In artificial lighting, the interior is illuminated by the luminous flux of a lamp. The light of a light source reaches the surfaces of the room through the luminair. The surfaces of the interior more or less reflect this light, and they illuminate each other in the process. Each instance of reflection reduces the amount of available light, so finally it is absorbed. At a given moment, a given surface is illuminated at the same time both by light directly radiated from the lamp and by light indirectly reflected from the surfaces.
It follows from what has been said above that every element of surface of a room is practically illuminated by a hemisphere: the hemisphere that is "seen" by the element. So a given surface of the interior is illuminated by the luminaires and surfaces it sees, in other words, the surface is exposed to the luminous flux of a hemisphere, to light coming from different directions and at different intensities.
There are different ways by which the light of a light source can reach a given surface of the room. These ways as well as the quantity and quality of the utilized luminous flux of the light source vary depending on the luminaire and the surfaces. If we want to take into account all the above, and we want to plan the illumination of the interior, we must clarify the details of this chain of effects. That is why it is advisable to follow the route of light from the light source to the
reference plane.
The light output of a light source is characterized quantitatively by Φo luminous flux, and qualitatively by T colour temperature and Ra colour rendering.
Light sources are always built into luminaires. Luminaires serve several technical and lighting functions.
Their most important technical functions are:
- to fix the lamp and to supply it with energy ,
- to protect the lamp from the environment and the environment from the lamp.
Their lighting functions as follows:
The lamp radiates only part of the luminous flux into the room, and it absorbs the rest. The effectiveness of a luminaire from this point of view is called the efficiency of the luminaire.
The efficiency of a luminaire is the ratio of the luminous flux emitted by the luminaire ΦL and the luminous flux generated by the lamp Φo. Its symbol is ηL, its unit is [%].
From the point of view of lighting, the most important function of a luminaire is to distribute light in the room in the required stereoscopic manner.
Luminaires can distribute and direct the light of lamp into space in different ways.
Light distribution, 3D distribution of light emitted from a surface can be described by luminous intensity.
Luminous intensity is the luminous flux per unit solid angle in question, its symbol is I, its unit is candela, [cd].
The distribution of a luminaire's light is characterized by so-called candle-power curves.
A candle-power curve shows I luminous intensity in different directions on a plane laid through the luminair. As luminous intensity is a vector, each point of the candle-power curve represents the value of the luminous intensity vector pointing at the direction indicated by the point.
Luminaires are classified according to their manner of lighting based on their light distribution:
the ratio of luminous flux radiated up and down an endless horizontal plane lain through the luminaire.
There is considerable difference between the possible utilization of the two parts of the luminous flux, since the part radiated down by the luminaire can, theoretically, reach the reference plane -in an infinitely large room-, but then the part of the luminous flux radiated up to an infinite plane can reach the reference plane only after one or more reflections, therefore only a fraction can be utilized.
The ratio of illumination of a certain surface of the room is determined by the location of the luminaires in the interior and by their light distribution.
Luminaires can affect the utilization of the light of a lamp in yet another way. If the transparent and/or reflecting part of the luminaire is coloured, the luminaire modifies the quality of the light of the light source.
The transparent or reflecting parts of luminaires that are used for general lighting are not coloured, so they do not change the light of the light source.
The light emitted from the luminaire and illuminating the surfaces of the interior, can be described by illuminance.
Illuminance is the luminous flux collected by unit of surface. Its symbol is E, its unit is lux [lx].
Illuminance is an effect the surface is exposed to.
Illuminance of a surface depends on the direction of the incident light. Luminous flux produces the largest illuminance on a surface that is at right angle to it. Light will not illuminate a surface that it is parallel to.
It follows from the above that a luminaire close to a surface can only illuminate effectively the part of the surface closest to it.
Illuminances coming simultaneously from different sources add up.
Surfaces change the characteristics of light in the following way:
- Each surface reflects only part of the luminous flux, and absorbs the rest, - Surfaces change the direction of light - more or lessdispersing it,
- Coloured surfaces change the quality of light.
The Φo luminous flux reaching a surface is - partly reflected (Φρ),
- partly absorbed (Φα) and
- partly transmitted (Φτ), if the surface is transparent.
The ratio of these parts are ρ reflection factor
α absorption factor
τ transmission factor
The ρ reflected part of the luminous flux is the part that is visually perceptible. It is this part that can be used to illuminate the surfaces of a room.
Surfaces of different quality reflect and disperse light in different ways. Surfaces can be divided into three main classes as shown in the following figure. The dispersion of reflected light can be characterized with the distribution of luminous intensity.
The figure illustrates the following:
- mat surfaces reflect light evenly irrespective of the direction of the incident light,
- mirrors and shiny surfaces reflect light highly unevenly and depending on the direction of the incident light.
In mirrors, the illuminated point can only be seen from direction "B".
In shiny surfaces, the illuminated point is darker from direction "A" than "B".
In mat surfaces, the illuminated point is equally bright viewed from every direction.
The quality i.e. the spectral distribution of Φo(λ), the light illuminating the surface, and of Φρ(λ), the light reflected from the surface, may be the same or more or less different depending on the ρ(λ) reflection function of the reflecting surfaces, according to the following equation
Φρ(λ) = ρ(λ) * Φo(λ)
With white, grey and black surfaces, the reflected and the illuminating light are nearly identical in quality.
Coloured surfaces mainly absorb their respective colours, consequently the reflected and the incident lights differ greatly in quality.
A reference plane is illuminated by the luminous flux coming directly from the luminaires, and indirectly by the luminous flux coming from some of the room's surfaces. A point of the reference plane is illuminated by all the points of the room which it "sees", i.e. every surface element is illuminated by a hemisphere, whose elements can be luminaires or surfaces illuminated and reflecting light to various degrees.
The reference or working plane is the part of the room the visual task refers to. Normally, it is a horizontal plane 0.85m above the floor, or in communicating areas it is the floor.
What the spectator sees is that parts of his or her field of view varies in brightness and colour.
How is a surface seen ? An observer viewing a surface from a given direction perceives an I* luminous intensity coming from an A* virtual size of surface. It is this that makes a surface look bright
to some degree. What is seen is the luminance of the surface.
Luminance is the luminous intensity of an element of surface seen as a unit. Its symbol is L its unit is cd/m2.
It follows from the above that
- the luminance of mirrors and shiny surfaces varies with the direction of viewing,
- the luminance of mat surfaces are constant independent of the direction of viewing.
From the point of view of the visual environment, using mat surfaces is more advantageous.