Glossary 2--Lighting words and specification explaination
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