LIGHTING DEVICE

Abstract

An illumination device that can adjust the chromaticity and brightness of the illuminating light in a broad range using a simple construction includes a first LED and a second LED having mutually different color temperatures, and a control circuit arranged to perform lighting and extinguishing control of the first and second LEDs. The control circuit performs periodic lighting and extinguishing control of the first and second LEDs such that during a lighting period, the first and second LEDs are lit and extinguished in a complementary manner, and during an extinguishing period, both the first and second LEDs are extinguished.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a preferred embodiment of the illumination device of the present invention.

FIG. 2 is a longitudinal sectional view schematically illustrating the structures of a first LED and a second LED.

FIGS. 3A-3C are diagrams illustrating the chromaticity adjustment operation of the illuminating light.

FIG. 4 is a diagram illustrating the color temperature variation corresponding to the ratio of the “on” period T2 of the LED 2 with respect to the lighting period Ton (=T1+T2).

FIG. 5 is a chromaticity diagram illustrating the variation in chromaticity corresponding to the ratio of the “on” period T2 of the LED 2 with respect to the lighting period Ton (=T1+T2).

FIGS. 6A-6C are diagrams illustrating one example of the brightness adjustment operation of the illuminating light.

FIG. 7 is a diagram illustrating the variation in the brightness in accordance with the length of the extinguishing period Toff.

FIGS. 8A-8C are diagrams illustrating another example of the brightness adjustment operation of the illuminating light.

FIG. 9 is a diagram illustrating the variation in brightness corresponding to the ratio of the lighting period Ton with respect to the lighting and extinguishing period T.

FIG. 10 is a block diagram showing a conventional example of an illumination device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram showing one preferred embodiment of the illumination device of the present invention.

As is shown in this figure, the illumination device of the present preferred embodiment includes a first light-emitting element LED 1, a second light-emitting element LED 2, NPN-type bipolar transistors N1 and N2, resistors R1 and R2, and a control circuit CTRL.

Two types of white light-emitting diodes that have mutually different color temperatures are preferably used as the first light-emitting element LED 1 and second light-emitting element LED 2. Furthermore, in the present preferred embodiment, a white light-emitting diode having a color temperature of about 5,000 K is preferably used as the first light-emitting element LED 1, and a white light-emitting diode having a color temperature of about 2,600 K is preferably used as the second light-emitting element LED 2. However, these color temperatures are merely examples. It is desirable that two types of white light-emitting diodes whose color temperatures are as far apart as possible be used in order to adjust the color temperature (chromaticity) of the illuminating light in a broad range.

The collectors of the transistors N1 and N2 are both connected to the power supply line. The emitter of the transistor N1 is connected to the anode of the first light-emitting element LED 1. The emitter of the transistor N2 is connected to the anode of the second light-emitting element LED 2. The cathode of the first light-emitting element LED 1 is connected to the ground line via the resistor R1. The cathode of the second light-emitting element LED 2 is connected to the ground line via the resistor R2.

The control circuit CTRL is constructed and arranged to perform lighting and extinguishing control of the first light-emitting element LED 1 and second light-emitting element LED 2 by generating switching control signals S1 and S2 that are supplied to the bases of the transistors N1 and N2 on the basis of chromaticity control signals and brightness control signals.

To describe this in more specific terms, when the first light-emitting element LED 1 is to be lit, the switching control signal S1 is set at a high level, and the transistor N1 is switched “on.” As a result of such control, the supply of a driving current to the first light-emitting element LED 1 is permitted, so that the first light-emitting element LED 1 is lit. Conversely, when the first light-emitting element LED 1 is to be extinguished, the switching control signal S1 is set at a low level, and the transistor N1 is switched “off.” As a result of such control, the supply of a driving current to the first light-emitting element LED 1 is cut off, so that the first light-emitting element LED 1 is extinguished.

Similarly, when the second light-emitting element LED 2 is to be lit, the switching control signal S2 is set at a high level, and the transistor N2 is switched “on.” As a result of such control, the supply of a driving current to the second light-emitting element LED 2 is permitted, so that the second light-emitting element LED 2 is lit. Conversely, when the second light-emitting element LED 2 is to be extinguished, the switching control signal S2 is set at a low level, and the transistor N2 is switched “off.” As a result of such control, the supply of a driving current to the second light-emitting element LED 2 is cut off, so that the second light-emitting element LED 2 is extinguished.

Furthermore, the chromaticity adjustment operation and brightness adjustment operation of the illuminating light that are performed by the control circuit CTRL will be described in detail later.

Next, the structures of the first light-emitting element LED 1 and second light-emitting element LED 2 will be described in detail with reference to FIG. 2.

FIG. 2 is a longitudinal sectional view which schematically illustrates the structure of the first light-emitting element LED 1. Furthermore, since the structure of the second light-emitting element LED 2 is also the same, a redundant description is omitted.

As is shown in this figure, the first light-emitting element LED 1 preferably includes a blue light-emitting diode 1 which emits blue light and a fluorescent layer 2 which covers the blue light-emitting diode 1. Furthermore, the fluorescent layer 2 is preferably formed by uniformly mixing a red fluorescent body 2a which is excited by blue light, and which emits red light, and a green fluorescent body 2b which is excited by blue light, and which emits green light, with a transparent resin 2c.

In the first light-emitting element LED 1 constructed as described above, white light with high color rendering properties can be produced by mixing the red light that is emitted by the red fluorescent body 2a, the green light that is emitted by the green fluorescent body 2b, and the portion of the blue light that is not absorbed by these two fluorescent bodies.

Furthermore, in cases where priority is given to the improvement of the light emission efficiency over improvement of the color rendering properties, a construction may be used in which the fluorescent layer 2 is formed by uniformly mixing a yellow fluorescent body which is excited by blue light, and which emits yellow light, with a transparent resin.

Next, the chromaticity adjustment operation (color temperature adjustment operation) of the illuminating light performed by the control circuit CTRL will be described in detail with reference to FIGS. 3A-3C.

FIGS. 3A-3C are diagrams illustrating the chromaticity adjustment operation of the illuminating light. The symbols S1 and S2 in (a) through (c) of this figure respectively indicate the logical states of the switching control signals S1 and S2 (and by extension, the lit or extinguished states of the LED 1 and LED 2).

As is shown in FIGS. 3A-3C, the control circuit CTRL has a construction which performs periodic lighting and extinguishing control of the first light-emitting element LED 1 and second light-emitting element LED 2 so that this control has a lighting period Ton in which the first light-emitting element LED 1 and second light-emitting element LED 2 are lit and extinguished in a complementary manner (in other words, so that the “on” duty of both light-emitting elements is a total of 100%), and an extinguishing period Toff in which both the first light-emitting element LED 1 and second light-emitting element LED 2 are extinguished.

Furthermore, the control circuit CTRL has a construction which variably controls the ratio of the lighting period T1 of the first light-emitting element LED 1 and the lighting period T2 of the second light-emitting element LED 2 within the lighting period Ton in accordance with input chromaticity control signals.

To describe this in more specific terms, in cases where the color temperature of the illuminating light is to be lowered, the ratio of the lighting period T2 of the second light-emitting element LED 2 with respect to the lighting period Ton (i.e., the “on” duty of the second light-emitting element LED 2) may be successively raised (FIG. 3C→FIG. 3B→FIG. 3A); conversely, in cases where the color temperature of the illuminating light is to be raised, the “on” duty of the second light-emitting element LED 2 may be successively lowered (FIG. 3A→FIG. 3B→FIG. 3C).

FIG. 4 is a diagram illustrating the color temperature variation corresponding to the ratio of the “on” period T2 of the second light-emitting element LED 2 with respect to the lighting period Ton (=T1+T2). Furthermore, FIG. 5 is a chromaticity diagram illustrating the variation in chromaticity corresponding to the ratio of the “on” period T2 of the second light-emitting element LED 2 with respect to the lighting period Ton (=T1+T2). The solid line in FIG. 5 shows the black body radiation curve.

As is shown in FIG. 4, in the illumination device of the present preferred embodiment, the color temperature of the illuminating light can be continuously adjusted from the color temperature (about 5,000 K) of the first light-emitting element LED 1 to the color temperature (about 2,600 K) of the second light-emitting element LED 2. With regard to the chromaticity of the illuminating light, furthermore, a broad adjustment range of approximately about 0.06 to about 0.14 in terms of chromaticity coordinates can be obtained as shown in FIG. 5.

Accordingly, in the illumination device of the present preferred embodiment, white illumination of various hues can be accomplished using a single module.

] Moreover, in the illumination device of the present preferred embodiment, lighting and extinguishing control of the first light-emitting element LED 1 and second light-emitting element LED 2 is performed so that the total of the “on” duty of both light-emitting elements is 100%. Accordingly, even if the ratio of the lighting period T1 of the first light-emitting element LED 1 and the lighting period T2 of the second light-emitting element LED 2 within the lighting period Ton is variably controlled, the device is constantly lit as viewed from the total lighting period Ton. Consequently, the brightness of the illuminating light can be maintained at a constant value.

Furthermore, chromaticity adjustment of the illuminating light can also be performed using a construction in which one of the light-emitting element LEDs, i.e., the first light-emitting element LED 1 or second light-emitting element LED 2, is constantly lit, and only the “on” duty of the other light-emitting element LED is varied (in other words, a construction in which the “on” duty of the first light-emitting element LED 1 and second light-emitting element LED 2 is not maintained at a total of 100%). However, in the case of such a construction, the adjustment range of the chromaticity is narrowed, and the brightness of the illuminating light cannot be maintained at a constant value. Accordingly, it is desirable to use the construction of the preferred embodiment described above.

Next, the brightness adjustment operation of the illuminating light performed by the control circuit CTRL will be described in detail with reference to FIGS. 6A-6C.

FIGS. 6A-6C are diagrams illustrating one example of the brightness adjustment operation of the illuminating light. The symbols S1 and S2 in FIGS. 6A-6C respectively indicate the logical states of the switching control signals S1 and S2 (and by extension, the lit and extinguished states of the LED 1 and LED 2).

As is shown in FIGS. 6A-6C, the control circuit CTRL has a construction which maintains the length of the lighting period Ton at a constant value, and which variably controls the length of the extinguishing period Toff in accordance with brightness control signals.

To describe this in more specific terms, in cases where the brightness of the illuminating light is to be lowered, the extinguishing period Toff may be made successively longer (FIG. 6A→FIG. 6B→FIG. 6C); conversely, in cases where the brightness of the illuminating light is to be raised, the extinguishing period Toff may be made successively shorter (FIG. 6C→FIG. 6B→FIG. 6A).

FIG. 7 is a diagram illustrating the variation in the brightness in accordance with the length of the extinguishing period Toff.

As is shown in this figure, the brightness of the illuminating light is lowered as the length of the extinguishing period Toff increases; conversely, the brightness of the illuminating light becomes higher as the length of the extinguishing period Toff becomes shorter.

FIGS. 8A-8C are diagrams illustrating another example of the brightness adjustment operation of the illuminating light. The symbols S1 and S2 in FIGS. 8A-8C respectively indicate the logical states of the switching control signals S1 and S2 (and by extension, the lit and extinguished states of the LED 1 and LED 2).

As is shown in FIGS. 8A-8C, the control circuit CTRL has a construction which variably controls the ratio of the lighting period Ton and the extinguishing period Toff within a specified lighting and extinguishing period T in accordance with brightness control signals.

To describe this in more specific terms, in cases where the brightness of the illuminating light is to be lowered, the ratio of the lighting period Ton with respect to the lighting and extinguishing period T (the total “on” duty combining the first light-emitting element LED 1 and second light-emitting element LED 2) may be successively lowered (FIG. 8A→FIG. 8B→FIG. 8C); conversely, in cases where the brightness of the illuminating light is to be raised, the total “on” duty may be successively raised (FIG. 8C→FIG. 8B→FIG. 8A).

FIG. 9 is a diagram illustrating the variation in the brightness corresponding to the ratio of the lighting period Ton with respect to the lighting and extinguishing period T.

As is shown in this figure, the brightness of the illuminating light is lowered with a decrease in the total “on” duty; conversely, the brightness of the illuminating light increases with an increase in the total “on” duty.

Furthermore, with regard to the lighting and extinguishing period T described above, it is desirable to set this period at a length (around several hundred ms) which is such that there is no sense of flickering as seen by the naked eye.

As was described above, the illumination device according to a preferred embodiment of the present invention is an illumination device preferably including a first light-emitting element LED 1 and a second light-emitting element LED 2 having mutually different color temperatures, and a control circuit CTRL which performs lighting and extinguishing control of the first light-emitting element LED 1 and second light-emitting element LED 2, with this control circuit CTRL having a construction which performs lighting and extinguishing control of the first light-emitting element LED 1 and second light-emitting element LED 2 so that this control has a lighting period Ton in which the first light-emitting element LED 1 and second light-emitting element LED 2 are lit and extinguished in a complementary manner, and an extinguishing period Toff in which both the first light-emitting element LED 1 and second light-emitting element LED 2 are extinguished.

By using such a construction, it is possible to arbitrarily adjust the chromaticity of the illuminating light without any effect on the brightness of the illuminating light by appropriately selecting the ratio of the lighting period T1 of the first light-emitting element LED 1 and the lighting period T2 of the second light-emitting element LED 2 within the lighting period Ton. Furthermore, it is possible to arbitrarily adjust the brightness of the illuminating light without any effect on the chromaticity of the illuminating light by appropriately selecting the length of the extinguishing period Toff, or the ratio of the lighting period Ton and the extinguishing period Toff within the lighting and extinguishing period T. Moreover, because the chromaticity control and brightness control of the illuminating light according to a preferred embodiment of the present invention are not accompanied by driving current control of the first light-emitting element LED 1 and second light-emitting element LED 2, the control in the control circuit CTRL can be realized in a simple manner.

With regard to the construction according to the present invention, furthermore, besides the preferred embodiments described above, various alterations may be added in a range that involves no departure from the gist of the invention. For example, field effect transistors may be used instead of the bipolar transistors N1 and N2 in FIG. 1. Moreover, a constant current source may be used instead of the resistors R1 and R2 in FIG. 1.

Preferred embodiments of the present invention provide a technique that is suitable for use in common illumination devices used in various types of applications, beginning with the backlighting of liquid crystal displays.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.

Claims

1. An illumination device comprising: a first light-emitting element;a second light-emitting element having a different color temperature than the first light-emitting element; anda control circuit that is arranged to perform lighting and extinguishing control of the first light-emitting element and second light-emitting element; whereinsaid control circuit is arranged to perform periodic lighting and extinguishing control of the first light-emitting element and second light-emitting element such that, during a lighting period, the first light-emitting element and second light-emitting element are lit and extinguished in a complementary manner, and during an extinguishing period, the first light-emitting element and second light-emitting element are both extinguished.

2. The illumination device according to claim 1, wherein said control circuit variably controls a ratio of the lighting period of the first light-emitting element and the lighting period of the second light-emitting element within said lighting period in accordance with chromaticity control signals.

3. The illumination device according to claim 1, wherein said control circuit variably controls a length of said extinguishing period in accordance with brightness control signals while maintaining a length of said lighting period at a constant value.

4. The illumination device according to claim 1, wherein said control circuit variably controls a ratio of said lighting period and said extinguishing period within a specified lighting and extinguishing period in accordance with brightness control signals.

5. The illumination device according to claim 1, wherein each of the first light-emitting element and second light-emitting element includes a blue light-emitting diode that emits blue light and a fluorescent layer that covers said blue light-emitting diode, and said fluorescent layer includes a uniformly mixed red fluorescent body and green fluorescent body that are excited by blue light and that respectively emit red light and green light with a transparent resin, or uniformly mixed yellow fluorescent body that is excited by blue light and that emits yellow light with a transparent resin.