In Search of Luminance | Lighting Analysts
Illuminance is called light level and is measured in footcandles (fc). fixture efficiency; lamp lumen output; the reflectance of surrounding surfaces Contrast is the relationship between the luminance of an object and its background. .. The lighting distribution that is characteristic of a given luminaire is described using the. Daylight factor (at a point) is best described as: The relationship between illuminance and luminance for an opaque surface is best described. With a moist, smooth corneal surface and clear crystalline lens in place, it may At that time, either an opaque disc may be placed on the corneal surface to summarizes the categories, and illuminance and luminance are described in In the relationship between the point light source and the light incident on a surface, .
Recalling that the surface can be real or imaginary, we can imagine placing an imaginary surface that is perpendicular to the beam direction i.
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? Imagine that you are looking at a blank sheet of matte white paper. Being an approximately ideal diffuser except at very oblique anglesthis paper will scatter incident light equally in all directions.
In accordance with the inverse square law, the luminous flux density of this light will decrease with the square of the distance from the point source.
To answer this, we need to look at the eye itself, which basically consists of a lens that focuses images onto the cones and rods of the retina. Each cone and rod has a finite width, and so it receives light from a finite area of the surface of the paper. This area of the paper is dependent on the distance of the paper from the eye. Moreover, it is proportional to the square of the distance … which exactly cancels out the inverse square law for a single point source.
Therefore, we perceive the luminance of a finite area surface as being constant regardless of its distance from the eye. There is a counterexample that emphasizes this point: Even though the actual diameter of a star may be a million miles or so, it is so far away that we perceive its light as a parallel beam that is focused onto a single rod or cone of our retina.
The luminance of this beam is constant, and so we see the star as having a specific perceived brightness or visual magnitude. 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. 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.
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. Often used to replace the lens in an installed troffer to enhance its appearance. The paracube is visually comfortable, but the luminaire efficiency is lowered. Also used in rooms with computer screens because of their glare-reducing qualities.
Brightness, Luminance, and Confusion
A light sensing device used to control luminaires and dimmers in response to detected light levels. A photometric report is a set of printed data describing the light distribution, efficiency, and zonal lumen output of a luminaire.
This report is generated from laboratory testing. The ratio of AC volts x amps through a device to AC wattage of the device. Some utilities charge customers for low power factor systems. A compact fluorescent lamp with a double twin tube configuration.
Interference to the radio frequency band caused by other high frequency equipment or devices in the immediate area. Fluorescent lighting systems generate RFI.
This ballast quickly and efficiently preheats lamp cathodes to start the lamp. Uses a "bi-pin" base. A ratio of room dimensions used to quantify how light will interact with room surfaces. A factor used in illuminance calculations.