Compact light mixing and diffusing apparatus

Details Compact light mixing and diffusing apparatus Compact light mixing and diffusing apparatus

1999-03-29

2001-05-29

Husar, Stephen (Department: 2875)

Illumination

Light fiber, rod, or pipe

Diffuser or diffusing of incident light

C362S026000, C362S276000, C362S555000, C362S560000

Reexamination Certificate

active

06238076

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a compact light mixing and diffusing apparatus typically used for small low power sources of illumination. More particularly it relates to a compact apparatus for providing diffused light of substantially uniform brightness and substantially uniform color at an exit surface of a visual indicator of an appliance.
BACKGROUND OF THE INVENTION
Visual indicators are often used in small electronic appliances, such as razors, modems, computers, audio and video equipment etc, to indicate that the appliance is powered ON or is in some given state. The visual indicator, which the user sees on the appliance, is generally of small size and of simple circular or square geometry. Often the appliances are powered by batteries for ease of use or to minimize cost or power requirements; thus, small low power drawing sources of illumination are used, such as light emitting diodes (LEDs). However, given the small size, illumination characteristics and illumination power, these light sources require enhancement to meet visual and aesthetic requirements for the appliance. Thus, to enhance these visual indicators the appliances use light diffusing apparatuses with the light source.
The visible surface of the light diffusing apparatus is usually located flush with the surface of the appliance enclosure. The light diffusing apparatus can be either attached to the enclosure or to a printed circuit board (PCB) along with the illumination source, typically a light emitting diode (LED). The apparatus is generally made of optically clear polymers (acrylic, polycarbonate, etc) and is shaped in such a way that the output of the LED is distributed uniformly across the visible exit surface which is grounded to enhance visibility.
A light pipe is a commonly used light diffusing apparatus which provides significant manufacturing advantages. Light pipes economically adapt the emission pattern of a standard off-the-shelf LED. LEDs usually have a radiation pattern of circular or elliptical symmetry with a maximum intensity on the optical axis (an axis which is usually normal to the exit window of the LED) which decays to zero at a direction of 90 degrees from the axis of illumination. A light pipe adapts the emission pattern of an LED to the esthetic requirement of a visual indicator by guiding the LEDs output to create a uniform illumination across the entire surface of the visual indicator. Light pipes also increase the area over which the light from the LED is spread allowing the visual indicator to be significantly larger than the LED which provides the illumination. Light pipes offer other advantages in that they provide a link between the LED, preferably soldered to the PCB, and the indicator position on the surface of the appliance's enclosure. The light pipe also facilitates the appliance's assembly and, in the case where it is attached to the enclosure, no mechanical contact to the PCB is required.
In many instances, it is necessary to have an indicator which can vary the color it displays. One standard method involves using an off-the-shelf dual color LED (two LEDs mounted in a single package) and fitted with a glass filled clear epoxy lens. The exit surface illumination and color uniformity is good. However, there are many limitations. The glass filled epoxy is a bulk diffuser i.e. light is diffused in all directions through the solid and is visible from all surfaces, thus lowering illumination efficiency. The LED has to be soldered to the PCB, which limits the packaging flexibility, and the indicator shape which the user of the appliance sees at the surface of the appliance is limited to the standard catalog shapes (usually square, rectangular or round). Moreover, there are no dual color LED packages with large-size diffusing lenses, that can extend through enclosures walls for instance, which are available in Surface Mount Technology (SMT) packages (with no leads).
SMT LED packages are very small, varying in size from a fraction of a mm up to about 6 mm for each side. The distance between the optical axis of two individually packaged SMT LEDs mounted side by side is about 0.5 mm to 1 mm. For two or three LED's mounted in the same package, the distance between their optical axis is around 0.7 mm to 0.8 mm. The most common two LEDs packages are bicolor (some packages have two LEDs of the same color), with orange, red and various tints of green being the most common colors. In the case of a red and green bicolor LED, varying the ratios of red and green creates a color, where the LED beams overlap, with a tint which can be continuously adjusted from green to red with the intermediate tints of yellowish green, yellow, amber, and orange. Three LED packages are usually tricolor with one of each LED of red, green, and blue color (blue LEDs devices are currently significantly more expensive than red and green devices); mixing all colors results in a white output and a partial mix of the three colors can generate almost any visible spectrum color. One obvious market for the tricolor LEDs is imaging displays where the representation of pictures or video content requires a full color capability. The color mixing capabilities of the light pipe then becomes a critical performance factor. However, even with such small distances between the optical axis as the ones described previously, good overlap is not obtained with most commercially available light pipes, which were designed for the more common single color LED packages, and multiple colors can be viewed on the exit surface instead of one solid uniform color.
Given the current state of the art, light pipes used for single color indicators on most small electronic appliances have a small visible exit surface. In most cases, the exit surface
15
is located at the end of a relatively long section
14
of FIG.
1
. The long section
14
is required to homogenize the LED output and the relative size of the exit surface
15
is small compared to the length of the mixing section
14
. Another example with similar characteristics is U.S. Pat. No. 5,581,683 granted to Bertignoll et al on Dec. 3, 1996, which contains a description of a uniform light pipe with an exit surface of 3.3 mm×51.5 mm; illumination is provided by two LEDs
16
located at the end of a U-shaped plate 50 mm away from the exit surface.
The example of the preceding paragraph both use Total Internal Reflection (TIR) as the mechanism to bend the light rays emitted from the source in order to obtain uniform illumination at the light pipe exit surface. TIR, a well know scientific principle, will be present whenever a light ray strikes a surface at an angle less than a critical angle, measured relative to a tangent at the surface encountered by the light ray, when going from a high refractive index medium to a low refractive index medium, in this case from the light pipe polymer to air. If the angle is larger than the critical angle with respect to a tangent at the point of the surface the light ray strikes, the light ray will be refracted and will escape from the light pipe. However, if the light ray strikes at a smaller or shallower angle than the critical angle it will be reflected. Thus, in constructing a light pipe based on TIR it is thus important to eliminate all abrupt angular changes on the light pipe structure, which factor results in the long structures of the prior art.
A large area linear light pipe is proposed by Simms in U.S. Pat. No. 5,590,945. The device is essentially a flat plate illuminated from its end and which has an accurate rear reflecting surface covered with triangular reflecting ridges which deflect the light toward the front surface. It is said that the design can also be used with multicolored light sources such as bicolor or tricolor LEDs. However, this apparatus cannot be scaled down because the triangular ridges become almost impossible to manufacture with precision, in a common plastic injection process, when features dimensions need to be 0.5 mm and smaller. Such small ridges would be required fo

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