribution of the emitted light on the visible part of the spectrum. From the scientific point of

view, shows the temperature at which a “black body” must be heated in order to make it emit a light of the same colour that the light source under test. It being a temperature, is measured in Kelvin degrees (ºK).When on a light source the predominant colour is red, it is said that the light is “warm”. On the contrary, if the predominant colour is blue it is said that the light is “cold”. In practice in order to obtain a “cozy” ambient, the “cold” light sources require a higher level of lighting than the “warm” sources..

Most common lamps are divided in three groups of colour temperatures:

- “Warm” white (ww): Colour temperature under 3.300 ºK

- “Neutral” white (nw). Between 3.300 ºK and 5.000 ºK.

- “Day” light (tw). Over 5.000 ºK.

As extreme examples, a standard incandescent lamp has a colour temperature of 2.700 ºK against a colour temperature of 6.000 ºK of “day” light fluorescent tubes.

It must be remembered that two light sources with the same colour temperature may have different colour rendering properties because of their different spectral distribution..