WINDOW ANGLES

Usually, architects are concerned about solar position in relation to the position of a window or solar collector. Without a good idea of the geometry involved, it is not possible to reliably design a fixed shading device or optimize exposure.

Sun Angles
Benefits of Summer and Winter Sun Angles on a South Facing Elevation

"Window Azimuth" is much like the solar azimuth. But where solar azimuth is concerned with the bearing of the sun from true south, the window azimuth measures the bearing of the sun on the horizon from normal (perpendicular in plan) to the window. Before the sun's rays are normal to the window, the angle is expressed in negative degrees. Afterwards, the angles are positive.

"Profile Angle" is the vertical angle of the sun measured in section cut normal (perpendicular) to the window in question. When the window azimuth angle is 0, the profile angle is equal to the solar altitude angle; otherwise the profile angle is greater because it is visually foreshortened by the oblique viewing angle (turn a 45 deg triangle away from a flat elevation and it becomes progressively steeper until it reaches 90 degrees.

"Incidence Angle" is measured between the angle of the sun's direct rays and a line normal (perpendicular in all directions) to the surface of the window. At high angles of incidence, the path length of solar energy through glazing becomes very long and more energy is reflected and absorbed. Thus the angle of incidence has an effect on the shading coefficient and visible light transmittance of windows and skylights.

 

Incidence Angles

Incidence Angles and Glazing Types

The type of glass selected will greatly affect the penetration of solar energy into the interior space. For maximum solar heat gain, clear glazing should be selected. This type of glass can be manufactured to have a very low iron content to maximize solar penetration. Where solar heat gain is not desired, as in the case of interior load dominated buildings, other types of glazing should be specified that minimize heat transfer through the glass. Spectrally selective glazing has the ability to screen for certain wavelengths of light.