Glossary - S

S 

scoop a floodlight consisting of a lamp in an ellipsoidal or paraboloidal matte reflector, usually in a fixed relationship, though some types permit adjustment of the beam shape. 

scotopic vision mediated essentially or exclusively by the rods. It is generally associated with adaptation to a luminance below about 0.034 cd/m2. See photopic vision. 

self-ballasted lamps any arc discharge lamp of which the current-limiting device is an integral part. 

semi-direct lighting involving luminaires that distribute 60 to 90% of the emitted light downward and the balance upward. 

semi-indirect lighting involving luminaires that distribute 60 to 90% of the emitted light upward and the balance downward. 

service period the number of hours per day for which daylighting provides a specified illuminance level. It often is stated as a monthly average. 

set light in theatrical lighting, the separate illumination of background or scenic elements. 

shade a screen made of opaque or diffusing material that is designed to prevent a light source from being directly visible at normal angles of view. 

shielding angle (of a luminaire) the angle between a horizontal line through the light center and the line of sight at which the bare source first becomes visible. See cutoff angle (of a luminaire). 

side light lighting from the side to enhance subject modeling and place the subject in depth, apparently separated from the background. 

signal shutter a device that modulates a beam of light by mechanical means for the purpose of transmitting intelligence. 

sky factor the ratio of the illuminance on a horizontal plane at a given point inside a building due to the light received directly from the sky, to the illuminance due to an unobstructed hemisphere of sky of uniform luminance equal to that of the visible sky. 

sky light† visible radiation from the sun redirected by the atmosphere. 

soft light (1) diffuse illumination that produces soft-edged, poorly defined shadows on the background when an object is placed in its path; (2) a luminaire designed to produce such illumination. 

solar efficacy the ratio of the solar illuminance constant to the solar irradiance constant. The current accepted value is 94.2 lm/W. 

spacing-to-mounting-height ratio, S/MH_wp† the ratio of the actual distance between luminaire centers to the mounting height above the workplane. Also, an obsolete term that described a characteristic of interior luminaires. See luminaire spacing criterion. 

special color rendering index, Ri measure of color shift of various standardized special colors, including saturated colors, typical foliage, and Caucasian skin. It also can be defined for other color samples when the spectral reflectance distributions are known. 

specular surface one from which the reflection is predominantly regular. See regular (specular) reflection. 

speed of light† the speed of all radiant energy, including light, is 2.9979258 × 10⁸ m/s in vacuum (approximately 186,000 mi/s). In all material media the speed is less and varies with the material's index of refraction, which itself varies with wavelength. 

sphere illumination illumination on a task from a source providing equal luminance in all directions about that task, such as an illuminated sphere with the task located at the center. 

spotlight any of several different types of luminaires with narrow beam angle designed to illuminate a well-defined area. In motion pictures, generic for Fresnel lens luminaires. Also, a form of floodlight, usually equipped with lenses and reflectors to give a fixed or adjustable narrow beam. 

standard source an electric light source having the same spectral power distribution as a specified standard illuminant. 

starter a device used in conjunction with a ballast for the purpose of starting an electric-discharge lamp. 

state of chromatic adaptation the condition of the chromatic properties of the visual system at a specified moment as a result of exposure to the totality of colors of the visual field currently and in the past. 

stray light (in the eye) light from a source that is scattered onto parts of the retina lying outside the retinal image of the source. 

street lighting luminaire a complete lighting device consisting of a light source and ballast, where appropriate, together with its direct appurtenances such as globe, reflector, refractor, housing, and such support as is integral with the housing. The pole, post, or bracket is not considered part of the luminaire.  Note Modern street lighting luminaires contain the ballasts for high-intensity discharge lamps where such lamps are used; a photocontrol can be mounted on the luminaire. 

street lighting unit the assembly of a pole or lamp post with a bracket and a luminaire. 

striplight (theatrical) once an open trough reflector containing a series of lamps; now usually a compartmentalized luminaire with each compartment containing a lamp, reflector, and color frame holder, wired in rotation in three or four circuits and used as borderlights, footlights, or cyclorama lighting from above or below. Often in short 0.9- to 2.4-m [3- to 8-ft] portable sections. 

stroboscopic lamp (strobe light) a flash tube designed for repetitive flashing. 

supplementary lighting used to provide an additional quantity and quality of illumination that cannot readily be obtained by a general lighting system and that supplements the general lighting level, usually for specific work requirements. 

surface-mounted luminaire a luminaire that is mounted directly on a ceiling. 

suspended (pendant) luminaire a luminaire that is hung from a ceiling by supports. 

Systeme Internationale (SI)† a measurement system used throughout the world, commonly referred to as the metric system. Public Law 100-418 designated the metric system as the preferred system of weights and measures for the United States.