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Home > Vol 8, No 1 (2008) > Shekari

JRHS 2008; 8(1): 46-55

Copyright © Journal of Research in Health Sciences

Estimation of Illuminance on the South Facing Surfaces for Clear Skies in Iran

Shekari Sh (Msc)a, Golmohammadi R (PhD)b ,Mahjub H (PhD)c,  Mohammadfam I (PhD)d, Motamedzadeh M (PhD)d

a School of Public Health, Hamadan University of Medical Sciences, Iran

b Center for Health Research, School of Public Health, Hamadan University of Medical Sciences, Iran

c Department of Biostatistics, Hamadan University of Medical Sciences, Iran

dDepartment   of Occupational Health, Hamadan University of Medical Sciences, Iran

*Corresponding author: Dr Rostam Golmohammadi, E- mail: Golmohamadi@Umsha.Ac.Ir

Received: 24 August 2007; Accepted: 5 February 2007

Abstract

Background: Daylight availability data are essential for designing effectively day lighted buildings. In re­spect to no available daylight availability data in Iran, illuminance data on the south facing vertical sur­faces were estimated using a proper method.

Methods: An illuminance measuring set was designed for measuring vertical illuminances for standard times over 15 days at one hour intervals from 9 a.m. to 3 p.m. at three measuring stations (Hamadan, Eshte­hard and Kerman). Measuring data were used to confirm predicted by the IESNA method.

Results: Measurement of respective illuminances on the south vertical surfaces resulted in minimum val­ues of 10.5 KLx, mean values of 33.59 KLx and maximum values of 79.6 KLx.

Conclusion: In this study was developed a regression model between measured and calculated data of south facing vertical illuminance. This model, have a good linear correlation between measured and cal­culated values (r= 0.892).

Keywords: Light, South, Vertical illuminance,  Iran

Introduction

Daylight is part of energy spectrum of elec­tromagnetic radiation emitted by the sun within the visible wave-band that is received at the surface of the earth after absorption and scattering in the earth's atmosphere. Sunlight is the direct component of light, while daylight is the total light from the sky dome (1). Daylight consists of direct (or beam), diffuse and ground reflected compo­nents. To accurately estimate daylight in the interiors it is required to estimate daylight

availability outdoors a room. Daylight avail­ability can be defined as the average amount of skylight and sunlight available during typi­cal period (day, month, season or year) taking in to account average turbidity (water vapor and particle content of the atmosphere and cloud type and cover) (2). This could be represented by an hourly, daily or monthly data. Such data can be obtained either by meas­urements or by calculation from other meteorological quantities (3).

In respect to no available data regarding ir­radiance, sky luminance and illuminance in Iran, external illumination was estimated by equa­tions proposed by "Illuminating En­gi­neer­ing Society of North America" (IESNA). Based on IESNA equations basic data and beam normal illuminance and solar al­titude and azi­muth were calculated for each station using fol­lowing equations (4).

ts=tr-1          [1]

       [2]                                                                                                                        

       [3]                                                            

       [4]

         [5]

         [6]

         [7]

Edn=Ext.e-cm         [8]

Edh=Edn.sinαt        [9]

αi=arcos(cosαt.cosαz)        [10]

Ekh=A+Bsincαt        [11]                                        

Where:

ET: Equation of time (the difference between solar time and clock time) expressed in deci­mal hours (for example, 1:30 p.m.= 13.5).

J: Julian date, a number between 1 and 365.

ts: standard time in decimal hours.

td: daylight time in decimal hours.

t: solar time in decimal hours.

SM: standard meridian for the time in rad.

L: site longitude in rad.

δ: solar declination in rad.

l: site latitude in rad.

αt: solar altitude in rad

Ext: extraterrestrial solar illuminance in KLx.

Esc: solar illumination constant in KLx (cur­rent standard 128 KLx).

as:solar azimuth in rad.

m: optical air mass (dimensionless).

c: atmospheric extinction coefficient (0.21 for clear, 0.8  for partly cloudy and very high for cloudy sky so Edn= 0 ).

Edn: direct normal solar illuminance in KLx

Edh: direct horizontal solar illuminance in KLx

Ekh: horizontal illuminance due to unob­structed skylight in KLx

A: sunrise/sunset illuminance in KLx (0.8 for clear, 0.3 for partly cloudy and 0.3 for cloudy sky).

B: solar altitude illuminance coefficient in KLx (15.5 for clear, 45 for partly cloudy and 21 for cloudy sky).

C: solar altitude illuminance exponent (0.5 for clear, 1 for partly cloudy and 1 for cloudy sky).

Estimation of illuminance on south facing vertical surfaces was simply achieved by following equations (4):

αz =αs - αe        [12]

Edv=Edh.cos αi        [13]

Edv=Edh.cos αi        [14]

Ekv=A+BcosαiC         [15]

Eg=p.(Edh+ Ekh)        [16]

EVsouth=Edv+ Ekv+  Eg        [17]

Where:

αz: Solar- elevation azimuth in rad

αe: Elevation azimuth in rad

αi: Incident angle in rad

Edv: Direct vertical solar illuminance in KLx

Ekv :Diffuse vertical illuminance in KLx

A: sunrise/sunset illuminance in KLx (0.8 for clear, 0.3 for partly cloudy and 0.3 for cloudy sky).

B: solar altitude illuminance coefficient in KLx (15.5 for clear, 45 for partly cloudy and 21 for cloudy sky).

C: solar altitude illuminance exponent (0.33 for clear, 1 for partly cloudy and 1 for cloudy sky).

Eg: Ground reflected illuminance in KLx

Evsouth: Global illuminance on south facing vertical surface in KLx

Electrical lighting directly shares 20% of elec­tricity use in a building and contributes 20% of cooling load to the air conditioning system. Air conditioning shares 60% of electricity use, so electric lighting also contributes indirectly another 12% of electricity use through the air conditioning system. Nevertheless, a num­ber of studies have shown the daylight inte­grated electric lighting in commercial build­ings can help reduce more than 50% of en­ergy and power use from those due to light­ing (5).

For the purpose of showing the potentiality of having a certain external average illumi­nance during a full working year, mean hourly and then mean monthly illuminances on south facing vertical window using cor­respondent linear model were obtained. Sub­sequently frequencies of days in a working year (294 d) which a given outdoor vertical illuminace is exceeded were determined.

Methods

Daylight measurements were carried out in three measuring stations having different geo­graphical coordinate and climatic conditions. Table 1 shows the characteristics of measur­ing stations. Illuminance values on south fac­ing vertical surfaces were taken using a sim­ple illuminance measuring equipment Lutron Lx-101. A measuring set which consists of two adjustable plans to support and keep the photo sensor of measuring equipment in a ver­tical position on any desired orientation. Vertical illuminance values were taken over 15 d at one hour intervals between 12 July and 1 August 2007 from 9 a.m. to 3 p.m. gen­erally 105 sets of measurements were taken at each station which are referred to standard time. All of the collected data were entered in statisti­cal sheet of SPSS software. Multiple re­gression models were applied to develop a model between calculated and measured variables of illumination.

Table 1: Characteristics of daylight measuring stations

Results

Measurement of illuminance on the south oriented surface was carried out for the purpose of confirming calculated data. In order to study the frequency of occurrence of cloud cover, the sky conditions were determined experimentally and clear skies were selected as those without any cloud. Generally from 315 times measurement of vertical illuminance on the south orientation, respective frequency of occurrence of clear, partly cloudy and cloudy skies found to be 277, 6 and 32. The frequency of occurrence of clear skies is shown in Table 2.

Table 2: Sky condition frequency at stations

Table 3 shows the frequency of clear days in a working year which a given outdoor vertical illuminance is exceeded.

Table 3: frequency of clear days in a working year which a given outdoor vertical illuminance is exceeded

Figure 1 shows the percent of clear days in different locations. In respect to more common clear skies at reporting period, data related to partly cloudy and cloudy sky conditions were eliminated and solely clear skies were considered.

Figure 1: percent of clear days in different locations on July

In accordance with Table 4, descriptive analysis of data exhibits that values of field measured and calculated illuminance at total station range from 10.5 to 79.6 KLx and from 7.05 to 54.9 KLx, respectively. Also mean respective values of measured and calculated illuminances exceed 33.5 KLx and 33.9 KLx. Table 5 exhibits that the maximum value of measured illuminances in Iran (79.6 KLx) compare to Saudi Arabia (3), Thailand (5), India (6) Hong Kong (7), San Francisco (8) and France (9) at corre­sponding measuring time.  

Frequency analysis of total data of measured and calculated south facing vertical illumi­nances were performed by dividing them in to 9 categories. In accordance with Table 6, maximum frequencies of measured and cal­culated illuminances on the south facing sur­face found to be 56(20.3%) and  46(16.7%), respectively. related values of these maxi­mum frequencies range from 33.41 KLx to 41.1 KLx and from 28.43 KLx to 33.72 KLx.

Table 4: Comparisons of measured and calculated values of south facing vertical surfaces

Note: Uupper and lower data at each station are related in mean measured and mean calculated illuminances respectively.

Mean measured and calculated hourly values on south vertical surface were determined at all stations for entire period. In respect to Table 5, minimum values of mean hourly meas­ured illuminances on the south oriented surface at total station touch 21.54 KLx.  Table 5 exhibits that the maximum mean hourly values in Iran (41.75 KLx) is con­siderably higher than Thailand, France, San Francisco, India and Saudi Arabia (3).

Table 5: Compare of mean, maximum mean hourly and maximum of south facing vertical illuminance in different locations

Results of this study

Table 7 shows the mean hourly values of measured and calculated on south facing vertical illuminance at standard time.

Table 8 exhibits comparison of mean hourly and mean monthly values of the south facing vertical surfaces in Iran and other locations at corresponding measuring period.

Table 6: Category and frequency of measured and calculated values on south facing vertical surface

Table 7: Mean hourly values of measured and calculated on south facing vertical illuminances

Note: Uupper and lower data at each station are related in mean measured and mean calculated illuminances respectively.

Table 8: mean hourly and monthly measured illuminances on the south facing vertical surfaces in different locations

* - Results of this study

Patterns of comparative curves of mean measured and mean calculated hourly illuminances on the south facing surfaces illustrate that IESNA equations are not able to estimate vertical illuminances exactly as the measured values at corresponding stan­dard times. Figure 2 illustrates comparative curves of measured and calculated mean hourly values on the south fact surfaces. In respect to coefficients of determination of total data, the correlation appears to be reasonable for measured and calculated values on the south facing vertical surface (r = 0.529).

Figure 2: Comparison of mean measured and means calculated values on the south facing vertical surface (Evs) at total stations

Measured values of the south facing illumi­nances plotted related calculated values; exhibit a good regression as shown in Figure 3. A simple regression model fitted between measured and calculated values on the south surfaces (r2= 0.796). This model suggested for predicting vertical illuminance by calcu­lated values using following equations:

Figure 3: Relation between measured and calculated values of south facing vertical illuminance (r2= 0.795).

Evsm=0.773Evsc+6.912        [20]

Where:

Evsm: measured south facing vertical illuminance in KLx

Evsc: calculated south facing vertical illuminance in KLx 

Table 9 shows the prediction of mean hourly and monthly illuminance on the south facing vertical surface for working year based on developed calculation model for Iran (Eshtehard location).

Table 9: Prediction of mean hourly and monthly illuminances on the south facing vertical surface for working year in Iran

Discussion

Objective of this paper was to estimate illu­minance on the south facing vertical surface based on IESNA method in Iran and com­pare to other world location to obtain a de­veloped regression model.

In accordance with Table 5, Descriptive analy­sis of data exhibits that the maximum mean hourly value of measured illuminances in Iran (79.6 KLx) is higher than Saudi Ara­bia (3), Thailand (5), Thailand (5), India (6) Hong Kong (7), France(9) and San Francisco (8) at corresponding measuring time. Also, mean val­ues of measured illuminances in Iran are mark­edly higher than Thailand, France, India and San Francisco (5-8). 

Frequency of occurrence of clear skies (87.9%) in Iran is 1.33, 2.6 and 14 times more than San Francisco Thailand and France, respec­tively. This means that Iran, as other sub­tropi­cal locations has good daylight­ing potential. Also Results showed that the percent of clear days in Iran was con­siderably higher than San Francisco, Thai­land and France at cor­responding measur­ing period (7-9). The cli­mate of the subtropics and much of the tropical area is dry and clear most of the year with an annual average direct sun component typically about 8 hour per day (10).

Although measured and calculated values of total data are pretty close in mean values, maximum measured values tend to higher lev­els in the morning than calculated illumi­nances whereas at the rest of time calculated values are higher than measured values. On the other hand calculated values are more con­centrated and have smaller standard de­via­tion rather than measured values. The rea­son of this difference could be restricted ability of IESNA method in identification of real sky conditions. recently Kittler et al(2) 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 illuminance model have been adopted by CIE (international com­mission on illumination) as General Stan­dard Skies (11) and completely de­scribed by Kambedezidis (12). Whereas in IESNA method there are only three sky condi­tions of clear, partly cloudy and cloudy and one distinct constant is considered for each sky condition so this limitation results in calculating concentrated vertical illumi­nances rather than measured illuminances.

Maximum value of measured south facing ver­tical illuminances at total station in Iran (79.6 KLx) found to be 1.06, 0.8 and 1.3 times more than France, Hong Kong and Thai­land, respectively. The respective ratios of maximum mean hourly values on south fac­ing surface of Iran (41.7KLx) to Thai­land, France and San Francisco and India found to be 2.2, 1, 1 and 1.4. Mean monthly south facing vertical illuminance in Iran was measured 2 and 1.2 times more than Thai­land and India, respectively and nearly equal to correspondent values in France and San Francisco.

A regression model was developed between measured and calculated data of south facing vertical illuminance. This model, have a good linear correlation between measured and calculated values (r2 = 0.796).

Conclusively, the suggested model could nicely predict of south facing illuminance in Iranian locations.

Acknowledgements

This paper is based on the first author Ms the­sis which was conducted in Hamadan Uni­ver­sity of Medical Sciences, Iran in 2007-2008. 

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