I hate to provide two answers, but this term exists as well.

Vertical Readability Angle: 90 degrees. Minimum Viewing Distance: 45'. Contrast Enhancement: Non-reflective black louvers and module face grooves disperse..
http://www.daktronics.com/en-us/products/message-displays/al...
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Readability angle: 150°. Resistant to damage from balls as per DIN 18032-3 and UNI 9554:1989 standards.
http://www.favero.com/en2_multisport_scoreboard_electronic_s...
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Readability Angle – The angle at which the LEDs on a sign can still be seen and/or
read. The readability angle, which is dependent on ambient light and site conditions,
is much larger than the viewing angle. For instance, a 30 degree readability angle
LED sign can easily be read at angles greater than 90 degrees at night. In an instance
where the LEDs are dimmed to less than 50 percent, for example, a viewer would still
be able to see or read the LEDs, hence the greater angle than in viewing angle.
Readability angle is largely subjective and difficult to measure. (See viewing angle)

Viewing Angle – The area will provide optimum viewing of a display. The angle is
determined by the horizontal and vertical points at which the measured light
intensity is 50 percent of that measured directly in front of the display. Expressed another way, the maximum viewing angle is defined by the vertical and horizontal points where the information displayed can no longer be interpreted by the viewer.
(This does not mean that the LED cannot be seen outside the viewing angle. Viewing angle is an industry-accepted term and used by the LED manufacturers themselves.)
(See readability angle)

http://www.bunbury.wa.gov.au/pdf/Council/Agendas and Minutes...

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Note added at 44 mins (2017-06-17 17:15:44 GMT)
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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.
https://en.wikipedia.org/wiki/Viewing_angle