Glancing Light

When specifiers, builders and home owners are considering the type of finish they require from plasterboard, it is important to understand how the overall appearance is likely to be affected by glancing light and the choice of decoration.

Although a flat finish is the common requirement this is not always possible with present building technology. No matter how flat a surface may appear, there are always undulations and deviations from flatness. What is possible is the appearance of flatness, though this is dependent on the level of finish and the way the lining is perceived.

Light Variations

Finished joints are subjected to many different lighting conditions. Light intensities are constantly changing throughout the day, depending on the position of the sun, sky conditions, window, door or the location of light source, reflections from surrounding buildings, ground etc., subjecting the ceilings to diffused light from all directions.

At night, the intensity varies again.

Glancing light is the light that shines obliquely across the surface of the wall or ceiling. The worst instances of glancing light occur with single unshaded light sources fixed directly to the ceilings or walls or in rooms with windows up to ceiling level or windows adjacent to walls.

Because of the angle of the light, it illuminates one side of even the most minute undulation and creates a shadow on the other side. This highlights imperfections which under more diffused light would not be visible.

Natural Glancing Light

The best way to illustrate circumstances that accentuate glancing light effect is by illustrating common conditions.

• The location of windows that extend up to (or close to) the corner of the room. This condition allows natural light to fall upon the side wall at a very shallow angle. The angle of light will, of course, vary according to the time of day. Accordingly, defects will be more apparent at certain times.

• A similar situation to that described above, occurs with windows that extend to the ceiling surface. Obviously, dawn or dusk will produce the most pronounced glancing light but reflection from an outside surface can extend the time of the effect considerably.

• Another classic situation is the raked ceiling coupled with high level windows such as southlight windows or louvred windows. In this case, the problem is invariably complicated by reflections from adjacent roof surfaces.

Where the physical situation producing glancing light effect exists the situation can be improved by appropriate soft furnishings, pelmets, curtaining, blinds or redecorating with light matt finishes.

Surface Finishes

When the problem cannot be designed out, an alternative solution is to adopt one of the variety of textured finishes currently available. If textures are not favoured, the problem can be limited by selection of light, matt colours.

Artificial Lighting - Ceilings

The most common way of providing artificial lighting is by a surface mounted ceiling fitting in the centre of the ceiling. This also provides the worst characteristic for producing glancing light effect. The light source is close to the ceiling surface and therefore the angle of incidence onto the ceiling is very shallow. Placing the fitting centrally in a room means that the shallow angle of light occurs in all directions.

Clearly, from the point of view of reducing apparent defects the ideal lighting technique is to use a series of fittings hung as low as possible below the ceiling. The angle of light striking the surface is as great as possible and the multiple light sources cancel out each others shadows.

Artificial Lighting Walls

The same effect applies to wall fittings. Wall mounted lights may well show up a ceiling in a kindly manner, because they are some distance below it. They will, however, tend to accentuate defects in the walls to which they are necessarily very close. Similarly, high output light sources are more severe in their effect because they create deeper shadows. The whiter the light, the stronger the contrast, the greater the apparent defect.

The most severe case occurs when a surface mounted fluorescent batten fitting is mounted in the centre of a surface. It produces high output white light at a shallow angle of incidence.

The position of feature lighting using small spots or flood lights can be considered. The fittings should allow a generous angle of incidence to the featured surface if they are to minimise defects. In this case, however, the reverse effect can be employed to highlight a heavily textured surface by deliberately lighting at a shallow angle and therefore highlighting "deliberate" defects.

In general terms, recessed fittings are kind to ceiling surfaces, though they should be kept away from walls. This is assuming that they are deeply recessed and do not have a flush mounted diffuser which transmits light to the ceiling surface.

Lighting design is very much a matter of cause and effect. By understanding the role that fittings play and the consequences of their position it is possible to minimise adverse effects on quite normal standards of workmanship.