Working with Critical Light

Lighting plays an equally big role in enhancing the beauty of a room as the paint work itself. However, the effect of lighting on the paint poses a major challenge for painters. Read on to learn more about these effects and the possible solutions.

Today’s buildings benefit from many innovations that have increased the speed of construction and allow almost unlimited design freedom. The use of lightweight framing and plasterboard wall and ceiling linings has been a major component in achieving this. Thanks to these, we now enjoy more open plan spaces and larger window areas that encourage natural light into homes and work environments, enhancing the sense of well-being.

Builders, plasterers and painters work hard to achieve the appearance of a flat surface when installing walls and ceilings. However some surface variation is inevitable due to the following factors:

  • Natural variations in the framing
  • The hand-finished nature of a plasterboard wall or ceiling
  • Subtle differences between the textures of plasterboard and the jointing compounds

Under the majority of lighting conditions a plasterboard surface finished to a Level 4 standard, as defined in AS/NZS 2589:2007 ‘Gypsum Linings – Application and finishing’, will appear flat. In critical lighting conditions, an effect referred to as ‘glancing light’, will highlight any surface variations.

What is Glancing Light?

Glancing 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


Critical and Glancing Light

One of the major issues affecting the use of plasterboard and its associated paint covering is consumer expectation in regard to the visual appearance. This visual appearance is significantly affected by lighting conditions both natural and artificial. HDL believes that consumers should be aware and consider some of these issues.

Natural Glancing Light

The intensity of natural lighting changes constantly throughout the day depending on the position of the sun. Sunlight can be reflected from adjacent buildings and water surfaces to produce unwanted rippling effects on walls and ceilings.

Artificial Glancing Light

The positioning of windows greatly influences how much glancing light will enter a room. Windows placed close to a ceiling or corner will allow light to fall across a surface and show up undulations on what would otherwise appear as a perfectly flat surface. Experience has shown that many joints that become visible under glancing light are difficult to see under normal lighting conditions. These joints are in many cases, well within industry tolerances. The most common form of artificial lighting is the surface mounted batten holder. Centrally mounted in a room and close to the ceiling without a shade, this style of lighting provides the worst conditions for glancing light.

Some Solutions

  • Avoid locating windows close to wall and ceiling surfaces during design phase
  • Introduce drapes where windows are close to wall and ceiling surfaces
  • Hang mesh curtains to diffuse light entering a room
  • Avoid mounting lights directly on surfaces and lot use shades to point light away from surfaces.
  • Use recessed light fittings in preference to surface mounted fittings
  • Position down lights so that they don’t shine down the face of the wall
  • Reduce shadows by fitting multiple light fittings of less intensity rather than one bright light.
  • Ensure plasterboard sheets are fixed horizontally in passage ways
  • Use flat paint finishes on ceilings – white for best results.
  • Increase the level of plasterboard finish specified.

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