1. Field of the Invention
The present invention relates generally to optics and the transmission of light, and in particular but not exclusively, relates to the measurement of light attenuation through a window.
2. Background Information
Window design is an important consideration in the architectural planning of a building. In addition to affecting the temperatures within the building""s interior spaces, the size and number of windows influence the aesthetic appearance of the building. Indeed, many skyscrapers in crowded urban environments are identifiable by large multi-colored panes of glass that make up the skyscrapers"" siding.
To help keep such buildings cool during hot sunny days, the panes of glass and the windows are typically tinted or coated with a reflective material. The reflective coating acts to reflect some of the sun""s incident light rays, while the tinting acts to absorb or filter certain wavelengths. That is, depending on the type of material used, some windows can be very opaque at certain wavelengths. By reflecting or filtering some of the sun""s incident light rays, the amount of energy used to cool the building during the summer can thus be reduced, thereby resulting in more efficient operation and maintenance.
Measurement of light attenuation through an existing or installed window to determine the window""s reflectiveness or absorption is particularly difficult. One technique is to place a photodetector on one side of an installed window (e.g., on one side of its glass pane), sending light through the glass pane from the other side, and then using the photodetector to measure the amount of light that passes through the glass pane.
Such a technique is impractical for a number of reasons. Most pre-existing or installed windows, particularly those in skyscrapers, are difficult to reach/access because of the skyscraper""s height, and also cannot be opened to install/retrieve the light-measuring equipment. Thus, measurement of light attenuation through an installed window is most easily taken at ground-level windows, which are not always made of the same material as windows at higher levels of the skyscraper, and thus this technique does not provide an accurate determination of light attenuation performance of all windows as a whole.
A second technique is to measure and compare ambient light, which is a technique that is less accurate than the first technique described above. With the second technique, a photodetector having a filter (such as a bandpass filter) in front of it is placed adjacent to the interior side of the window at a specific angle. The photodetector-filter is then taken outside as fast as possible and held at the same angle. The difference in measured light is subsequently compared to obtain a very rough approximation of light attenuation through the window.
As with the first technique, the second technique is very impractical and inaccurate. The photodetector-filter unit needs to be held at the same angle at both sides of the window, which is itself difficult to do in an exact way. More significantly, it is impractical to use this technique for a skyscraper""s installed windows that do not open, since a person performing the measurement has to xe2x80x9cdashxe2x80x9d from an upper floor of the skyscraper after taking the interior measurement, to the outside of the skyscraper to obtain an exterior measurement at ground level. The time taken to obtain the measurement from the outside of the skyscraper reduces the accuracy of this technique.
Accordingly, improvements are needed in the measurement of light attenuation through a window.
An aspect of the invention obtains a first value indicative of a power level of a first light signal incident on a window. A second value indicative of a power level of a second light signal reflected from a first surface of the window is obtained. A third value indicative of a power level of a third light signal reflected from a second surface of the window is obtained. A fourth value indicative of light transmission through the window is determined based on the obtained first, second, and third values.