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.\" RCSid "$Id: gendaylit.1,v 1.2 2009/06/06 20:22:49 greg Exp $" |
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.TH GENDAYLIT 1 4/12/94 "RADIANCE ISE/ADEME EXTENSIONS" |
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.SH NAME |
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gendaylit - generates a RADIANCE description of the daylight sources using Perez models for direct and diffuse components |
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.SH SYNOPSIS |
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.B "gendaylit month day hour [-P|-W|-L|-G|-E] input_value(s) " |
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[ |
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.B options |
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] |
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.br |
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.B "gendaylit -ang altitude azimuth [-P|-W|-L|-G|-E] input_value(s) " |
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[ |
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.B options |
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] |
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.SH DESCRIPTION |
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.I Gendaylit |
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produces a RADIANCE scene description based on an angular distribution of the |
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daylight sources (direct+diffuse) for the given atmospheric conditions |
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(direct and diffuse component of the solar radiation), date and |
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local standard time. The default output is the radiance of the sun (direct) and the sky (diffuse) |
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integrated over the visible spectral range (380-780 nm). We have used the |
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calculation of the sun's position and the ground brightness models which |
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were programmed in |
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.I gensky. |
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|
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The diffuse angular distribution is calculated using the Perez et al. |
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sky luminance distribution model (see Solar Energy Vol. 50, No. 3, pp. 235-245, 1993) which, quoting Perez, |
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describes "the mean instantaneous sky luminance angular distribution patterns for all sky |
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conditions from overcast to clear, through partly cloudy, skies". The correctness of the |
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resulting sky radiance/luminance values in this simulation is ensured through the normalization of the modelled |
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sky diffuse to the measured sky diffuse irradiances/illuminances. |
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|
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As described below, the radiation can be defined with the pairs direct-normal and diffuse-horizontal irradiance |
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(-W option), direct-horizontal and diffuse-horizontal irradiance (-G option), direct-normal and diffuse-horizontal |
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illuminance (-L option) or global-horizontal irradiation alone (-E option). The direct-normal radiation |
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is understood here as the radiant flux coming from the sun and an area of approximately 3 degrees around the sun |
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(World Meteorological Organisation specifications for measuring the direct radiation. |
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The aperture angle of a pyrheliometer is approximately 6 degrees). |
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To simplify the calculations for the direct radiation, the sun is represented as a disk and no |
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circumsolar radiation is modelled in the 3 degrees around the sun. This means that |
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all the measured/evaluated direct radiation is added to the 0.5 degree sun source. |
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|
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The direct and diffuse solar irradiances/illuminances are the inputs needed for the calculation. |
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These quantities are the commonly accessible data from radiometric measurement centres or from the |
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Test Reference Year. The use of such data is the recommended method for achieving the most accurate |
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simulation results. |
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|
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The atmospheric conditions are modelled with the Perez et al. parametrization |
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(see Solar Energy Vol. 44, No 5, pp. 271-289, 1990), which is dependent on the values for |
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the direct-normal and the diffuse-horizontal irradiances. The three parameters |
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are epsilon, delta and the solar zenith angle. "Epsilon variations express the transition from |
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a totally overcast sky (epsilon=1) to a low turbidity clear sky (epsilon>6); delta |
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variations reflect the opacity/thickness of the clouds". Delta can vary from 0.05 |
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representing a dark sky to 0.5 for a very bright sky. Not every combination of |
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epsilon, delta and solar zenith angle is possible. For a clear day, if |
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epsilon and the solar zenith angle are known, then delta can be determined. For intermediate or overcast |
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days, the sky can be dark or bright, giving a range of possible values for delta |
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when epsilon and the solar zenith are fixed. The relation between epsilon and delta |
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is represented in a figure on page 393 in Solar Energy Vol.42, No 5, 1989. Note that the |
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epsilon parameter is a function of the solar zenith angle. It means that a clear day |
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will not be defined by fixed values of epsilon and delta. Consequently the input |
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parameters, epsilon, delta and the solar zenith angle, have to be determined on a graph. |
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It might be easier to work with the measured direct and diffuse components (direct normal irradiance/illuminance |
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and diffuse horizontal irradiance/illuminance) than with the epsilon and delta parameters. |
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|
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The conversion of irradiance into illuminance for the direct and the diffuse |
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components is determined by the luminous efficacy models of Perez et al. (see |
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Solar Energy Vol. 44, No 5, pp. 271-289, 1990). To convert the luminance values |
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into radiance integrated over the visible range of the spectrum, |
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we devide the luminance by the white light efficacy factor of |
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179 lm/W. This is consistent with the RADIANCE calculation because the luminance |
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will be recalculated from the radiance integrated over the visible range by: |
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|
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luminance = radiance_integrated_over_visible_range * 179 or |
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luminance = (RED*.263 + GREEN*.655 + BLUE*.082) * 179 with the capability |
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to model colour (where radiance_integrated_over_visible_range == (RED + GREEN + BLUE)/3). |
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|
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From |
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.I gensky, if the hour is preceded by a plus sign ('+'), then it is interpreted as local solar time instead of standard time. |
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The second form gives the solar angles explicitly. The altitude is measured in degrees above the horizon, and the |
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azimuth is measured in degrees west of South. The x axis points east, the y axis points north, and the z axis |
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corresponds to the zenith. The actual material and surface(s) used for the sky is left up to the user. |
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In addition to the specification of a sky distribution function, |
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.I gendaylit |
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suggests an ambient value in a comment at the beginning of the description to use with the |
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.I \-av |
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option of the RADIANCE rendering programs. (See rview(1) and rpict(1).) This value is the cosine-weighted |
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radiance of the sky in W/sr/m^2. |
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.PP |
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.I Gendaylit |
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can be used with the following input parameters. They offer three possibilities |
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to run it: with the Perez parametrization, with irradiance values and with illuminance values. |
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.TP 10n |
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.BR \-P |
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.I epsilon |
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.I delta |
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(these are the Perez parameters) |
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.TP |
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.BR \-W |
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.I direct-normal-irradiance |
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(W/m^2), |
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.I diffuse-horizontal-irradiance |
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(W/m^2) |
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.TP |
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.BR \-G |
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.I direct-horizontal-irradiance |
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(W/m^2), |
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.I diffuse-horizontal-irradiance |
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(W/m^2) |
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.TP |
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.BR \-L |
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.I direct-normal-illuminance |
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(lm/m^2), |
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.I diffuse-horizontal-illuminance |
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(lm/m^2) |
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.TP |
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.BR \-E |
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.I global-horizontal-irradiance |
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(W/m^2) |
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.PP |
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The -E option calculates the diffuse irradiance fraction with the model of Erbs, Klein and Duffie (Solar Energy 28/4, 1982), |
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being followed by the calculation of the -G option. Due to the high uncertainty of the model, the results have to be handled |
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with care. A second irradiance value, if available, is definitely recommended. |
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.PP |
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The output can be set to either the radiance of the visible radiation, the solar radiance (full spectrum) or the luminance. |
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.TP 10n |
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.BR \-O [0|1|2] |
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(0=output in W/m^2/sr visible radiation (default), 1=output in W/m^2/sr solar radiation, 2=output in lm/m^2/sr luminance). |
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.PP |
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.I Gendaylit |
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supports the following options. |
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.TP 10n |
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.BR \-s |
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The source description of the sun is not generated. |
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.TP |
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.BR \-w |
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Suppress warning messages |
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.TP |
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.BI -g \ rfl |
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Average ground reflectance is |
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.I rfl. |
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This value is used to compute |
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.I skyfunc |
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when Dz is negative. |
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.PP |
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The following options do not apply when the solar |
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altitude and azimuth are given explicitly. |
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.TP |
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.BI -a \ lat |
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The site latitude is |
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.I lat |
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degrees north. |
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(Use negative angle for south latitude.) |
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This is used in the calculation of sun angle. |
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.TP |
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.BI -o \ lon |
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The site longitude is |
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.I lon |
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degrees west. |
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(Use negative angle for east longitude.) |
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This is used in the calculation of solar time and sun angle. |
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Be sure to give the corresponding standard meridian also! |
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If solar time is given directly, then this option has no effect. |
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.TP |
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.BI -m \ mer |
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The site standard meridian is |
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.I mer |
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degrees west of Greenwich. |
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(Use negative angle for east.) |
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This is used in the calculation of solar time. |
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Be sure to give the correct longitude also! |
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If solar time is given directly, then this option has no effect. |
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.TP |
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.BI -i \ time_interval [min] |
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If gendaylit is used with weather files, the specified instantaneous points of time may be incorrect. This error occurs |
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due to the fact that measurement results are frequently defined for time intervals, not for specific points of time. |
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Although gendaylit is working correctly, this may lead to wrong outputs especially at low sun altitudes. |
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The -i option allows to specify the time interval of the measurements in minutes, causing the solar position to be corrected for low sun |
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altitudes. A warning message is returned if a correction has been performed. |
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|
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.SH EXAMPLES |
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A clear non-turbid sky for a solar altitude of 60 degrees and an azimut of 0 degree might be defined by: |
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.IP "" .2i |
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gendaylit -ang 60 0 -P 6.3 0.12 or gendaylit -ang 60 0 -W 840 135 |
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This sky description corresponds to the clear sky standard of the CIE. |
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.PP |
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The corresponding sky with a high turbidity is: |
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.IP "" .2i |
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gendaylit -ang 60 0 -P 3.2 0.24 or gendaylit -ang 60 0 -W 720 280 |
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.PP |
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The dark overcast sky (corresponding to the CIE overcast standard, see CIE draft standard, |
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Pub. No. CIE DS 003, 1st Edition, 1994) is obtained by: |
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.IP "" .2i |
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gendaylit -ang 60 0 -P 1 0.08 |
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.PP |
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A bright overcast sky is modelled with a larger value of delta, for example: |
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.IP "" .2i |
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gendaylit -ang 60 0 -P 1 0.35 |
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.PP |
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To generate the same bright overcast sky for March 2th at 3:15pm standard time at a site |
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latitude of 42 degrees, 108 degrees west longitude, and a 110 degrees standard meridian: |
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.IP "" .2i |
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gendaylit 3 2 15.25 -a 42 -o 108 -m 110 -P 1 0.35 |
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.PP |
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.SH FILES |
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/usr/local/lib/ray/perezlum.cal |
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.SH AUTHOR |
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Jean-Jacques Delaunay, Jan Wienold, Wendelin Sprenger, Fraunhofer ISE (Freiburg i.B., Germany) ([email protected]) |
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.SH ACKNOWLEDGEMENTS |
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The first work on this program was supported by the German Federal Ministry for Research |
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and Technology (BMFT) under contract No. 0329294A, and a scholarship from |
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the French Environment and Energy Agency (ADEME) which was co-funded by Bouygues. |
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Many thanks to Peter Apian-Bennewitz, Arndt Berger, Christian Reetz, Ann Kovach, R. Perez, C. Gueymard and G. Ward for their help. |
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.SH "SEE ALSO" |
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gensky(1), rpict(1), rview(1), xform(1) |
