Najjar, L. J. (1989). Electroluminescent keyboard lamp. IBM Technical Disclosure Bulletin, 32(5B), 474-447.
This article describes an electroluminescent keyboard lamp to illuinate a full travel keyboard. A full travel keyboard is one in which the keys travel generally at least 2.5 mm. Full travel keyboards are preferred by typists because they provide obvious tactile feedback of key switch activation.
Some jobs require that a full travel keyboard be used in a dimly lit ambient environment. For example, to maintain their ability to see aircraft and runways at night, air traffic controllers will use a full travel keyboard in a dimly illuminated tower cab. Full travel keyboards are also used in the dim environments of specialized work stations on submarines and military aircraft.
Two techniques exist currently to illuminate a full travel keyboard in a dimly lit ambient environment: back lighting and front lighting. Back lighting requires that an incandescent light bulb or light-emitting diode (LED) be placed inside each keyboard key. When power is applied, the bulb or LED illuminates from the rear the legend on that key cap. Front lighting requires that a light source, typically a gooseneck desk lamp with an adjustable brightness incandescent light bulb, be placed above the keyboard. When power is applied, light from the lamp illuminates from the front the legends on the key caps.
Each keyboard-lighting technique has advantages and disadvantages. Back lighting has the advantage in that is is integrated with the keyboard to eliminate annoying cables and reach and vision obstructions onto and behind the keyboard. In back lighting, each key is illuminated to avoid finger shadonws on the key caps and provide good key cap legend readability. Also, the brightness of the keyboard can be easily adjusted to avoid display reflections and suit different lighting conditions and user preferences.
However, back lighting has disadvantages in that the keyboard itself is usually quite heavy and thick due to deeper keys necessary to accommodate incandescent light bulbs or LEDs and other support electronics. Back lit keyboards are also quite expensive due to low volume and unusual, customized electronics. They suffer from low reliability due to electronic complexity and relatively short life of incandescent light bulbs, and require relatively large amounts of power.
Front lighting has an advantage in that it is much lower in cost to implement because a commercially available lamp can be used. The lamp's incandescent light bulb can be adjusted if it is interfaced with a rheostat. A lamp can be adjusted if a flexible mount is provided.
Front lighting has disadvantages in that the lamp osbstructs the user's view and access to keys on the keyboard because, to get adequate light on the keyboard key caps, the lamp has to be positioned above the keyboard in the user's line of sight. Uneven lighting of keys occurs in front lighting the keyboard because the incandescent light bulb serves as a single, small, nonuniform source of light. The lamp is not integrated with the keyboard because the lamp is a separate physical device. The lamp also gets quite hot because activated incandescent light bulbs generate a great deal of heat.
Neither back lighting nor front lighting is an optimal keyboard lighting technique. As a result, the author invented an electroluminescent (EL) keyboard lamp that provides a keyboard-lighting technique that is superior to the back-lit and front-lit techniques described previously.
The EL keyboard lamp is shown in Figure 1. The dimensions may be revised to suit specific keyboard applications. To improve its strength and reliability, the EL keyboard lamp itself is glued to a plastic support. To provide adjustable tilt for the EL lamp, the plastic support has two extensions at the top. As shown in Figure 1, these extensions protrude through the pastic housing to allow the EL lamp tilt control knobs to be attached. The resistance of the EL lamp tilt control knobs allows the EL lamp to be tilted easily, yet remain in position after being adjusted. The EL keyboard lamp is attached physically to the keyboard via prongs. The prongs are slightly indented on the sides to allow them to click firmly into protrusions in the keyboard sockets. A side view of the EL keyboard lamp attached to a keyboard is shown in Figure 2.
Figure 1. Electroluminescent keyboard lamp.
Figure 2. Side view of electroluminescent keyboard lamp.
The EL keyboard lamp is integrated electrically with the keyboard to eliminate the requirement for a separate power cable. Electrical power is provided by two extra wires in the keyboard cable. The wires attach to the prong sockets on the left side of the EL keyboard lamp. The prongs on the right side of the EL keboard lamp are non-electrical. The EL keyboard lamp includes a potentiometer to adjust the brightness of the EL lamp. In inverter provides proper voltage to the EL lamp. The inverter uses the EL capacitance as part of the inverter oscillator circuitry. To compensate for the changes in EL capacitance and lamp brightness as the lamp ages, the inverter changes the oscillator frequency. As a result, the inverter extends the life of the EL keyboard lamp.
The solid EL portion of the lamp emits a soft white glow of light when activated. To control the amount of light on the keys, the lamp is tilt and brightness adjustable. The controls are easy to see and reach by the keyboard user. To accommodate right- and left-handed users, an EL lamp tilt control knob is located on each side of the EL keyboard lamp. To minimize wiring complexity, the EL lamp brightness potentiometer is located on the left side of the device, near the electrical prongs. The controls are located on the side of the EL keyboard lamp to reduce the likelihood of damage from objects being dropped onto the lamp or from the lamp being bumped into other objects such as displays, and to lower the chances of a user's shirt sleeves getting caught on the controls.
The use of electroluminescent technology allows the development of a keyboard lamp design with many desirable features. The lighting is physically integrated with the keyboard to allow the use of a more comfortable, low-profile keyboard and improves convenience, especially when moving the keyboard. Since the lamp is electrically integrated with the keyboard, it eliminates the requirementfor a separate power cable. The adjustable brightness control of the EL keyboard lamp provides excellent key cap readability, reduces potential glare, and allows the user to tailor brightness to different ambient light environments and personal preferences. The adjustable tilt control reduces potential glare, and allows the user to tailor the angle to suit personal preferences. Electroluminescent light is very evenly distributed; it is a soft white lgith which eliminates finger shadows on the keyboard and reduces potential glare. The EL lamp operates at low temperature, thus enhancing safety and comfort. It also uses a lower current than incandescent lighting to provide low operating cost.