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Glossary
| Term | Symbol | Unit | Formula | Meaning |
|---|---|---|---|---|
| Luminous flux | Ф | Lumen (lm) | Luminous flux is the radiated power of a light source including the spectral sensitivity of the human eye. | |
| Luminous intensity | I | Candela (cd) | I =Ф/ω | Luminous intensity is an expression of the amount of light power emanating from a point source within a solid angle of one steradian. |
| Illuminance | E | Lux (lx) | E =Ф/A | Illuminance is the total incident luminous flux on a surface per unit area. |
| Luminance | L | (cd/m²) | L =I/A L =I/A cosФ |
Luminance is the luminous intensity per unit area of light in a given direction and angle. |
| Luminance distribution | Luminance distribution is the ratio of different areas and their corresponding luminance. | |||
| Luminous colour | Cool white Natural white Warm white |
Luminous colour describes ranges of correlated colour temperatures (CCT). | ||
| Correlated colour temperature (CCT) | Kelvin (K) | The correlated colour temperature of a light source is equal to the temperature of an ideal black body radiator. | ||
| Colour rendering index (CRI) | Ra | Ra index | The Colour Rendering Index is a quantitative measure of the ability of a light source to reproduce the colours of various objects faithfully in comparison with an ideal or natural light source. The maximum Ra = 100 means the light source radiation is equal to an ideal black body radiator. | |
| Luminous efficacy | η | Lm/W | η =Ф/P | Luminous efficacy is the ratio of luminous flux to power. The power can be either the radiant flux of the source’s output, or it can be the total electric power consumed by the source. |
| Uniformity | Emin/Emid Lmin/Emid |
Ratio of different areas of Illuminances. | ||
| Glare | Glare influences the visibility of details and the eyesight. | |||
| Efficiency | η | % | The efficiency is the ratio between the total luminous flux emitted by a device and the total amount of input power. | |
| Lifetime | L70 | Time (h) | The lifetime describes the time of degradation of the light source, e.g. the time in which the light output decreased to 70%. |
Standards and Norms
LM 79 and LM 80
Light emitting diodes (LEDs) are a relatively new and unique source for outdoor lighting. They are more reliant upon effective thermal management than any previous source, more of a directional source, and have to be designed and tested as an entire lighting system. Therefore, LEDs require new guidelines and practices for testing. There also needs to be a correlation between how LED manufacturers test their LEDs and how fixture manufacturers test their LED fixtures. In response, the Illuminating Engineering Society of North America or IESNA developed LM-79-08 and LM-80-08 for LED fixture and LED device testing.
The IESNA
The IESNA is a 100+ year old lighting industry group with membership that includes manufacturers (both sources and fixtures), lighting in IESNA designers and architects, utilities, and others affiliated with lighting such as consultants, government, researchers and educators.
LM-80-08 for the LEDs themselves
LM-80-08 Approved Method: Measuring Lumen Maintenance of LED Light Sources was published by the IESNA Solid State Lighting (SSL) Subcommittee in the third quarter of 2008. Simply referred to as LM-80, this document covers lumen maintenance measurement for inorganic LED-based packages, arrays, and modules; it does not cover any other aspect of LED performance.
One of the key reasons for the development of LM-80 is due to differences in measuring LED performance criteria. LED manufacturers typically measure LEDs in pulse mode operation with no heat sink. The pulse is very short — typically 10 or 20 milliseconds (that is, thousands of a second) — which will not heat up the LED; therefore, no heat sink is required and Tj can be assumed to be equal to ambient temperature TA (typically held constant at 25°C). This is useful for doing high yield LED measurements quickly. This also explains why LED manufacturer data sheets typically show LED performance for Tj = 25°C.
In contrast, LED fixture manufacturers measure LED performance in situ, which means while it is in their fixture. Under these conditions, the LED is operated in constant DC mode and there are typically numerous LEDs configured together often in close proximity to one another, elevating Tj above 25°C. This elevated Tj affects the photometric and colorimetric performance of the LEDs. In order to compare “apples to apples”, a new testing criteria needed to be developed: LM-80-08.
LM-80-08 prescribes uniform test methods for LED manufacturers under controlled conditions for measuring LED lumen maintenance while controlling the LEDs TS or case temperature, the DC forward voltage and forward current to the LED. LM-80-08 requires 55°C, 85°C and one other TS chosen by the LED manufacturer. It also requires lumen maintenance data out to at least 6,000 hours of constant DC mode (not pulse mode) operation 4.
The data resulting from LM-80-08 measurements are matrices of lumen maintenance values. LED fixture manufacturers use this data in combination with their UL in-situ thermal testing to predict the lumen maintenance of the LEDs when used in their fixtures and, subsequently, the lumen maintenance of the LED fixtures themselves. For example, if we measure 85°C TS at the hottest LED LinearTM LED in one of our fixtures, then we look up that particular data set from our Japanese suppliers to determine the LED fixture’s lumen maintenance based upon and correlated with the LEDs lumen maintenance at that same TS. Fixture manufacturers also use the data to predict LED color stability over time at the various TS temperatures.
