Standard General Sky Research Articles

Pixel-Based Image Processing for CIE Standard Sky Classification through ANN

Digital sky images are studied for the definition of sky conditions in accordance with the CIE Standard General Sky Guide. Likewise, adequate image-processing methods are analyzed that highlight key image information, prior to the application of Artificial Neural Network classification algorithms. Twenty-two image-processing methods are reviewed and applied to a broad and unbiased dataset of 1500 sky images recorded in Burgos, Spain, over an extensive experimental campaign. The dataset comprises one hundred images of each CIE standard sky type, previously classified from simultaneous sky scanner data. Color spaces, spectral features, and texture filters image-processing methods are applied. While the use of the traditional RGB color space for image-processing yielded good results (ANN accuracy equal to 86.6%), other color spaces, such as Hue Saturation Value (HSV), which may be more appropriate, increased the accuracy of their global classifications. The use of either the green or the blue monochromatic channels improved sky classification, both for the fifteen CIE standard sky types and for simpler classification into clear, partial, and overcast conditions. The main conclusion was that specific image-processing methods could improve ANN-algorithm accuracy, depending on the image information required for the classification problem.

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.

Average daylight factor for the ISO/CIE Standard General Sky

The average daylight factor is one of the most widely used methods for predicting the internal illuminance due to daylight. It is a simple tool that can be used in the early stages of design and can give information about the luminous environment inside a room. This method is supposed to be used under overcast sky conditions. Expanding the use of the average daylight factor to other sky conditions will encourage the use of daylight in buildings. Several methods have been proposed for the prediction of daylight based on the CIE Standard General Sky. However, most of these methods aimed for a detailed estimation of the internal illuminance pattern. This paper aims to investigate the potential for using the average daylight factor method under the ISO/CIE Standard General Sky.

Implementation of CIE General Sky Model Approach in Ukraine and Effects on Room Illuminance Mode

New standard CIE S 011/E:2003 “CIE standard general sky” was accepted in Ukraine in 2010. With its adoption the question of its appropriate usage with national documents became an important event. Feasible way of standard’s utilization with Ukrainian daylighting standards and effects of its implementation are given in the paper.

Cloud Cover Measurement from All-Sky Nighttime Images

The site quality of astronomical observatory critically depends on cloud coverage, and the measurement of cloudiness is particularly important for site survey. A method to deal with all-sky images in no-moon nights is described. By identifying the positions of bright reference stars and making photometry for a set of all-sky images in clear nights, we can set up a reference image with median smoothing differential magnitude values. The standard image can be taken as the threshold for clear nights, and the detectivity of stars on other images can be utilize to reveal cloud coverage. Four types of all-sky images, clear night, icy lens, part of cloud, and full of cloud, are selected to check up the method. For the moonlight image, the formula of the CIE (Commission International de l'Eclairage ) standard general sky model are used to fit the luminance distribution of the image, then to estimate the cloud cover of the image from the difference between the fitting image and the real image.

基于空间模型测量点的照度比值的CIE标准一般天空类型选择

基于空间模型测量点的照度比值的CIE标准一般天空类型选择

Improving the All Sky Model for the luminance and radiance distributions of the sky

Energy conservation is a fundamental requirement in buildings today. It is essential to use technology that integrates the thermal environment and the luminous environment to ensure indoor environmental quality. It is important to understand as accurately as possible the distribution of luminance and radiance of the daytime sky. Though the CIE Standard General Sky for sky luminance distributions has been recommended, a method of selecting 15 Sky Types was not shown.In this paper, the author tries to improve the previous All Sky Model based on IDMP measurement data from Osaka with the aim of a high accuracy estimation method for luminance and radiance distributions of the sky. The appearances of the Types of CIE Standard General Sky and the assignment in estimating types are shown based on the measurement data in Osaka. The improved All Sky Model and the validation of this improved model using the measurements in Osaka and Tokyo, Japan was introduced.

Algorithm for determining CIE Standard General Sky occurrence from digital sky images

The use of digital cameras to capture sky images for measuring sky brightness is an inexpensive and proven method from previous research. However, a digital image has numerous pixels each containing colour data that are impossible to calculate into luminance values manually. Therefore, in this article we describe the algorithm of a computer program that can automate the calculation process and simultaneously compare the measured sky luminance with ISO/CIE Standard Sky luminance, to determine the occurrence frequency.

A method for selecting the CIE standard general sky model with regard to calculating luminance distributions

It is known that the best predictions of sky luminances are obtainable by the CIE 15 standard skies model, but the predictions by this model need knowledge of the measured luminance distributions themselves, since a criterion for selecting the type of sky starting from the irradiance values has not found until now.The authors propose a new simple method of applying the CIE model, based on the use of the sky index Si. A comparison between calculated luminance data and data measured in Arcavacata of Rende (Italy), Lyon (France) and Pamplona (Spain) show a good performance of this method in comparison with other methods of calculation of luminance existing in the literature.

Opinion: The CIE Standard General Sky and local climate

Opinion: The CIE Standard General Sky and local climate

Blurring the boundaries between Standard General Sky types due to multiple scattering of light

The diffuse light reaching the ground varies with instantaneous physical state of the atmosphere, specifically with atmospheric turbidity, vertical stratification of attenuating and scattering constituents, cloud coverage and many others. Despite this complexity, a set of empirical models based on so-called homogeneous sky types is widely used to simulate the real lighting conditions. However, the sky type classification can be influenced by multiple scattering of light. Even if multiple scattering is essentially important under low elevation angles and high turbidity conditions, it is quite often ignored – e.g. when modelling the luminance patterns. As a consequence, the interpretation of experimental data can suffer from the omission of multiple scattering effects. Since multiple scattering smoothes the luminance patterns, the neighbouring sky types may be subject to blurring, and thus it might be difficult to distinguish them in automated data processing. In this paper we introduce a model that lowers the uncertainties caused by multiple scattering, so the subsequent numerical routines will definitely profit from more accurate identification of sky types.

天空放射輝度分布及び周囲遮蔽物を考慮した壁面日射量計算法の開発

On the calculation of the values of heating/cooling loads based on more real state of a building in the city, influences of the sky radiance distribution and the surroundings around the building should be take into account. However they have not been considered to be important factors. In this paper, with the CIE standard general sky and the approximations that have analytic solution, we intend to present a new calculation method of surface irradiance value of the building surrounded by the other buildings. In the development, the accuracy of calculation and the processing time are made much account.

天空輝度・放射輝度分布を推定するAll Sky Modelの改良

Energy conservation is an indispensable assignment in the building now. It is demanded to establish the technology that integrates the thermal environment and the luminous environment securing the indoor environmental quality. We should understand the phenomenon as minutely as possible that originates in the radiation from the sun and the sky that is a large factor of turbulence.CIE recommended CIE Standard General Sky about the sky luminance distribution. Even the method of selecting 15 Sky Type is not shown. Authors tried the improvement of previous All Sky Model based on the measurement data of IDMP in Osaka aiming the high accurate estimation method of luminance and radiance distributions of sky.In this paper, the appearance circumstance of Types of the CIE Standard General Sky and the assignment in estimating Types is shown based on the measurement data in Osaka and the improved All Sky Model is introduced.

Correlating Indian measured sky luminance distribution and Indian Design clear sky model with five CIE Standard clear sky models

This paper aims to find out representative CIE (International Commission on Illumination) Standard Clear Sky model(s) for three different seasons - winter solstice, equinox, and summer solstice applicable for prevailing clear sky climatic conditions in India [Roorkee]. Indian measured sky luminance distribution database is available only for Roorkee [29°51′ N; 77°53′ E]. To find out the best match between Indian measured sky luminance distribution and each of five CIE Standard Clear sky models, sky component of spatial illuminance distribution over the working plane of a room was simulated by MATLAB for three different seasons. The effectiveness of using Indian design clear sky model proposed in IS: 2440:1976 was also evaluated side by side. Daylight Coefficient method has been applied for the simulation using Indian sky luminance database. The simulation has been done for the room with eight different window orientations ranging from 0° to 315° with an interval of 45° to generate data for the entire sky vault. To find out the best fit CIE model(s), statistical test of significance (95% confidence level) has been carried out for the spatial illuminance distributions taking results obtained with Indian measured sky luminance distribution data as reference. Analysis revealed that CIE Standard General Clear sky type 15 described as “White-blue sky, turbid with a wide solar corona effect” is the best-fit clear sky model for both summer and equinox seasons and sky type 11 described as “White blue sky with a clear solar corona” is the best-fit clear sky model for winter season at Roorkee. Hence these sky types can be used for daylight prediction instead of Indian Design Clear Sky Model which shows large deviations.

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.

Predicting sky types and zenith luminance from the cloud index derived from geostationary satellite data

This paper presents methods to predict sky types and hourly zenith luminance from cloud index n derived from the visible channel data of the geostationary satellites. Three sky conditions based on the CIE standard general sky can be classified by cloud index: n > 0.38, 0.13 ≤ n ≤ 0.38 and n < 0.13, corresponding to overcast, partly cloudy and clear sky conditions, more specifically to sky 1, sky 8 and sky 13 of the ‘Hong Kong Representative Sky’. Hourly zenith luminance can be derived as a function of the combination of relative standard overcast sky zenith luminance and relative standard clear sky zenith luminance determined by cloud index. The mean bias error and the root mean square error of hourly zenith luminance are 0.49 and 3.37 kcd/m2, respectively.

An Implementation of the Method of Aperture Meridians for the ISO/CIE Standard General Sky

The use of the new ISO/CIE Standard General Sky offers some new challenges for lighting designers. The MAMmodeller package described in this study provides a user-friendly tool for visualizing each of the SSLD 15 models in simple design contexts. The method of aperture meridians (MAM) provides a relatively simple geometric framework to estimate the appearance of the sky as a luminance map in the window opening, as seen from a point of interest inside the room. In addition, the total horizontal illuminance at the same point is estimated. This provides the user or designer with a quick tool for undertaking preliminary design investigations. MAMmodeller is not a complete lighting design tool, it considers only sunlight and the diffuse light component from the sky entering through the window opening. It does not account for internal reflections, artificial lighting, or external reflections.

Outdoor Illuminance Levels in the Tropics and their Representation in Virtual Sky Domes

This paper presents two ways to predict outdoor illuminance levels in the tropics. The first one is based on calculations mainly following the recent CIE/ISO Standard General Sky. The second one applies Virtual Sky Domes (VSD) and Virtual Sky and Sun Domes (VSSD) generated for use with standard light simulation software. The accuracy of the VSD and VSSD methods in predicting illuminance levels with and without sunlight is evaluated and shown to be very close to the calculations. Thus, VSD and VSSD can eventually become useful tools to be added into standard lighting simulation software.

Comment 1 on ’The method of aperture meridians: a simple calculation tool for applying the ISO/CIE Standard General Sky’ by R Kittler and S Darula

Comment 1 on ’The method of aperture meridians: a simple calculation tool for applying the ISO/CIE Standard General Sky’ by R Kittler and S Darula

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.