Flip-dot displays are electromechanical devices. Electrical switching is used to rotate individual dots on the display so that either the black side is visible, for a dot that is "off," or the colored side (which is usually bright yellow) is visible, for a dot that is "on." The dots can be either round or square. Once an image is set, the flip-dot display presents a fixed message with characters made up of a dot matrix. Flip-dot displays are predominantly used for destination signs on buses.

The following characteristics of flip-dot displays affect their performance:

Dot Pitch and Dot Size - The dot pitch is the distance between dot centers on the flip-dot display. As with LED displays, dot pitch is related to the size of the dot. Dot pitch is equal to the dot size plus the dot spacing. Greater spacing between dots produces a reduction in readability. Reduced spacing between dots produces an improvement in readability.

Character Formation - To form a character, a minimum dot matrix of 7 x 9 is preferred. Characters must be double stroke (made up of two adjacent rows of dots).

Advantages and Disadvantages of Flip-Dot Displays

Advantages of flip-dot displays, compared to other transit system usable displays, are as follows:

• They are lower in cost than LCDs and LED displays while still a "flat" design, which is suitable for use in limited space situations.

Disadvantages of flip-dot displays, compared to other transit system usable displays, are as follows:

• They are more subject to partial failure than purely electronic displays because of the mechanical element of physically rotating the dots.

• They are limited to a dot matrix character, thus resolution and readability are not as good as those displays that form characters appearing more like fixed print.

In summary, flip-dot displays are most suitable for onvehicle displays where space limitations and cost limitations exist.

A light emitting diode (LED) is a semiconductor device. A small current is passed through the semiconductor material. The current causes electrons in the material to be temporarily excited (raised in energy) such that they move to a higher level energy band than their normal position. When the electrons return to their normal energy band, photons (specific quantities of light energy) are emitted. The type of semiconductor material in the LED determines the color of the light emitted.

The liquid crystal display uses the property of certain crystals to change their orientation and their effect on light when an electric current is applied. Liquid crystals can be used to make dot matrices or mosaic tiles. When current is applied to the dots or tiles, alphanumeric characters are produced.

Advancements in LED technology include the development of brighter devices that emit green, orange, and yellow. Currently, only red LEDs are bright enough for outdoor use. With two of the three primary colors (red and green) currently available, the development of a blue LED is a high priority. That addition would bring increased color capability for LED displays.

Modern LED displays do not use separate LEDs placed into holes to make the display matrix. Instead the display is made up of component blocks. Each block is a square matrix with flat top cavities for each individual dot of semiconductor material. The result is that the luminous elements are right on the display surface, providing the widest possible viewing angle. The viewing angle referred to here, which is the angle between a line that is perpendicular to the display surface and a line drawn from the display to the viewer is relative to the display. Characteristics of LED displays that affect their performance are:

LED Size—The diameter (or width if the display is a matrix of square LED elements) of one LED is referred to as the "dot size" of the display. The prevalent dot size for transit system displays is currently 5 mm (0.197 in.).

Dot Pitch—The dot pitch, or distance between dot centers, which is currently prevalent in transit system displays is 6 mm (.236 in.). Greater spacing between dots produces a reduction in readability. This is due to the loss of a cumulative effect whereby adjacent LEDs act together to form an image, rather than as individual dots. At a dot pitch of 6 mm. (0.236 in.), this effect enhances display readability down to 1.8 m (6 ft.). Dot pitch is related to dot size because there is a minimum spacing required between LED adjacent elements. To make a display more readable at very close viewing distances the dot size could be reduced, which would allow a reduction in dot pitch, but the cost of the display would be greater. The current prevalent use of a 6-mm dot pitch is due to the fact that it produces good readability at an acceptable minimum viewing distance of 6 ft (1.8 m) within reasonable cost.

Character Formation—To form a character, a minimum dot matrix of 7 x 9 is preferred. Characters must be double stroke (made up of two adjacent rows of dots).

Display Luminance—The display must be capable of enough brightness to be visible in the intended environment. If lighting conditions are variable, this would make the display too bright for the lower illumination levels. Therefore, dimming controls or sensors should be used for displays with varied ambient conditions.

Current indoor, semi-outdoor, and ultra bright versions of LED blocks for different illumination levels are available.

Advantages and Disadvantages of LED Displays

Advantages of LED displays, compared to other transit system usable displays include

• LED display panels display text in ADA-compliant character heights,

• Lower cost than LCDs,

• Solid state design resists vibration (making LEDs suitable for on-vehicle use),

• Flat configuration suitable for use in limited space situations.

• Animation capability (thus more suitable for advertising).

Disadvantages of LED displays, compared to other transit system usable displays include the following:

• They are more subject to glare than some types of LCD displays (thus they are not as suitable as an outdoor display).

• Their readability is distorted when viewed at an angle.

In summary, LED displays are most suitable for on-vehicle or vehicle stop displays where space limitations, vibration, and the desire for advertising revenue exist.