Lighting A to Z

GlossaryDirect Current (DC) Flow of electricity continuously inone direction.Direct Glare Glare resulting from high luminances orinsufficiently shielded light sources in the field of view.Direct Lighting Lighting by means of fixtures or luminairesthat have 90 to 100 percent of the light reaching the workingplane directly.Directional Lighting Lighting in which the light on theworking plane or on an object is coming predominately from oneparticular direction.Discharge Lamp Lamp in which the light is produced,directly or indirectly, by an electric discharge through a gas, ametal vapor, or a mixture or several gases and vapors.Discomfort Glare Glare that causes discomfort withoutnecessarily impairing the vision of objects.Dispersion To split light into the colors of the rainbow.Distance (D) The distance from the light source to theworking plane.Downlight (Can, High Hat) Small luminaire, whichdistributes the light downward, usually recessed in the ceiling.Efficacy Of a source. Efficacy is the rate at which lamp is ableto convert electrical power (watts) into light (lumens), expressedas lumens per watt. Divide light produced (lumens) by the powerconsumed (watts). Lumens/Watts = LPW. Units lumen per wall,LPW, lm/WEfficiency Often misused term in lighting, to describe lumensper watt; the correct term is efficacy. See efficacy.Electric Discharge The passage of an electric currentthrough gases and vapors. This results in the emission of electromagneticradiation (light).Electromagnetic Spectrum A continuum of electric andmagnetic radiation that can be characterized by wavelength orfrequency. Visible light encompasses a small part of theelectromagnetic spectrum in the region from about 380nanometers (violet) to 770 nanometers (red) by wavelength.46 Product Knowledge