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All Things Lighting . Eng., FIESChief Scientist, Lighting Analysts Inc. The light of day.
Apart from having a wonderfully circular definition in most English- language dictionaries, daylight really is just another form of illumination. As such, most people would expect lighting designers to be able to simulate daylight with the same ease that we simulate electric lighting .
The daylight factor metric is exceedingly simple to calculate, but it is not very useful in understanding how daylight illuminates an interior space. All it really tells us (and our clients) is whether there will be sufficient daylight to read a newspaper indoors on an overcast day. We have, in other words, good reason to blush. We know better, of course. Given the architectural plans of a building, including both its geographical coordinates and orientation, we know we can use historical weather data to determine the typical distribution of daylight within the building for every hour of every day of the year. We even have a name for this: climate- based daylight modeling (CBDM), an expression introduced a decade ago at a CIBSE lighting conference appropriately called “Engineering the Future” (Mardaljevic 2.
With CBDM, we can calculate daylight metrics such as spatial Daylight Availability and Annual Sunlight Exposure (IES 2. Useful Daylight Illuminance (Nabil et al. Daylight Glare Probability (Wienold et al.
As consultants to architectural firms, lighting design professionals have an obligation to provide these metrics. Simatic Key Installer Free Download. Installer Ubuntu Sur Disque Dur Externe Depuis Windows Xp. Glare metrics take this one step further, offering the ability to identify potential design problems with large expanses of glazing. Going further still, we can design and validate daylight harvesting systems for energy savings, and address building energy modeling issues involving solar insolation.
Most important, we can work with architects during the conceptual design phase to take full advantage of what daylighting has to offer. From a lighting design perspective, CBDM expands the consulting services we can provide. Until recently, the only software capable of performing CBDM calculations has been Lawrence Berkeley National Laboratory’s Radiance . It is not a monolithic program, but rather a set of one hundred or so Unix programs that can be linked together using command- line scripts.
Radiance, however, is first and foremost a research tool. It is reasonable to assume that most architects and engineers will prefer not to learn Unix, with command- line scripts such as: Fortunately, there are a number of free and commercial architectural- engineering design applications that are available, and which provide graphical user interfaces to the Radiance toolset. Those that support climate- based daylight modeling include DAYSIM and DIVA for Rhino (www. This article is not, however, about Radiance and its derivatives; it is about climate- based daylight modeling. More particularly, it is about a unique radiosity- based approach to CBDM calculations that does not involve the Radiance daylight calculation engine. It is the culmination of over twelve years of research and development, beginning with the paper “Modeling Daylight for Interior Environments” (Ashdown 2.
The details are disclosed herein for those interested in understanding how it works. Follow the Light.
In order to fully understand the radiosity approach, it is necessary to follow the light from source to receiver. The source is the combination of direct sunlight and diffuse daylight; the receiver is a room or similar naturally illuminated space within a building. Modeling direct sunlight is straightforward. The solar position in the sky can be readily calculated from the equations presented in Section 7. Solar Position, of the IESNA Lighting Handbook, Tenth Edition (IES 2. The solar disk is only 0. Modeling diffuse daylight is more challenging.
Some of the extraterrestrial radiation from the sun is scattered by the Earth’s atmosphere, resulting in the hemispherical diffuse light source that is the sky. The sky luminance (colloquially, “brightness”) spatial distribution varies with geographic location, site altitude, time of day, time of year, and weather conditions, including clouds and aerosols such as smoke and airborne dust, and also the dew point temperature. It is clearly not practical to deterministically model realistic weather conditions, especially partly cloudy weather where the sky luminance distribution may vary on a time scale of minutes. Modeling diffuse daylight therefore requires a simplifying mathematical model, which in turn requires measured weather data. Typical Meteorological Year.
There are over 2,1. North America) that measure weather data on an hourly basis. Through a complex set of empirical rules (Wilcox et al. Typical Meteorological Year (TMY3) data set for the station’s geographic location. Direct normal irradiance can be measured with a pyrheliometer, an instrument that measures the solar irradiance (including visible light and ultraviolet and infrared radiation from 3.
Muneer 2. 00. 4). The device is always aimed directly at the Sun, and it includes a narrow tube that limits its field of view to six degrees (e. Figure 1). Diffuse horizontal irradiance is usually measured with a pyranometer, an instrument (such as that shown in Figure 2) that measures irradiance from the sky incident upon its horizontal thermopile sensor (Muneer 2. A shadow band may be positioned over the sensor to obscure a six degree- wide band following the path of the Sun, although it must be moved on a regular basis throughout the year. Alternatively (and more accurately), the global horizontal irradiance can be measured without a shadow band, and the measured direct normal irradiance measurement subtracted from it to determine the diffuse horizontal irradiance (ibid). Perhaps surprisingly, these two measurements are all that are needed to model diffuse daylight.
Perez Sky Model. Based on some 1. Berkeley, California, Perez et al. The only two measured input parameters are direct normal and diffuse horizontal irradiance. Perez sky model is better than most.
More important, it has been implemented in the Radiance tool gendaylit to generate a single sky luminance distribution, and in gendaymtx to generate a set of hourly sky luminance distributions for the year from a TMY3 weather data file. It should also be noted that where TMY3 weather data include direct normal and diffuse illuminance values, they have likely been calculated from the corresponding irradiance measurements using the Perez sky model. When direct normal and diffuse horizontal illuminance values are submitted to gendaylit, it uses an undocumented iterative algorithm to estimate the original measured irradiance measurements that the Perez sky model requires. Sky Luminance Distribution. Prior to the introduction of calibrated all- sky digital cameras with fisheye lenses (e. Figure 3), mechanical scanners were used to measure the sky luminance distribution. Still manufactured by EKO Instruments, these instruments consist of a luminance meter mounted on an alt- azimuth platform with stepper motors, and measure the sky luminance at 1.
They were previously used to obtain data for, among other purposes, the validation of various all- weather sky models, including the Perez sky model. One legacy of these scanners has been the Tregenza sky subdivision (Tregenza 1.
Figure 4). FIG. 4 – Tregenza sky subdivision. Apart from the circumsolar region within a few degrees of the solar disk, the luminance of the sky dome in any direction can be interpolated with reasonable accuracy from these values. Daylight Coefficients. Another advantage of the Tregenza subdivision scheme comes from a paper published over three decades ago, simply titled “Daylight Coefficients” (Tregenza et al. The researchers observed that each sky patch can be thought of as a separate and independent area light source that potentially illuminates a room through a window or opening (Figure 5).