The International Sign Association commissioned Dr. Ian Lewin of Lighting Sciences to develop scientifically researched recommendations for the brightness levels of electronic message centers (EMCs). Lewin is a past chair of the Illuminating Engineering Society of North America (IES).
In the simplest terms, following his research, Lewin recommends a nighttime brightness level for on-premise EMCs of 0.3 footcandles above ambient light conditions when measured at an appropriate distance. Note that this recommendation doesn't apply to channel letters and exposed neon. Lewin was aided in his research by referencing the IES publication about light trespass, (TM-11-00).
The study explains the distinction between the two accepted measures of brightness -- illuminance and luminance -- and that the former is preferred. It measures the amount of light that intercepts an object at a given distance from a light source, and is measured in footcandles or its metric equivalent, lux. The study includes a chart of more than 30 sign sizes, and the corresponding distance from which its illumination should be measured. The study then outlines the six steps for measuring the luminance.
The expression "more is better" clearly doesn't apply. Illuminating EMCs at the appropriate brightness has several advantages:
Increases life expectancy of components
Promotes community goodwill
A 10-page Executive Summary of the study can be seen at http://www.signs.org/pdf/EMC_Brightness_Recommendations.pdf