GENDAYLIT(1)                                                      GENDAYLIT(1)



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

SSYYNNOOPPSSIISS
       ggeennddaayylliitt  mmoonntthh  ddaayy  hhoouurr  [[--PP||--WW||--LL]]  ddiirreecctt__vvaalluuee  ddiiffffuussee__vvaalluuee  [
       ooppttiioonnss ]
       ggeennddaayylliitt --aanngg aallttiittuuddee aazziimmuutthh [[--PP||--WW||--LL]] ddiirreecctt__vvaalluuee ddiiffffuussee__vvaalluuee [
       ooppttiioonnss ]

DDEESSCCRRIIPPTTIIOONN
       _G_e_n_d_a_y_l_i_t 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 radi-
       ation),  date  and local standard time. The default output is the radi-
       ance of the sun (direct) and the sky (diffus) integrated over the visi-
       ble  spectral  range  (380-780 nm). We have used the calculation of the
       sun's position and the ground brightness models which  were  programmed
       in _g_e_n_s_k_y_.

       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/eval-
       uated direct radiation is added to the 0.5 degree sun source.

       _T_h_e  _d_i_r_e_c_t  _a_n_d  _d_i_f_f_u_s_e _s_o_l_a_r _i_r_r_a_d_i_a_n_c_e_s_/_i_l_l_u_m_i_n_a_n_c_e_s _a_r_e _t_h_e _i_n_p_u_t_s
       _n_e_e_d_e_d _f_o_r _t_h_e _c_a_l_c_u_l_a_t_i_o_n_.  These quantities are the commonly accessi-
       ble  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 irradi-
       ances.  The  three  parameters  are epsilon, delta and the solar zenith
       angle. "Epsilon variations express the transition from a totally  over-
       cast  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 effi-
       cacy  factor of 179 lm/W. This is consistent with the RADIANCE calcula-
       tion because the luminance will be recalculated from the radiance inte-
       grated over the visible range by :

       luminance = radiance_integrated_over_visible_range * 179   or

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

       From  _g_e_n_s_k_y , 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 corre-
       sponds 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, _g_e_n_-
       _d_a_y_l_i_t suggests an ambient value in a comment at the beginning  of  the
       description  to  use with the _-_a_v option of the RADIANCE rendering pro-
       grams.  (See rview(1) and rpict(1).)  This value is the cosine-weighted
       radiance of the sky in W/sr/m^2.

       _G_e_n_d_a_y_l_i_t  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.

       --PP        _e_p_s_i_l_o_n _d_e_l_t_a (these are the Perez parameters)

       --WW        _d_i_r_e_c_t_-_n_o_r_m_a_l_-_i_r_r_a_d_i_a_n_c_e  (W/m^2), _d_i_f_f_u_s_e_-_h_o_r_i_z_o_n_t_a_l_-_i_r_r_a_d_i_-
                 _a_n_c_e (W/m^2)

       --LL        _d_i_r_e_c_t_-_n_o_r_m_a_l_-_i_l_l_u_m_i_n_a_n_c_e (lm/m^2),  _d_i_f_f_u_s_e_-_h_o_r_i_z_o_n_t_a_l_-_i_l_l_u_-
                 _m_i_n_a_n_c_e (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.

       --OO[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)

       _G_e_n_d_a_y_l_i_t supports the following options.

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

       --gg _r_f_l    Average  ground  reflectance  is  _r_f_l_.  This value is used to
                 compute _s_k_y_f_u_n_c when Dz is negative.

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

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

       --oo _l_o_n The site longitude is _l_o_n 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.

       --mm _m_e_r The  site  standard  meridian  is _m_e_r 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.

EEXXAAMMPPLLEESS
       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  stan-
       dard 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


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

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

AACCKKNNOOWWLLEEDDGGEEMMEENNTTSS
       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.

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



RADIANCE ISE/ADEME EXTENSIONS       4/12/94                       GENDAYLIT(1)
