When an LED light source serves as a white light source utilizing an LED device and a fluorescent material in combination, for example, a blue LED element that can emit blue light can be resin sealed with a sealing resin. In this case, the sealing resin can include a base transparent resin and a yellow fluorescent material dispersed in the transparent resin. The yellow fluorescent material can be excited by blue light, thereby being capable of wavelength-converting the light to emit yellow light which has a complementary color to blue. When the above configured LED light source is turned on, the blue LED element can emit blue light. Part of the blue light from the blue LED element can excite the yellow fluorescent material so that the yellow fluorescent material can wavelength convert the blue light to yellow light. Then, the remaining part of the blue light can be mixed with the yellow light from the yellow fluorescent material to generate pseudo white light by means of additive color mixing.
Alternatively, instead of the yellow fluorescent material, a green fluorescent material and a red fluorescent material can be used to be dispersed in the transparent resin for resin sealing the LED light source. The green fluorescent material can be excited by blue light, thereby being capable of wavelength-converting the light to emit green light. The red fluorescent material can be excited by blue light, thereby being capable of wavelength-converting the light to emit red light. When the above configured LED light source is turned on, the blue LED element can emit blue light. Part of the blue light from the blue LED element can excite the green fluorescent material so that the green fluorescent material can wavelength convert the blue light to green light. Another part of the blue light from the blue LED element can excite the red fluorescent material so that the red fluorescent material can wavelength convert the blue light to red light. Then, the remaining part of the blue light can be mixed with the green light and the red light from the respective fluorescent materials to generate pseudo white light by means of additive color mixing.
In either case, the fluorescent material can emit faint light at a certain intensity even when the LED element is turned off. This is because the fluorescent material can be excited by any light of a wavelength range from ultraviolet to blue, which may be contained in outside light including natural light such as sunlight and artificial light such as light from an incandescent lamp, a fluorescent lamp, and the like. Accordingly, when the sealing resin sealing the LED element is observed while the LED element is turned off, the sealing resin containing the yellow fluorescent material dispersed therein may be observed as a yellowish member while the sealing resin containing the red and green fluorescent materials dispersed therein may be observed as an orangish member.
In particular, when the white LED light source with the above configuration is used together with optical devices such as a reflector, a lens, and the like, the sealing resin may be enlarged by the optical devices, thereby being remarkably conspicuous with the fluorescent material color while being turned off. This may degrade the appearance of the overall optical system.
In order to cope with this type of problem, a white light diffusion layer may be provided over the surface of the sealing resin, or a white light diffusion member may be disposed in front of the sealing resin. In this way, the outside light such as natural light or artificial light can be shielded by the layer or member to suppress the excitation of the fluorescent material, thereby improving the outer appearance of the overall optical system.
However, the above configuration has the problem in which the light emitted from the LED element may be diffused or absorbed by the white light diffusion layer or member provided in front of the device, which can result in light loss and a decrease in the entire light intensity or axial direction luminous intensity. Furthermore, such a white light diffusion layer or member may have optical characteristics such that it can easily absorb visible light at a shorter wavelength range. Accordingly, in the above configuration, the light emitted from the light source device may be shifted in wavelength by the white light diffusion layer or member.
In view of these problems, an illumination apparatus has been proposed in, for example, Japanese Patent Application Laid-Open No. 2006-251396 (corresponding to U.S. Pat. No. 7,859,193B2) to prevent the fluorescent material color from being observed from outside. The illumination apparatus can be configured to provide a standby current flow through an LED chip when the system does not function as a light source so that the LED chip can emit faint light to excite a fluorescent material to some extent. This excitation can generate faint light from the fluorescent light mixed together with the original faint light. Accordingly, the mixed light can generate faint white light to hinder the fluorescent material color itself.
In the above proposed illumination apparatus, a standby current that is to flow through the LED chip can be controlled according to a driving signal based on the brightness of external light detected by an external light sensor, thereby reducing the power consumption. However, the technique does not clearly show the configuration and functions relating to the LED light source, the external light sensor, and their positional relationship and the like as well as the relationship between the brightness of the LED light source and that of the external light. This may make concrete modes achieved by this technique difficult to conceptualize.