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displays. Electrical impulses were applied to a grid of disks with reflective material on one side of the disk, and a contrasting finish on the other side. The electrical impulses would position each disk within the grid to either reveal or conceal the reflective portion of the device as required, to produce an image or spell out a message. These technologies were energy efficient, but due to the mechanical nature of the product, failures were an issue. Shortly after the introduction of the reflective products, new incandescent lamps emerged. The new "wedge base' Xenon gas-filled lamps featured many positive qualities. Compared to the larger incandescent lamps that had been used for several years, the wedge base lamps were very bright, required less power to operate and had much longer lifetimes. These smaller lamps allowed electronic display manufacturers to build displays that featured tighter resolutions, allowing users to create more ornate graphic images. Next in the evolution of the changeable message sign was the LED. LED (light emitting diode) technology had been used for changeable message displays since the mid 1970s. Originally, LEDs were available in three colors: red, green and amber, but were typically used for indoor systems because the light intensity was insufficient for outdoor applications and the durability of the diodes suffered in the changing temperatures and weather conditions. As technology improved, manufacturers were able to produce displays that had the intensity and long life required for outdoor use, but were limited in the viewing angle from which they could be effectively seen. Recently, breakthroughs in this field have made available high intensity LEDs in red, green, blue and amber. These LEDs have made it possible to produce displays bright enough for outdoor use with viewing angles that are equal to, or better than, other technologies currently available. They are energy-efficient, can be programmed and operated remotely, and require little maintenance. In addition, the computer software has evolved such that a broad range of visual effects can be used to display messages and images. The spacing of the LEDs can be manipulated to achieve near-television resolution. Earlier "flip disk" and incandescent technologies have become nearly obsolete as a result. Types of Changeable lvlessage Signs Changeable message signs can be placed into two basic categories: manually- changed and electronically-changed. The most common form of manually- changed sign involves a background surface with horizontal channels. Letters and numerals are printed on individual plastic cards that are manually fitted into the channels on the sign face. Abroad range of letter styles and colors are available. The manually-changed sign is relatively inexpensive and is somewhat versatile. Some discoloration has been experienced in the 'oacl<ground surface materials 22