Glancing Light on Plasterboard


Glancing light (or critical light) is a condition which exists when light hits the plasterboard surface at an acute angle and casts shadows that highlight any surface irregularities. On plasterboard walls and ceilings this can make the surface look uneven and highlight the appearance of joints.
This is most commonly found in situations where there are:
• Floor to ceiling windows.
• Windows directly adjacent to walls.
• Unshaded batten holder ceiling lights.
• Ceiling mounted fluorescent lights.
• Wall lights and downlights close to walls.
• Windows at the end of long corridors.
• Brightly lit rooms.
• Lights installed just below skillion/raked ceilings.
• Reflections of light from water features.

Consideration to Minimise
Glancing Light

The best time to consider potential glancing light issues is during the design phase, which allows choices to be made that can greatly reduce the impact of glancing light.
Large window areas are a popular feature of modern design and the preference for open plan living and working often results in ceilings and walls that extend through a number of different spaces. These features can lead to challenging lighting conditions for wall and ceilings surfaces.
When designing a project it is important to consider the effect of both natural and artificial light and how it will fall on the walls and ceilings across the whole day.
In particular, attention should be given to light entering the building in mornings and evenings when the sun is lower in the sky and casts elongated shadows that can highlight any surface variations in walls and ceilings.


For windows that are positioned where glancing light can be an issue, the use of external shading or vertical louvres may help to mitigate any problems. Curtains or interior blinds are also helpful in this situation.


Ideally windows should not abut walls or ceilings and should be oriented away from the east and west. External reflective surfaces, such as pools or neighbouring buildings, can reflect light into the space, should also be considered as they can exacerbate the problem.


The installation of plasterboard walls and ceilings should also be considered as there are a number of design and installation choices which can significantly impact the appearance of the surface.
Running the plasterboard so that the long joints are parallel to the direction of the light will help reduce the effects of glancing light. The use of longer sheets to reduce the number of butt joints is also beneficial.


Any imperfection in a completed lining installation will be made obvious by a condition called critical lighting or glancing light, where the incident light from an artificial or natural light source is nearly parallel to the surface. Glancing light also greatly exaggerates the size of imperfections making them glaringly obvious.
The worst result is achieved by an unshaded light source located directly on a ceiling or wall where the light shines parallel to the surface.

Cases where this situation may exist include:

• Unshaded batten holder light fittings.
• Fluorescent lights mounted on the ceiling.
• Wall mounted up lights and downlights.
Methods to minimise glancing/critical lighting effects from artificial lighting sources

The following lighting solutions will provide diffused light and reduce the appearance of surface variations:

• Shaded batten holder light fittings.
• Ceiling mounted pendant lights.
• Recessed ceiling lights such as downlights and recessed fluorescents (although recessed lights are more likely to be associated with glare problems).
• Consider the use of more lights of lower intensity at regular spacings, ensuring lit areas overlap. This will improve ambiance and reduce the visible effects of glancing light, and minimise shadows that can occur from a single row or single light source.
• Allow a generous angle of incidence to the surface for feature lighting such as spotlights, to minimise the highlighting of imperfections.
• Do not locate a single or isolated unshaded light source close to a wall or ceiling in a space which has generally low levels of light.
• Do not use uplights, wall-washers and spotlights in areas with a smooth wall finish to eliminate light being emitted at a glancing angle to the surface.
• Preferably, locate fluorescent lights about 450mm below the ceiling as this will give a more even distribution of light.
• When installing ceiling mounted fluorescent lights it is recommended to position the light fittings over the long edge joints.

Methods to minimise glancing/critical lighting effects from natural lighting sources

• Do not take window glazing right up to the ceiling level.
• Avoid placing windows or glass doors immediately adjacent to the end of a wall.
• Provide sun shades over the windows and glass doors.
• Recess the window to stop the sunlight reaching the wall.

Attribution: CSR Residential Installation Guide. The RED Book.

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