Glossary - M

M 

maintenance factor (MF)† a factor formerly used to denote the ratio of the illuminance on a given area after a period of time to the initial illuminance on the same area. This term is obsolete and is no longer valid. See light loss factor. 

matte surface a surface from which the reflection is predominantly diffuse, with or without a negligible specular component. See diffuse reflection. 

mean spherical luminous intensity the average value of the luminous intensity in all directions for a

mercury lamp a high-intensity discharge (HID) lamp in which the major portion of the light is produced by radiation from mercury operating at a partial pressure in excess of 105 Pa (approximately 1 atm). Includes clear, phosphor-coated (mercury-fluorescent), and self-ballasted lamps. 

mercury-fluorescent lamp (phosphor mercury lamp) an electric-discharge lamp having a highpressure mercury arc in an arc tube and an outer envelope coated with a fluorescing substance (phosphor) that transforms some of the ultraviolet energy generated by the arc into light. 

mesopic vision vision with fully adapted eyes at luminance conditions between those of photopic and scotopic vision, that is, between about 3.4 and 0.034 cd/m2. 

metal halide lamp a high-intensity discharge (HID) lamp in which the major portion of the light is produced by radiation of metal halides and their products of dissociation--possibly in combination with metallic vapors such as mercury. Includes clear and phosphor-coated lamps. 

metamers lights of the same color but of different spectral power distribution.  Note The term "metamers" is also used to denote objects that, when illuminated by a given source and viewed by a given observer, produce metameric lights. 

modeling light illumination that reveals the depth, shape, and texture of a subject; key light, cross lighting, counter-key light, side light, back light, and eye light are types of modeling light. 

monocular visual field the field for a single eye. See binocular portion of the visual field. 

mounting height (roadway)† the vertical distance between the roadway surface and the center of the apparent light source of a luminaire. 

mounting height above the floor (MHf)† the distance from the floor to the light center of the luminaire, or to the plane of the ceiling for recessed equipment. 

mounting height above the workplane (MHwp)† the distance from the work plane to the light center of the luminaire, or to the plane of the ceiling for recessed equipment. 

multiple-beam headlamp a headlamp so designed as to permit the driver of a vehicle to use any one of two or more distributions of light on the road.