Recent advances in lighting calculations, based on an integral-equation formulation for interflections have led to the practical development of highly efficient and glare-free luminous ceilings. The exact equations, however, are formidable and even the calculated tables are somewhat clumsy for engineering use.
A slide rule is therefore devised for simplifying the design of ceiling lighting. With a single setting, the rule gives average lumens per unit area at table level, the coefficient of utilization, and the helios (brightness) ratios for walls, ceiling, and floor. The device is applicable only when standardized reflectances are employed in the room. For other values of reflectance, similar slide rules may be constructed or simple approximate formulas of an exponential type may be used.
PARRY MOON and DOMINA EBERLE SPENCER, "A Slide Rule for Lighting Calculations," J. Opt. Soc. Am. 41, 98-100 (1951)