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 [
gendaylit -ang altitude azimuth [-P|-W|-L] direct_value diffuse_value [
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
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
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-
-L direct-normal-illuminance (lm/m^2), diffuse-horizontal-
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, (email@example.com)
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)