Having dealt with light quantity, conside-ration must be given to the quality of light, the difference between diffuse light and directed light being one of the most important aspects. We are familiar with these different forms of light through our everyday experience with daylight – direct sunlight when the sky is clear and diffuse light when the sky is overcast.
Characteristic qualities are the uniform, almost shadowless light we experience under an overcast sky, in contrast to the dramatic interplay of light and shade in bright sunlight.
Diffuse light is produced by extensive areas that emit light. These may be extensive, flat surfaces, such as the sky in the day-time, or, in the field of artificial lighting, luminous ceilings. In interior spaces diffuse light can also be reflected from illuminated ceilings and walls.
This produces very uniform, softlighting, which illuminates the entire space and makes objects visible, but produces reduced shadows or reflections.
Directed light is emitted from point light sources. In the case of daylight this is the sun,in artificial lighting compact light sources. The essential properties of directed light are the production of shadows on objects and structured surfaces, and reflections on specular objects. These effects are particularly noticeable when the general lighting consists of only a small portion of diffuse light.
Daylight, for example, has a more or less fixed ratio of sunlight to sky light (directed light to diffuse light) of 5:1 to 10:1. In interior spaces, on the other hand, we can determine the ratio of directed and diffuse light we require or prefer.
The portion of diffuse light decreases when ceiling and walls receive too little light, or when the light falling on a surface is absorbed to a large extent by the low reflectance of the environment.
This can be exploited for dramatic effects through accent lighting. This technique is often applied for the presentation of objects, but is only used in architectural lighting when the concept intends to create a dramatic spatial effect.
Directed light not only produces shadows and reflections; it opens up new horizons for the lighting designer because of the choice of beam angles and aiming directions that he has at his disposal.
Where as the light emitted by diffuse or exposed light sources always has an effect on the entire space, in the case of tightly controlled light, the effect of the light relates directly to the position of the luminaire.
Here lies one of the most progressive aspects of lighting technology. Whereas in the era of the candle and the oil lamp the light was bound to the immediate vicinity of the luminaire, it is now possible to use light in other parts of the space at any distance from where the light source is located.
It is possible to use lighting effects at specific illuminance levels on exactly defined areas from practically any location within a space.
Another basic feature of the world around us, and one that we take absolutely for granted, is its three-dimensional quality.
One essential objective regarding visual perception must therefore be to provide information about this aspect of our environment. Three-dimensionality comprises a number of individual areas, from the extension of the space around us to the location and orientation of objects within the space, down to their spatial form and surface structure.
Perception of the three-dimensional character of our environment involves processes that relate to our physiology and perceptual psychology. The shaping of our environment through light and shade is of prime importance for our perception of spatial forms and surface structures.
This has been referred to, but the significance for human perception must be analysed.
If we view a sphere under completely diffuse light we cannot perceive its spatial form. It appears to be no more than a circular area. Only when directed light falls on the sphere – i.e. when shadows are created, can we recognise its spatial quality.
The same applies to the way we perceive surface structures. These are difficult to recognise under diffuse light. The texture of a surface only stands out when light is directed onto the surface at an angle and produces shadows.
Only through directed light are we able to gain information about the three-dimensional character of objects. Just as it is impossible for us to retrieve this information when there is no directed light at all, too much shaping can conceal information. This happens when intensely directed light casts such stark shadows that parts of an object are concealed by the darkness.
The task of lighting design is therefore to create a suitable ratio of diffuse light to directed light to meet the requirements of each individual situation. Specific visual tasks, where the spatial quality or the surface structure is of prime importance, require lighting that emphasises shapes and forms. Only in situations where spatial quality and surface structure are of no importance, or if they are disturbing factors, can completely diffuse lighting be used.
As a rule suitable proportions of diffuse light and directed light are required.
In some standards for workplace lighting there is a criterion for the modelling effect of a lighting installation. It is referred to as the modelling factor, which is defined as the ratio of cylindrical illuminance to horizontal illuminance. When planning the application of directed and diffuse light it is advisable to rely on our fundamental experience of daylight with regard to the direction and colour of the light.
Direct sunlight either comes from above or from the side, but never from below. The colour of sunlight is clearly warmer than that of diffuse sky light. Consequently, lighting that comprises directed light falling diagonally from above with a lower colour temperature than the diffuse general lighting will be felt to be natural.
It is, of course, possible to apply light from other directions and with other colour temperature combinations, but this will lead to effects that are especially striking or strange.
Another feature of directed light alongside its modelling effect is brilliance.
The light source itself will be seen as a brilliant point of light. A good example of this is the effect of a candlelight in evening light. Objects that refract this light are perceived as specular, e.g. illuminated glass, polished gems or crystal chandeliers. Brilliance is also produced when light falls on highly glossy surfaces, such as porcelain, glass, paint or varnish, polished metal or wet materials.
Since sparkling effects are produced by reflections or refraction, they are not primarily dependent on the amount of light applied, but mostly on the luminous intensity of the light source. A very compact light source (e.g. a low-voltage halogen lamp) can create reflections of far greater brilliance than a less compact lamp of greater luminous power.
Brilliance can be a means of attracting attention to the light source, lending a space an interesting, lively character.
When applied to the lighting of objects brilliance accentuates their spatial quality and surface structure – similar to modelling – because sparkling effects are mainly evident along edges and around the curves on shiny objects.
Accentuating form and surface structure using brilliance enhances the quality of the illuminated objects and their surroundings. Sparkling effects are in fact generally used in practice to make objects or spaces more interesting and prestigious. If an environment – a festival hall, a church or a lobby – is to appear especially festive, this can be achieved by using sparkling light sources: candlelight or low-voltage halogen lamps.
Directed light can also be applied with sparkling effect for the presentation of specific objects – making them appear more precious. This applies above all for the presentation of refractive or shiny materials, i.e. glass, ceramics, paint or metal. Brilliance is effective because it attracts our attention with the promise of information content. The information we receive may only be that there is a sparkling light source.
But it may also be information regarding the type and quality of a surface, through the geometry and symmetry of the reflections.
If the brilliance possesses no informative value, then it is found to be disturbing. Disturbing brilliance is referred to as glare. This applies in particular when it arrises as reflected glare.
In offices, reflections on clear plastic sleeves, computer monitors or glossy paper are not interpreted as information (brilliance), but as disturbing glare, disturbing as it is felt that the information we require is being concealed behind the reflections.
Resource: Handbook of Lighting Design – ERCO Edition (Rüdiger Ganslandt, Harald Hofmann)