BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to a display system, and more particularly to a head-up display (HUD).

2. Description of the Related Art

For a display device needing to show both an image and the background therebehind, a transparent or translucent reflecting screen (glass window or windshield) is usually used for reflecting images. For example, the display device is a head-up display (HUD) or shows a commercial advertisement, and the image is provided by a projecting device or a display. However, an observer may receive the same image reflected by different surfaces of the reflecting screen, which generates a ghost image caused by overlapping the images and greatly affects the image quality.

Please refer to FIG. 1 illustrating a conventional head-up display. As shown in FIG. 1, a projecting device 4 generates an image light M1 projected toward a transparent screen 2, such as a glass window or a windshield. A portion of the image light M1 is directly reflected by a reflecting surface 2a and produces a reflecting image light M2. Another portion of the image light M1 enters the transparent screen 4 and is reflected by a reflecting surface 2b. Then, the portion of the image light M1 passes through the reflecting surface 2a and produces a reflecting image light M3. When the observer receives both the reflecting image light M2 and M3, an overlapping ghost image is generated, which causes difficulty in identifying the image.

Therefore, it is important to reduce the ghost image resulted from multi-reflection for improving the image quality of such kind of display device.

SUMMARY OF THE INVENTION

The invention is directed to a display system for changing the polarizing directions of the reflecting image light reflected by different surfaces and the multi-reflected image light. As a result, the image quality is improved.

According to the present invention, a display system including an image device, an image reflecting element and a polarizing element is provided. The image device is used for producing a polarized image light. The image reflecting element includes a substrate and a phase modulating element. The phase reflecting element is adjacent to the substrate and has a reflecting surface for receiving the polarized image light. After the polarized image light is projected to the reflecting surface, the reflecting surface reflects a portion of the polarized image light to produce a first reflecting polarized image light. Another portion of the polarized image light is projected into the phase modulating element, reflected by the substrate and then projected out of the reflecting surface to produce a second reflecting polarized image light. The phase difference between the first reflecting polarized image light and the second reflecting polarized image light is substantially equal to nÏ€, wherein n is a positive odd number. The polarizing element is for receiving and allowing the first reflecting polarized image light to pass through, and blocking the second reflecting polarized image light.

According to the present invention, a display system including an image device, a transparent substrate and a phase modulating element is provided. The phase modulating element is adjacent to the transparent substrate and has a reflecting surface for receiving a polarized image light. After the polarized image light is projected to the reflecting surface, the reflecting surface reflects a portion of the polarized image light to produce a reflecting polarized image light. Another portion of the polarized image light is projected to the phase modulating element and becomes an incident polarized image light having a desired polarizing direction. Thus most of the incident polarized image light is transmitted into the transparent substrate.

The invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional head-up display;

FIG. 2 illustrates a display system according to a first embodiment of the present invention;

FIG. 3 illustrates a display system according to a second embodiment of the present invention; and

FIG. 4 shows reflectance of the ghost image reducing device of the second embodiment of the invention for TE electromagnetic wave and TM electromagnetic wave under different incident angles.

DETAILED DESCRIPTION OF THE INVENTIONFirst Embodiment

Please refer to FIG. 2 illustrating a display system according to a first embodiment of the present invention. As shown in FIG. 2, the ghost image reducing device 100 includes an image reflecting element 110 and a polarizing element 120. The image reflecting element 110 includes a substrate 112 and a phase modulating element 114. The phase modulating element 114 is adjacent to the substrate 112 and has a reflecting surface 114a.

As shown in FIG. 2, the ghost image reducing device 100 and an image device 10 compose a display system 1, that is, a head-up display. The image device 10 generates a polarized image light P1. For example, the image device 10 can be a liquid crystal display panel, and the polarized image light P1 can be generated by the liquid crystal display panel. Or, the image device 10 can be an image generating device incorporated with an image polarizing element, and the polarizing image light P1 can be generated by an image light generated by the image generating device and then passing through the image polarizing element. The reflecting surface 114a is for receiving the polarized image light P1. After the polarized image light P1 is projected to the reflecting surface 114a, a portion of the polarized image light P1 is reflected by the reflecting surface 114a for producing a first reflecting polarized image light P2. The first reflecting polarized image light P2 and the polarized image light P1 have the same polarizing direction. After projected into the phase modulating element 114 and reflected by the substrate 112, another portion of the polarized image light P1 is projected out of the reflecting surface 114a for producing a second reflecting polarized image light S2.

Preferably, the polarized image light P1 is a linearly-polarized image light, such as a p-type linearly-polarized image light, which means the polarizing direction of the light is parallel to plane defined by the propagation directions of the incident light and the reflecting light. Or, the polarized-image light P1 can be an s-type linearly-polarized image light, which means the polarizing direction of the light is perpendicular to the plane defined by the propagation directions of the incident light and the reflecting light. In the present embodiment, the polarized image light P1 is exemplified by a p-type linearly-polarized image light. But the present embodiment is not limited thereto. The polarized image light can be a linearly polarized image light categorized neither p-type nor s-type linearly-polarized image light. The polarized image light can be a circularly polarized image light or an elliptically polarized image light, too.

Preferably, the substrate 112 is a transparent substrate, such as a transparent glass plate or a transparent plastic plate. The phase modulating element 114 is for example a quarter-wave plate made of polymer material and attached to the substrate 112. Or, the phase modulating element 114 is a quarter-wave coating layer disposed on the substrate 112. Therefore, when a portion of the polarized image light P1 enters and then is projected out of the phase modulating element 114, a phase retardation or a phase difference of 180 degree (Ï€) is generated between the second reflecting polarized image light S2 and the polarized image light P1. Otherwise, a phase retardation or a phase difference of nÏ€ is generated between the second reflecting polarized image light S2 and the polarized image light P1, wherein n is a positive odd number selected from 1, 3, 5 . . . . The polarizing direction of the first reflecting polarized image light P2 and that of the second reflecting polarized image light S2 are substantially perpendicular to each other. In other words, the second reflecting polarized image light S2 is a s-type linearly-polarized image light.

As shown in FIG. 2, the first reflecting polarized image light P2 and the second reflecting polarized image light S2 are projected to the polarizing element 120 altogether. Preferably, the polarizing element 120 is a p-type polarizer, which allows the first reflecting polarized image light P2 passing through to from a received polarized image light P3 and meanwhile blocks the second reflecting polarized image light S2. Likewise, even a portion of polarized image light P1 passes through phase modulating element 114, and then reflected by the reflecting surface 112a of substrate 112 and passes through phase modulating element 114 to produce a third reflecting polarized image light S3, the polarizing direction of the third reflecting polarized image light S3 and that of the first reflecting polarized image light P2 are substantially perpendicular to each other. Therefore, the third reflecting polarized image light S3 can not pass through polarizing element 120. Though the quarter-wave plate is used in the present embodiment as an example, the present invention is not limited thereto. As long as the retardation wavelength of the wave plate satisfies the formula (1), the same phase retardation can be generated.

$\begin{matrix} \frac{1}{4} î¢ž Î» + \frac{1}{2} î¢ž n Â\cdot Î» & (1) \end{matrix}$

n is zero or a positive integer. Î» is the wavelength of the second reflecting polarized image light.

Furthermore, even the polarized image light is not categorized either p-type or s-type linearly-polarized image light, the polarizing direction of the first reflecting polarized image light and that of the second reflecting polarized image light are still perpendicular to each other. When the first reflecting polarized image light is reflected by the reflecting surface 114a, the first reflecting polarized image light will be different in phase angle by 180 degrees compared with the polarized image light. Therefore, as long as the polarizing axis of the polarizing element 120 is consistent with the polarizing direction of the first reflecting polarized image light, the polarizing element can still allow the first reflecting polarized image light pass through and block the second reflecting polarized image light.

Besides, if the polarized image light is circularly or elliptically polarized image light, the rotating direction of light of the first reflecting polarized image light and that of the second polarized image light which passed through the quarter-wave plate will be opposite. Therefore, as long as a circular polarizing plate whose rotating direction of light is consistent with that of the first reflecting polarizing light is adopted as the polarizing element, the circular polarizing plate can still allow the first reflecting polarized image light pass through and block the second reflecting polarized image light.

Furthermore, though the present embodiment uses a linearly-polarized image light as an example, the present embodiment can be applied to a circularly-polarized image light or an elliptically-polarized image light. As long as the first reflecting light and the second reflecting light have different polarizing directions when passing through the polarizing element 120 so that the polarizing element 120 allows the first reflecting polarized image light passing through and blocks the second reflecting polarized image light, the present invention encompasses all of such modifications.

Therefore, as long as the reflecting polarized image light passes through the phase modulating element 114, the polarizing element 120 is able to block such light to avoid ghost image resulted from multi-reflection.

Moreover, when the polarized image light P1 is a light of color, the wavelength of the phase modulating element 114 can be taken in the middle band, such as the wavelength of green light, for reducing color shift.

Second Embodiment

Please refer to FIG. 3 illustrating a display system according to a second embodiment of the present invention. As shown in FIG. 3, the ghost image reducing device 200 includes a transparent substrate 210 and a phase modulating element 220. The phase modulating element 220 is adjacent to the transparent substrate 210 and has a reflecting surface 220a.

As shown in FIG. 3, the ghost image reducing device 200 and the image device 20 composes a display system 3, that is, a head-up display. The image device 20 generates the polarized image light TE1. The reflecting surface 220a is for receiving the polarized image light TE1. The reflecting surface 220a reflects a portion of the polarized image light TE1 to produce a reflecting polarized image light TE2. Another portion of the polarized image light TE1 is projected into the phase modulating element 220 and becomes an incident polarized image light TM2. The incident polarized image light TM2 is adjusted to have a desired polarizing direction, such that most of the incident polarized image light TM2 is incident to the transparent substrate 210. The image device can be a liquid crystal display panel, and the polarized image light TE1 is generated by the liquid crystal display panel. Or, the image device can be an image polarizing element incorporated with an image generating device, and the polarized image light TE1 is generated by an image light generated by the image generating device and then passing through the image polarizing element. Preferably, the polarized image light TE1 is a linearly-polarized image light. The polarized image light TE1 of the present embodiment is also TE electromagnetic wave, which means the polarizing direction thereof is perpendicular to the incident plane.

Preferably, the transparent substrate 210 is a glass plate or a transparent plastic plate. The phase modulating element 220 is a half-wave plate disposed on the transparent substrate 210. Or, the phase modulating element 220 is a half-wave coating layer disposed on the transparent substrate 210. As shown in FIG. 3, after a portion of the polarized image light TE1 is projected into the phase modulating element 220, phase retardation or phase difference of 180 degrees (Ï€) is generated between the incident polarized image light TM2 and the reflecting polarized image light TE2. Otherwise, a phase retardation or a phase difference of nÏ€ is generated between the incident polarized image light TM2 and the reflecting polarized image light TE2, wherein n is a positive odd number selected from 1, 3, 5 . . . . Therefore, the polarizing direction of the reflecting polarized image light TE2 and that of the incident polarized image light TM2 are substantially perpendicular to each other. The polarizing direction of the reflecting polarized image light TE2 and that of the polarized image light TE1 are substantially the same. In other words, when the reflecting polarized image light TE2 is a TE electromagnetic wave, the incident polarized image light TM2 is a TM electromagnetic wave, which means the polarizing direction is parallel to the incident plane.

As shown in FIG. 3, the incident polarized image light TM2 enters the transparent substrate 210 at an incident angle Î¸. When the difference between the incident angle Î¸ and the Brewster's angle of the phase modulating element 220 and the transparent substrate 210 is less than 15 degrees such that the reflectance is extremely low, the incident polarized image light TM2 enters the transparent substrate 210 and is projected out of the transparent substrate 210. Therefore, most of the incident polarized image light TM2 entering the phase modulating element 220 is not reflected by the transparent substrate 210 and is not received by the observer. As a result, the ghost image caused by redundant reflecting image light is reduced effectively.

Preferably, when the incident angle Î´ is close to the Brewster's angle of the phase modulating element 220 and the transparent substrate 210, the incident polarized image light TM2 passes through the transparent substrate 210 with almost none reflection. Please refer to FIG. 4. FIG. 4 shows reflectance of the ghost image reducing device of the second embodiment of the invention for TE electromagnetic wave and TM electromagnetic wave under different incident angles. As shown in FIG. 4, zone I represents the scenario that light enters the phase modulating element 220 from the transparent substrate 210. Zone II represents the scenario that light enters the transparent substrate 210 from the phase modulating element 220. In the present embodiment, the phase modulating element 220 is made of polymer material, and the transparent substrate 210 is a transparent glass plate. In the scenario of zone II, as a curve 401 shows, the Brewster's angle Î¸_Bof TM electromagnetic wave is about 56 degrees. When the incident angle Î¸ is between 40 and 70 degrees, the reflectance of the incident polarized image light TM2 entering the transparent substrate 210 from the phase modulating element 220 is less than 10%. Therefore, the location of the image device 20 can be adjusted to make the incident angle Î¸ of the incident polarized image light TM2 close to the Brewster's angle Î¸_B, for reducing the reflectance of the incident polarized image light TM2. While the refractive index of the medium of incident side is n_i, and the refractive index of the medium of leaving side is n_t, the Brewster's angle Î¸_Bcan be computed by formula (2):

$\begin{matrix} {Î¸}_{B} = \tan^{- 1} î¢ž \frac{n_{t}}{n_{i}} & (2) \end{matrix}$

Though the phase modulating element is a half-wave plate as an example in the present embodiment, the present invention is not limited thereto. As long as the retardation wavelength of the wave plate satisfies the formula (3), the same phase retardation can be caused.

$\begin{matrix} \frac{1}{2} î¢ž Î» + n Â\cdot Î» & (3) \end{matrix}$

n is zero or a positive integer. Î» is the wavelength of the incident polarized image light.

Furthermore, if the polarized image light is a circularly polarized image light or an elliptically polarized image light, the phase modulating element can be a quarter-wave plate disposed on the transparent substrate 210, or a quarter-wave coating layer disposed on the transparent substrate 210. Therefore, the incident polarized image light resulted form a portion of the polarized image light projected into the phase modulating element is transformed into a linearly polarized image light, and the polarizing direction thereof is adjusted to become a TM electromagnetic wave. Therefore, as long as the incident angle of the incident polarized image light entering the transparent substrate 210 is close to the Brewster's angle of the phase modulating element and the transparent substrate 210, the effect of reducing the reflecting rate can also be achieved.

Moreover, when the polarized image light TE1 is a light of color, the wavelength of the phase modulating element 114 can be in the middle band, such as the wavelength of green light, for reducing color shift. Meanwhile, the reflectance of most of the incident polarized image light is kept low.

In the display systems of the above embodiments, the phase modulating element is used for changing the phase of the polarized image light entering the phase modulating element. The phase of the polarized image light reflected by other surfaces is changed and then filtered by the polarizing element or is projected out of the transparent substrate directly. The present invention effectively reduces redundant reflecting image light and reduces ghost image for improving the display quality.

While the invention has been described by way of example and in terms of preferred embodiments, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.