Economic lifetime

In a lighting installation consisting of a sufficient sample of lamps for statistical analysis, economic lifetime is the time period which elapses for approximately 30 % depreciation of the total luminous flux due to failure of lamps and reductions in luminous flux.

Driver efficiency varies depending on the load rate.

Average lifetime

In a lighting installation consisting of a sufficient sample of lamps for statistical analysis, average lifetime is in normal conditions the time period which elapses for 50 % of the lamps to fail. The average lifetime of LED chips varies according to junction temperatures. LEDs driven at high currents have high temperatures and their lifetimes are reduced. LM 80-08 "Approved Method: Measuring Lumen Maintenance of LED Light Sources" tries to define the luminous flux depreciations of LED light sources in time and requires LED light source manufacturers to provide a minimum of 6000 hours of luminous flux (lumens) measurements. It is recommended that the luminous flux measurements are made for minimum 6000 hours , ideally 10,000 hours, collecting the data every 1000 hours. For periods longer than 10000 hours, the standard TM 21-11 "Approved Method: 90 Measuring Lumen Maintenance of LED Light Sources" is used. In TM 21-11, a method is proposed for estimating the data obtained from the measurements 85 according to LM 80-08 in the long run. With the measured values, an attempt 80 at estimating the time that elapses until the luminous flux value drops down to 70%, in other words, economic lifetime, is made. The estimated lifetime of 75 a sample LED chip driven in different current levels are given below.

Color Temperature

It is the temperature of a "blackbody" which has the same spectrum with the evaluated light source. Its unit is Kelvin(oK). The body which can absorb all of the radiation at different wavelengths falling on it is called as blackbody and the spectral absorption factor of blackbody is assumed as 1 theoretically. When heat energy is given to a blackbody, it will begin to warm up first and then it will emit yellowish, yellow, yellow-and-white and blue-and-white light. The light color of light sources varies depending on the differences in the radiation intensity of their spectrum. The temperature at which a "body with a color temperature", which emits light like a blackbody at a temperature higher than its actual temperature, is referred to as color temperature.

Color Rendering Index

Light sources' ability to distinguish the colors of objects that they illuminate is called the color rendering index. Color Rendering Index - CRI is used to compare the color characteristics of different light sources. These characteristics of various light sources can be measured using a reference source. In these measurements, daylight, with its continuous spectrum is used as the reference. The color rendering index is denoted with Ra and it has no unit. Its value is between 0 and 100. If the color rendering index of a light source has the maximum value of 100 (Ra = 100), this means that the spectrum of that source is identical to the reference source.


Luminaries are used to control the distribution of light emitted from light sources and to shape it as desired, to protect the lamp and electric circuits against physical effects, to limit glare and to respond to aesthetic feelings and the need for comfort. For the anticipation and control of the lighting design to be realized, luminaires are the most important data sets necessary to conduct the "lighting calculations". The photometric data of the luminaires signify the luminous intensity distribution surfaces or curves, luminaire efficiencies and luminance and therefore glare.

Luminous Intensity Distribution Surfaces and Curves

The geometric position of the end points of luminous intensities of a luminaire in a variety of directions in the space constitute a surface. This surface is referred to as the luminous intensity distribution surface of a luminaire. To obtain these surfaces, measurements should be conducted in an infinite number of directions. As a more practical means of application, the luminous intensity distribution curves which are cross-sections of different planes passing through the luminaire axis and the luminous intensity distribution surface of the luminaire, or luminous intensity tables are provided instead. In order to perform lighting calculations accurately, luminous intensity tables of luminaires are necessary. Luminous intensity distribution curves and luminous intensity tables are scaled down to correspond to 1000 lm (cd/klm). According to CIE, the luminous intensity distribution curves of luminaires can be given for three different planes of A, B and C.

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