Lighting Terms

Accent Lighting - Directional lighting designed to emphasize a particular object or to draw attention to a part of the field of view.

Ambient Light - Electric and/or natural lighting throughout a space that produces uniform general illumination.

Beam Angle – The angle between the two directions for which the intensity is 50% of the maximum intensity as measured in a plane through the nominal beam centerline.

Beam Spread - The angle between the two directions in the plane in which the candlepower is equal to a stated percent of the maximum candlepower in the beam.

Chromaticity - Refers to the dominant or complementary wavelength and purity aspects of the color taken together.

Coefficient of Utilization (CU) - The ratio of lumens from a luminaire received on the work plane to the total quantity of lumens emitted by the lamps of that luminaire.

Color Rendering Index (CRI) - A measure of a lamp’s ability to render colors accurately. The scale ranges from 1 (low pressure sodium) to 100 (the sun). A CRI of 85 is considered to be very good.

Contrast - The ratio of the luminance of an object to that of its immediate background.

Cut-off Angle - The critical viewing angle beyond which a source can no longer be seen because of an obstruction.

Daylight Factor (DF) - The ratio of daylight illumination at a given point on a given plane due to the light received directly or indirectly from the sky of known luminance distribution. Direct sunlight is excluded for both values of illumination. The daylight factor is the sum of the sky component, the external reflected component, and the internal reflected component.

Diffusing (surface) - Those surfaces and glazing that redistribute some of the incident flux by scattering in all directions.

Filament - A tungsten wire that incandesces or lights up when an electric current runs through it.

Hue - The attribute of a color that allows it to be classified as red, yellow, blue, and so on.

Illuminance - The density of incident luminous flux on a surface; illuminance is the standard metric for lighting levels, and is measured in lux (lx) or foot-candle (fc).

Incandescent Lamp – Lamp in which light is produced by means of an element heated to the point of incandescence by the passage of an electric current.

Lamp - An electrically energized source of light, commonly called a bulb or tube.

Lamp Depreciation - The decrease over time of the lamp lumen output, caused by bulb wall blackening, phosphor exhaustion, filament depreciation and other factors.

Lens – A glass or plastic element used in luminaries to change the direction and control the distribution of light rays.

Light - Radiant energy that is capable of exciting the retina and producing a visual sensation.

Light Loss Factor (LLF) - A factor used in calculating the illuminance after a given period of time and under given conditions. It takes into account temperature and voltage variations, lamp depreciation (of electric luminaries), and dirt accumulation on luminaire and room surfaces, maintenance procedures and atmosphere conditions.

Luminance - The luminous intensity of a surface in a given direction per unit area of that surface as viewed from that direction.

Matte Surface - Dull Surface from which the reflection is predominantly diffuse.

Photocell - A device that measures the amount of incident light present in a space.

Power - The rate at which energy is taken from an electrical system or dissipated by a load, expressed in watts (W); power that is generated by a utility is typically expressed in volt-amperes (VA).

Rated Life - The rated life of a lamp signifies the time at which 50% of a large quantity of these lamps will have burned out. That means that 50% of these lamps will burn out BEFORE the rated life and 50% will burn out AFTER the rated life. The rated life does NOT mean that every one of the lamps will last at least that long. Also, please note that the Actual Life of a Lamp = the Rated Life of that Lamp x (Rated Voltage/Operating Voltage) raised to the 12th power. For example, the Actual Life of a certain Lamp that is designed to be used with 130 volts is equal to the Rated Life of that Lamp (let's say 1000 hours) x (130 volts/ 120 volts) raised to the 12th power. The Actual Life of this Lamp is, therefore, equal to (1000 hours) x (1.083) raised to the twelfth power = (1000 hours) x (2.61) = 2610 hours. That means that an incandescent lamp that has a rated life of 1000 hours and is designed to be used with 130 volts BUT is used with 120 volts instead will have an Actual Life of 2610 hours.

Refraction - The process by which the direction of light changes as it passes obliquely from one medium to another in which its speed is different.

Specular Angle - The angle of mirror reflection (angle of incidence equals angle of reflectance).

Specular Reflection - The process by which incident light is redirected at the specular (mirror) angle.

Specular Transmission - The process by which incident flux passes through a surface or medium without scattering.

Ultraviolet Radiation (UV) – Invisible to the naked human eye and is generally considered to be that electromagnetic radiation (radiant energy) which has a wavelength of less than 400 nanometers (nm) and greater than 100 nm.

Visual Comfort Probability (VCP) - The rating of a lighting system expressed as a percentage of the people who, when viewing from a specified location and in a specified direction, will be expected to find it comfortable in terms of discomfort glare.

Voltage Drop - The loss of voltage caused by the electrical resistance of the wire and the lamps. Voltage drop may be minimized by using a lower gauge wire and shortening the distance between the transformer and the lamps.