06-27-2007 Council Agenda - Laserfiche WebLink

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PRELIMINARY DRAFT FOR REVIEW BY CITY OF MINNETONKA Further changes are anticipated following Signage Workshop *Preliminary Report is specific to City of Minnetonka issues and may not be sufficient to address concerns in other communities*" Appendix B — Outdoor Advertising Sign Brightness Definitions This appendix defines various technical terms that are used to describe the operational aspects of electronic billboards. Billboard Illuminance Billboard illumination is typically discussed using two terms: illuminance and luminance. Because this section includes some technical jargon, a glossary that further defines terms used in outdoor advertising is provided in Appendix C. Illuminance: The amount of light that is incident to the surface of an object. This is the method for describing ambient light levels or the amount of light that is projected onto a front -lit sign. This parameter is typically measured in lux (footcandles x meters). For the purposes of dimming, illuminance is discussed to describe the ambient light that hits the photocell. Luminance: The amount of Tight that emanates from an internally illuminated sign. This parameter is measured in nits. The nit levels necessary for the sign to be legible vary with the ambient light conditions. On a sunny day, the nit levels must be very high, while at night, the levels must be very low to prevent the image from distorting and to prevent glare. Billboard Luminance (Brightness) Luminance is measured in nits (candelas /square meter) and describes how bright the image is. In essence, it is the amount of light that is radiated from the sign divided by the amount of surface area of the sign. No matter how big the sign is, the luminance of the sign is consistent. For example, the brightness of computer monitors is also measured in nits. The European standard "EN 12966" specifies that at certain ambient light levels, the sign should output a given number of nits. There are different tables for each color due to the properties of how the human eye interprets each color. The color that is most often used to set dimming levels is white. The FHWA has developed recommended practices for dynamic message signs installed within the roadway right -of -way. The standard is NEMA's TS -4 "Hardware Standards for Dynamic Message Signs (DMS) With NTCIP Requirements." Note that these standards were prepared for message signs deployed within the roadway right -of -way and should not be taken as recommended luminance levels for advertising signs. Table A -1 provides a simplified version of the NEMA TS -4 standard for the color white.