The relationship between illuminance and luminance for an opaque surface is best described as


Figure 7: Measurement grid for Luminance Values at Receptor Site A . . Figure Modeled interior illuminance of a section of future roof top. .. This term describes light intensity on a surface. In regards .. The diagram indicates the relationship between candelas (cd), foot- candles. The relationship between illuminance and luminance for an opaque surface is The basic principle of operation of an incandescent lamp is best described as. White. The relationship between illuminance and luminance for an opaque surface is best described as: luminance is illuminance as modified by reflectance.

The angle from a fixture's vertical axis at which a reflector, louver, or other shielding device cuts off direct visibility of a lamp. It is the complementary angle of the shielding angle. A dimming system controlled by a photocell that reduces the output of the lamps when daylight is present. As daylight levels increase, lamp intensity decreases. An energy-saving technique used in areas with significant daylight contribution. Term describing dispersed light distribution.

Refers to the scattering or softening of light. A translucent piece of glass or plastic sheet that shields the light source in a fixture. The light transmitted throughout the diffuser will be redirected and scattered. Glare produced by a direct view of light sources. Often the result of insufficiently shielded light sources.

A type of ceiling luminaire, usually fully recessed, where most of the light is directed downward. A metric used to compare light output to energy consumption.

Efficacy is measured in lumens per watt. Efficacy is similar to efficiency, but is expressed in dissimilar units. For example, if a watt source produces lumens, then the efficacy is 90 lumens per watt. A light source technology used in exit signs that provides uniform brightness, long lamp life approximately eight yearswhile consuming very little energy less than one watt per lamp.

A ballast that uses semi-conductor components to increase the frequency of fluorescent lamp operation typically in the kHz range. Smaller inductive components provide the lamp current control.

Fluorescent system efficiency is increased due to high frequency lamp operation. A variable output electronic fluorescent ballast. Abbreviation for electromagnetic interference. High frequency interference electrical noise caused by electronic components or fluorescent lamps that interferes with the operation of electrical equipment.

EMI is measured in micro-volts, and can be controlled by filters. A type of magnetic ballast designed so that the components operate more efficiently, cooler and longer than a "standard magnetic" ballast.

By US law, standard magnetic ballasts can no longer be manufactured.

Brightness, Luminance, and Confusion

A lower wattage lamp, generally producing fewer lumens. A light source consisting of a tube filled with argon, along with krypton or other inert gas. When electrical current is applied, the resulting arc emits ultraviolet radiation that excites the phosphors inside the lamp wall, causing them to radiate visible light. The English unit of measurement of the illuminance or light level on a surface. One footcandle is equal to one lumen per square foot.

English unit of luminance. The effect of brightness or differences in brightness within the visual field sufficiently high to cause annoyance, discomfort or loss of visual performance.

A harmonic is a sinusoidal component of a periodic wave having a frequency that is a multiple of the fundamental frequency. Harmonic distortion from lighting equipment can interfere with other appliances and the operation of electric power networks.

The total harmonic distortion THD is usually expressed as a percentage of the fundamental line current. Abbreviation for high intensity discharge. Generic term describing mercury vapor, metal halide, high pressure sodium, and informally low pressure sodium light sources and luminaires.

Pertains to the type of lighting in an industrial application where the ceiling is 20 feet or higher. Also describes the application itself. A lamp or ballast designed to operate at higher currents mA and produce more light. A ballast with a 0. A high intensity discharge HID lamp whose light is produced by radiation from sodium vapor and mercury.

The phenomenon of re-striking the arc in an HID light source after a momentary power loss. Hot restart occurs when the arc tube has cooled a sufficient amount.

A photometric term that quantifies light incident on a surface or plane. Illuminance is commonly called light level. It is expressed as lumens per square foot footcandlesor lumens per square meter lux.

SID Article - Brightness, Luminance, and Confusion

Glare produced from a reflective surface. A fluorescent circuit that ignites the lamp instantly with a very high starting voltage from the ballast. Instant start lamps have single-pin bases. The peak lamp current divided by the RMS average lamp current. An LCCF of 1.

A factor that represents the reduction of lumen output over time. The factor is commonly used as a multiplier to the initial lumen rating in illuminance calculations, which compensates for the lumen depreciation.

The LLD factor is a dimensionless value between 0 and 1. A fluorescent fixture; usually a 2' x 4' fixture that sets or "lays" into a specific ceiling grid. Abbreviation for light emitting diode. An illumination technology used for exit signs. Consumes low wattage and has a rated life of greater than 80 years. Transparent or translucent medium that alters the directional characteristics of light passing through it.

Usually made of glass or acrylic. Factors that allow for a lighting system's operation at less than initial conditions. These factors are used to calculate maintained light levels. LLFs are divided into two categories, recoverable and non-recoverable. Examples are lamp lumen depreciation and luminaire surface depreciation. The total costs associated with purchasing, operating, and maintaining a system over the life of that system.

Grid type of optical assembly used to control light distribution from a fixture. Can range from small-cell plastic to the large-cell anodized aluminum louvers used in parabolic fluorescent fixtures. Essentially, an uncorrected ballast power factor of less than 0. A low-pressure discharge lamp in which light is produced by radiation from sodium vapor. Considered a monochromatic light source most colors are rendered as gray.

A lamp typically compact halogen that provides both intensity and good color rendition. Lamp operates at 12V and requires the use of a transformer. A relay magnetically-operated switch that allows local and remote control of lights, including centralized time clock or computer control. A unit of light flow, or luminous flux. The lumen rating of a lamp is a measure of the total light output of the lamp. A complete lighting unit consisting of a lamp or lamps, along with the parts designed to distribute the light, hold the lamps, and connect the lamps to a power source.

Also called a fixture. The ratio of total lumen output of a luminaire and the lumen output of the lamps, expressed as a percentage. For example, if two luminaires use the same lamps, more light will be emitted from the fixture with the higher efficiency. A photometric term that quantifies brightness of a light source or of an illuminated surface that reflects light.

It is expressed as footlamberts English units or candelas per square meter Metric units. The metric unit of measure for illuminance of a surface. One lux is equal to one lumen per square meter. One lux equals 0. Refers to light levels of a space at other than initial or rated conditions. This terms considers light loss factors such as lamp lumen depreciation, luminaire dirt depreciation, and room surface dirt depreciation.

A type of high intensity discharge HID lamp in which most of the light is produced by radiation from mercury vapor. Emits a blue-green cast of light. Available in clear and phosphor-coated lamps.

A type of high intensity discharge HID lamp in which most of the light is produced by radiation of metal halide and mercury vapors in the arc tube. A low-voltage quartz reflector lamp, only 2" in diameter. Typically the lamp and reflector are one unit, which directs a sharp, precise beam of light. A reference direction directly below a luminaire, or "straight down" 0 degree angle. Abbreviation for National Electrical Manufacturers Association. Abbreviation for National Institute of Standards and Technology.

Control device that turns lights off after the space becomes unoccupied. May be ultrasonic, infrared or other type. A term referring to the components of a light fixture such as reflectors, refractors, lenses, louvers or to the light emitting or light-controlling performance of a fixture.

A parabolic aluminized reflector lamp. An incandescent, metal halide, or compact fluorescent lamp used to redirect light from the source using a parabolic reflector.

Lamps are available with flood or spot distributions. A popular type of fluorescent fixture that has a louver composed of aluminum baffles curved in a parabolic shape.

The resultant light distribution produced by this shape provides reduced glare, better light control, and is considered to have greater aesthetic appeal. A metallic coated plastic louver made up of small squares. More than a mathematical definition, professional lighting designers need to understand what it is that we see. Luminance Understood To understand luminance, we begin with a parallel beam of light. Ignore any thoughts of surfaces or light sources; just imagine a beam of light traveling through empty space in a given direction.

Imagine also that this beam has a finite width; say, a rectangular beam one meter on a side. If we take a cross-section of this beam at any point along its length, we can measure so many lumens of light i.

Being parallel, the beam does not diverge or converge, and so the luminous flux density remains constant along the length of the beam. Now, what happens if the beam illuminates a real or imaginary surface at an angle?

Conceptually, as the angle of incidence becomes greater i. This is basic high school algebra! Ignore the symbols and concentrate on the underlying physical concept.

See the previous article Solid Angles for an explanation of this concept. Recalling that the surface can be real or imaginary, we can imagine placing an imaginary surface that is perpendicular to the beam direction i.

Daylight analysis for Glare, Luminance & Illuminance

What this means is that the luminance of a parallel beam of light is constant along its length. In other words, luminance is not an intrinsic property of the surface, but of the beam itself. As an example, the sky has a measurable luminance when viewed from the ground, but it has no real surface. Dispensing with the mathematics, we can therefore say: Luminance is the amount of luminous flux per unit area as measured in a parallel beam of light in a given direction.

Photometry is traditionally taught using the concept that luminance is a property of real or imaginary surfaces. The problem with this approach is that you cannot easily explain why participating media such as the atmosphere, smoke, fog, colloidal suspensions in water, and so forth have measurable luminance.

Thinking of luminance as a property of a beam of light rather than of surfaces eliminates this difficulty. Luminance Perceived How do we perceive luminance?