Zenith Luminance Research Articles

Analysis of zenith luminance data for all sky conditions

Two different procedures are explored in the present work to estimate the zenith luminance at the International Daylight Measurement Program station in Madrid, when all the data available are used independently of the sky type. On the basis of the results obtained, correlations for the seasonal dependence of the zenith luminance on solar elevation are reported. However, it is observed that polynomials obtained for different ranges of the luminous clearness index, the ratio of global to extraterrestrial illuminance, can predict the zenith luminance with a more reasonable accuracy. The results show that the luminous clearness index is an important parameter for zenith luminance characterization, to be used when global illuminance values on horizontal surfaces are available, but diffuse illuminance is not measured

Daylight levels during the solar eclipse of 11 August 1999

Solar eclipses are unique phenomena not only for astronomical and space observations but also for terrestrial; they create unique conditions of sunbeam blockage which cause not only the reduction of direct sunlight but also the dimming of skylight from the whole sky vault. Very favorable conditions were met during the recent August 1999 solar eclipse in Athens, Greece and Bratislava, Slovakia. General class daylight stations operate within the International Daylight Measurements Program in the two cities. One-minute data of global/diffuse illuminance and zenith luminance from those stations have been used to provide information about their levels and the daylight reduction rate during the eclipse. An approximate formula for the estimation of sunlight and skylight illuminance levels as well as zenith luminance using relative luminance sky patterns is also presented in this work. To achieve this, recently developed sky standards together with their parameterizations are utilized.

A study on zenith luminance on Madrid overcast skies

The zenith luminance has been measured at Madrid for skies with a cloud cover of 7–8 oktas and experimental mean 15-min values of L z have been obtained. Among the most accurate fits to the data, a 2nd degree polynomial for L z against tan( α) is the most simple. When only data for a cloud cover of 8 oktas are used a similar fit is obtained, but for a smaller coefficient of correlation, due to the smaller amount of data. A linear, second degree, or power dependence for L z against sin( α) obtained by other researchers, does not give the best fit for our data.

A study of zenith luminance on Madrid cloudless skies

The zenith luminance ( L z) has been measured for cloudless skies at the IDMP station in Madrid. Mean values obtained at every 15 min have been fitted vs. solar elevation ( α). A 5th degree polynomial connecting log ( L z) against ( α) gives the best correlation in terms of the correlation coefficient ( r=0.9358). A possible explanation of the dependence of the zenith luminance on solar elevation at low values is given. An exponential model obtained by other authors ( Vázquez and Bernabeu, 1997) for a nearby temporary station in the same city does not account for the qualitative dependence of L z vs. α for α<≈15°, and also overestimates experimental values for high solar elevations. The effect of turbidity on L z is studied categorizing available data in three specified groups of T v, the visual or luminous/illuminance turbidity. Best fits with 5th degree polynomials connecting log ( L z) against ( α) are given for: T v<3, 3< T v<5, and T v>5.

All-sky zenith luminance models for the UK

Sky luminance distribution data are of great importance to the lighting designer considering daylighting as an option for provision of internal illuminance. Previous studies have shown that the sky vault can be considered as a series of separate sky patches, each with its own individual degree of luminance. Such luminance values can be determined as a function of the corresponding zenith luminance. A lack of long-term zenith luminance data has led to the creation of a variety of zenith luminance models, from those that are complex in construction to those that are operable only under certain sky conditions. This paper presents an evaluation of Moon and Spencer's overcast sky model and the model of Perez et al., which has previously been shown to be efficient for all-sky conditions. Two new zenith luminance models are also proposed in an attempt to simplify the process by which zenith luminance estimations are obtained under any prevailing sky type. In doing so, an opportunity has been created to incorporate more routinely recorded data, such as cloud cover and sunshine fraction, into the design procedure.

絶対値で表す天頂輝度と天空輝度分布 : 天空輝度分布の分類法の研究(その4)

A numerical equation has been proposed by the authors in the previous papers. It gives the relative values of the sky luminance distribution for all sky conditions. Though the numerical equations for the zenith luminance in absolute value on the various standard skies have been proposed by several researchers to get the sky luminance distribution in absolute value, they do not bring the suitable value for the new equation. A numerical equation on the zenith luminance using the same indices as those of the newly proposed equation has been composed. It has made it possible for the sky luminance distribution in absolute value to be estimated by one of the easiest measurable values, the global illuminance. The result of this research work convinced the authors of its contribution to the progress and development of the prediction technique in the daylit environment. The new equation of the zenith luminance in absolute value is introduced, the process of composing it is explained in detail and the examples of the sky luminance distribution for all sky conditions in absolute values are illustrated in this paper.

Calibration of grey values of hemispherical photographs for image analysis

The hemispherical photography method is widely used for the estimation of solar radiation in forests or agricultural crops. However, calibration problems repeatedly arise concerning the grey-value information of the photographs. This article introduces a method which may help to solve these calibration problems. The method takes into consideration two information transport media (black-and-white film and digital image analysis). This method is based on the exposure of the photographs to the zenith luminance of an overcast sky and the determination of the gamma function of the film by means of an optical density wedge photograph. By taking these characteristics into account, it is possible to obtain reproducible and bias-free results from the diffuse site factor even with different film materials. Moreover, the modelling of penumbral effects is shown. Limitations are discussed.

Parametrization problems of the very bright cloudy sky conditions

When exterior global/direct and diffuse illuminance, as well as zenith luminance, is regularly measured and recorded, a selection and specification of the most light efficient skylight conditions is needed. A routine representation of all 1- or 5-minute data on the P–G–D diagram can show extreme diffuse, i.e. skylight ratios normalized to the simultaneous extraterrestrial horizontal illuminance level, which indicate the bright cloudy sky conditions. A new method of cluster analysis was developed to select sky conditions associated with sunlight under specific turbidity or cloudiness conditions at actual solar altitudes, while a rough classification of the sky luminance pattern is possible using the ratio of zenith luminance to diffuse illuminance. Some characteristic data recorded in Bratislava, Slovakia during 1995 and during the three year period 1994–1996, demonstrate the parametrization and selection method used, and document detailed dynamic changes occurring during the brightest days.

Evaluation of luminous efficacy models against Japanese data: Technical note

SYNOPSIS This note presents the evaluation of Perez and other simpler luminous efficacy models for Japan using the data now emerging from the CIE International Daylight Measurement Proaramme. The results indicate that the perez model, although exhibiting a low scatter, has a tendency to consistently underpredict global and diffuse illuminance while it heavily overpredicts zenith luminance. The average-values for global and diffuse luminous efficacies for Fukuoka data have been found to be significantly different from the ones reported for other geographical locations.

DAYLIGHT MEASUREMENT IN INDONESIA

Raw data from sunrise till sunset were summarized after quality control tests. Methods of processing data followed those used in the IDMP (International Daylight Measurement Programme) in Japan [2].(1) Daily average values, standard deviations and the numbers of the data classified by solar altitude in 6-degree intervals separately in the morning and afternoon, (2) Monthly average values, standard deviations and the numbers of the data classified by solar altitude in 6-degree intervals separately in the morning and afternoon, (3) Daily average values, standard deviations and the numbers of the data classified by half-hourly intervals, (4) Monthly average values, standard deviations and the numbers of the data classified by half-hourly intervals.Tables 1 to 3 show examples of monthly processed data in May 1992 by solar altitude in 6-degree intervals. The total numbers of processed data were 15, 627 on illuminance and 17, 646 on zenith luminance.

Design sky proposal for the warm, humid tropics

Extensive studies have already been carried out by other workers on the CIE Standard Overcast Sky and the Clear Blue Sky as daylighting design skies for temperate regions and other areas in the hot, dry tropics. The present study reports on the partly cloudy or cloudy skies prevalent in the warm, humid tropics and proposes a design sky for this region. For daylighting design in the humid tropics a completely overcast sky is proposed whose average zenith luminance is 7 000 cd m-2 with a corresponding average total illuminance of 17 000 lux from an unobstructed sky. These values are compared with those by other workers.

Zenith luminance and total horizontal illuminance of the tropical clear sky

Data on zenith luminance and total horizontal illuminance obtained for tropical clear sky at Delhi showed strong turbidity-related seasonal dependance. A comparison of the variability of these parameters with solar height for the Delhi sky with variabilities for other locations, as given by data-based empirical equations and as calculated theorectically, revealed significant differences. The best approximation to zenith luminance and its variability for different locations was obtained with the formula of Dogniaux. A similar approximation was obtained for the total horizontal illuminance of different locations with the values and variabilities as derived, combining Dogniaux's formula for zenith luminance with the luminance distribution of Kittler. Exceptions to this type of approximation may be noticed for locations with special atmospheric conditions and ground reflectance.

Modeling daylight availability and irradiance components from direct and global irradiance

This paper presents the latest versions of several models developed by the authors to predict short time-step solar energy and daylight availability quantities needed by energy system modelers or building designers. The modeled quantities are global, direct and diffuse daylight illuminance, diffuse irradiance and illuminance impinging on tilted surfaces of arbitrary orientation, sky zenith luminance and sky luminance angular distribution. All models are original except for the last one which is extrapolated from current standards. All models share a common operating structure and a common set of input data: Hourly (or higher frequency) direct (or diffuse) and global irradiance plus surface dew point temperature. Key experimental observations leading to model development are briefly reviewed. Comprehensive validation results are presented. Model accuracy, assessed in terms of root-mean-square and mean bias errors, is analyzed both as a function of insolation conditions and site climatic environment.

A Study on Zenith Luminance in Montreal, Canada

A Study on Zenith Luminance in Montreal, Canada

Variations of the luminous efficacy of global and diffuse radiation and zenith luminance with weather conditions—description of a potential method to generate key daylight availability data from existing solar radiation data bases

The Atmospheric Sciences Research Center and the New York State Energy Research and Development Authority have undertaken a comprehensive daylight resource assessment program which includes the coincident measurement of 25 radiometric and photometric quantities[1]. This paper contains selected results of the analysis performed on collected data. These include the study of the variations of global and diffuse luminous efficacy, and of zenith luminance with insolation conditions. These conditions are described using a parameterization based on two widely available radiative quantities (global and direct irradiance). Based on three 45-day monitoring periods, it appears that many of the observed variations can be accounted for from the knowledge of these two quantities.

Zenith luminance and sky luminance distributions for daylighting calculations

We derive an equation for zenith luminance as a function of turbidity and solar altitude based on analysis of large quantities of luminance and illuminance data measured in San Francisco, California, between September 1979 and August 1982. Using only average turbidity values to predict hourly zenith luminance as a function of solar altitude can produce large errors. We compare the equation derived from our data, which is valid over a wide range of turbidity and solar altitude, to other published models. We also compare the relationship between horizontal illuminance and zenith luminance from the clear sky and conclude that, when ideal clear days are compared, this relationship is similar to earlier work based on measurements in European climates. Finally, we compare our sky luminance distribution measurements to previous published luminance distributions using the diffusion indicatrix. Our results are intended to help improve daylight availability prediction techniques and define additional requirements for data collection.

Measurements of Sky Luminance, Sky Illuminance, and Horizontal Solar Radiation

This paper presents initial findings of a sky measurement program currently in progress at the National Bureau of Standards. Measurements are reported on sky luminance and illuminance and how they relate to horizontal solar radiation and sun position. Correlations are presented relating horizontal illuminance to horizontal solar radiation, and zenith luminance to solar altitude. These empirical models are particularly suitable for use in daylighting energy studies since they are based on existing solar data currently available on standard weather tapes.

Quantitative data on daylight for illuminating engineering

Data are provided for the extraterrestrial radiation of the Sun, also for those quantities which are of interest to the illuminating engineer, namely luminance distribution, zenith luminance, illumination, spectral distribution and luminous efficacy of the overcast sky, clear sky and sunlight. Formulae for luminance and illuminance due to several authors are given and shown graphically. CIE recommendations are included.