CIE Standard General Skies Research Articles

Analytical expressions for estimating sky diffuse irradiance and illuminance on tilted planes for the CIE Standard General Skies

This work proposes a set of analytical expressions for the calculation of the relationship between sky diffuse illuminance on a tilted plane and horizontal plane corresponding to the 15 ISO/CIE standard sky types. Given the parallelism in the angular distribution of sky diffuse radiance and luminance, these expressions can also be used to calculate the diffuse sky irradiance on an inclined plane. The proposed analytical expressions are obtained by adjusting the results of a numerical calculation whose methodology is described in detail in the paper. The use of these analytical expressions significantly reduces the calculation time. It is verified that the relative root-mean-square errors obtained with the analytical expressions are small compared with the use of numerical calculation, ranging from 0.03% to 4.09%, and with a tendency to increase with clear skies and high solar zenith angles.

The use of horizontal sky illuminance to classify the CIE Standard General Skies

The CIE Standard General Skies defines the luminance distribution of skies into 15 types, from overcast sky to clear sky. Finding the frequency distributions of such skies is not a simple task, for two reasons. The first is the need for accurate luminance measurements over a long period of time. Such data are not available in many regions around the world. The second is the difficulty in obtaining accurate readings for the zenith luminance in low latitude climates. This paper presents a method in which the horizontal sky illuminance can be used to classify sky type.

Analysis of vertical sky components under various CIE standard general skies

Vertical sky diffuse illuminance data are important for architecture and daylighting designs. In 2003, the International Commission on Illumination (CIE) adopted a range of 15 standard skies representing the actual sky conditions in the world. This paper studies an approach to computing the vertical sky component (VSC) under the 15 CIE Standard Skies. Regression equations for calculating the VSC under various CIE skies based on the scattering angles (χ) were developed to estimate the vertical daylight illuminance on building façades. The performance of the proposed numerical approach and the regression equation are assessed against the results obtained by computer simulations. Our findings show that the VSC data determined by the numerical method and the regression equation are in good agreement with the simulated results. Our research has provided a simple tool for estimating the vertical daylight illuminance under various sky conditions.

The use of sky luminance and illuminance to classify the CIE Standard General Skies

Optimum use of natural light in buildings requires knowledge of the luminance distribution of the sky. Since the introduction of the CIE Standard General Skies, several methods have been proposed to find the frequency distribution of sky types. This paper presents a method whereby the ratio of the luminances of two sky elements and the ratio of the luminance of one sky element to the horizontal illuminance can be used in the determination of sky type.

A review of the CIE general sky classification approaches

Recently, the International Commission on Illumination (CIE) has adopted a range of 15 standard sky distributions representing the whole probable spectrum of actual skies in the world. Each sky standard has its own well-defined sky luminance pattern which can be conveniently used to calculate the sky radiance and luminance for a given sky patch and the solar irradiance and daylight illuminance on inclined surfaces facing various orientations. The crucial issues are whether the skies could be correctly identified. This paper presents the work on the classification of the CIE Standard General Skies using various climatic parameters and indices. Meteorological variables namely luminance distribution for the whole sky including zenith luminance, global, direct-beam and sky-diffuse illuminance on a horizontal surface and vertical sky illuminance, and horizontal and vertical solar irradiance data are adopted for analysis. The results demonstrate that there are a number of appropriate climatic parameters for sky classification and the selection depends on their availability, accuracy and sensitivity. The approaches could contribute to the estimation of solar irradiance and daylight illuminance which are essential to the renewable and sustainable developments and energy-efficient building designs.

Estimation of average daylight factor under obstructed CIE Standard General Skies

Recently, the CIE has adopted a range of 15 standard sky distributions covering the whole spectrum of usual skies found in nature. In a dense urban region, the external environment plays a significant role in daylighting designs. This paper studies an approach to calculating the average daylight factor for rooms facing heavily obstructed environments under overcast and non-overcast skies. The modelled vertical sky components under the various unobstructed and obstructed CIE Standard General Skies were determined. The performance of the proposed method was evaluated against the results obtained by computer simulations. The proposed approach provides a simple alternative for comparing daylighting designs for buildings facing different obstructed environments under various sky conditions.

Finding frequency distributions of CIE Standard General Skies from sky illuminance or irradiance

The CIE Standard General Sky divides the luminance distributions of skies into 15 types, ranging from overcast sky to clear sky. Finding the type of sky from the set of CIE Standard General Skies is a difficult task. This is due to the need for accurate luminance measurements over a long period of time because such data are not available in many regions around the world and to the difficulty in obtaining accurate readings for the zenith luminance. This study presents a method whereby the ratio of the vertical sky illuminance (or irradiance) on two surfaces or the ratio of vertical sky illuminance to the horizontal sky illuminance can be used to classify a sky into one of the CIE Standard General Skies.

Identifying CIE standard skies using vertical sky component

This paper studies the classification of the International Commission on Illumination (CIE) Standard General Skies using vertical sky component ( VSC) defined as the procedures for identifying the Standard General Skies are proposed. The performance of the recommended sky classification process was evaluated against measured data in terms of percentage error (PE) and root-mean-square error (RMSE). The results revealed that the method produced the RMSE of not more than 20% for most of classified Standard General Skies. The proposed approach provides an alternative for defining CIE Standard General Skies when the measured vertical and horizontal daylight illuminance data are available.

A dynamic sky recognition method for use in energy efficient lighting design based on CIE standard general skies

This paper innovatively proposes an “active green lighting model” for control and optimization of building luminous environment. It is expected to optimize the utilization of daylight dynamically in energy efficient lighting design from the viewpoint of intelligent control. As one primary work of implementing the overall model, the luminance distribution of CIE standard general Skies is addressed, and a dynamic sky recognition method for utilization optimization of daylight is proposed based on swarm intelligence. The experiment results of instance field data show the proposed intelligent sky recognition method is feasible and effective.

Analysing sky luminance scans and predicting frequent sky patterns in Singapore

A thorough analysis of sky distributions can help in the prediction of daylight patterns and thus provides a good understanding of architectural window and building envelope design. This paper introduces three methods of analysis for sky distributions, namely the relative indicatrix and gradation classification, and two other independent methods established by Kittler and Tregenza. All these three methods were verified for their feasibility for analysing the extreme daylight conditions in Singapore, which is located at the equator. The most suitable approach, from Tregenza, was eventually adopted in analysing the measured data from Singapore IDMP station. The prediction of frequent sky patterns in Singapore was based on the CIE Standard General Skies, as well as a complete version considering all 36 theoretical sky types.

Estimating depth of daylight zone and PSALI for side lit office spaces using the CIE Standard General Sky

The use of daylight has energy saving implications. But to harness daylight in buildings requires detailed analysis into its penetration. The paper predicts the depth of daylight zone and Permanent Supplementary Artificial Lighting of Interiors (PSALI) in a generalized high-rise office space in Kuala Lumpur, Malaysia. Using CIE Standard General Skies, two experiments are conducted to find the probable upper and lower limits of daylight penetration throughout the year. As CIE Standard General Skies are not widely incorporated into simulation software, therefore, a combination of Daylight Coefficient method and Waldram Diagrams is used for daylight calculation. The lowest daylight penetration is taken as the depth of daylight zone while the difference between the lowest and highest daylight penetration is considered for the depth of PSALI zone.

Frequency Distribution of CIE Standard General Skies for Subang, Malaysia

Standard skies are very important in analysing the daylight climate of a particular region or country. Studies of their frequency distribution can help in identifying the most reccurring typologies, which can then be used by researchers in modelling daylight in buildings. The paper describes the process of determining the occurrence of CIE Standard General Skies for Subang, Malaysia, utilizing the daylight coefficient method and global illuminance values converted from global irradiance measurements available for the location.

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.

Comment 1 on ‘Analysing sky luminance scans to obtain frequency distributions of CIE Standard General Skies’ by PR Tregenza

Comment 1 on ‘Analysing sky luminance scans to obtain frequency distributions of CIE Standard General Skies’ by PR Tregenza

Analysing sky luminance scans to obtain frequency distributions of CIE Standard General Skies

The CIE Standard General Sky consists of a family of luminance distributions which can be matched to measured sky brightness patterns. The daylight climate of a site can be described concisely and effectively by the statistical distribution of the General Sky types that best fit the sky luminance patterns that occur there. This paper gives a procedure for deriving the frequency of occurrence of General Sky types from International Daylight Measurement Programme sky scan data. Measured luminances are normalized with respect to horizontal illuminance and solar elevation, then the sky types giving the best least-squares fit are found. A data structure for daylight climate simulation is proposed.

Comment 2 on ‘Analysing sky luminance scans to obtain frequency distributions of CIE Standard General Skies’ by PR Tregenza

Comment 2 on ‘Analysing sky luminance scans to obtain frequency distributions of CIE Standard General Skies’ by PR Tregenza