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


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


       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

       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.


       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

       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

         gendaylit -ang 60 0 -P 1 0.08

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

         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




       Jean-Jacques Delaunay, FhG-ISE Freiburg, (


       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.


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