Contrast Ratios in Outdoor LED Displays
Real-world viewing situations for LED displays in an outdoor environment are highly variable in terms of light-source intensity, light-source angle, and angle of measurement. Thus, accurate specifications can be difficult to determine.
by Ryan Hansen
THE dominant display technology used in outdoor digital signage is direct-view light- emitting-diode (LED) technology. With this technology comes a variety of interesting challenges in measuring and evaluating contrast ratio. Along with a few other key display parameters, contrast ratio is a critical component in device selection among various display technologies and manufacturers for a given application.
Most display insiders would agree that few end users truly understand the impact of contrast ratio on perceived luminosity, color depth, and picture fidelity. This realization, however, has done little to slow the contrast ratio wars raging among consumer liquid-crystal-display (LCD) TV manufacturers. Different measurement approaches combined with a variety of testing variables make it possible to publish and promote almost any contrast-ratio specification. The sky is truly the limit, especially as dynamic-contrast-ratio specifications become more common and accepted.
Fig. 1: Nearly 6000 LED modules make up this HD display at a major-league baseball field. Photo courtesy Daktronics.
Consumer-display manufacturers are not alone in this regard. The LED-display industry is experiencing contrast-ratio pressure as well, driven largely by the marketing push in the consumer-display market. This is especially challenging for outdoor LED displays, as end users are quick to search for a contrast-ratio specification without fully understanding the variables involved in both the testing environment and the real-world viewing of an outdoor display.
Components of Large-Screen LED Displays
In exploring this topic, it is helpful to understand that small LED modules are the basic building block of any large-screen LED display. By stacking the modules within predesigned frames, and then by stacking together those populated frames, it is possible to interconnect power and signal to produce a display of any size or resolution. For instance, the mega-high-definition (HD) display for the Kansas City Royals baseball team (Fig. 1) measures approximately 85 ft. wide by 105 ft. tall and is comprised of 36 sections containing a total of 5940 LED modules, resulting in a final resolution of 1584 x 1800 pixels.
The front-face mechanical characteristics of an LED module are the key influencers of the critical off-state measurement of the display. While designs and subtleties vary across LED-display manufactures, most major companies invest significant sums working to improve their "video-black" states across a number of areas (Fig. 2).
The louver assembly is the final overlay component of most LED modules, placed and secured after construction of the base module assembly. This assembly is the central focus regarding measured off-state luminance performance. It gets its name from the horizontal shaders that provide sunlight protection for the LED face. Generally speaking, the less sunlight reflecting directly off the display face, the higher the overall contrast ratio. In most outdoor installations, the sun is usually overhead for a large portion of the day, and manufacturers must carefully evaluate the louver design to ensure it prevents washout on the display face. Louver design is a balancing act between contrast ratio and display viewing angles.
In addition to the LED module shader design, the texturing approach across the louver assembly will affect how much ambient light is reflected back to the eye of the viewer. Generally speaking, and dependent on approach, a rougher texturing will disperse ambient light more effectively than a semi-smooth surface, creating the visual perception of a blacker off-state, thereby providing more consistent contrast across all angles of the display. In contrast, smooth finishes appear darker from the angle of incidence, theoretically allowing manufacturers to claim a higher contrast ratio. However, the same finish will produce a severe glare at the angle of reflection that can compromise side-angle viewing.
Complementing the shader design and the texturing method is the coating approach. Many, but not all, LED-display manufacturers treat the surface of their LED-module louver assemblies with an anti-reflective UV-resistant dark paint that is, again, intended to minimize light reflection from the face of the LED module. Environmental conditions also contribute to the overall contrast of a display. A seasoned display, without occasional cleaning, collects dust and dirt. Over time, this begins affecting overall contrast, making the black state appear brown in color. Methods to repel dust and dirt from displays have been introduced, but are still unproven. Other common challenges for display manufacturers and owners are the effect UV rays have on the front of the display. UV-resistant paint and plastics for the display front help to minimize this effect.
Fig. 2: This LED-display module features the louver assemblies often used to improve contrast ratio in outdoor signage. Image courtesy Daktronics.
Last, the individual LEDs themselves – protruding from the face of the display by 1–3 mm – also reduce contrast performance. The amount of ambient light that is reflected from each LED depends on the size of the LED, as well as the tinting of the LED's epoxy shell. Each LED contains a small reflector cup that is intended to focus light from the package; how-ever, it also reflects any ambient light that may enter the LED. As a result, each and every LED has the potential to detract from the overall display contrast ratio. Fortunately, LED man-ufacturers have increased both power efficiency and luminous intensity to allow for the optimal one-red, one-green, and one-blue pixel design. Past generations of LED-display products required 12, 8, 6, and 4 LEDs per pixel to produce adequate intensity – regardless of considerations of louver design and face texturing (Fig. 3). Advances in LED technology have enabled improvements in contrast ratio.
A new marketing approach has recently appeared within the LED video industry that uses the term "black LED package" to promote improved contrast on an outdoor, lamp-style LED video display. Most red, green, and blue outdoor lamp-style LEDs have red, green, and blue tinting added to the lamp epoxy to aid in identification and improve display contrast. The "black package" LEDs simply have a little more tinting, making them slightly darker but they are not black. Because of the heavier tinting, the black LED packages do provide a modest improvement in overall display contrast, but do so at the expense of power consumption and display brightness. Figure 4 depicts the difference between standard LEDs and black LEDs.
As a side note, all contrast-enhancing methods must coordinate in a uniform manner across not only a batch of LED modules, but across all modules within a reasonable time frame. Some customer segments will routinely require matching modules 5–10 years after the initial sale. The functional lifetime of an LED video display is typically 75,000–100,000 operational hours. On the high end, that's more than 11 years of company support and contrast continuity that must be planned for from the onset of each project.
Fig. 3: Pixel layouts evolved over about a 10-year span to include fewer and fewer LEDs. Image courtesy Daktronics.
Fig. 4: There is a subtle difference between "black LEDs" (bottom row) and conventional LEDs (top).
Understanding the basic design of an outdoor LED-display module as it affects contrast ratio, it is equally useful to gain insight into the variables that affect contrast-ratio testing of such products. As noted, the contrast ratio of an outdoor large-screen LED video display is largely dependent on the module construction methods previously described: louver design, texturing, coating, and LED density.
For a meaningful and repeatable contrast measurement, a true dark room protected from unintended light interference is required. The three-primary components interacting with one another for contrast testing are the LED module, the measurement instrument, and the light source. Most LED-display manufacturers perform a basic on–off testing procedure with "off" being video black. However, there is little industry-wide consistency in measurement techniques. While prescribed standards exist to measure contrast ratio for some display technologies, such as the VESA FPDM (flat-panel display measurement) process, there is little conformance across the LED-display industry. Unlike the flat-panel-display measurement, the ambient light source (sun) and the measurement tool (viewer) shift over the course of the day and, in the case of a motorist or walking pedestrian, over the course of a minute. This is difficult to replicate in testing and to explain on a product specification sheet.
It should also be noted that under testing conditions it is only practical to test individual LED modules or collections of modules due to the relatively large size of the fully constructed LED video displays.
Within the testing environment, ambient lighting is provided through an artificial light source intended to reproduce a true-to-life outdoor viewing experience. Of course, it is easy to drastically impact the testing results for promotional advantage by adjusting the lighting levels (expressed in lux) within the testing environment. Virtually any decision can be justified due to the variable nature of sunlight hitting the display face. Display manufacturers currently publish outdoor specifications based on ambient lighting levels ranging from as low as 40 lux to as high as 50,000 lux.
The angle of the light source to the display module can also be manipulated to increase contrast-ratio testing results. As mentioned previously, LED modules have horizontal louvers – or light shaders – running between rows of LEDs or pixels. Increasing the angle of the light source in relation to the LED module results in more shaded area across the module face – and thus an improved black state.
Lastly, and in combination with the previously listed point, adjusting the angle of the measurement instrument to the module face will also provide display manufacturers with control of the final contrast-ratio specification. Again, this is largely possible due to the interaction between the light source and the horizontal shading louvers.
These three basic variables – light source intensity, light source angle, and measurement instrument angle – work together to produce the published contrast ratio of an LED display. Unless a test is conducted identically from display-to-display, one cannot be absolutely certain that the comparison is correct. As a result, display-manufacturers' specifications on contrast are rarely meaningful (Fig. 5).
Real-world viewing situations somewhat correspond to the testing variables of light-source intensity, light-source angle, and angle of measurement. Sky conditions will have a substantial impact on the amount of sunlight striking the display face. The time of day and time of year will impact the angle of the sun to the display face. And, lastly, the position of the viewer in front of the display face will interact with the prior two variables to produce a contrast ratio accurate for only that specific viewing experience. This environment is substantially different from that of the indoor home theater, the birthplace of the problematic contrast-ratio war, where lighting conditions and viewing positions can be predicted with at least a modest amount of certainty.
Fig. 5: (a) This illustration shows one possible setup scenario for testing contrast on an LED module. (b) This graph shows how the contrast ratio will change relative to a fixed light-source location. This curve will change based on module design. (c) This graph shows how the contrast ratio will change at a fixed light-source and observation location based on the ambient light conditions. This curve will change based on module design.
Manufacturers of LED displays will continue to publish contrast-ratio specifications to compete for customer attention; that is the reality of the marketplace. However, the industry has a responsibility to educate customers on the variables involved. As contrast-ratio specifications climb higher, customer education becomes increasingly difficult. Given the current pace of marketing in the LED-display industry, it is likely we will see outdoor contrast specifications at 100,000:1 and beyond in the near future. However, it is highly unlikely that these numbers will be substantiated in any meaningful way. •