Planning indirect lighting: diffuse light

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Indirect lighting is a type of lighting in which light is directed to the target surface due to reflection via another surface. If, for example, the light from a luminaire is directed towards the ceiling or walls and then reflected from there into the room, this is called indirect lighting. The relevant category of luminaires is called ceiling washlights or wallwashers. Because ceilings and walls usually have a matt surface, indirect lighting generates diffuse, uniformly scattered light that does not create harsh shadows. Indirect lighting is often used in combination with direct lighting to create a balanced mix of diffuse and directional light.

What is indirect lighting?

Indirect lighting uses the ceiling, walls or other surfaces as secondary reflectors. If it is sufficiently bright, general illumination with diffuse light is created. The brightening of the room's boundary surfaces creates an open spatial impression that visually expands the room.

The diffuse light leads to low shadowing and weak modelling. Shapes and surface structures are only slightly emphasised. Exclusive use of indirect lighting results in little spatial differentiation. Compared to direct lighting, indirect lighting requires a much higher luminous flux to produce the same illuminances on the working plane. The secondary reflector should have high reflectivity. Direct and reflected glare is largely avoided.

How can indirect lighting be planned?

Indirect lighting is achieved by the light from a primary light source being scattered by a much larger, usually diffusely reflecting surface, which thus takes on the character of a planar secondary luminaire. The architecture itself can initially be a reflector surface – the light is directed onto the walls, onto the ceiling and possibly even onto the floor, and reflected from there into the room. The luminaire is shielded from the viewer, and the light emission should be above eye level in order to avoid glare.

Good to know:
For uniform lighting, the distance between wallwashers and the wall should be around one third of the room height. The ideal distance between a pendant luminaire emitting light onto the ceiling and the ceiling itself should be at least 0.5 metres.

What is diffuse lighting suitable for?

Diffuse, indirect lighting is suitable wherever soft and uniform light is required. This is the case in work areas for example, but also in multifunctional spaces, museums, exhibitions and circulation zones. Diffuse lighting ensures a uniform as well as cosy and relaxed atmosphere, but also sufficient safety in circulation areas.

This type of lighting can be implemented with the following luminaire groups:

Lighting structures, such as track or continuous line lighting

Recessed luminaires

Wall luminaires

Pendant luminaires

Light ceilings

What is diffuse light?

In lighting design, diffuse light refers to a rather "soft", uniformly distributed light. Rooms and surfaces thus appear uniformly bright, with low contrasts and shadowing. To create diffuse light, diffuse distribution large-area luminaires are used, or reflection via the room surfaces. If the surface is rough or matt or has unevenness, the light is reflected in many different directions. This scatters the light and spreads it over a wide angle. Examples in nature are clouds that scatter sunlight or, indoors, illuminated walls or ceilings that distribute the light evenly throughout the room.

In short:
Diffuse light emanates from large, luminous surfaces, producing uniform, soft illumination with little modelling due to contrasts and without brilliance. The opposite of diffuse light is directional light.