An integrating sphere is a spherical chamber whose internal surface is coated with highly reflective diffusing material. It has an input port where lamps can be inserted and an exit port where detectors can be installed. There are several applications that require the use of an integrating sphere and there are also standards of measurement such as IES’s LM-79 which are described in the following article.
a) Integrating sphere to measure total luminous flux: In order to measure total luminous flux from a lamp, which is measured in units of “Lumen”, the lamp is placed inside an integrating sphere. Rays of light that emanate from the lamp are diffusely reflected from the walls of the sphere multiple times, until they find their way into the detector. The detector therefore collects the total luminous flux emanating from the lamp. The technique originated at the turn of 20th century as a simple and fast method of comparing the lumen output of different lamp types and remains the dominant method in the lamp industry for manufacturing quality control. Figure 1 shows the set-up.
Using this set-up the sphere acts as a photometer. The lamp is mounted at the center of the integrating sphere and is baffled from the viewing port that is equipped with a smaller diffuser sphere and a photopic response detector. The function of this detector is to emulate the human eye response. The Baffle is usually mounted at 2/3 the radius of the sphere. Firstly, a lamp of known luminous power is placed inside the sphere to calibrate the detector. After calibration the lamp to be tested is placed inside the sphere and measured. It is also possible to replace the photodetector with a spectro-radiometer to measure spectral radiant flux. Obtaining spectral information from integrating sphere provides two key advantages:
The spectral response of the detector and spectral responsivity of the sphere wall do not affect the measurement of luminous efficacy of the lamp since one can correct for them. (luminous efficacy is defined as the ratio of output luminous power to input electrical power measured in lumen/watt) as well as lumens one can obtain important color information such as chromaticity coordinates, correlated color temperature and color rendering indices.
b) LM-79 standard: The Illuminating Engineering Society of North America (IES) published LM-79-08, an approved method for the electrical and photometric measurements of solid state lighting products. LM-79 testing measures performance characteristics of Solid-State Lighting (SSL) products such as Light-Emitting Diodes (LEDs). The measurements are usually done under controlled conditions at the beginning of a lifetime of a product. LM-79 data enables objective product comparisons, allows for evaluation relative to performance requirements, and is required by labelling programs such as LED Lighting Facts. Measuring the total luminous flux in Lumen and the luminous efficacy in Lumen/Watt is essential for LM-79 standard reports. The content of a given LM-79 report partly depends on the apparatus used for measurements. Integrating spheres can measure the total luminous flux and colorimetric data. However, they are not capable of measuring luminous intensity at different locations around the sample. The spatial distribution of beam around a lamp is also an important part of the LM-79 report but only gonio-photometers are capable of measuring the spatial distribution. However, gonio-photometers cannot measure colorimetric data. Given the different capabilities of the two methods, both methods are covered in the LM-79 standard.
As an example, if only the integrating sphere method was used, the report will not characterize the spatial distribution of light. Similarly, if only the gonio-photometry method was used, the report is unlikely to include the colorimetric data. Some reports will contain both aspects. Figure 2 shows a mechanized integrating sphere which is used for testing lighting product samples.