"Critical light" is a term pros use for a common wall problem. It refers to the way light hits a flat wall surface. A table lamp sitting next to a wall shines critical light on the nearby surface and it will show even the smallest imperfections. Lumps, bumps, paint runs and nail heads, even tiny specks throw big shadows when critical light spills flush across a wall. Pros often use critical light to catch flaws. A drywall finish job typically looks great until it's painted and illuminated with critical light; then the awful truth comes out, especially if the paint is glossy. Pros avoid this by sanding and continually checking with a critical light from various angles, they use a high-power flashlight or a high-intensity lamp that projects hard-edged shadows. They then sand and tweak until it's super smooth. Then they paint. And that's the On The House tip for today.