man/man1/gendaylit.1

.\" RCSid "$Id: gendaylit.1,v 1.6 2007/09/04 17:36:40 greg Exp $"
.SH NAME
gendaylit - generates a RADIANCE description of the daylit sources using Perez models for diffuse and direct components
.SH SYNOPSIS
.B "gendaylit month day hour [-P|-W|-L] direct_value diffuse_value "
[
.B options
]
.br
.B "gendaylit -ang altitude azimuth [-P|-W|-L] direct_value diffuse_value "
[
.B options
]
.SH DESCRIPTION
.I 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
.I 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. 

.I The direct and diffuse 
.I solar irradiances/illuminances 
.I are the inputs needed 
.I 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 
.I 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.
.PP
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.
.PP
In addition to the specification of
a sky distribution function,
.I gendaylit
suggests an ambient value in a comment at the beginning of the
description to use with the
.I \-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.
.PP
.I 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.
.TP 10n
.BR \-P 
.I epsilon 
.I delta  
(these are the Perez parameters)
.TP
.BR \-W 
.I direct-normal-irradiance
(W/m^2), 
.I diffuse-horizontal-irradiance
(W/m^2)
.TP
.BR \-L 
.I direct-normal-illuminance
(lm/m^2), 
.I diffuse-horizontal-illuminance
(lm/m^2)
.PP
The output can be set to either the radiance of the visible radiation (default), the solar radiance (full spectrum) or the luminance.
.TP 10n
.BR \-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)
.PP
.I Gendaylit
supports the following options.
.TP 10n
.BR \-s
The source description of the sun is not generated.
.TP
.BI -g \ rfl
Average ground reflectance is
.I rfl.
This value is used to compute
.I skyfunc
when Dz is negative.
.PP
The following options do not apply when the solar
altitude and azimuth are given explicitly.
.TP
.BI -a \ lat
The site latitude is
.I lat
degrees north.
(Use negative angle for south latitude.)
This is used in the calculation of sun angle.
.TP
.BI -o \ lon
The site longitude is
.I 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.
.TP
.BI -m \ mer
The site standard meridian is
.I 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.
.SH EXAMPLES
A clear non-turbid sky for a solar altitude of 60 degrees and an azimut of 0 degree might be defined by:
.IP "" .2i
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.
.PP
The corresponding sky with a high turbidity is:
.IP "" .2i
gendaylit -ang 60 0 -P 3.2 0.24 or gendaylit -ang 60 0 -W 720 280
.PP
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:
.IP "" .2i
gendaylit -ang 60 0 -P 1 0.08 
.PP
A bright overcast sky is modelled with a larger value of delta, for example:
.IP "" .2i
gendaylit -ang 60 0 -P 1 0.35 
.PP
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:
.IP "" .2i
gendaylit 3 2 15.25 -a 42 -o 108 -m 110 -P 1 0.35
.PP 
.SH FILES
/usr/local/lib/ray/perezlum.cal
.SH AUTHOR
Jean-Jacques Delaunay, FhG-ISE Freiburg, ([email protected])
.SH 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.
.SH "SEE ALSO"
gensky(1), rpict(1), rview(1), xform(1)
Revision:	1.1
Committed:	Sat Jun 6 20:20:04 2009 UTC (15 years, 11 months ago) by greg
Branch:	MAIN
Log Message:	Added Jean-Jacques Delaunay's gendaylit program to distribution
#	Content
1	.\" RCSid "$Id: gendaylit.1,v 1.6 2007/09/04 17:36:40 greg Exp $"
2	.SH NAME
3	gendaylit - generates a RADIANCE description of the daylit sources using Perez models for diffuse and direct components
4	.SH SYNOPSIS
5	.B "gendaylit month day hour [-P\|-W\|-L] direct_value diffuse_value "
6	[
7	.B options
8	]
9	.br
10	.B "gendaylit -ang altitude azimuth [-P\|-W\|-L] direct_value diffuse_value "
11	[
12	.B options
13	]
14	.SH DESCRIPTION
15	.I Gendaylit
16	produces a RADIANCE scene description based on an angular distribution of the
17	daylight sources (direct+diffuse) for the given atmospheric conditions
18	(direct and diffuse component of the solar radiation), date and
19	local standard time. The default output is the radiance of the sun (direct) and the sky (diffus)
20	integrated over the visible spectral range (380-780 nm). We have used the
21	calculation of the sun's position and the ground brightness models which
22	were programmed in
23	.I gensky.
24
25	The diffuse angular distribution is calculated using the Perez et al.
26	sky luminance distribution model (see Solar Energy Vol. 50, No. 3, pp. 235-245, 1993) which, quoting Perez,
27	describes "the mean instantaneous sky luminance angular distribution patterns for all sky
28	conditions from overcast to clear, through partly cloudy, skies". The correctness of the
29	resulting sky radiance/luminance values in this simulation is ensured through the normalization of the modelled
30	sky diffuse to the measured sky diffuse irradiances/illuminances.
31
32	The direct radiation is understood here as the radiant flux coming from the sun
33	and an area of approximately 3 degrees around the sun (World Meteorological Organisation specifications
34	for measuring the direct radiation. The aperture angle of a pyrheliometer is approximately 6 degrees).
35	To simplify the calculations for the direct radiation, the sun is represented as a disk and no
36	circumsolar radiation is modelled in the 3 degrees around the sun. This means that
37	all the measured/evaluated direct radiation is added to the 0.5 degree sun source.
38
39	.I The direct and diffuse
40	.I solar irradiances/illuminances
41	.I are the inputs needed
42	.I for the calculation.
43	These quantities are the commonly accessible data from radiometric measurement centres, conversion models
44	(e.g. global irradiance to direct irradiance), or from the Test Reference Year. The use of such
45	data is the recommended method for achieving the most accurate simulation results.
46
47
48	The atmospheric conditions are modelled with the Perez et al. parametrization
49	(see Solar Energy Vol. 44, No 5, pp. 271-289, 1990), which is dependent on the values for
50	the direct and the diffuse irradiances. The three parameters
51	are epsilon, delta and the solar zenith angle. "Epsilon variations express the transition from
52	a totally overcast sky (epsilon=1) to a low turbidity clear sky (epsilon>6); delta
53	variations reflect the opacity/thickness of the clouds". Delta can vary from 0.05
54	representing a dark sky to 0.5 for a very bright sky. Not every combination of
55	epsilon, delta and solar zenith angle is possible. For a clear day, if
56	epsilon and the solar zenith angle are known, then delta can be determined. For intermediate or overcast
57	days, the sky can be dark or bright, giving a range of possible values for delta
58	when epsilon and the solar zenith are fixed. The relation between epsilon and delta
59	is represented in a figure on page 393 in Solar Energy Vol.42, No 5, 1989,
60	or can be obtained from the author of this RADIANCE extension upon request. Note that the
61	epsilon parameter is a function of the solar zenith angle. It means that a clear day
62	will not be defined by fixed values of epsilon and delta. Consequently the input
63	parameters, epsilon, delta and the solar zenith angle, have to be determined on a graph.
64	It might be easier to work with the measured direct and diffuse components (direct normal irradiance/illuminance
65	and diffuse horizontal irradiance/illuminance) than with the epsilon and delta parameters.
66
67
68	The conversion of irradiance into illuminance for the direct and the diffuse
69	components is determined by the luminous efficacy models of Perez et al. (see
70	Solar Energy Vol. 44, No 5, pp. 271-289, 1990). To convert the luminance values
71	into radiance integrated over the visible range of the spectrum,
72	we devide the luminance by the white light efficacy factor of
73	179 lm/W. This is consistent with the RADIANCE calculation because the luminance
74	will be recalculated from the radiance integrated over the visible range by :
75
76	luminance = radiance_integrated_over_visible_range * 179 or
77
78	luminance = (RED.263 + GREEN.655 + BLUE.082) 179 with the capability
79	to model colour (where radiance_integrated_over_visible_range == (RED + GREEN + BLUE)/3).
80
81	From
82	.I gensky
83	, if the hour is preceded by a plus sign ('+'), then it is interpreted
84	as local solar time instead of standard time.
85	The second form gives the solar angles explicitly.
86	The altitude is measured in degrees above the horizon, and the
87	azimuth is measured in degrees west of South.
88	.PP
89	The x axis points east,
90	the y axis points north, and the z axis corresponds to the zenith.
91	The actual material and surface(s) used for the sky is left
92	up to the user.
93	.PP
94	In addition to the specification of
95	a sky distribution function,
96	.I gendaylit
97	suggests an ambient value in a comment at the beginning of the
98	description to use with the
99	.I \-av
100	option of the RADIANCE rendering programs.
101	(See rview(1) and rpict(1).)
102	This value is the cosine-weighted radiance of the sky in
103	W/sr/m^2.
104	.PP
105	.I Gendaylit
106	can be used with the following input parameters. They offer three possibilities
107	to run it: with the Perez parametrization, with the irradiance values
108	and with the illuminance values.
109	.TP 10n
110	.BR \-P
111	.I epsilon
112	.I delta
113	(these are the Perez parameters)
114	.TP
115	.BR \-W
116	.I direct-normal-irradiance
117	(W/m^2),
118	.I diffuse-horizontal-irradiance
119	(W/m^2)
120	.TP
121	.BR \-L
122	.I direct-normal-illuminance
123	(lm/m^2),
124	.I diffuse-horizontal-illuminance
125	(lm/m^2)
126	.PP
127	The output can be set to either the radiance of the visible radiation (default), the solar radiance (full spectrum) or the luminance.
128	.TP 10n
129	.BR \-O [0\|1\|2]
130	(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)
131	.PP
132	.I Gendaylit
133	supports the following options.
134	.TP 10n
135	.BR \-s
136	The source description of the sun is not generated.
137	.TP
138	.BI -g \ rfl
139	Average ground reflectance is
140	.I rfl.
141	This value is used to compute
142	.I skyfunc
143	when Dz is negative.
144	.PP
145	The following options do not apply when the solar
146	altitude and azimuth are given explicitly.
147	.TP
148	.BI -a \ lat
149	The site latitude is
150	.I lat
151	degrees north.
152	(Use negative angle for south latitude.)
153	This is used in the calculation of sun angle.
154	.TP
155	.BI -o \ lon
156	The site longitude is
157	.I lon
158	degrees west.
159	(Use negative angle for east longitude.)
160	This is used in the calculation of solar time and sun angle.
161	Be sure to give the corresponding standard meridian also!
162	If solar time is given directly, then this option has no effect.
163	.TP
164	.BI -m \ mer
165	The site standard meridian is
166	.I mer
167	degrees west of Greenwich.
168	(Use negative angle for east.)
169	This is used in the calculation of solar time.
170	Be sure to give the correct longitude also!
171	If solar time is given directly, then this option has no effect.
172	.SH EXAMPLES
173	A clear non-turbid sky for a solar altitude of 60 degrees and an azimut of 0 degree might be defined by:
174	.IP "" .2i
175	gendaylit -ang 60 0 -P 6.3 0.12 or gendaylit -ang 60 0 -W 840 135
176	This sky description corresponds to the clear sky standard of the CIE.
177	.PP
178	The corresponding sky with a high turbidity is:
179	.IP "" .2i
180	gendaylit -ang 60 0 -P 3.2 0.24 or gendaylit -ang 60 0 -W 720 280
181	.PP
182	The dark overcast sky (corresponding to the CIE overcast standard, see CIE draft standard,
183	Pub. No. CIE DS 003, 1st Edition, 1994) is obtained by:
184	.IP "" .2i
185	gendaylit -ang 60 0 -P 1 0.08
186	.PP
187	A bright overcast sky is modelled with a larger value of delta, for example:
188	.IP "" .2i
189	gendaylit -ang 60 0 -P 1 0.35
190	.PP
191	To generate the same bright overcast sky for March 2th at 3:15pm standard time at a site
192	latitude of 42 degrees, 108 degrees west longitude, and a 110 degrees standard meridian:
193	.IP "" .2i
194	gendaylit 3 2 15.25 -a 42 -o 108 -m 110 -P 1 0.35
195	.PP
196	.SH FILES
197	/usr/local/lib/ray/perezlum.cal
198	.SH AUTHOR
199	Jean-Jacques Delaunay, FhG-ISE Freiburg, ([email protected])
200	.SH ACKNOWLEDGEMENTS
201	The work on this program was supported by the German Federal Ministry for Research
202	and Technology (BMFT) under contract No. 0329294A, and a scholarship from
203	the French Environment and Energy Agency (ADEME) which was co-funded by Bouygues.
204	Many thanks to Peter Apian-Bennewitz, Arndt Berger, Ann Kovach, R. Perez, C. Gueymard and G. Ward for their help.
205	.SH "SEE ALSO"
206	gensky(1), rpict(1), rview(1), xform(1)