In display technology parlance, viewing angle is the maximum angle at which a display can be viewed with acceptable visual performance. In a technical context, this angular range is called viewing cone defined by a multitude of viewing directions.
The image may seem garbled, poorly saturated, of poor contrast, blurry or too faint outside the stated viewing angle range, the exact mode of "failure" depends on the display type in question. For example, some projection screens reflect more light perpendicular to the screen and less light to the sides, making the screen appear much darker (and sometimes colors distorted) if the viewer is not in front of the screen. Many manufacturers of projection screens thus define the viewing angle as the angle at which the luminance of the image is exactly half of the maximum. With LCD screens, some manufacturers have opted to measure the contrast ratio, and report the viewing angle as the angle where the contrast ratio exceeds 5:1 or 10:1, giving minimally acceptable viewing conditions.
The viewing angle is measured from one direction to the opposite, giving a maximum of 180° for a flat, one-sided screen. Some display devices exhibit different behaviour in horizontal and vertical axis, requiring users and manufacturers to specify maximum usable viewing angles in both directions. Usually the screens are aligned and used to facilitate greater viewing angle in horizontal level, and smaller angle in the vertical level, should the two of them be different in magnitude.
Narrow viewing cones of some types of displays have also been used to bring a measure of security in businesses, where employees handle private information in the presence of customers, banks being one example. Rectangular filters fitting to the computer monitors have also been sold as accessories for these kind of purposes.
LEDs are measured so that the line along half the viewing-angle from directly forward is half the brightness as at directly forward.