Lumes

Lumes are luminescent materials used to create visible glow on surfaces such as watch dials, instrument gauges, and signage. The term is commonly encountered in horology, where lume is applied to numerals and hands to improve legibility in darkness. Etymologically, it derives from lumen, a unit of light, and from luminous materials that emit or store light.

Historically, lumes varied in composition. Early formulations often relied on radioactive compounds, such as radium-based paints, which produced strong afterglow but posed health and environmental concerns. By the mid-20th century these materials were phased out in favor of safer alternatives. In the late 20th century, tritium gas tubes provided self-luminous markers without external charging, using beta decay to continuously excite a phosphor.

Modern lumes predominantly use non-radioactive phosphorescent pigments, with strontium aluminate doped with europium and dysprosium as a common standard. These pigments absorb light and re-emit it gradually, offering longer afterglow and greater stability than earlier zinc sulfide formulations. Performance depends on pigment particle size, binder compatibility, and exposure to light prior to use; charging by ambient light is usually sufficient for several hours of glow.

Applications extend beyond watches to aviation and automotive instruments, safety signage, and decorative fixtures. Maintenance often involves re-luming or repainting worn markers to restore legibility, especially on vintage pieces. While modern lumes are generally considered safe, historical radium-based materials are now regulated and avoided in consumer products. See also luminous paint, photoluminescence, and watchmaking.