xenial (1) gendaylit.1.gz

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NAME

       gendaylit  -  generates  a  RADIANCE description of the daylit sources using Perez models for diffuse and
       direct components

SYNOPSIS

       gendaylit month day hour [-P|-W|-L] direct_value diffuse_value [ options ]
       gendaylit -ang altitude azimuth [-P|-W|-L] direct_value diffuse_value [ options ]

DESCRIPTION

       Gendaylit produces a RADIANCE scene description based on an angular distribution of the daylight  sources
       (direct+diffuse)  for  the  given  atmospheric  conditions  (direct  and  diffuse  component of the solar
       radiation), date and local standard time. The default output is the radiance of the sun (direct) and  the
       sky (diffus) integrated over the visible spectral range (380-780 nm). We have used the calculation of the
       sun's    position    and    the    ground    brightness    models    which     were     programmed     in
       gensky.

       The  diffuse  angular distribution is calculated using the Perez et al.  sky luminance distribution model
       (see Solar Energy Vol. 50,  No.  3,  pp.  235-245,  1993)  which,  quoting  Perez,  describes  "the  mean
       instantaneous  sky luminance angular distribution patterns for all sky conditions from overcast to clear,
       through partly cloudy, skies". The correctness of the resulting sky  radiance/luminance  values  in  this
       simulation  is  ensured through the normalization of the modelled sky diffuse to the measured sky diffuse
       irradiances/illuminances.

       The direct radiation is understood here as  the  radiant  flux  coming  from  the  sun  and  an  area  of
       approximately  3  degrees  around the sun (World Meteorological Organisation specifications for measuring
       the direct radiation. The aperture angle of a pyrheliometer is approximately 6 degrees).  To simplify the
       calculations  for  the direct radiation, the sun is represented as a disk and no circumsolar radiation is
       modelled in the 3 degrees around the sun. This means that all the measured/evaluated direct radiation  is
       added to the 0.5 degree sun source.

       The  direct  and diffuse solar irradiances/illuminances are the inputs needed for the calculation.  These
       quantities are the commonly accessible data from radiometric measurement centres, conversion models (e.g.
       global  irradiance  to  direct  irradiance), or from the Test Reference Year. The use of such data is the
       recommended method for achieving the most accurate simulation results.

       The atmospheric conditions are modelled with the Perez et al. parametrization (see Solar Energy Vol.  44,
       No  5,  pp.  271-289, 1990), which is dependent on the values for the direct and the diffuse irradiances.
       The three parameters are epsilon, delta and the solar  zenith  angle.  "Epsilon  variations  express  the
       transition  from  a  totally  overcast  sky  (epsilon=1)  to a low turbidity clear sky (epsilon>6); delta
       variations reflect the opacity/thickness of the clouds". Delta can vary from 0.05 representing a dark sky
       to 0.5 for a very bright sky. Not every combination of epsilon, delta and solar zenith angle is possible.
       For a clear day, if epsilon and the solar zenith angle are known,  then  delta  can  be  determined.  For
       intermediate or overcast days, the sky can be dark or bright, giving a range of possible values for delta
       when epsilon and the solar zenith are fixed. The relation between epsilon and delta is represented  in  a
       figure  on  page  393  in  Solar  Energy  Vol.42,  No 5, 1989, or can be obtained from the author of this
       RADIANCE extension upon request. Note that the epsilon parameter is a function of the solar zenith angle.
       It  means  that  a  clear  day will not be defined by fixed values of epsilon and delta. Consequently the
       input parameters, epsilon, delta and the solar zenith angle, have to be determined on a graph.  It  might
       be  easier  to work with the measured direct and diffuse components (direct normal irradiance/illuminance
       and diffuse horizontal irradiance/illuminance) than with the epsilon and delta parameters.

       The conversion of irradiance into illuminance for the direct and the diffuse components is determined  by
       the  luminous  efficacy  models  of  Perez et al. (see Solar Energy Vol. 44, No 5, pp. 271-289, 1990). To
       convert the luminance values into radiance integrated over the visible range of the spectrum,  we  devide
       the  luminance  by  the  white  light  efficacy  factor of 179 lm/W. This is consistent with the RADIANCE
       calculation because the luminance will be recalculated from the  radiance  integrated  over  the  visible
       range by :

       luminance = radiance_integrated_over_visible_range * 179   or

       luminance  =  (RED*.263  +  GREEN*.655  +  BLUE*.082) * 179    with the capability to model colour (where
       radiance_integrated_over_visible_range == (RED + GREEN + BLUE)/3).

       From gensky , if the hour is preceded by a plus sign ('+'), then it is interpreted as  local  solar  time
       instead  of  standard time.  The second form gives the solar angles explicitly.  The altitude is measured
       in degrees above the horizon, and the azimuth is measured in degrees west of South.

       The x axis points east, the y axis points north, and the z axis corresponds to the  zenith.   The  actual
       material and surface(s) used for the sky is left up to the user.

       In addition to the specification of a sky distribution function, gendaylit suggests an ambient value in a
       comment at the beginning of the description to  use  with  the  -av  option  of  the  RADIANCE  rendering
       programs.   (See  rview(1)  and  rpict(1).)   This  value  is  the cosine-weighted radiance of the sky in
       W/sr/m^2.

       Gendaylit can be used with the following input parameters. They offer three possibilities to run it: with
       the Perez parametrization, with the irradiance values and with the illuminance values.

       -P        epsilon delta (these are the Perez parameters)

       -W        direct-normal-irradiance (W/m^2), diffuse-horizontal-irradiance (W/m^2)

       -L        direct-normal-illuminance (lm/m^2), diffuse-horizontal-illuminance (lm/m^2)

       The output can be set to either the radiance of the visible radiation (default), the solar radiance (full
       spectrum) or the luminance.

       -O[0|1|2] (0=output in W/m^2/sr visible radiation, 0=output in  W/m^2/sr  solar  radiation,  2=output  in
                 lm/m^2/sr luminance)

       Gendaylit supports the following options.

       -s        The source description of the sun is not generated.

       -g rfl    Average ground reflectance is rfl.  This value is used to compute skyfunc when Dz is negative.

       The following options do not apply when the solar altitude and azimuth are given explicitly.

       -a lat The site latitude is lat degrees north.  (Use negative angle for south latitude.)  This is used in
              the calculation of sun angle.

       -o lon The site longitude is lon degrees west.  (Use negative angle for east longitude.)  This is used in
              the  calculation of solar time and sun angle.  Be sure to give the corresponding standard meridian
              also!  If solar time is given directly, then this option has no effect.

       -m mer The site standard meridian is mer degrees west of Greenwich.  (Use negative angle for east.)  This
              is  used  in the calculation of solar time.  Be sure to give the correct longitude also!  If solar
              time is given directly, then this option has no effect.

EXAMPLES

       A clear non-turbid sky for a solar altitude of 60 degrees and an azimut of 0 degree might be defined by:

         gendaylit -ang 60 0 -P 6.3 0.12 or gendaylit -ang 60 0 -W 840 135 This sky description  corresponds  to
         the clear sky standard of the CIE.

       The corresponding sky with a high turbidity is:

         gendaylit -ang 60 0 -P 3.2 0.24 or gendaylit -ang 60 0 -W 720 280

       The  dark  overcast sky (corresponding to the CIE overcast standard, see CIE draft standard, Pub. No. CIE
       DS 003, 1st Edition, 1994) is obtained by:

         gendaylit -ang 60 0 -P 1 0.08

       A bright overcast sky is modelled with a larger value of delta, for example:

         gendaylit -ang 60 0 -P 1 0.35

       To generate the same bright overcast sky for March 2th at 3:15pm standard time at a site latitude  of  42
       degrees, 108 degrees west longitude, and a 110 degrees standard meridian:

         gendaylit 3 2 15.25 -a 42 -o 108 -m 110 -P 1 0.35

FILES

       /usr/local/lib/ray/perezlum.cal

AUTHOR

       Jean-Jacques Delaunay, FhG-ISE Freiburg, (jean@ise.fhg.de)

ACKNOWLEDGEMENTS

       The  work on this program was supported by the German Federal Ministry for Research and Technology (BMFT)
       under contract No. 0329294A, and a scholarship from the French  Environment  and  Energy  Agency  (ADEME)
       which  was  co-funded  by  Bouygues.   Many thanks to Peter Apian-Bennewitz, Arndt Berger, Ann Kovach, R.
       Perez, C. Gueymard and G. Ward for their help.

SEE ALSO

       gensky(1), rpict(1), rview(1), xform(1)