Blackout Is a Gap Problem: Light at the Eye from Blind Edge Leakage as a Function of Gap Width, and the Overlap That Replaces Expensive Fabric

Expensive blackout fabric is specified by its transmittance, but the light that reaches a sleeper's eye in a real installation is dominated by the gaps at the edges, not by the fabric. This paper models a 1.4 by 1.3 metre window lit at a stated 300 lux, comparing fabric transmission with perimeter gap leakage as gap width grows from 0 to 20 mm. With the gaps sealed, blackout fabric holds the eye to 0.15 lux; a gap of only 2.0 mm around the perimeter breaches a stated 1 lux design criterion, and a 10 mm gap admits 4.6 lux — more than an ordinary curtain with sealed edges delivers through its fabric at 7.5 lux. A simple overlap strip that forces gap light through two reflections cuts the 10 mm case to 0.32 lux. Darkness, in short, is bought at the perimeter, not in the fabric shop. This study is simulated, not measured.