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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

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

-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


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

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