CIE 84 Luminous Flux Measurement

The “CIE 84 Luminous flux measurement” standard, developed by the International Commission on Illumination (CIE), defines the terminology required for luminous flux measurements, covers the principles of luminous flux measurements, and describes methods for evaluating illumination distribution, measuring luminous flux by means of an integrating sphere photometer, and determining luminous flux by means of luminous intensity, luminous density, and illuminance measurements.

This standard is based on procedures for measuring the luminous flux of discharge lamps and their calibration as working standards and the results of the CIE Light and radiation measurement symposium 81.

The purpose of this standard is to review the main methods used for luminous flux measurements. These methods include:

• Calculation from illumination or light distribution (used by standards laboratories)
• Measurement with integrating sphere (widely used in industry)
• Measuring light intensity distributions (can be performed in a limited number of industrial laboratories with access to a goniophotometer)

Although each user group is primarily interested in the method used in its own field, a reference work on luminous flux measurements should cover all the main methods used for this purpose and place them in perspective with respect to each other.

The luminous flux of a light source can be calculated or measured by different methods:

• Calculation from light intensity distribution
• Calculation from lighting distribution
• Measurement with a sphere photometer with photometric or spectral measurements
• Measurement with a box photometer
• Relative measurements via illumination, light intensity or brightness

Calculating the luminous flux from the luminous intensity distribution is suitable if measurements of this distribution are already available (e.g. for luminaires). Deriving the luminous flux from the measurement of the illuminance distribution of a lamp is the method used in many national standards laboratories to determine the basic standards of luminous flux. An accurate measurement of the colorimetric properties of light sources and the spatial variation of their spectral power distribution can also be made using this method.

Luminous flux measurements using a sphere photometer are suitable for:

• Measurements made in industrial laboratories for production control.
• Measurements made by test facilities and users.
• Calibration of standard lamps (e.g. working standards) against higher grade standard lamps, with additional corrections for errors resulting from geometric, spectral and light distribution differences between the lamps to be compared.
• Measurement of light sources whose luminous flux varies with time (e.g. adjustable lamps, flash lamps).
• Measurement of luminous flux as a function of time.

Luminous flux measurement with a box photometer only shows a direct relationship between the luminous flux of the light source and the indirect illumination at an arbitrary point on the inner surface of the box if the reference light source and the light source to be measured have the same spatial light intensity distribution, the same spectral distribution and the same dimensions.

Spectral radiant flux measurements can be made with an integrating sphere photometer for light sources where the spectral power distribution varies with direction (e.g. metal halide lamps). This method gives all the information necessary to calculate:

• Spectral power distribution
• Luminous flux
• Radiant flux
• Color
• Colour rendering indices

The determination of the luminous flux of light sources by measuring illumination, light intensity or brightness is often done in practice to determine the influence of certain parameters (e.g. aging, temperature, location). It usually takes the form of a relative measurement. The method can also be used to measure the luminous flux of fluorescent lamps in lighting installations.

The method used to measure luminous flux depends on the equipment available. The equipment and method used are affected by:

• The task of the photometric laboratory
• Finances
• Time consumption
• Acceptable measurement uncertainty

The light intensity distribution can be measured with a goniophotometer. The measurement of the light intensity distribution is described in a separate technical report from the CIE. There are different types of goniophotometers used to measure the illumination distribution. In all of them, the light source to be measured is operated in the intended burning position:

• Goniophotometer with the light source in a fixed position
• Goniophotometer in which the light source is rotated around a spatially fixed light center
• Goniophotometer in which the light source is rotated about a vertical axis with a moving light center

The accuracy of luminous flux measurements made by evaluating the illuminance distribution is determined to a large extent by the quality of the illuminance meter used. This device must be of very high quality.

In principle, the luminous flux of a light source can be measured in a sphere photometer by comparing it with a luminous flux standard lamp. When measuring, the light source and the standard lamp are placed sequentially at the same location on the integrating sphere. The indirect illumination on the sphere surface is taken as a measure of the luminous flux. A sphere photometer consists of an integrating sphere, a photometer head with a readout unit, and an electrical source for data acquisition and measuring equipment.

Our organization has a strong staff that closely follows the developments in the world in the field of science and technology and constantly improves itself. Among the numerous test, measurement, analysis and evaluation studies provided for businesses in various sectors, there are also test services in accordance with the "CIE 84 Luminous Flux Measurement" standard.