Light Emitting Devices

Abstract

Light emitting arrays in which the light source is positioned between two opposed reflector surfaces. The light from the source is reflected back past the source by a first reflector and then re-reflected by a second reflector back past the light source to the first reflector. A very thin assembly is obtained. The arrangement is particularly suitable for light emitting diodes and can be used for character displays.

Description

This invention relates to light emitting devices, and in particular though not exclusively to light emitting devices as integers of a display, for example for displaying numerals, letters and symbols, and also to such displays.

Various forms of devices are used for the display of numerals, letters and symbols in the so-called "match-stick" style or code. It has been proposed to use a small filament lamp for such a device. The light from a lamp is directed to a parabolic reflector, which reflects the light back past the lamp to issue from a viewing face. Because of the shape of the reflector and other requirements such as a device is relatively large, being of a substantial thickness in the direction of light emission from the size of the display. There is also the associated disadvantage of the relatively high power consumption of a filament lamp. Light emitting diodes have also been used, either with a plurality of the diodes extending for the form of the segment, for example side by side, or with a diode mounted at the focus point of a parabolic reflector. The use of a plurality of diodes not only negates the low power consumption of individual diodes but may increase the power consumption above that of a filament lamp. Using a diode with a parabolic reflector means that the device has substantial thickness.

The present invention provides a device which is thin, has a very low power consumption and permits a reduction in the number of light sources. In the broadest aspect the invention provides a light emitting device comprising a semiconductor light emitting element or diode having a reflector on a first side and a reflective surface on a second side, light from the element being reflected by the reflector to the reflective surface and then reflected back past the element and reflector by the reflective surface. Particularly there is provided a light emitting device having a substrate onto which is positioned a reflective layer. The reflective layer may be diffusing, and is substantially flat. The light emitting element is mounted on the reflective layer and a reflector positioned over the element such that light from the element first passes to the reflector, the reflector reflecting the light back to the reflecting layer. A light transmitting encapsulating material encapsulates the element and the reflective surface and the light is further reflected by the reflective layer through the encapsulating material to issue from a viewing surface thereof.

For a segment of a display array, the reflector is arranged symmetrical to the element and has two portions arranged to reflect the light from the element back past the element, on either side of the element. The reflective layer is flat and again reflects the light back past the element to form a lighted segment on either side of the element. Conveniently the device is encapsulated with a black or non-light transmitting outer portion or frame and the light transmitting material within the frame. The reflector can be mounted on the encapsulating material.

A display array is composed of a plurality of devices arranged in a predetermined pattern to meet the display requirements. The shape of individual devices can be varied to provide for effective assembly of the array.

The invention will be readily understood by the following description of certain embodiments, by way of example, in conjunction with the accompanying drawings, in which:

FIG. 1 is a plan view of a device;

FIG. 2 is a cross-section on the line II--II of FIG. 1;

FIG. 3 is a plan view of a display using devices in accordance with the invention;

FIG. 4 illustrates the conductor pattern and circuit for the arrangement of FIG. 3; and

FIG. 5 illustrates one form of multi-digit display using devices in accordance with the present invention.

As seen in FIGS. 1 and 2 a light emitting device 10 comprises a substrate 11 having a conducting layer or pattern 12 on which is mounted a semiconductor light emitting diode 13, hereinafter referred to as an LED, for example a gallium phosphide chip providing a p-n junction in the known manner. In addition to acting as a conductor the layer 12 serves as a reflective layer as will be described. The LED 13 is encapsulated in a light transmitting plastic material 14. The encapsulating material is formed to provide sloping sides 15, the sides inclined upwardly and inwardly relative to the substrate 11 and the LED 13. Immediately over the LED 13 the encapsulating material is shaped as a trough 16 having sides 17 which incline upwards and outwards relative to the LED 13, being on either side of the axis of the LED. A flat top surface 18 acts as the surface through which light is emitted.

A layer of reflecting material 19 is formed on each of the sides 15 and 17 so that a reflector is formed entirely around the periphery of the encapsulating material. On energization of the LED 13 light is emitted from the top 20 of the LED and from the sides 21. The light emitted from the top is reflected downwardly and outwardly to the layer 12. Layer 12, which conveniently is slightly diffusing, reflects the light upwards and light will issue through the surface 18 on either side of the trough 16. Some light will be reflected by the layer 12 upwardly and outwardly for further reflection by the reflecting layer 19. Also light emitted from the sides 21 will be reflected by the layer 19, in this instance downwards toward the reflective layer 12, and then upwards. The angles of the various sides 15 and 17 are chosen so that the distribution of the light emission from the surface 18 is approximately even to provide an appearance of uniform illumination at differing viewing angles. A typical value for angle .theta., the inclination of the sides 15, is approximately 74.degree., and for angle .phi., the inclination of the sides 17 of the trough 16, is approximately 32.degree.. Any light emitted from the LED 13 is reflected repeatedly until it issues through the surface 18. The encapsulating material 14 is usually of clear plastic material, which can be coloured and can also contain a small addition of a light diffusing filler. Paths of rays are indicated by lines 22.

The reflecting layer 12, as previously stated, is conveniently part of the conducting layer or pattern for the LED 13 and, for example, is of gold.

FIGS. 1 and 2 illustrate a single light emitting device which can be used whenever a lighted device is required. Although shown as having the LED 13 mounted centrally with light issuing through two areas, one on each side of the central trough 16, the LED can be mounted at one end of a device to present one lighted area. Further, the particular shape of the device and particularly the shape or shapes of the area or areas through which light issues can be varied to suit requirements.

FIG. 3 illustrates an array or display using light emitting devices in accordance with the present invention, the array being one which can be arranged to produce characters, in the present example figures. As shown, a plurality of devices 30 are arranged in the form of a rectangle with a central transverse bar. The positions of the LEDs are indicated by the dotted circles 31 while the surfaces through which light issues are indicated at 32. A decimal point is produced by a device 33 which is single ended, that is, the LED indicated by the dotted circle 34 is at one end of the device, the light issuing surface indicated at 35. By suitable energization of particular LEDs it will be seen that figures and letters can be produced.

An array, as for example illustrated in FIG. 3, is made as a complete unit. For example the individual devices 30 are themselves encapsulated in a black plastic material. Either the black material can be moulded having cavities formed for the eventual filling with the light transmitting plastic encapsulating the LEDs or, alternatively, the LEDs are first encapsulated in the light transmitting plastic to form the individual devices and the devices are positioned in a mould and the black plastic material moulded around the devices. If the black material is first moulded, the reflecting layers 19 are formed by deposition by any suitable process on the related surfaces before the light transmitting plastic material is moulded into the black moulding. When the individual devices are first moulded the reflecting layers 19 are then deposited on the related surfaces prior to encapulsation into the black material. Other colours than black can be used, the main intention being to provide contrast.

The array for a character is generally mounted on a common substrate, which can be of ceramic for example. The substrate will then usually form the substrate 11 as in FIG. 1. A plan view of such a substrate showing the conductor circuitry is illustrated in FIG. 4. In this figure the devices are not shown, for clarity, but the positions of the surfaces through which light issues are indicated at 32, as in FIG. 3. Similarly the positions of the LEDs are indicated at 31 and 34 as in FIG. 3.

As seen in FIG. 4 the ceramic substrate is at 40. A layer of conducting material, for example gold, is deposited at 41. The LEDs 31 and 34 are mounted directly onto the gold layer. The conducting material 41 is deposited in such a pattern that conductors 42 are formed, separated from the main layer 41 by spaces 43. Connections are made to the LEDs from the conductor pattern 42 by leads 44. An MOS decoder/driver 45 is also mounted on the gold layer 41 and is connected to the conductor pattern 42 by leads 46.

A multiple array is shown in FIG. 5, comprising five character arrays 50 and a positive, negative and overflow array 51. A fixed decimal point is provided at 52 but it will be appreciated that a device for a decimal point can be provided at each character array, for a movable decimal point.

In the arrays described and illustrated, seven devices are used for each array. For a larger character size a larger number of devices can be used, for example nine. Also differing arrangements of the devices can be used to provide arrays capable of differing or more complex characters.

Although devices in accordance with the invention have been described in use for the production of character arrays, other uses exist for such devices, either used singly or in multiplicity. Devices can be mounted on a printed circuit board instead of a ceramic substrate, and can be a component of a printed circuit device.

The devices in accordance with the invention are very thin. A character array as in FIGS. 3 and 4 can be produced with overall dimensions of approximately 0.25 ins. .times. 0.20 ins. and with a thickness from the substrate of approximately 0.020 ins. Individual devices have overall dimensions of approximately 0.120 ins. .times. 0.050 ins. by 0.020 ins. thick. These devices in accordance with the invention are thinner than conventional devices. The use of LEDs reduces the power requirements as compared with conventional devices, using other forms of light source. What 1s claimed is:

Claims

1. A light emitting device for producing widely diffused light comprising:

a substantially planar substrate having a reflecting surface thereon;

a semiconductor light emitting element mounted on the reflecting surface of said substrate, said light emitting element emitting light laterally in a direction parallel to said substrate and forwardly in a direction normal to said substrate;

a body of encapsulating material enclosing said light emitting element, said body of encapsulating material having a front surface and at least one side surface, said side surface being tapered inwardly from said substrate to said front surface;

a layer of reflecting material on the side surface of said body of encapsulating material for reflecting the laterally emitted light from said light emitting device downwardly onto the reflecting surface of said substrate;

a layer of reflecting material on a portion of the front surface of said body of encapsulating material, said layer of reflecting material on said portion of said front surface being substantially in alignment with said light emitting element and being tapered inwardly from said front surface toward the reflecting surface of said substrate to reflect the forwardly directed light downwardly onto the reflecting surface of said substrate;

the remaining portion of the front surface of said body of encapsulating material being light emitting to allow emission of the light reflected by the reflecting surface of said substrate.

2. A light emitting device as claimed in claim 1, said inwardly tapered layer of reflecting material on said portion of said front surface comprising two intersecting reflecting surfaces.

3. A light emitting device as claimed in claim 1 including an electrical conductor pattern on said substrate, the reflecting surface of said substrate comprising part of said electrical conductor pattern.