An Evidence Based Model Sign Code - College of Design ...

The third bullet point illustrates a key aspect of establishing
luminance levels. The optimal luminance level is closely related
to the environment in which a sign is viewed. A sign in a
dark rural environment can be read with lower luminance
than one in a brightly lit urban environment. Furthermore,
since most on-premise signs are illuminated internally, it is
possible to provide much higher luminance levels than can
be achieved with traffic signs.
In 1986, Mace et al. provided an excellent literature summary
based on the determination of minimum brightness standards
for sign legibility (Mace et al. 1986). The findings related to
minimum luminance requirements for legibility (MLRL) for
overhead signs were:
• MLRL increase as the ratio of letter stroke width to letter
height decreases.
• MLRL increase as the level of internal contrast decreases.
• The published data are inconsistent on the effect of sign
luminance and surround luminance.
• MLRL are not influenced by glare, unless the glare source
is very bright and immediately adjacent to the sign.
• MLRL increase with observer age.
In an effort documented in 1985, Schmidt-Clausen reported
on minimum luminance levels needed for sufficient and
optimal performance (Transportation Research Circular 297).
The investigation was carried out on a scale model and compared
to those values found in real-world situations. The study
showed that a legend luminance of 3.5 to 10 cd/m 2 is sufficient.
Luminance values between 10 and 35 cd/m 2 are optimal. The
maximum luminance was determined to be about 60 cd/m 2 .
The actual value of the minimum and maximum luminance
depends upon the level of critical detail in a sign, the contrast
ratios, the level of ambient lighting, and the visual complexity
of the viewing background. As indicated in the technical report,
there is general agreement that 3.4 cd/m 2 is the absolute
minimum luminance for a white sign legend. Luminance
levels between 34 and 1 cd/m 2 are optimal, and the maximum
luminance is 340 cd/m 2 . These luminance levels are for a white
legend on a traffic sign, which uses positive contrast (light
legend on a darker background). In comparison, on-premise
signs often use a wider range of colors for the sign legend and
background. In addition, many on-premise signs are internally
illuminated, which is not a common practice for traffic signs.
The ability to transfer research findings from traffic sign
research is somewhat limited due to the wider range of letter
forms, colors, sign layouts, and illumination technologies
and levels associated with on-premise signs. However, the
information for traffic signs provides a valuable comparison
for sign legibility performance.
This report does not recommend that illuminance be a major
criterion in regulating sign lighting design. Illuminance level
varies with distance from the lighting source and depends on
the type of and power output of the lighting facility. Furthermore,
there are no standardized methods or equipment for
measuring illuminance of on-premise signs that will exclude
the impact of ambient lighting (Garvey 2004). Furthermore,
this report recommends that luminance levels for static signs
be based on legibility needs. There is currently insufficient
scientific evidence to establish maximum luminance levels
for on-premise static signs.
Contrast
In addition to size and luminance, a message must also
possess sufficient contrast with the background to be visible.
According to Olson, contrast is defined as characteristics of a
scene that cause an object to appear distinct from its background.
At higher levels of illumination, contrast can be
provided by such things as differences in color, pattern,
shading, texture, and brightness. At night, generally only
brightness contrast is available (Olson 1996).
The concept of contrast is best illustrated in Figure 5. In this
figure, the color of the word “contrast” is the same shade of
gray for the entire word. However, the shade of the background
changes from a light gray to near black. As a result,
the letter “R” is not visible due to a lack of contrast between
the word and the background. Furthermore, the visibility of
the first and last parts of the word has different characteristics.
In the first part of the word, the object is darker than the
background; this is referred to as negative contrast. In the last
part of the word, the object is brighter than the background;
this is referred to as positive contrast. The minimum design
criteria for sign visibility can depend upon whether the sign
message is presented with positive or negative contrast.
Figure 5. Example of Contrast
A Legal and Technical Exploration of On-Premise Sign Regulation: An Evidence Based-Model Sign Code 21