CIE Overcast Sky Research Articles

Sunshine duration and daily courses of illuminance in Bratislava

AbstractThe occurrences of typical sky conditions during overcast days are close to the CIE Overcast Sky Standard in the Bratislava area; their variation is specified by the diffuse coefficient kdd = Dv/Ev where Dv is the horizontal diffuse illuminance and Ev is the extraterrestrial illuminance. Luminance distributions on cloudless days are close to the CIE Clear Sky Standard. Complex daylight conditions can be expected during cloudy days when moving clouds cause frequent sun disk shading with abrupt changes in illuminance levels. Since sunshine duration is one of the basic climatic parameters that describes sunny situations, the present study attempts to investigate its relation to daily illuminance levels. However, the average daily value of sunshine duration does not exactly correspond to the same illuminance course, as it depends on the temporal distribution of sunny periods within the day. Anyhow, the estimation of daytime illuminance levels is mainly influenced by the presence or absence of sunshine with frequent sky luminance patterns. Copyright © 2004 Royal Meteorological Society

Predicting daylight illuminance by computer simulation techniques

,!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. Traditionally, the daylight performance of a building is often evaluated in terms of daylight factors with the calculations being based on the CIE overcast sky. In general, the daylight factor approach is quite simple to use but it cannot predict the dynamic variations in interior illuminance as sky conditions and the sun’s position change. The recently introduced concept of the daylight coefficient, which needs sky luminance data, provides an alternative approach. With the advances in computer technology, the computation of daylight illuminance using the two prediction methods can be conducted by a simulation program. This paper presents a study of the daylight factor and daylight coefficient approaches. The interior daylight illuminance data measured in a scale model and a classroom were compared with the simulated results using RADIANCE computer simulation software. It was found that, in general, the daylight coefficient approach performs better than daylight factor approach. Both methods tend to estimate the daylight illuminance less accurately when the measurement points in the classroom are far away from the window facade. It seems that the parameters for inter-reflection calculations including the furniture layout and external obstructions cannot be input into the RADIANCE program with sufficient detail for accurate simulation.

Surface Reflectance Distributions and their Effect on Average Daylight Factor Values in Atrium Buildings

The aim of this research was to study parametrically the effect of different distribution patterns of atrium wall reflectances on average daylight factor (ADF) values in the well of a square, four-sided, top-lit atrium model that was placed inside an artificial sky. The artificial sky replicated the luminance distribution of a CIE overcast sky. To achieve the variability in reflection distribution the atrium surfaces were painted in alternating bands of white and black horizontal stripes. For each of the experiments the widths of these bands were altered but the overall split between white and black bands was always maintained at 50 % white and 50% black. Experiments were also conducted for atrium wall surfaces painted all black (0% wall reflectance) and all white (100% wall reflectance). This study was able to investigate the impact on ADF values in atria of more realistic distributions of light reflecting surfaces. Results from the study showed that the ADF values on the floor of the atrium, compared to the standard calculated ADF, were affected by the reflectance distributions of the atrium surfaces. Wide bands of different reflectances displayed the largest variations. For narrow bands the different reflectance distributions did not affect the ADF levels as much. The results of this study demonstrate the importance of the reflective properties of the atrium surfaces and the variations in reflectance distributions on the daylight reaching the atrium floor.

Analytical tests for investigating the accuracy of lighting programs

To help potential users test and compare lighting computer programs, this paper defines some basic reference test cases with analytical solutions. These can be used to verify the accuracy and the capacity of lighting programs to correctly simulate different aspects of lighting calculations; daylight entry inside buildings, influence of incidence angle on glass transmittance, light distribution from luminaires having photometric files, inter-reflections between diffuse surfaces with influence of surface transmittance and finally daylight factor calculations under CIE overcast sky. To illustrate the use of these tests they were conducted with a software package available on the market, and the results compared with the analytical solutions. The tests demonstrate the software capabilities and limitations with regard to inter-reflection calculations, to simulation of luminaries and to daylight penetration through unglazed or glazed apertures.

Algorithms for calculation of daylight factors in streets

This paper presents algorithms for calculation of sky components and daylight factors in linear building structures using the solid angle projection method. In order to estimate the precision of calculations two sky models; the uniform sky and CIE overcast sky, both giving equal illuminance on the horizontal plane, are used. Daylight factors calculated for CIE overcast sky are also compared to daylight factors measured in a physical scale model of a linear atrium in an artificial sky of the mirror-box type.

Evaluation of the CIE overcast sky model against Japanese data

The present study was undertaken to evaluate the CIE overcast sky model for Japan using the data now emerging from the CIE International Daylight Measurement Programme. Kyushu University in Japan is the national and international co-ordinating centre for this programme. Using data from only those days which experienced a complete overcast it has been shown herein that the above model always underestimates the average vertical illuminance. The analysis indicates that a uniform sky is a better representation of the overcast luminance distribution.

Mathematical and Physical Simulation of Indoor Daylighting: A Case Study

Modern lighting technology aims at the developing of energy efficient lighting system, that is, the objective is to provide good visual environment to the occupants as well as to conserve energy by consuming minimum electricity. With the increasing electricity cost there is a renewed interest of the use of daylight.A fair scope of energy conservation is also available by proper utilisation of daylight. The variable nature of daylight makes the daylighting design a complex one. Hence during design stage of a building-project both quantitative and qualitative evaluation of daylighting performance within interior is essential. In this paper, the total glazing area for an office room was calculated for required area-weighted average illuminance recommended by Bureau of Indian Standard using BRE window design formula. The daylighting performance of the room was studied by Mathematical and Physical simulation. The Mathematical simulation was done using CIE Standard overcast sky and Indian clear sky. The physical simulation was carried out under Artificial Sky setup at Illumination Eng. Section of the E.E. Department which can produce sky luminance distribution pattern very close to that of the CIE Overcast sky. The experimental and theoretical results were discussed.

Daylighting contribution in interior lighting: Experimental verification of software simulation results

A study was carried out in an office building to investigate daylight impact comparing SUPERLITE 1.01 and LUMENMICRO 5 simulation results with measured data.Referring to the CIE standard Clear (with direct sun) and Overcast sky luminance distributions, calculated illuminance and daylight factor data for the work plane are compared with on-site data collected under real sky conditions. The main aim of the paper is not a comparative evaluation of simulation programs, but to define the meaning and/or uncertainty attached to the quoted results from the programs.

天窓に入射する天空光の室内各表面への分布 : CIE曇天空および一様天空の場合

This paper deals with the distribution of light incident from sky through skylight window onto interior surfaces of a room with a shape of rectangular prism. As for sky conditions, this paper assumes CIE overcast sky and uniform sky. This paper presents a series of equations for calculating direct illuminance at an arbitrary point on an arbitrary interior surface and luminous flux incident directly on an arbitrary interior surface. In addition, this paper presents a couple of graphs that illustrate ratio of luminous flux distributed on an interior surface to total luminous flux incident inside through skylight window.

Effective glass transmission factors under a CIE sky

Daylight availability in interiors is reduced because the window glazing is not perfectly transparent. The reduction factor will vary throughout a room according to the angles of incidence of the incoming light on the glazing. This note outlines a new method to calculate the reduction of daylight from a CIE overcast sky due to glazing, which gives accurate results throughout the room. The method is compared with existing methods, and it is shown to be especially appropriate when average daylight factor is calculated by averaging values of daylight factor computed at points on a grid extending throughout the room.

The luminance distribution of an average sky

Daylight in buildings can save energy by reducing artificial lighting energy consumption. An accurate estimate of internal daylight availability is required for energy use predictions and to include the effect of window orientation this must be based on an adequate model of sky luminance distribution. For such energy calculations the 'average sky' (based on measurements for a wide range of real skies) is recommended as a replacement for the CIE overcast sky. This paper analyses luminance data obtained by Wegner to show that the luminance distribution of an average sky is of the form L = ae-by + c where y is the angle in degrees between the sun and the element of sky under consideration, b is approximately 0.025 and a and c are functions of solar altitude alone, and, significantly, do not depend on the altitude of the element of sky. Using this formula together with a direct solar component the paper derives expressions for horizontal and vertical illuminances, and the average illuminance inside a side-lit room; and briefly outlines further work, under way at BRS, to evaluate the model for weather conditions in the UK and to apply it to problems of daylighting design.