A Methodology for Designing and Calibrating an Artificial Sky to Simulate ISO/CIE Sky Types with an Artificial Sun

Abstract

Artificial skies are now commonplace in laboratories for daylighting studies in university schools of built environment studies. However, these research and educational devices are often not used to their full potential because they are not calibrated to model even location-specific International Oganisation for Standardisation (ISO)/CIE sky types. Nevertheless, sky luminance patterns and parallel sunbeam produced by an artificial sun in a sky simulator are required for realistic physical modeling. These facilities must be designed and calibrated in welldocumented protocols to achieve the scale ratios that proportionally model real outdoor skylight and sunlight conditions. Such exacting tasks can be accomplished by respecting several technical imperatives that this article was written to elaborate and discuss.

Development of Sky Luminance and Daylight Illuminance Prediction Methods for Lighting Energy Saving in Office Buildings

“…Thus, to calculate sky luminance distribution, the CIE standard sky luminance distribution provided by CIE [14] and a theory regarding the CIE general sky standard [20][21][22][23] were examined. The sky luminance distribution of CIE overcast and CIE clear sky was reproduced with 145 sky luminance patches [36] through the algorithm of sky luminance distribution.…”

“…From the ISO 15469/2004 standard [20], CIE general sky is classified into two groups: indicatrix and gradation. CIE general sky is also divided into different classifications and includes sky luminance distribution in Kittler's study [22][23][24][25]. The position of the Sun, the arbitrary sky element, and parameters a, b, c, d, and e, which describe atmospheric conditions, are used as input calculation quantities.…”

“…Figure 2 presents a diagram of sky relative luminance distribution based on the location of the sun and arbitrary sky elements. The ratio of the luminance L γα in an arbitrary sky element to the zenith luminance L z is expressed as shown in Equation (3) following the current CIE clear sky standard [15,21,[23][24][25]:…”

“…where when 0 ≤ Z ≤ π/2, and at the horizon, φ(π/2) = 1. By using the formal study of Kittler [15,[21][22][23][24][25], the zenith luminance and sky luminance, which were based on the data collected from the illuminance sensor, can be calculated. Each coefficient values (A, B, C, D, E, and T v ) were proposed in the prior study.…”

The experience with the design and refurbishment of the artificial sky in the SAS daylight laboratory

“…It is important to model standard sky types in their homogeneous or quasi-homogeneous nature as in their mathematically defined set as proposed for daylight calculation purposes and basis for computer-aided design and evaluation tools. Therefore the simulation of the fifteen ISO/CIE luminance patterns in an artificial sky has to follow certain basic requirements and simulation rules such as: -A genuine evenness and fluent luminance patterns of all the fifteen ISO/CIE sky types have to be reproduced [6] in a certain model scale because real luminance in absolute units cannot be achieved in the artificial sky. -In the same scale have to be simulated also illuminance level on the measuring model table where architectural model buildings or room with windows will be placed.…”