scope:

SCOPE AND PURPOSE

This guide provides the IES required uniform method for determining and reporting the photometric characteristics of skylights and tubular daylighting devices that incorporate a means to diffuse the natural hemispherical daylight as the daylight passes through the daylighting system. It describes the procedures followed and the precautions observed in obtaining uniform and reproducible measurements of tubular daylighting devices and skylights with glass or plastic glazing. This guide identifies the components and the structure type needed to adequately measure daylighting devices. The procedures, calibration of the equipment, and determination of sun angles and sky conditions are also discussed. This method is not recommended for daylight devices with clear glazing (see Annex B).

Unlike electric luminaires, that require only one photometric test, skylights require a separate test to characterize their performance at different sun angles as well as a separate measurement to determine the sky conditions. Skylights are very often large, some exceed the size capabilities of typical intensity distribution goniophotometers, and the sky conditions are always changing. Therefore, a method was developed to quickly and accurately measure daylight data and place the results in a standardized photometric file format such as IESNA LM 63-02 (R2008).¹ The collection of results can be used by lighting design software programs to simulate how skylights under the desired test conditions will fill a space with daylight. This equipment tested also proved to be accurate with regard to the effects of test distance.

This is only one method of achieving luminous intensity distribution curves and efficiency data for skylights and tubular daylighting devices with diffusing properties. In addition, this method mirrors the proved method used to measure electric lights but includes many adjustments because of the very intense movable light source, the sun. Since 2002 this method has proved to produce consistent repeatable results for measuring skylights and tubular daylighting devices under actual sky conditions.

The procedures contained in this document are a benefit to the lighting industry in characterizing the performance of daylighting products and are backed by significant experience using this measurement method. However, daylight photometry is in infancy stages of research and further research and development of these and other methods will continue

In addition, further research is being conducted to compare measurement methods to simulation methods. Simulations of daylight system performance use predicted sky models from either the CIE or IES and allow for the rapid comparisons of multiple design options or sky conditions. Further research will also be conducted to improve methods of achieving accurate skylight luminous intensity distribution curves and efficiency data.