Critical Lighting

Robert Grupe / June 2021

Q: What is meant by the phrase “critical lighting”?

A: In general, critical lighting can be considered as any lighting condition that tends to magnify surface anomalies on a finished surface. The finish could be an interior painted gypsum panel wall or ceiling surface. It could also be an exterior wall covered with cement plaster. There are several recognized definitions that will be provided here.
ASTM C840, Standard Specification for Application and Finishing of Gypsum Board, defines it as “a condition where interior surfaces are flooded by natural or artificial lighting at an oblique angle.”
The Gypsum Association, in GA-214, The Finishing & Decorating of Interior Gypsum Panel Surfaces, has a similar definition: “Strong side lighting from windows or surface-mounted light fixtures.”
This definition is expanded in the comments section: “Critical (Severe) Lighting Areas. Examples include wall and ceiling areas that are illuminated or flooded with artificial and/or natural light. Strong oblique light from windows or surface-mounted light fixtures may exaggerate minor surface differences.
USG Corp., in the Gypsum Construction Handbook, defines critical lighting this way: “Strong, angular or harsh light that can show imperfections in reflecting surfaces. Most common sources are skylights, wall sconces and directed track lights as in art galleries.”
The Technical Services Information Bureau, in Technical Bulletin 60.100 Procedures for Judging Finished Portland Cement Plaster, states: “No plaster wall is perfectly flat. Strong light casting across the surface at just the right angle will make a good wall look bad, sometimes really bad. Sconce or up-lighting can have the same effect to the wall.”
In the report “Illumination and Decoration of Flat Surfaces” published by the Division of Building Research Commonwealth Scientific and Industrial Research Organization in Australia, the term “glancing light” is used. It is the same concept simply that light, depending on its location and direction, can have a very different and potentially serious negative effect on the overall finished appearance of a wall or ceiling, be it interior or exterior.
There are several points to be gleaned from all these definitions. This glancing light affects all surfaces, not just gypsum panels and plaster, for example, poured-in-place concrete. The affected surface can be otherwise sound, with no other defects. This is purely an aesthetic issue.
Surfaces over gypsum panels, plaster, be it gypsum or cement based, and even cast-in-place concrete, are intended to give the illusion of a flat plane. The reality is that the intended flat plane is unachievable. There will always be surface undulations. Even metal spandrel panels cannot provide that flat plane, meaning surface undulations are visible under certain and critical lighting conditions.
Mitigating the effects of critical lighting starts in the design stage. Controlling the placement and type of lighting can help. This is impossible on exterior applications, where based on the building’s orientation and time of day, the sun will cause shadows on the finished surface revealing all abnormalities. The location of interior partitions in relation to the exterior fenestration can be problematic. A long corridor that terminates normal to or 90 degrees into a glass curtain wall is an issue. The streaming light coming from the windows will highlight all abnormalities in both a gypsum panel or plaster surface. As mentioned in the definitions, skylights and track lighting can challenge the appearance of a wall or ceiling surface.
Surface texture can also help. Texture on the finished surface can help minimize the impact of critical lighting. Diffusing the light can be an aid.
In lighting the wall artificially, there is a difference between the terms “wall washing” and “wall grazing.” In wall washing, the source of the light is placed a specified distance away from the wall to uniformly “wash” the wall with light. This distance away from the wall is to minimize the potential for creating shadows on the wall surface. Wall grazing is to create the opposite effect. Here the concept is to create the shadows to highlight the texture of the wall. In this situation, the light source is purposely set close to the wall to create the desired effect. It is advisable that a lighting consultant be brought in to analyze critical conditions.
It is important for the contractor to understand critical lighting before physically starting the project. A review of the plans and specification looking for critical lighting conditions should be undertaken. Determining where there are potential critical lighting conditions, and matching the desired finish on the wall or ceiling, is a prudent exercise. Discussions with the design team early on to manage expectations may be an important step. Critical lighting may be the final desired condition to enhance the wall surface, but in most cases it is not. Knowing the difference and communicating any potential issues is critical for the success of the project.

Robert Grupe is AWCI’s director of technical services. Send your questions to, or call him directly at (703) 538.1611.