Glossary

Accent lighting
The highlighting of individual features
Adaption
The process whereby eyesight adjusts to relative levels of brightness, e.g. daytime or night time
Asymmetric reflector
A reflector that distributes light differentially in specific directions

Baffle
A shield type of apparatus to prevent light spill from a luminaire
Beam angle
The angle over which the intensity from a reflector drops to half of the peak value

Candela
A measure of luminous intensity from a lamp
Colour appearance
The relative perception of warmth of light, warm, intermediate or cool. (See also correlated colour temperature)
Colour rendering index (CRI)
The relative ability of a light source to enable the identification of colour
Correlated colour temperature (CCT)
The numeric expression of colour appearance, expressed in Kelvin
Contrast
The subjective appearance of different levels of lighting within the visual field

DALI
Stands for Digital Addressable Lighting Interface and is a standardised communication protocol used with electronic control gear such as LED Drivers. Many CMS systems use DALI interface to communicate with street lighting equipment
Diamond Optic®
Award winning adjustable reflector system that tailors light distribution to the area to be lit. 
Diffuse lighting
Lighting that emanates in many directions with no peak intensity in any given direction
Diffuser
Translucent or frosted glazing that engenders a more even light distribution
Direct entry luminaire
Luminaire where the connection between it and the column is via an offset spigot
Direct lighting
Lighting where the target area is lit directly from the luminaire rather than via another surface
Disability glare
Light that adversely affects one’s ability to undertake a given visual task
Discomfort glare
Light that makes a given visual task uncomfortable

Easy-fit™ connector
The preferred connector between a pendant luminaire and its bracket that resolves all the problems associated with the more dated screw type connectors

Full cut off luminaire
Expression for luminaires where there is an absolute cut off of light above the horizontal. For further information on G4, G5 and G6 glare ratings see EN 13201

Glare
That light emitted by a luminaire that has an adverse effect on a given visual task

Integrating sphere
Testing equipment which measures various aspects of light output including total luminous flux, colour temperature and colour rendering index. It allows more accurate data to be collected from products which utilise LED as their light source
IP rating
Is a two digit number that defines the relative ingress protection of a product. 
Illuminance
The measure of light arriving at a given point. Measured in lux and represented by the symbol E
Isolux diagram
A diagram showing the light distribution on a surface by means of contour type lines where each contour joins points of like illuminance

Junction temperature (Tj)
Is the temperature measured at the point where an LED creates the emitted light. It is important to keep the junction temperature as low as possible in order to gain the expected longevity stated for the product

Lamp
The generic term for a traditional light source
LED
Light Emitting Diode. A solid state light source
LED driver
Required by LED products to control the electrical supply to banks of LEDs. This can be adapted to provide specific pre-set drive currents which allow controllability of the products light output and wattage
Light
That part of the electromagnetic spectrum that is visible to humans
Light output ratio (LOR)
The proportion of light emitted by the luminaire against that emitted by the light source within the luminaire
Light pollution
More properly known as obtrusive light. It is that light that is spilled in undesired directions, and/or causing undesired effects
Longitudinal uniformity ratio (Ul)
The minimum point luminance divided by the average luminance in any row of calculated points between two lighting positions in a street lighting design. Under BS5489-1:2003 the lowest of these is used in assessing the project
Louvre
An optical controller consisting of a number of blades arranged to obstruct and/or redirect light output from a luminaire
Luminaire
Technical term for a light fitting or lantern
Luminance
The measure of brightness of a surface. Measured in Candelas per metre square (cd/m2) and represented by the symbol L
Luminous efficacy
The ratio of light output of a light source against the electricity consumed. Measured in lumens per watt [lm/w]
Lux
Unit of measurement for illuminance
L90
Is the term used for predicted lifetime of an LED chip to the point at which it still produces 90% of the initial luminous flux. This figure is used to calculate the yearly light loss from the product and is a good guide as to the general longevity of the complete product

Maintained illuminance
Measurement of light arriving at a given point after maintenance factors have been taken into account
Maintenance factor
A multiplier used to downgrade the calculated light output from a luminaire that takes into account light degradation from the light source and the increasing accumulation of dirt on the luminaire
Maintained lighting principle
Current best practice calls for lighting design to simulate the results that would be expected in a project at the worst snapshot in time
Mesopic vision
A state of human visual performance that equates to brightness levels between scotopic and photopic vision. i.e. at levels where colour and focussed vision begins to fail and monochromatic vision is starting up

NaaS
Network as a Service
Narrow beam reflector
A symmetrical reflector with a beam spread typically less than 25°

Obtrusive light
More accurate terminology for light pollution
Obtrusive light shields (OLS)
A range of shields that can be applied to luminaires to reduce or eliminate light distribution in given directions
Overall uniformity ratio (Uo)
The minimum point illuminance or luminance divided by the average illuminance or luminance in a project or target area

Pendant luminaire
A luminaire that is suspended via an entry in its top surface
Photoelectric control unit (PECU) or photocell
A light sensitive switch
Photopic vision
Human visual response at relatively high brightness levels, i.e. daytime adapted vision. This is colour vision and is also associated with focussed vision
Post top luminaire
A luminaire with the mechanical entry for the column or bracket being on the under surface
Projection
The distance from the centre of the column (for column brackets or surface of wall for wall brackets) to the point of entry of the luminaire except for pendant or post top type luminaires where it is to the centre point of the luminaire

Reflectance
The relative degree to which a surface reflects light
Remote monitoring
Technology that allows luminaires to be assessed and usually switched from a location remote from the luminaires themselves
RGB
A method of colour changing which combines red, green and blue colours mixed in differing amounts to create the various colours available

SaaS
Software as a Service
Semi cylindrical illuminance
Measure of light that falls on a vertically oriented half cylinder. This is usually measured at 1.5m above ground level to simulate light falling on a human face.
Scotopic vision
Human visual response associated with very low levels of light. It is also monochromatic vision and mostly peripheral rather than focussed. See also photopic and mesopic vision.
Side entry luminaire
Luminaire where the support bracket enters the luminaire from one side
Spill light
Light from a luminaire that has no intended illuminating purpose, usually associated with light pollution, although can be used intentionally for decorative purposes
Surround ratio
Under BS EN13201 this is a specific term for the ratio of a 5m wide strip of land outside either side of the road compared to 5m strips within each side of the roadway (for roadways of less than 10m overall width the strips are reduced to be accommodated)
Symmetric reflector
Reflector where the light reflected is symmetrically distributed about a vertical axis through it

Threshold increment [ti %]
Relative measure of loss of visibility in a roadway caused by glare

Uniformity
Measure of evenness of the light distribution on a lit surface. Mathematically it is the minimum point divided by the average values
Uplighter
A type of luminaire designed to emit light in an upwards direction
Upward light output ratio (ULOR)
That proportion of light emitted by a luminaire above the horizontal compared with the light output of the light source (sometimes the term Upward Waste Light Ratio, UWLR, is used)

Windage area
The projected area, usually of the luminaire plus brackets used to calculate wind resistance when specifying an appropriate column