The Dangers of Nitpicking

The ANSI/IES RP-39-19 recommendations for electric-sign luminance present some serious issues for the industry.
Electric signs are promotional devices and should not be regulated as “lighting.”

Regulatory changes impacting the sign industry exhibit a stealth quality that often enables them to sneak up on the unsuspecting. Thus, I was surprised to read the sidebar “Nit Guns – The New Sheriff in Town for Brightness.”

During my stint as director of technical and regulatory affairs for the International Sign Association (ISA), I was deeply involved in the issue of sign luminance. At that time, ISA conducted laboratory testing of internally illuminated electric signs in cooperation with Underwriters Laboratories (UL). We also conducted extensive field surveys of existing internally illuminated electric signs using luminance meters (aka “nit guns”). Several years prior to this research, I represented the ISA team working with Southern California Edison (SCE) to develop sign-lighting standards for the California Energy Commission (CEC).

Concurrently, ISA conducted a comprehensive study of all available historical materials pertaining to the topic of sign illumination and brightness (i.e. luminance). Based on the findings from this body of previous research, the ANSI/IES RP-39-19 Recommended Practice: Off-Roadway Sign Luminance standard raises some issues which would adversely impact the performance properties of electric signs if its guidelines are adopted by local jurisdictions.

Published for decades in the IESNA/IES Lighting Handbook, this chart describes the appropriate luminance range for internally illuminated signs.
Published for decades in the IESNA/IES Lighting Handbook, this chart describes the appropriate luminance range for internally illuminated signs.

It’s normally expected that developing a national standard will entail due process and consensus, defined by ANSI as “substantial agreement reached by directly and materially affected interests.” Unfortunately, there’s no evidence the RP-39-19 document references the sign industry’s comprehensive knowledge about the essential performance properties of electric signs. 

Following are three key reasons why adoption and enforcement of ANSI/IES RP-39-19 would severely impair the electric-sign industry:

First, the specified, maximum luminance limits in RP-39-19 are far lower than the documented, baseline requirements for internally illuminated (plastic) signs. The maximum luminance allowed under RP-39-19 is 150 nits (candelas per square meter). But this limit applies strictly to the LZ4 classified lighting zone, defined as areas having very high densities of signage and lighting (e.g. Times Square). The LZ3 zone, identified as the default category, characterizes most urban and suburban areas of the country. The sign-luminance limit for LZ3 is only 80 nits. This limit is drastically lower than the original, recommended range for sign luminance (700-1,000 nits) published in the IESNA/IES Lighting Handbook in various editions for several decades prior to adoption of RP-39-19. 

An important result of the ISA luminance research was to confirm the accuracy of this range by conducting lab testing of new electric signs built to conventional specifications. Additionally, the 2005 study High Efficacy Signage conducted by Southern California Edison (in connection with its research for the CEC) determined the baseline luminance for the most common type of internally illuminated sign falls within the original IESNA/IES range. ISA’s sign-testing program at UL’s Allentown (PA) lab also confirmed a baseline luminance within this same range. 

Secondly, the scope of RP-39-19 mischaracterizes the functional requirements for sign lighting. The scope states, “Lighting should maintain the minimum luminance required for visibility.” While this statement might apply to outdoor lighting devices, it’s inappropriate with respect to sign luminance. 

Signs are messaging devices that serve an entirely different function than lighting fixtures. Whereas general outdoor lighting is designed to illuminate specific areas to facilitate nighttime activities, signs are designed to project their messages in all directions from which the sign may be viewed. Because electric signs are promotional devices for businesses, imposing limits based on the “minimum luminance required for visibility” greatly inhibits the core advertising function of signage. That’s true because the luminance level at which a sign is detectable (and legible) is far below the level at which the sign is prominent

Third, RP-39-19 adopts a “one-standard-fits-all” approach to sign luminance. The RP-39-19 scope states that it includes “on- and off-premise, internally and externally illuminated, and electronic signs.” But key distinctions between these different types of illuminated signs make this regulatory scheme inappropriate. For example, whereas electronic signs typically incorporate unshielded light sources (LEDs) exposed directly to the human eye, internally illuminated signs emit light diffused and reduced in intensity via transmittance through various plastic face materials. There’s simply no rational basis for applying the same luminance limits to all electric signs regardless of type.

The center bars in this chart show baseline luminance values for lab-measured signs incorporating two common face thicknesses (source: SCE High Efficacy Signage Study, 2005).
The center bars in this chart show baseline luminance values for lab-measured signs incorporating two common face thicknesses (source: SCE High Efficacy Signage Study, 2005). 

Historical data and modern testing confirm that the sign industry adheres to well-defined guidelines governing the brightness of electric signs and limiting escalation of sign brightness over time. These guidelines have existed for decades and they’re essential to the proper functioning of electric-sign products under a broad range of environmental conditions.

Likewise, the unique nature, function and variety of electric signs makes the image of a “new sheriff in town” packing a nit gun highly impractical as a means of enforcement. Undoubtedly, enforcing the RP-39-19 recommendations in this manner (or any other) would severely impair the performance properties of electric-sign products coast to coast.


Bill Dundas is a 40-year veteran of the electric-sign industry, and former executive for both a national sign association and industry foundation.