You may have heard the term or you may have read about the "viewing angle" in data-sheets of LCD-monitors and TVs, a measure that is supposed to inform you about how good your screen performs when observed from oblique directions. The term is a slobby expression and more, it remains meaningless in most of the cases even when you understood that it is supposed to be the angle of inclination, measured from the normal of the display surface, at which ... hummm ? Yes, that's the point, it remains meaningless as long as no limiting value is specified together with it. A reasonable maximum angle of inclination could be specified e.g. by the inclination at which the contrast ratio has dropped to a value of 10.

Since in some data sheets of LCD-monitors we read reports about the "viewing angel", we went out to search for theses beings ... and, we finally discovered some of them.

Things being not so easy, the maximum angle of inclination for a certain limiting value of the contrast (might be the luminance or changes of chromaticity) alone does not tell us enough. We also want to know for which azimuth this angle applies, is it for the vertical direction (12:00 and 6:00) or for the horizontal direction (9:00 and 3:00). Taking the face of a clock we can easily specify the azimuth in terms of the time. Looking at a spot on the screen from 12:00 at an angle of inclination of e.g. 20° specifies the viewing-direction perfectly (see figure below).

There are lots of "definitions" available on the viewing angle (e.g. ".. is the direction from which the display looks best" !), why don't you give it a try and ask your favorite search engine about "viewing angels" in the web and ... have fun !

Meet the real viewing angel !

The observer, looking at a spot on the object (i.e. a visual display), here located at the center of the coordinate system, is represented by one single eye and the viewing direction (= the direction of propagation of the light emerging from the spot under observation and entering the eye of the observer) is represented by the green arrow. This viewing direction is usually specified in a polar coordinate system by an angle of inclination, theta and an angle of rotation, phi (also called azimuth). The angle phi is shown in the the above figure by the shadow of the viewing direction (which is the projection of the viewing direction onto the plane of the object) and usually specified by the angle between the positive x-axis and the projection of the viewing direction. Phi can also be conveniently specified by indication of the corresponding time, i.e. in the figure above 10:30.