Our skies are defined according to the theoretical models provided by the CIE. Those models give the radiance for every point of the sky depending on the position of the sun, be it visible or hidden by clouds. Therefore, we first need to say where the sun should be positioned. This can be done directly by the azimuth and height angle, or by specifying the time of day and time of year.
Notice: Sky definitions are global to the project. This means that if you edit a sky definition within the current variation and there are any other variations in the project that use the same sky definition, then future simulations in the affected variations will all use the new settings.
is given in degrees around the vertical axis, with south defining zero, and counting in positive angles to the west. Negative angles consequently point to the east.
is given in positive degrees above the theoretical flat horizon. The rising or falling sky is at zero degrees, a sun position exactly in the zenith (as possible in tropical areas) has a height of 90°.
Those values represent the exact time that should be simulated. The program doesn't check for nonsensical dates (like February the 31th), but will assume a the day that could be most reasonably be represented by such a value (in our example, that would normally be March the 4th).
Normally, the time is assumed to be relative to the Timezone as set in the Location Properties. If this checkbox is activated, then local solar time is assumed, which defines noon exactly at the highest solar position.
Edit the color of the sky, and assign an image map to the sky hemisphere.
Edit the color of the ground, and assign an image map to the ground hemisphere.
The following are the parameters that are specific to the gensky(1) program that actually constructs the sky for Radiance.
The type of the sky can be selected as one of the following:.
A completely clear sky with full sun impact.
Sunny Sky without Sun
The same clear sky with just the sun left away (imagine a solar eclipse minus the show).
A sky with full cloud cover, and therefore no direct sun.
Intermediate Sky with Sun
The CIE defines an intermediate sky as having a cloud coverage between 30 and 70 %. This setting results in sky radiance values that average that amount of cloudyness.
Intermediate Sky without Sun
The same above, just with the sun left away.
Uniform Cloudy Sky
This is a synthetic model, where all the sky has constant radiance values.
Average Ground Reflectance
Our "sky" defines a full sphere around the scene as modelled with geometrical elements. The lower half of that sphere will get radiance values equal to those that a ground material of the specified reflectance would have.
Greater turbidity factors correspond to greater atmospheric scattering. A turbidity factor of 1.0 indicates an ideal clear atmosphere (i.e. a completely dark sky). Values less than 1.0 are physically impossible.
The overall brightness of the sky can be calibrated by specifying one of the following:
Default Zenith Brightness
The sky radiance values will be completely determined by the theoretical model, depending on the solar position.
Diffuse Zenith Radiance
The radiance values of the complete sky will be modified relative to the value you set for the zenith point. (The default value of 22 W/sr/m2 is set to approximate a ground level illuminance of 10'000 lx.)
Horizontal Diffuse Irradiance
The radiance values of the complete sky will be modified to produce this irradiance value on an unobstructed horizontal surface. (The default value of 55.9 W/m2 is set to approximate a ground level illuminance of 10'000 lx.)
There are several ways to define the brightness of the sky:
Default Solar Radiance
The solar radiance values will be completely determined by the theoretical model, depending on the solar position.
Direct Solar Radiance
The radiance of the sun will be adjusted to this value. (The default value of 1.078e6 W/sr/m2 is set to approximate 1.93e8 cd/m2 of solar irradiance.)
Horizontal Direct Irradiance
The radiance of the sun will be adjusted to produce the irradiance specified on an unobstructed horizontal surface. (The default value of 558.6 W/m2 is set to approximate 100'000 lx of solar illuminance.)
Ok - use the entered values and close the dialog.
Cancel - discard the entered values and close the dialog.
Revert - discard the entered values and reset all fields to the values they had when the dialog opened.
Help... - display this information.
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