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‘Glancing light’ describes light that shines obliquely across the surface of a wall or ceiling. It generally occurs when a single unshaded light bulb is fixed directly to a ceiling, wall or in a room with windows up to ceiling level or windows adjacent to walls.
The phenomenon is caused when the angle of the light illuminates an undulation and creates a shadow on the other side of the undulation. The light reveals perceived imperfections or surface texture variations (caused by joints and patches) which, under more diffused light, would not be visible.
Section 3 of AS/NZS 2589:2007 Gypsum linings – Application and Finishing sets out a series of levels for the surface finish of plasterboard lining. The levels range from 3 to 5 (5 being the highest).
A Level 5 finish is generally required for use where gloss / semi-gloss or dark paint colours are used, especially where critical lighting conditions occur on flat, satin or low-sheen paints. Compared to a Level 4 finish, a Level 5 finish requires greater tolerances of substrate straightness, which will require additional costs in terms of time and materials so as to minimise the risk of any effects from ‘glancing light’ on the painted surface.
This ultimately means that any expectations of a Level 5 finish providing a perfect flat surface are unrealistic.
More information is available in CSIRO’s Illumination and Decoration of Flat Surfaces leaflet.
Finished joints in plasterboard can vary in appearance depending on the lighting conditions (natural and artificial). The characteristics of the surface texture tend to show variations (joints, patches, etc.), which are generally minimised in diffused lighting.
Ceiling and wall joints should run in the direction of the light source – e.g. at right angles to windows or large openings. Butt joints should be avoided where possible.
Situations in which glancing light occurs can be improved by installing appropriate soft furnishing, pelmets, curtaining or blinds, or by redecorating with light matte finishes.
Ceiling-mounted light fittings can provide the greatest glancing light effect because the light source is close to the ceiling surface. Therefore, the angle of incidence onto the ceiling is very shallow. A centrally located light fitting in a room allows for a shallow angle of light to be broadcast in all directions, reducing apparent defects. The ideal lighting technique is to use a series of fittings hung as low as possible below the ceiling. Multiple light sources cancel out the shadows cast by each individual light source.
The same principle applies to wall fittings. Wall-mounted lights tend to accentuate minor imperfections not normally seen in the walls. In addition, high-output light sources can be more severe in their effect because they create deeper shadows. Soft, low-wattage diffused lighting provides the best results.
The HIA Guide to Materials and Workmanship [LINK TO RELEVANT RESOUCE IN THE HIA SHOP] contains information on glancing light and plaster finishes that may assist the installer.
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