13.2.10 Daylighting

Visual can perform daylighting calculations and provide daylighting renderings in a single-instance mode; that is, for a particular place at a particular time. The basic calculation procedure is that same as that described for electric lighting; with the sky and sun considered as additional light sources.

Additional data

To add the sky and sun as light sources, additional user data is required; used to find either the appropriate weather data from the Visual Weather Database, or to calibrate a CIE sky specified by the user. In addition, the diffuse reflectance of the surrounding ground plane A two-dimensional and flat object. must be specified, as well as glazing information.

Project location

Location is specified by Longitude and Latitude, specified in degrees. Positive and negative values of Latitude specify north and south the equator, respectively. Positive and negative values of Longitude specify east and west values from the Prime Meridian at Greenwich England. Longitude and Latitude are input by the user of come automatically from the Visual user interface Site Locator.

Project orientation

The default orientation of the Visual project site is that sky and sun North (geographic North) corresponds to +y in local Visual coordinates Location information usually provided in terms of (X,Y) or (X,Y,Z) components along Cartesian axes.. A project orientation angle changes the relative angular position of the sky and sun with respect to the project. Positive values, in degrees, rotate the site clockwise when viewed from above. NB: this does NOT change the significance of the local Visual coordinates Location information usually provided in terms of (X,Y) or (X,Y,Z) components along Cartesian axes. nor does it rotate the Visual drawing A graphical representation of a view of the Design Environment placed on a Page in the Print Editor..

Date and time of analysis

Date and time are local civil time. If in effect, daylight saving time should be indicated.

Weather data

The Visual Weather Database is derived from all of the more than 2100 EnergyPlus data sets that cover most of the globe. For each location with a data set, only required radiometric or photometric Referring to a data file containing information related to the photometric distribution of lighting equipment. Valid files of this type typically have a .IES extension and adhere to the format outlined in IES LM-63. data has been extracted from the hourly data. If only radiometric data is available at a particular site, photometric Referring to a data file containing information related to the photometric distribution of lighting equipment. Valid files of this type typically have a .IES extension and adhere to the format outlined in IES LM-63. data is derived using a process devised by Perez. See: "Modeling Daylight Availability and Irradiance Components from Direct and Global Irradiance," R. Perez, P. Ineichen, R. Seals, J. Michalsky, and R. Stewart. Solar Energy. Vo. 44, No.5. pp 271-289.

In all cases, the primary photometric Referring to a data file containing information related to the photometric distribution of lighting equipment. Valid files of this type typically have a .IES extension and adhere to the format outlined in IES LM-63. data that is extracted or generated for every available site are hourly values of direct solar illuminance The area density of the luminous flux incident on a surface. and total horizontal sky illuminance The area density of the luminous flux incident on a surface. for each data of the year. Data in the EnergyPlus weather file for a particular site are usually constructed from several years of measurements, aggregated together to establish a Typical Meteorological Year for that site.

Sun and Sky

The fundamental luminous properties of sun and sky are the direct solar illuminance The area density of the luminous flux incident on a surface. and the total horizontal illuminance The area density of the luminous flux incident on a surface. produced by the sky. These are either: 1) derived from weather data, or 2) calculated using IES standard sun and sky parameters. See: IES Lighting Handbook, Chapter 7, Section 7.9 Formulary.

The sun is modeled as a luminous disc, ½-degree in diameter. Its luminous power is expressed as the direct, unoccluded illuminance The area density of the luminous flux incident on a surface. produced on a surface with its normal pointed to the sun. Solar position is determined from the location and local time. See: IES Lighting Handbook, Chapter 7, Section 7.1.5 Solar Position.

The sky is modeled as a luminous dome, with a relative luminance The area density of the luminous flux leaving a surface through a given solid angle. distribution determined according to ISO/CIE Standard 15469, 2nd edition. The parameters that determine the distribution are either derived from the appropriate weather data or come directly from the standard set of parameters if the user specifies a specific CIE sky. See: "All-weather Model The entire lighting system composed of Luminaires, Solid Objects (perhaps), and Calculation Zones. for Sky Luminance The area density of the luminous flux leaving a surface through a given solid angle. Distribution – Preliminary Configuration and Validation," R. Perez, R. Seals, and J. Michalsky, Solar Energy, Vol 50, No 3, pp 235-245.

NB: Standard weather data aggregates the illuminance The area density of the luminous flux incident on a surface. from a circum-solar 5-degree circular patch of the sky with the direct illuminance The area density of the luminous flux incident on a surface. from the sun. Therefore, the sky is modeled with a 5-degree hole centered on the sun.

The Visual calculation engine establishes a distance, based on the maximum extents of the project that defines the distance to the sun and the radius Related to arcs, circles, and spheres. The radius may be used to define the curvature by virtue of fact that all points on the object are equidistant from a center point. Any straight line from the center to a point on an arc, circle, or sphere. of the sky dome. The distance is such that that parallax error over the extent of the project to any point on the sky dome is less than ½-degree. The sky is discretized into planar Two-dimensional in nature. All components lie within a single plane. rectangles, accounting for luminance The area density of the luminous flux leaving a surface through a given solid angle. gradient. Using the luminance The area density of the luminous flux leaving a surface through a given solid angle. distribution, element size, and sky dome radius Related to arcs, circles, and spheres. The radius may be used to define the curvature by virtue of fact that all points on the object are equidistant from a center point. Any straight line from the center to a point on an arc, circle, or sphere., each discretized sky element is assigned a luminous power defined by the direct normal illuminance The area density of the luminous flux incident on a surface. it produces at the center of the Visual project.

Sky dome approximated with planar Two-dimensional in nature. All components lie within a single plane. rectangles, sized and distributed according to the luminance The area density of the luminous flux leaving a surface through a given solid angle. distribution of the sky. Notice the 5-degree hole left for the sun and its circum-solar component.

Ground

A ground plane A two-dimensional and flat object. is automatically established around the project and horizontally positioned at Visual local z=0.0. Its reflectance The ratio of the flux actually reflected by a sample surface to that which would be reflected into the same reflected-beam geometry by an ideal, perfectly diffuse standard surface irradiated in exactly the same way as the sample. is user-specified. The plane A two-dimensional and flat object. is automatically discretized into elements that are truncated wedges surrounding the project.

Apertures, windows, and skylights

Skylight and sunlight illuminate any outward-facing user-specified surfaces and the elements of the ground plane A two-dimensional and flat object.. Occlusion by any other surfaces in taken into account. Skylight and sunlight illuminate any surface of an otherwise inward-facing or closed set of surfaces if admitted by an aperture, window or skylight. An aperture is a user-specified opening in an otherwise opaque surface. A window is an aperture into which a user has specified an image-preserving transmittance Transmission is when light passes through a material. less than 100%. A skylight is an aperture into which a user has specified either an image-preserving or a diffuse transmittance.

Glass transmittance

The image-preserving transmittance Transmission is when light passes through a material. specified by the user is assumed to be the perpendicular or normal transmittance Transmission is when light passes through a material.. The Visual calculation engine accounts for the reduction from this value due to increased incident angles. The Fresnel Laws of Reflectance The ratio of the flux actually reflected by a sample surface to that which would be reflected into the same reflected-beam geometry by an ideal, perfectly diffuse standard surface irradiated in exactly the same way as the sample. and Transmission are used to determine this transmittance Transmission is when light passes through a material. value. This calculation is done automatically whenever flux is passing through a surface with an image-preserving transmittance Transmission is when light passes through a material. and uses the normal transmittance Transmission is when light passes through a material., the incidence angle, the number of glazing layers, and the assumed index of refraction for glass of 1.5.

Example of transmittance Transmission is when light passes through a material. as a function of incidence angle for single, double, and triple glazing.