Patent Application
20060039129
This invention relates to indirect lighting for displays and, in particular, to a display back lit by reversely mounted light emitting diodes (LEDs).
“Point” is not used in the mathematical sense of vanishingly small. A point source of light is a bright source in a small, finite space, “small” being relative to the size of the surrounding structure. Some people may quibble that a point source of light radiates uniformly in all directions. That quibble is not true in practice and does not apply here. As such, incandescent lamps, LEDs, some gas discharge lamps, and others are point sources of light even though, as in the case of LEDs, they radiate in a preferred direction.
Strictly speaking, all non-luminous objects, except black holes, reflect light, otherwise nothing would be visible. A reflecting surface is either specular (a mirror-like or polished surface), uniformly diffuse, or somewhere in-between. At a microscopic level, even a highly polished, front surface mirror is not perfectly specular, nor is any diffuse reflector perfectly lambertian. Mathematical minutiae are of no concern here. Rather the concern is with a macroscopic, practical, diffuse reflector that is reasonably, if not perfectly, lambertian. Many surfaces fulfill this criterion, such as a sheet of white paper or a sheet of white plastic. Obviously, colored paper or plastic filters the light in addition to reflecting the light.
Although the invention is described in the context of an instrument cluster for a vehicle, the invention relates to backlighting any form of display, from something as simple as a switch to something as complicated as the backdrop for a pinball machine. In other words, “display” is meant broadly and “Instrument cluster” is not intended to limit the kinds of display in which the invention can be used.
A “luminous” object emits light. Light incident upon a subject “illuminates” the subject. “Luminance” refers to the amount of light emitted from a source. “Illuminance” refers to the amount of light incident upon a subject.
A “graphic” can be text, a symbol, an arbitrary shape, or some combination thereof. A graphic can be translucent, shaded, colored, a silhouette or outline, or some combination thereof.
As used herein, a “flex circuit” is any type of substrate including conductive traces for including LEDs and other devices in an electrical circuit. As such, a flex circuit includes printed circuit boards. The flexibility of the substrate has no bearing on the invention.
As used herein, an electroluminescent (EL) “panel” is a single sheet including one or more luminous areas, wherein each luminous area is an EL “lamp.” An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is typically transparent. The dielectric layer can include a phosphor powder or there can be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using relatively little current.
In the particular display known as an instrument cluster, one either illuminates a dial, e.g. U.S. Pat. No. 2,172,765 (Kollsman) or back lights a mask defining translucent areas corresponding to the dials for gauges or to graphics, such as turn signal indicators; e.g. U.S. Pat. No. 5,578,985 (Cremers et al.).
It is known in the unrelated art of astronomy to make a flat field projector by sandblasting an aluminum plate and illuminating the plate with four LEDs; see Simon Tulloch, Design and Use of a Novel Flat Field Illumination Light Source, Technical Note 108, Instrument Science Group, Royal Greenwich Observatory, 1996.
A diffuse light source, such as an EL panel, is often used for backlighting graphics but is not as luminous as an LED. Some indicators are preferably bright and vivid in color. An LED is generally preferred to an incandescent lamp as a source of light because the LED produces light more efficiently while producing much less heat.
For back lighting, one wants as uniform a light source as possible, and therein lies a problem. LEDs have numerous advantages over incandescent lamps but, like incandescent lamps, are point sources of light. Various forms of light guides or light channels are used to diffuse the light but the fact remains that a point source of light is often visible through the object being backlit. A result is non-uniform lighting. Light from a source that is viewed directly is “glare” and is undesirable.
The need for light guides and the like requires complex structures that are expensive to manufacture, at least for initial tooling.
Another problem with point sources of light, and schemes for diffusing and redirecting the light, is “leakage”; i.e., light from one area being visible in or affecting light in another area. The problem is especially critical for indicators, where only the desired indicator should be back lit while other indicators remain unlit. Related to this is a large, relative to the size of the graphic, minimum separation for indicators to prevent leakage. The minimum separation limits the design of instrument clusters and other displays.
The electronics for most instrument clusters are mounted on flex circuits, where circuit cost is proportional to area, among other factors. Reducing area and simplifying design changes can significantly reduce costs. Design changes can be simplified, for example, if one could change graphics only while using the same flex circuit for the new design.
In view of the foregoing, it is therefore an object of the invention to provide a display, or portion thereof, that is substantially uniformly backlit from a point source of light.
Another object of the invention is to provide an indicator that is substantially uniformly and brightly backlit and vivid in color.
A further object of the invention is to provide a display having areas that are substantially uniformly backlit by LEDs.
Another object of the invention is to provide a display combining EL lamps and reversely mounted LEDs for substantially uniform backlighting.
A further object of the invention is to provide a backlight that is less prone to light leakage and simplifies the construction of complex displays.
The foregoing objects are achieved in this invention wherein a display is back lit by reflection of light from a surface illuminated by at least one point source of light, such as an LED. The reflecting surface can be far from the source, near to the source, or even a coating on a package containing the light source. The uniformity of light from the reflecting surface can be changed by shaping the reflecting surface. The point source projects light rearwardly, i.e. away from a viewer. That is, the axis along which light emission is greatest extends from the source away from a viewer. It has been found that the LED itself obscures the point source of light and that the reflected light seen by a viewer does not create a perceptible shadow on the viewer's side of the display.
A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which:
In
Display 11 further includes LED 21 backlighting turn indicator 22 and LED 23 backlighting turn indicator 24. The LEDs are not shown in proportion to the indicators but are somewhat enlarged to show the leads. As indicated by the position of the leads, LEDs 21 and 23 are reversely mounted; i.e. the direction of greatest light emission is away from the viewer, into the plane of the drawing of
As illustrated in
Semiconductor die 34 in LED 22 emits light predominantly upwardly, as oriented in
The unexpected result of all this is that turn indicator 24, or any other indicator, is substantially uniformly and brightly backlit. In other words, LED 22 provides high luminance over a wide area (several diameters) without the appearance of a point source. The indicators shown in area 16 (
Shell 41 is made from any suitably reflective material. A molded plastic shell is extremely inexpensive and effective. As shown in
Optionally, a shell can be molded as part of back cover 47 or attached to the back cover by strut 48.
As also shown in
In
In one embodiment of the invention, an LED was coated with “white out” or correcting fluid for painting over printed characters or lines on a sheet of paper. The LED functioned as a diffuse backlight. Many other materials can be used instead, such as boron nitride, which is commercially available in the form of a white powder.
The second change is that coating 87 does not extend down to the plane of the flex circuit. It is preferred that the entire package be coated but this is not required. It is also preferred that the reflector cover substantially 2π steradians of the space around die 85, centered on the axis of greatest emission. Below the die, or below the leads, coating 87 is reflecting reflected light. In some configurations, covering more than 2π steradians is not necessary.
There is a third difference between
Light output is substantially uniform almost to a radius of 5 mm, where brightness is about half. Other simulations were run with various shapes for the reflecting surface. An axial depression in the hemisphere produced a doughnut shaped illumination pattern (circular brighter area surrounding and surrounded by dimmer areas.) To enhance the graphic being backlit, the uniformity of the light could be adjusted by shaping the reflector.
The invention thus provides a backlight that is substantially uniformly despite using point sources of light, such as LEDs. The reflecting surface can be far, near, or a coating on the LED. The LED faces rearwardly, i.e. away from a viewer, thereby eliminating glare. Despite the presence of the LED in the field of view, the LED can be positioned to avoid perceptible shadow. Alternatively, an LED can be positioned laterally away from a translucent area and be outside the field of view. An advantage of the shell (
Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, the diffusion coating on an LED can include cascading color materials, such as dyes or phosphors, for enhancing the visual appeal of the display. Large areas can be backlit by plural LEDs or by one or more electroluminescent lamps. The reflecting surface need not be white or of uniform reflectivity.
