# Lighting Basics: Measuring Light

Light measurement lightmeter Measuring Light

There is a lot of terminology used in the lighting industry to compare, rate and measure the effectiveness, efficiency of luminaires and light sources and also to describe and measure light output, light intensity and generally to discuss and quantify lighting. We cover these commonly used terms below.

Amperes (Amps): A measure of the flow of electrical current. In incandescent bulbs, current is related to voltage and power with the formula: Watts (power) = Volts x Amps (current)

Ballast Factor (BF): The BF is a ratio used to calculate the expected performance of a lamp. It describes ballast’s actual lumen output versus the rated lumen output when a specific commercially available ballast is used. BF is expressed as a percentage, for example a ballast with a ballast factor of 0.95, will result in the lamp emitting 95% of its rated lumen output.

Beam Angle: Is the angle at which the intensity of light emitted by a light source drops to 50% of the maximum intensity. The beam angle is usually used to describe reflector lamps such as R and PAR types.

Beam Lumens: Is the total lumens contained within the portion of the beam in the beam angle.

Candela (cd): Is the measure of luminous intensity or power emitted by a light source in a given direction. The term has been retained from the early days of lighting when a standard candle was defined as producing light with a luminous intensity of the equivalent of one candela in every direction.

Candlepower: Is a term often used in place of luminous intensity used to express the luminous intensity of a source of light. One candlepower is equal to 0.981 candelas.

Candlepower Distribution Curve: The distribution of light intensity of a given source of light can be represented in a graph. This plot is referred to as candlepower distribution curve.

Centre Beam Candlepower (CBCP): Center beam candlepower is the luminous intensity at the center of a beam, expressed in candelas (cd).

Coefficient of Utilization (CU): This is the ratio of luminous flux that reaches the work plane to the luminous flux produced by the source of light. It is a function of the room shape, surface reflectance, and luminaire efficiency.

Cost of Light: A term referring to the cost of operating and maintaining a lighting system.

Crest Factor (Max Current): The ratio of the maximum lamp current (peak lamp current) to the root mean square current.

Driver Efficiency: A measure of a luminaire efficiency that focuses on the performace of the driver or power supply components. It is measure as a ratio of the power delivered the driver, divided by the power required to operate the driver.

Efficacy: Efficacy is a commonly used measure of how effective a light source is in converting electrical energy to lumens of visible light. Efficacy is measure in lumens per watt (LPW).

Footcandle (fc): Is a unit of illuminance which measures the amount of light reaching a subject. Abbreviated as fc or ftc, one footcandle is equivalent to the illuminance produced by a standard candle onto a surface that is one foot from it. One footcandle is equal to approximately ten lux.

Illuminance: It is the density of light falling on a surface. It is measured in lux or footcandles.

Initial Light Output: Also known as initial lumens, it is lamp’s light output in lumens after the first 100 hours of operating. Is a way to measure the output generated by a lighting system.

Kilowatt Hour (kWh): The unit of measuring the power consumed by an electrical device over a period of time. It is a product of power (in kilowatts) and time (in hours). The Formula is: Kilowatt Hour (kWh) = Power (kW) x time (h).

Light Loss: The reduced light output caused by a circuit-level power reducer expressed as a percentage of the light output without the circuit-level power reducer. (Full system output minus reduced output with a lighting-circuit power reducer divided by the full system output times 100.)

Light Loss Factor: A product of voltage fluctuations, lamp degradation over time, dirt, reflector degradation, and other factors that contribute in one way or another to the lowering of the illumination level.

Lumen: A measure of the quantity of light or luminous flux produced by a light source (lamp) that is visible to the human eye. A standard 100-watt incandescent lamp has 1,700 lumens.

Lumen Depreciation: The process where the lumen output reduces over time is referred to as lumen depreciation. While LED and fluorescent technology tend to last longer, they still lose brightness over time. Lumen depreciation can be accelerated depending on lighting conditions such as heat or incorrect voltage.

Luminance: Also called brightness, it is a measure of how bright a surface will appear to an observer who is focused in the direction of the illuminated surface. The SI unit for luminance is candelas per square foot or candelas per square metre (cd/m2).

Luminaire Efficiency: It is the ratio of the luminous flux produced by a luminaire to that produced by the light source (lamp). Alternatively we can express efficiency in terms of power consumed by a lamp. That is, efficiency is the watts of light emitted by a light source for every watt of electrical power consumed.

Luminous Emittance: The number of lumens given off by a surface per square metre.

Luminous Intensity: A measure of the strength or visibility of a light source in a particular direction, expressed in Candles or Candelas per solid angle (steradian) in a given direction.

Luminous Flux: Measure, usually given in lumens, of the total amount of visible light.

Lumen Maintenance: The quantity of light produced by a source lowers with time. Lumen maintenance is a measure of how good a light source is in  maintaining the quantity of light it produces over time.

Lux: Is a standard measure of light falling on a surface. One lux is equal to
one lumen per square metre. Ten lux equals roughly one footcandle.

Mean Lumens: It refers to the average light emitted by a light source over the rated life provided by the manufacturer. It is also referred to as design lumens.

Power Factor (PF): It is a measure of the difference between the actual power dissipated by a circuit and theproduct of root mean square (R.M.S) values of voltage and current. The electrical circuit could be a ballast, motor, et cetera.

Rated Lamp Life: It is the length of time that it takes for half the light sources of a statistically large batch to fail/ die.

Room Cavity Ratio (RCR): This calculation considers the room’s area and shape based on dimensions of length (L), width (W) and height (H) to summarize a room’s cavity proportions. RCR is not based on square footage since different rooms can have more walls which absorb light. The formula is calculated as follows: Room Cavity Ratio = 5H(L+W)/(LxW) or Room Cavity Ratio = (2.5) Total Wall Area/ Floor Area

Spacing to Mounting Height Ratio (S/MH): This ratio helps to calculate the maximum the distance that light fixtures can be spaced whereby they can still provide uniform illumination.

Spectral Power Distribution (SPD): It is a plot of the radiant power against wavelength of a given source of light. The SPD curve provides a means of precisely describing any source of light.

Steradian (sr): A unit of measure equal to the solid angle subtended at the center of a sphere by an area on the surface of the sphere equal to the square of the sphere radius.

Toxicity Characteristic Leaching Procedure Test: The TCLP test is used to classify fluorescent lamp waste either as hazardous or non-hazardous waste.

Total Harmonic Distortion (THD): Total harmonic distortion is a measurement of
the distortion of a signal. In electrical systems, this measure describes the distortion of an electrical signal.

Ultraviolet (UV) Radiation: The Ultraviolet radiation refers to the radiant energy between 100 and 380 nm. The UV band, according to the International Commission on Illumination, is defined as UV-A (315-400nm).

Veiling Reflection: Sometimes referred to as reflected glare, they are reflections of relatively large luminance that totally or partially obscure task details by reducing contrast. For instance, when reading a shiny magazine in a place with bright light, you have to tilt it accordingly to avoid glares.

Visible Spectrum: The portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 700 nm.

Visual Comfort Probability (VCP): Consider a group of people standing at a specific point and facing a particular direction. VCP is a measure of the fraction of people who would find the discomfort glare acceptable. VCP is usually expressed as a percentage.

Voltage (Volts): A measurement of the electromotive force in an electrical circuit or device expressed in volts. Line voltage in the North America is 120V.

Wattage (Watts): The amount of electricity or power consumed by a light source.

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