What is critical light?

Critical light is when you look at a surface from an angle that reflects on a surface in a way that it shows up defects in the substrates. Examples of this might be along a hallway or stairwell that has a window at the end of it, ceilings that lookout to a great view but also a lot of light. Glancing light can be reflections, examples of this would be reflections from water, swimming pools, mirrors etc.

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For more information, see this Master Painters Fact Sheet.