Standard Sky Luminance Distribution Research Articles

The sky characterization according to the CIE Standard General Sky: Comparative analysis of three classification methods

Since the publication of the standard sky luminance distributions (SSLD) that was consolidated in the ISO 15469:2004(E)/CIE S 011/E:2003, numerous procedures have emerged for the characterization of the sky condition according to that standard. Precisely, the use of different procedures for the classification of the skies of a certain place according to the ISO/CIE standard can lead to obtain different frequencies of sky types. The existing uncertainties in the characterization of the sky condition according to the CIE Standard General Sky as a consequence of the classification procedure used are analyzed in this study. For this, three different classification procedures are used to characterize the sky radiance and luminance distribution measurements made by means of a sky-scanner in Pamplona (Spain) from 2007 to 2013. That is, (1) a method focused on determining the relative gradation and indicatrix functions, (2) a method based on the comparison of measured and standard luminances normalized against the horizontal diffuse illuminance, and (3) a new high-spatial-resolution approach that compares measured and standard luminances relative to zenith. In general terms, it is concluded that there is some uncertainty in the classification depending on the procedure used to characterize the sky.

Seasonal caracterization of CIE standard sky types above Burgos, northwestern Spain

Outdoor daylight conditions in Burgos, Spain, are studied throughout a full year. The CIE standard sky type is selected in accordance with the lowest RMSD (Root Mean Square Deviation) following the comparison of both the theoretical and the experimental luminance distributions in the sky hemisphere. The selection is based on luminance distribution data, recorded every 30 min, from 145 patches of the sky hemisphere. The original criterion to determine the type of sky, the SSLD (the Standard Sky Luminance Distribution), is difficult to apply in certain places and at times when the solar elevation is higher than 35°. In consequence, two alternative procedures are used and compared in this study: the Tregenza method and the Normalization Rate (NR) introduced by Littlefair. The selection was taken from luminance distribution data of 145 patches of the sky hemisphere recorded between June 2016 and May 2017. The most frequent sky type observed in Burgos was V.5. (cloudless polluted with a broad solar corona), with a frequency of occurrence close to 20%. Notwithstanding that observation, the group of clear skies exhibited a higher frequency (in almost 50% of the cases under study, using both methods). The skies above Burgos were of an overcast sky type in less than 25% of cases, a situation with a higher likelihood in winter and in autumn, while in spring and summer the skies tended to be clear and cloud free. Both of the methodologies showed similar results in percentage terms and in confusion matrixes with almost insignificant differences when compared on a monthly, a seasonal, and an annual basis. Nevertheless, some mismatches were located in the highest solar elevation values.

Application of Evolutionary Computation for Identification of Parameters of Sky Luminance Distributions

The International Commission on Illumination has adopted a set of fifteen standard sky luminance distributions (SSLD). One of our previous papers uses a least squares method to sort sky luminance measurements from a station located north of Bangkok into fifteen slots, each corresponding to each type of SSLD, to appraise the statistics of occurrence of each sky type at the location. The paper also uses Kittler's method (Kittler; and others, 1997) to prepare paired values from the measurements sorted and obtains parameter values of the gradation function and those of the indicatrix function separately from measurements in each slot. This paper applies evolutionary computation to obtain a set of parameter values for both gradation function and indcatrix function simultaneously under least squares criterion. This method enables all measurements in each slot to be used. The resulting luminance distribution function thus obtained for each sky type possesses lower bias error and lower mean square error than those of the corresponding type of SSLD when both are evaluated against measurements.

HOLIGILM: Hollow light guide interior illumination method – An analytic calculation approach for cylindrical light-tubes

A physical model for interior illuminance calculation HOLIGILM applied to cylindrical skylights was developed. Based on the ray-tracing between the diffuser of a tubular skylight and the sky vault including the Sun, the light transmission coefficient of the cover dome and of the interior diffuser with the high light-guide reflection, as well as taking into account the geometry of the interior, the resulting illuminance of the working plane can be analysed. As the model is strictly physical it allows to study influences of optical properties of tubular skylights (dome, tube and diffuser) on the interior daylight illuminance distribution specifying the luminance distribution on the hollow light-guide diffuser. The standard sky luminance distributions adopted ISO 15469:2004 and the new solar luminance and constant were included into the evaluation concept of tubular skylight. In addition to the original formulae some practical examples of the predicted illuminance on the working plane are also presented in this paper.

A new statistical methodology for classification of sky luminance distributions based on scan data

In the present work, the sky luminance distribution in Garston, south England, is studied using the multivariate statistical methods of Factor Analysis and Cluster Analysis. The database consists of sky luminance scan data for the year 1992. The whole methodology is applied for 5 solar altitude intervals (6°–18°, 18°–30°,…, 54°–66°). At first, Factor Analysis defines the main categories of sky conditions (overcast, partly cloudy, clear) and then Cluster Analysis gives a more detailed grouping of the available scans. For the final classification of the sky luminance distributions, the Cluster Analysis results along with the functions of “relative gradation” and “relative scattering indicatrix”, which have been introduced by Kittler et al., are taken into account. For an assessment of the methodology proposed, the results have been compared to the ones found by using the Standard Sky Luminance Distribution method (Kittler et al.). For solar altitudes up to 42° the differences in the frequency of occurrences, for the three main categories, is less than 10%. The statistical approach used in this paper is recommended as an additional means to “standard” methods for classification of sky luminance conditions, when scan measurements are available.

Method of segmentation affects leaf inclination angle estimation in hemispherical photography

This paper deals with the issue of segmentation methods in hemispherical photography determining diffuse site factor (DIFFSF) and leaf inclination angle distribution in forest stands simultaneously. Field inclination angle distributions were sampled in pure stands of European beech, hornbeam, sessile oak and silver birch on sites where towers facilitated crown access. In the same stands leaf litter traps were placed from which LAI values were derived after autumn leaf fall. Analogue hemispherical photographs were taken in summer before leaf fall. We combined the ellipsoidal G-function and Markov–Poisson model to invert leaf inclination angle functions (LIDF). Field leaf inclination angle distributions were planophile for three species and plagiophile for birch. The planophile distributions were well represented by ellipsoidal as well as beta distribution while plagiophile distribution of birch was exclusively represented by beta distribution. When segmentation methods were varied, LIDF parameter predictions were more in correspondence with field values by using sub-pixel methods than by using binary methods. When integrating vignetting properties into the segmentation algorithm or when accounting for standard sky luminance distribution, the accuracy of LIDF predictions varied; it is assumed that this is due to variation in sky properties during photography. Thus, different segmentation techniques in hemispherical photography did not yield constancy in deriving different required variables, i.e. DIFFSF and LIDF. Rather, method of segmentation affects leaf inclination angle estimation.

The method of aperture meridians: a simple calculation tool for applying the ISO/CIE Standard General Sky

Following the recent publication of the CIE Standard General Sky,1 adopted as an ISO Standard,2 an Application Guide is expected to be prepared by CIE Technical Committee 3-37 with the aim of explaining and simplifying its practical use. The advantage of the 15 Standard Sky Luminance Distributions (SSLD) adopted by CIE and ISO in relative terms normalized to zenith luminance is not only in the more versatile definition of skylight in various locales and daylight climate regions, but also in the possibility of simulating annual daylight profiles in absolute units based on typical luminance sky patterns. As well as complex computer programs for producing isoluminance contours in plan projections of the sky vault, a simpler tool is needed to calculate absolute sky luminance within the solid angle of arbitrarily dimensioned apertures, especially vertical windows. With this objective in mind, the Method of Aperture Meridians or MAM for any SSLD is introduced in this paper.

Average daylight factor for the 15 CIE standard skies

For daylighting calculations, the design criteria are often expressed in terms of average daylight factors (DFave) with the computations being based on the CIE (International Commission on Illumination) overcast sky, which is generally considered to provide the worst daylight condition. The daylight illuminance of a room is mainly influenced by the luminances and patterns of the sky in the direction of view of the window at any given time. Overcast skies may not always be the appropriate reference sky types for daylighting analysis. Recently, Kittler et al. have proposed a new range of 15 standard sky luminance distributions including five clear, five partly cloudy and five overcast sky types. These 15 sky luminance models have been adopted as the CIE General Standard Skies. This paper presents an approach to computing the DFave for the 15 standard skies. The techniques for calculating the two configuration parameters for the DFave determination, namely; (a) light received directly from the sky above the horizon on a vertical surface, and (b) light received directly from the ground below the horizon on a vertical surface are established and described. The performance of the proposed method was assessed against the results obtained by other independent calculation methods and computer simulations. It is shown that the results predicted by the proposed approach are in reasonably good agreement with those produced from the other two calculation tools. The findings provide architects and building designers with a reliable and simple method for estimating the interior daylight illuminance under various standard overcast and nonovercast sky conditions.

A simplified procedure for determining indoor daylight illuminance using daylight coefficient concept

The first step in evaluating the visual performance and energy efficiency provided by daylight requires an accurate estimation of the amount of daylight entering a building. The actual daylight illuminance of a room is mainly influenced by the luminance levels and patterns of the sky in the direction of view of the window at that time. The daylight coefficient concept, which considers the changes in the luminance of the sky elements, offers a more effective way of computing indoor daylight illuminances. Recently, Kittler et al. have proposed a new range of 15 standard sky luminance distributions including the CIE (International Commission on Illumination) standard clear sky. Lately, these 15 sky luminance models have been adopted as the CIE Standard General Skies. This paper presents a graphical method to calculate interior illuminance for the CIE standard clear sky using the daylight coefficient approach. The simplified techniques in the form of a nomograph and Waldram diagram were established and described. The performance of the proposed approach was evaluated against the results obtained by an independent calculation approach and a computer simulation program. It was shown that the daylight illuminances estimated by our graphical tool were in reasonably good agreement with those produced from the other two methods. The findings provide building professionals and students a reliable and simple alternative that incorporates the daylight coefficient concept to estimate the interior daylight illuminance and assess daylighting performance.

A Method to Construct Virtual Sky Domes for Use in Standard CAD-based Light Simulation Software

A Virtual Sky Dome (VSD) is a computational model of a hemisphere resembling the daylight conditions of a particular location, date and time. VSD's allow users of standard CAD-based simulation software to simulate architecture and assess daylight performance criteria making full use of the 15 standard sky luminance distribution models, which have just recently been accepted as a CIE standard [1]. This paper introduces the method to construct VSD. Prior to this any daylight simulation using standard CAD based light simulation software was limited to a smaller set of standard skies: clear, overcast and intermediate with non-changeable parameters as in the case of the software Lightscape. The all-weather-sky tool developed for the software Radiance follows the sky definitions defined by Perez. Neither of these tools considers the recently developed standard of 15 sky types, which is to replace previous standards[2] to further differentiate the skies to better match the real daylight conditions. This is of particular significance for predominately cloudy regions like Singapore, where a wide range of overcast and intermediate skies with significant different luminance distributions occur. A VSD is a CAD model of the hemisphere comprising 145 point lights of controllable parameters, each one sitting in the centre of a particular sky patch. Their luminous flux is derived from the luminance of the individual sky patch calculated using Kittler's equation[3]. A computational method has been developed that creates a VSD for a given sky type based on the input of solar azimuth and elevation, hence representing a particular time and location. This paper explains the method and shows results using VSD's to simulate daylight with the standard CAD-based light simulation software Lightscape.

All Sky Model as a Standard Sky Luminance Distribution

The aim of this research work is to establish a standard sky model for designing excellent daylighting schemes that cover all sky conditions from clear sky to overcast sky. In the previous paper (Part 1)1), the “normalized global illuminance” was defined as a function of the measured global illuminance and the solar altitude for estimating the sky luminance distribution. In this paper (Part 2), the “Relative All Sky Model” is introduced. It offers formulas to show the relative sky luminance distribution as a function of the normalized global illuminance. The equation of the zenith luminance concerning the Relative All Sky Model is also a function of the normalized global illuminance. It is called the “All Sky Zenith Luminance”. An absolute standard sky luminance distribution model called the “All Sky Model” is introduced that is a multiplication of the Relative All Sky Model and the All Sky Zenith Luminance. The All Sky Model can be calculated from the normalized global illuminance, i.e., from the measured global illuminance and the solar altitude or from the measured global illuminance, the horizontal diffuse illuminance, and the solar altitude. The global illuminance and horizontal diffuse illuminance are easily obtained from even the simplest daylight measurements. The All Sky Model can produce an excellent standard sky for daylighting design for any place where an advanced daylight environment is needed.

標準天空としてのAll Sky Modelの提案

標準天空としてのAll Sky Modelの提案

標準天空としてのAll Sky Modelの提案

標準天空としてのAll Sky Modelの提案