Principles of controlling light
The most essential task of a luminaire is to direct the lamp lumens; whereby, a light distribution is striven for that corresponds to the particular job of the luminaire for the best possible utilisation of the energy used. A step towards a targeted and specific light control was realised by the introduction of the reflector lamps and PAR lamps. The light is focused by reflectors integrated into the lamp and can be directed in the desired direction with defined beam emission angles. The demand for more differentiated lighting control, for enhanced luminaire efficiency and improved glare limitation led to the reflector being taken from the lamp and integrated into the luminaire. This means that it is possible to construct luminaires that are designed to meet the specific requirements of the light source and the task.
Reflectance
Luminous intensity distribution I in the case of diffuse reflection
Luminous distribution L in the case of diffuse reflection. It is the same from all angles of vision.
Luminous intensity distribution in the case of mixed reflection
Luminous intensity distribution in the case of specular reflection
Diffusion
In the case of reflection, the light incident on a surface is fully or partially reflected, depending on the reflection factor of the surface. Besides reflectance the degree of diffusion of the reflected light is also significant. In the case of specular surfaces there is no diffusion. The greater the diffusing power of the reflecting surface, the smaller the specular component of the reflected light, up to the point of completely diffused reflection where only diffuse light is reflected.
Specular reflection of parallel beams of light falling onto a flat surface (parallel optical path)
Concave surface
(converging beam)
Convex surface
(diverging beam)
Surface forms
Specular reflection is a key factor in the construction of luminaires; by using suitable reflector contours and surfaces, it enables a targeted control of light and is also responsible for the magnitude of the light output ratio.
Transmission
Luminous intensity distribution I in the case of diffuse transmission
Luminous distribution L in the case of diffuse transmission. It is the same from all angles of vision.
Luminous intensity distribution in the case of mixed transmission
Luminous intensity distribution in the case of mixed transmission through transparent material
Transmission describes how the light incident on a body is totally or partially transmitted depending on the transmission factor of the given body. The degree of diffusion of the transmitted light must also be taken into account. In the case of completely transparent materials there is no diffusion. The greater the diffusing power, the smaller the directed component of the transmitted light, up to the point where only diffuse light is produced. Transmitting materials in luminaires can be transparent. This applies to simple front glass panels or filters that absorb certain spectral regions but transmit others, thereby producing coloured light or a reduction in the UV or IR range. Occasionally diffusing materials, e.g. opal glass or opal plastics, are used for front covers in order to reduce lamp luminance and to help control glare.
Absorption
Absorption describes how the light incident on a surface is totally or partially absorbed depending on the absorption factor of the given material. In the construction of luminaires absorption is primarily used for shielding light sources; in this regard it is essential for visual comfort. In principle, however, absorption is not desirable since it does not direct but rather wastes light, thereby reducing the light output ratio of the luminaire. Typical absorbing elements on a luminaire are black multigroove baffles, anti-dazzle cylinders, barn doors or louvres of various shapes and sizes.
Refraction
When transmitted from one medium with a refractive index of n1 into a denser medium with a refractive index of n2, the rays of light are diffracted towards the axis of incidence (ε1> ε2). For the transition from air to glass the refractive index is approx. n2/ n1=1.5.
When transmitted through a medium of a different density, rays are displaced in parallel.
Introduction
When beams of light enter a clear transmitting medium of differing density, e.g. from air into glass and vice versa from glass into the air, they are refracted, i.e. the direction of their path is changed. In the case of objects with parallel surfaces there is only a parallel light shift, whereas prisms and lenses give rise to optical effects ranging from change of radiation angle to the concentration or diffusion of light to the creation of optical images. In the construction of luminaires refracting elements such as prisms or lenses are frequently used in combination with reflectors to control the light.
Prisms and lenses
Typical ray tracing of parallel incident light through an asymmetrical prism structure (top left), symmetrical ribbed prism structure (top right), Fresnel lens (bottom left) and collecting lens (bottom right).
Refractive index
There is an angular limit εG for the transmission of a ray of light from a medium with a refractive index of n2 into a medium of less density with a refractive index of n1. If this critical angle is exceeded the ray of light is reflected into the denser medium (total internal reflection). For the transition from glass to air the angular limit is approx. εG = 42°. Fibre-optic conductors function according to the principle of total internal reflection (right).
Interference
Interference is described as the intensification or attenuation of light when waves are superimposed. From the lighting point of view, interference effects are exploited when light falls on extremely thin layers that lead to specific frequency ranges being reflected and others being transmitted. By arranging the sequence of thin layers of metal vapour according to defined thicknesses and densities, selective reflectance can be produced for specific frequency ranges. The result can be that visible light is reflected and infrared radiation transmitted, for example - as is the case with cool-beam lamps. Reflectors and filters designed to produce coloured light can be manufactured using this technique. Interference filters, so-called dichroic filters, have a high transmission factor and produce particularly distinct separation of reflected and transmitted spectral ranges.
Mirror-finish reflectors with good material quality are free of interference.
