Dynamic Message Signs Legibility Parameters
According to NEMA TS-4, Legibility is the ability to discern the content of a display, while Visibility is the ability to recognize that a display exists.
The elements that influence legibility of a sign are:
• contrast ratio of the display,
• luminance level of the display,
• viewing angle of the display,
• color of the display,
• and uniformity.
Legibility distance is proportionally related to character size.
As the main purpose of a Dynamic Message Signs is delivering visual message to the drivers and travelers, the major Dynamic Message Signs requirement is good legibility and visibility throughout the required viewing range. This performance is defined through the main optical parameters – luminance, luminance ratio (contrast ratio) and viewing angle.
Legibility of Dynamic Message Signs is the most critical during daytime operation. When sun glare directly strikes the sign face, it severely reduces the brightness contrast between the sign display and the sign background, resulting in inability to decipher what is displayed on the sign.
The key parameter for assuring satisfying legibility in such cases is Contrast Ratio.
EXPLANATION: Contrast Ratio is the degree of pixel illumination over the degree of ambient illumination (daylight)
A good Contrast Ratio is achieved by a sign front face, able to absorb ambient light, and not to reflect it. This is achieved with a proper front face paint (deep black matt paint with ultra-low reflection ratio) and special pixel design. Pixels should absorb the ambient light, not to reflect it – this is achieved by placing specially crafted lenses in front of LED.
Poor contrast ratio
Good contrast ratio
IMPORTANCE: Contrast ratio should always be considered in conjunction with Luminance. Contrast ratio assures that LED Dynamic Message Signs display with sufficient Luminance (as per EN12966 or NEMA TS-4) features satisfactory legibility.
Without proper contrast ratio, even with sufficient luminance, LED display will be poorly legible.
On the other hand, Dynamic Message Signs with proper contrast ratio will be legible to the travelers.
USA-Europe COMPARISON: EN12966 defines 3 luminance ratio classes. NEMA TS-4 defines one luminance class which is equal to R2 European class.
Conclusion: EN12966 gives an option to require even more stringent contrast ratios than NEMA TS-4.
EXPLANATION: Luminance is the level of light that human eye perceives. Different colors require different luminance to be equally perceived by humans. Because of this, Luminance is defined for each basic color separately.
IMPORTANCE: Luminance should always be considered in conjunction with contrast ratio. Luminance assures that Dynamic Message Signs with sufficient contrast ratio (as per EN12966 or NEMA TS-4) features satisfactory legibility.
USA-Europe COMPARISON: EN12966 defines 3 luminance classes. NEMA TS-4 defines one luminance class which is equal to the strongest European L3 class (class for highway applications).
Conclusion: NEMA TS-4 and EN12966 require the same luminance performance.
Contrast Ratio (R) and Luminance (L) determine legibility of Dynamic Message Signs.
In order to achieve satisfactory legibility, satisfactory contrast ratio and luminance have to be achieved. If any of these parameters does not achieve required class, the DMS will not be sufficiently legible.
To achieve legibility, either the sign luminance has to be increased (sending more power through the sign to light up the LEDs brighter), or the sign's front face reflection has to be decreased.
As demonstrated in the previous paragraph, contrast ratio is achieved either by increasing sign’s luminance or decreasing front face reflection.
Decreasing front face reflection is more advanced as it results in the following:
- Lower energy consumption because the sign:
- Uses less luminance to achieve desired legibility
- Dissipates less heat due to lower LED currents (less luminance requires less current), meaning that it requires less energy for cooling
- Increased sign durability because it operates with lower LED current (less luminance requires less current), which consequently results in less LED wear and tear and prolongs LED life cycle
Beam Width (viewing angle, cone of vision) is the third parameter that determines legibility of the sign.
Beam Width is the visual region in which Dynamic Message Sign is still legible to the driver.
It is a set of three viewing angles from the center (horizontal left, horizontal right, vertical down), at which the luminance is at least 50% of the minimum required on-axis (L3, 0º horizontal, 0º vertical) luminance. No point in the viewing angle shall be less than 50% of the minimum luminance of the center point (0º horizontal, 0º vertical).
Both NEMA TS-4 and EN12966 define the same values and conditions for viewing angles. The only exception is B5 angle which does not exist in NEMA TS-4.
On most of today’s arterial and similar multi-lane roads the satisfactory cone of vision is class B6, or NEMA class.
The following is a simulation of B6 beam width on the road:
Over specifying cone of vision (for example B7/class f instead of B6/class e) can quadruple Dynamic Message Signs energy consumption without proper need.
Both NEMA and EN12966 define color coordinates of all primary colors (white, amber, red, green, blue) to make it compliant with traffic regulations. Today, most of LED manufacturers deliver LED’s which fulfill those requirements.
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