1. Field of the Invention
The invention relates to a lens system, and more particularly to a lens system including an IR absorptive lens.
2. Description of the Related Art
A digital still camera, a digital video camera, or a mobile phone with photograph function generally includes a lens system and an image sensor (e.g. CCD or CMOS), wherein the lens system converges the light which is emitted from an object to the image sensor, and the image sensor receives and converts the light to electrical signals for subsequent processing. In addition to the visible signals which are emitted from the object, the image sensor is capable of sensing the infrared (IR) signals of wavelengths greater than 750 nm. As a result, the captured image is susceptible to color aberration arising from the infrared signals. The image quality is seriously affected.
A known method of eliminating the above-mentioned color aberration caused by the infrared lights is shown in
The above-mentioned method can correct color aberration caused by infrared light. However, the transmission spectrum of the IR cut filter shifts to shorter wavelengths when the incident angle of the incident beam increases. The greater the incident angle of the incident beam is, the more the transmission spectrum of the IR cut filter shifts. A zero-degree beam 15 and a twenty-six-degree beam 16 are shown in
The invention provides a lens system to solve the above problems. The lens system includes an IR cut lens. One surface of the IR cut lens is coated with an IR cut thin film and the other surface of the IR cut lens is coated with an antireflection coating. Such an arrangement is capable of lessening the shift of the transmission spectrum to shorter wavelengths when an incident beam enters the lens system at a large incident angle, reducing the differences between all incident beams in the average transmission rate in the visible range (420 nm-680 nm), and reducing the average transmission rate for all incident beams in the infrared range (750 nm-1200 nm), so as to reduce color aberration and ghost image, eliminate infrared noise and improve the image quality. In addition, the invention does not use an IR cut filter, thus saving the cost of an IR cut filter as well as shortening the length of the lens system.
The lens system in accordance with an exemplary embodiment of the invention includes a first lens that is an IR absorptive lens, wherein the first lens includes a first surface facing an object side and a second surface facing an image side.
In another exemplary embodiment, the curvature of the second surface is smaller than the curvature of the first surface.
In yet another exemplary embodiment, the first lens further includes an IR cut thin film that is coated on the second surface.
In another exemplary embodiment, the first lens further includes an antireflection coating that is coated on the first surface, or the first lens further includes an IR cut thin film that is coated on the first surface, or the first lens further includes an antireflection coating that is coated on the second surface.
In yet another exemplary embodiment, the first lens is made of blue glass.
In another exemplary embodiment, the lens system further includes a second lens, wherein the first lens and the second lens are arranged in sequence from the object side to the image side.
In yet another exemplary embodiment, the second lens is made of plastic material or glass material.
A detailed description is given in the following embodiments with reference to the accompanying drawings.
The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
The following description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
Referring to
The first lens 31 is made of material that can absorb infrared light. For example, the first lens 31 is an IR absorptive lens that is made of blue glass, allows visible of the incident light to pass through, and absorbs infrared of the incident light, thereby avoiding the image sensor from receiving infrared noise. The IR cut thin film 3121 is capable of reducing the shift of the transmission spectrum to shorter wavelengths when the incident beam enters the lens system 3 at a large angle, whereby the color aberration is reduced, the infrared light which is not absorbed by the first lens 31 is filtered out, and the infrared noise is significantly eliminated. The antireflection coating 3111 is capable of reducing the differences between all incident beams in the average transmission rate in the visible range (420 nm-680 nm), thereby reducing the ghost image.
The advantage of the lens system of the invention is described by using the test data shown in
The average transmission rates for the zero-degree beam and twenty-six-degree beam in the visible range (420 nm-680 nm) are respectively 78% and 75%, and the variation value is 3%, as shown in
The average transmission rates for the zero-degree beam and the twenty-six-degree beam in the infrared range (750 nm-1200 nm) are respectively 0.73% and 0.16%, as shown in
As described, the invention is capable of enhancing the image quality. It is also understood that an infrared cut filter is not included in the lens system of the invention. Thus, the cost of infrared cut filter can be saved and the length of the lens system can be shortened.
In
In the above embodiment of the lens system for a mobile phone, the first lens 31 is made of blue glass, the second lens 32 is made of plastic or glass, and the third lens 33 is made of plastic or glass. However, it is understood that the invention is not limited to this embodiment. The material of each lens can be changed, depending on the practical situations.
The surface curvature of the lens is smaller (flatter), uniformly coating the IR cut thin film 3121 on the lens surface is easier. In the above embodiment, the curvature of the second surface 312 is smaller than that of the first surface 311. Therefore, the antireflection coating 3111 is coated on the first surface 311 and the IR cut thin film 3121 is coated on the second surface 312. However, it has the same effect and falls into the scope of the invention if the first surface 311 is coated with the IR cut thin film 3121 and the second surface 312 is coated with the antireflection coating 3111.
While the invention has been described by way of examples and in terms of embodiments, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
Number | Date | Country | Kind |
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101124876 | Jul 2012 | TW | national |