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Neon Visibility

Understanding the basics of light and color creates satisfied clients

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August is frequently a time for vacations and relaxing. Nobody wants to think about work because business will soon become stressful enough. Because this is the time when new neon installations are planned, it’s a good time to discuss design properties that affect appearance. To ensure that neon signs deliver maximum impact, sign builders need to put themselves in their customers’ shoes. This month, I focus on the physiological or human factors that influence neon-sign design.

Because physiology is not an exact science, I present statistics that represent averages of subjective responses. As a physicist accustomed to exact measurements, this makes me uncomfortable. Substantial human diversity, however, dictates that this type of perceptual study be based on averaging responses.

Visual perception

The sensory point of entry for a neon sign’s message is the human eye. Thus, the sign’s visibility depends on three principal factors:

  1. The viewer’s peculiar eyesight characteristics;
  2. The atmosphere’s ambient transparency and its effect on color; and
  3. The viewed object’s luminous properties.

Human-eye resolution and sensitivity are closely related. We know, for example, that the "average" eye can resolve two objects separated by a viewing angle of 1 arc/minute when the background has an average luminous density of 1 candela per square meter (Fig. 1). From this curve, we determine that nighttime viewing requires greater separation between various portions of a neon sign than would be necessary in daylight. The sign’s size requirement can be calculated using the normal, estimated viewing distance (Fig. 2).

The values given in Fig. 2, however, are valid only in good weather conditions. Since the 1920s, we have known that the atmosphere’s transparence is reduced in bad weather, but not equally for all wavelengths or colors (Fig. 3). The superior penetrating power of a red-neon discharge still stands as one of the neon industry’s principal marketing arguments.

Luminous properties of neon signs

When discussing the minimum brightness necessary to distinguish two separate points, we must also discuss the opposite effect: image blurring or irradiation. At high intensities, light activates surrounding optical nerves, as well as those at the spot where light is focused. This increases the visible light spot’s diameter. For sign installations where incandescent lightbulbs are used, this is a desirable effect (Fig. 4). But the effect is undesirable for neon signs because letter contours become blurred. In this case, halos form around the entire sign, decreasing visibility.

Without reducing a sign’s light output, a sign builder can counter irradiation by:

  • Increasing the letter height. This provides more dark space between critical letters (B,E,G,H,R,S). Thus, the eye is given a wider viewing angle to better distinguish the letter.
  • Aligning the tube layout properly. For longer viewing distances, neon lettering usually is created in multiple rows of tubing. Concentrating tubes in the center of the letter, however, does not give the font a very bright appearance because of irradiation. Therefore, when bright colors are used, single- or double-stroke neon letters may provide better readability than multiple rows of tubing.
  • Properly designing channel-letter shapes. The ability to restrict neon’s light-emission angle is one key reason for channel letters’ popularity. The typical reflection of light radiated sideways from exposed neon is reduced, which, in turn, reduces the halo effect for better readability.
  • Calibrating the sign’s brightness according to background brightness.

Because human-eye sensitivity comprises a wide range of visual impressions, "brightness" always depends on the ambient environment. For example, to avoid the irradiation effect, a neon sign cannot be more than 10-100 times brighter than the background surface. Because many signs must be readable by day and night, a designer encounters problems. Either the sign appears too dark in daylight or too bright after dark. One solution is to make the neon bright enough to be seen well by day, but to illuminate the sign’s surroundings with additional lights to avoid blurring at night.

Because the surface brightness of acrylic-face letters is approximately 10 times less than for exposed neon, irradiation is reduced greatly in enclosed channel letters. To provide visibility equivalent to exposed neon, enclosed neon signs must either be made considerably larger or equipped with more powerful illumination.

Color, brightness and power

We know the human eye is most sensitive to yellow/green colors. Thus, the blurring effect is much more likely when using greenish colors than, for example, a deep red. Both radiate the same power, but at different wavelengths. Furthermore, lighting efficiency (electrical input power vs. light output power) varies widely depending on the type of gases, fluorescent powders and glass colors used. Fig. 5 gives typical surface brightnesses for 15mm, exposed neon operated by a typical, 60mA transformer. This table also suggests that it might be unwise to place a ruby-red tube of equal size and operating current beside a standard green tube. The green tube will definitely overpower the red.

Figure 5
Color Brightness (cd/m2)
white 6,500 K 6503
clear red 2160
clear blue 900
standard green 10026
standard blue 2560
ruby red 800
cobalt blue 1600
noviol gold 6500
Fig. 5: Average values calculated from different manufacturers’ measurements for a 15mm tube operated at 50mA (or 60mA standard transformer).

Choice of colors is often dictated by the client company’s philosophy or existing logo. Some customers accept deviations, while others will not. Thus, a signmaker must calculate a sign’s size, as well as the brightness and operating current of each component color, to create a successful design.

Acrylic-face signs

When company logos or sign messages are too complex for exposed neon, they must be created as backlit acrylic signs or channel letters. Backlighting acrylic plastics with neon offers several advantages over fluorescent lamps, but achieving good results can be challenging. The principal advantages of neon backlighting include unlimited shapes and longevity of the lighting system. The ability to fabricate neon tubes in any shape allows signmakers to optimize brightness uniformity throughout the illuminated surface. Because brightness, shape and spacing are critical, sign builders should experiment before they quote jobs.

Specific face colors must be combined with the proper neon backlighting colors for optimum appearance. For example, if you’re backlighting a large-format, full-color digital print, you need a "Daylight" neutral white with a color rendering index (CRI) of at least 95. Otherwise, some colors may appear as dirty shades. When backlighting red acrylic, you can use clear red or, even better, a bright purplish red for maximum brightness. Conversely, using standard white (6,500K) neon in this application creates a dirty-brown shade, not red. In short, to avoid a mismatched appearance, you must choose a neon hue that provides the same colors of the light spectrum that exist in the acrylic face.

Because the acrylic face reduces surface brightness to less than 10% of it’s original value, the blurring effect I’ve discussed is negligible for enclosed neon signs. Daylight visibility of acrylic-face signs is rarely an issue because faces typically reflect daylight quite well. But to achieve good visibility at dusk (when most people view signs), the neon must be bright enough to overcome the 90% loss incurred when the light passes through the plastic. In this respect, more is usually better to create dynamic signs that meet your clients’ expectations.

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