Upgrading a Broad Area Illuminating Integrating Sphere and Solar Transmittance Measurement of a Sheer Blind
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Shading devices are frequently used to control solar gain through windows. Solar optical properties are very important in the energy analysis of windows. Transmittance, absorptance and reflectance are required to quantify solar heat gain through complex fenestration systems, which consist of combinations of glazing and shading layers. In this research the solar transmittance of a sheer blind was measured using a Broad Area Illuminating Integrating Sphere (BAI-IS). More specifically, the spectral directional-hemispherical transmittance was measured in the wavelength range of 0.4 µm to 2.0 µm. A “sheer blind” consists of soft fabric vanes, similar to a venetian blind, suspended between two vertical layers of sheer fabric. This arrangement is popular because it is attractive and it has potential application for daylighting. The vertical sheer fabric reduces the solar intensity and diﬀuses incident radiation; generally reducing solar gain and producing soft, natural illumination. The fabric vanes control the amount of light entering the room. Shading devices such as venetian blinds, sheer blinds and drapes have spatially non-uniform and light scattering surfaces. Hence, measurement error occurs if the solar optical properties are measured by traditional narrow-beam measurement techniques typically used in commercial spectrophotometers. To reduce this error, a BAI-IS is recommended. The BAI-IS apparatus consists of a 20-inch diameter integrating sphere, sample mounting system, monochromator, radiant source, lock-in-amplifier, photo sensor, optical chopper and various auxiliary devices. In order to improve reliability of the measurement the BAI-IS has recently been upgraded by replacing most of the key control and measurement equipment. The refurbishment of the BAI-IS apparatus was successful. The directionalhemispherical transmittance of a sheer blind from BAI-IS measurement was found to agree well with an analytical model.