Bright Ideas

Lighting Measures and Terminology Decoded

Posted by Joe Lightman on Mar 18, 2016 4:04 PM

Whether you are a lighting systems designer or a facility owner, it is important to ensure that every part of your facility has optimum illumination, both to enhance the ambience and avoid energy wastage. Figuring out the right number and mix of LED lamps, fixtures and lumens for a facility is both an art and a science. It helps if you understand the terminology used to talk about lighting, especially in terms of rating, measuring and quantifying the effectiveness, efficiency, intensity and quality of light output. In this post we cover some of the common language used to measure lighting.


First step, what are the main factors that determine the number of lamps and amount of lumens required for optimal illumination of your space? These include the following:

  1. Room Type - Lighting needs vary depending on the use of a space, whether it is a kitchen, warehouse, supermarket, balcony, showroom, hospital, library, restaurant, or office.
  2. Room Measurements - You should get accurate measurements of the room’s length, width, and height and any cavities.
  3. Illumination Intensity – What is the desired illumination intensity - high, medium, or low?
  4. Wall Colour - Are the walls light or dark in colour and what is the intended light colour?
  5. Light Placements - Will the lights be placed at the centre of the room, ceiling or near corners?

Lighting Terminology

Below are some of the most commonly used terminology used to measure and quantify lighting.

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 or Beam Spread: 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.

Candela: 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.

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.

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, measured in lux or footcandles.

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 Factor: 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.)

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: 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, incorrect voltage or

Lumen Method: The lumen method provides a simple approach of designing lighting systems. This method is suitable for spaces with reasonably simple geometry. By changing the subject of the formula, you can calculate the number of luminaires that your space requires, the number of lumens per lamp, and so on.


Where: E  represents the required illuminance; n represents the number of light sources (lamps) per luminaire; N represents the number luminaires; F represents the number of lumens emitted by each lamp; UF represents the utilisation factor (sometimes referred to as coefficient of utilisation); LLF represents the light loss factor; A represents the area of the working plane

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 the product 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.

For a complete eBook with all the lighting terminology you need to know, dowload RAB Design's Lighting Glossary by clicking on the button below.

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Topics: Lighting Essentials