Abstract:
The present invention provides a color wheel including a motor and a color filter module. The color filter module includes a first color filter having a first transmittance and a second color filter having a second transmittance that is smaller than the first transmittance. 50% of the first transmittance is corresponding to a first light wavelength and a second light wavelength that is larger than the first light wavelength. The second color filter and the first color filter have substantially same hues. 50% of the second transmittance is corresponding to a third light wavelength and a fourth light wavelength that is larger than the third light wavelength. The second color filter satisfies a first condition chosen from the third light wavelength being larger than the first light wavelength or the second light wavelength being larger than the fourth light wavelength.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a color wheel, and more particularly, to a color wheel in a projection apparatus. 
   BACKGROUND OF THE INVENTION 
   A color wheel is often disposed in a projection apparatus. The color wheel includes a motor and a color filter module. The color filter module is disposed on the motor and rotated by the motor. The color filter module includes a red, a green and a blue color filter for filtering three primary colors, and a digital micro-mirror device (DMD) is used together for adjusting the color contrast of projected images. 
   DMD belongs to a linear response display and therefore a gamma correction is needed for adjusting the color value. However, the gamma correction causes the gray scale to be quantified and then become discontinuous. In prior art, an error diffusion filter is used for solving the above drawback, but meanwhile apparent dither noise is shown in the gray scale of low brightness. 
   Please refer to  FIG. 1 . A conventional color filter module  10  includes two green color filters  12  with high transmittance, two red color filters  14  with high transmittance, two blue color filters  16  with high transmittance and further includes a green color filter  18  with low transmittance that is about 6%. Among the green gray scales displayed in the conventional color filter module  10 , a green gray scale with high brightness is displayed by the two green color filters  12  with high transmittance; a green gray scale with middle brightness is displayed by the two green color filters  12  with high transmittance and the green color filter  18  with low transmittance; a green gray scale with low brightness is mainly displayed by the green color filter  18  with low transmittance. In such disposition, the problem of dither noise is solved. 
   Please refer to  FIG. 2  that shows the spectrogram of each green color filter  12  with high transmittance and each green color filter  18  with low transmittance. Each green color filter  12  with high transmittance has a first transmittance  20 . 50% of the first transmittance  20  is corresponding to a first light wavelength  22  and a second light wavelength  24 , wherein the second light wavelength  24  is larger than the first light wavelength  22 . The green color filter  18  with low transmittance has a second transmittance  26  that is smaller than the first transmittance  20 . 50% of the second transmittance  26  is corresponding to a third light wavelength  28  and a fourth light wavelength  30 , wherein the fourth light wavelength  30  is larger than the third light wavelength  28 . In the conventional color filter module  10 , the gray scale is displayed by two color filters of different transmittances, and the first light wavelength  22  must be equal to the third light wavelength  28  while the second light wavelength  24  must be equal to the fourth light wavelength  30  for preventing the gray scale from variation. 
   In practice, the coating technique of color filters nowadays for making the first light wavelength  22  equal to the third light wavelength  28  while the second light wavelength  24  equal to the fourth light wavelength  30  has low yield rate, high cost and production difficulty. Therefore, the condition for light wavelengths needs to be improved. 
   SUMMARY OF THE INVENTION 
   One objective of the present invention is to provide a color filter module of a color wheel for solving the above problems in prior art. 
   In the present invention, the first color filter and the second color filter have similar hues, and the second transmittance of the second color filter is smaller than the first transmittance of the first color filter. The second color filter is conformed to a first condition. The first condition is chosen from that the third light wavelength is larger than the first light wavelength or the second light wavelength is larger than the fourth light wavelength. The color filter module in the present invention can solve the problems of dither noise and variation in gray scale. In practice, the cost is low and the production is easy. 
   The present invention is a color wheel including a motor and a color filter module. The color filter module is disposed on the motor and rotated by the motor. The color filter module includes a plurality of color filters, and each color filter is for filtering a specific color. The plurality of color filters includes a first color filter and a second color filter. The first color filter has a first transmittance corresponding to a first spectrum relation about the transmittance and the light wavelength. In the first spectrum relation, 50% of the first transmittance is corresponding to a first light wavelength and a second light wavelength, wherein the second light wavelength is larger than the first light wavelength. The second color filter has a second transmittance corresponding to a second spectrum relation about the transmittance and the light wavelength. In the second spectrum relation, 50% of the second transmittance is corresponding to a third light wavelength and a fourth light wavelength, wherein the fourth light wavelength is larger than the third light wavelength. The hue of the second color filter is similar to that of the first color filter. The second transmittance is smaller than the first transmittance. The second color filter is conformed to a first condition chosen from that the third light wavelength is larger than the first light wavelength or the second light wavelength is larger than the fourth light wavelength. 
   The above and other objectives, advantages and features of the present invention will be understood from the following detailed description of the present invention when considered in connection with the accompanying drawing below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view showing a conventional color filter module. 
       FIG. 2  is a spectrogram of each green color filter with high transmittance and each green color filter with low transmittance. 
       FIG. 3  is a schematic view showing the color filter module in the first preferred embodiment. 
       FIG. 4  is a spectrogram of the first color filter and the second color filter in the first preferred embodiment. 
       FIG. 5  is a schematic view showing the gray scale of the color filter module in the first preferred embodiment. 
       FIG. 6  is a schematic view showing the color filter module in the second preferred embodiment. 
       FIG. 7  is a spectrogram of the first color filter and the third color filter in the second preferred embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Please refer to  FIG. 3 . In the first preferred embodiment, the color filter module  40  includes a red color filter  42  with high transmittance, a green color filter  44  with high transmittance, a blue color filter  46  with high transmittance and a green color filter  48  with low transmittance. The red color filter  42  with high transmittance, the green color filter  44  with high transmittance, the blue color filter  46  with high transmittance and the green color filter  48  with low transmittance are sector plates with a same radius, and the central angles thereof are 108°, 104°, 118° and 30° respectively. The green color filter  44  with high transmittance is defined as a first color filter  44 , and the green color filter  48  with low transmittance is defined as a second color filter  48 . 
   Please refer to  FIG. 4  that shows the spectrogram of the first color filter  44  and the second color filter  48  in the first preferred embodiment. The first color filter  44  has a first transmittance  50  corresponding to a first spectrum relation  51  about the transmittance and the light wavelength. In the first spectrum relation  51 , 50% of the first transmittance  50  is corresponding to a first light wavelength  52  that is about 480 nm and a second light wavelength  54  that is about 580 nm. The second color filter  48  has a second transmittance  56  corresponding to a second spectrum relation  57 . In the second spectrum relation  57 , 50% of the second transmittance  56  is corresponding to a third light wavelength  58  and a fourth light wavelength  60 , and the fourth light wavelength  60  is larger than the third light wavelength  58 . The hue of the second color filter  48  is similar to that of the first color filter  44 . The second transmittance  56  is smaller than the first transmittance  50 . The second color filter  48  is conformed to the first condition chosen from that the third light wavelength  58  is larger than the first light wavelength  52  while the second light wavelength  54  is larger than the fourth light wavelength  60 . 
   Please refer to  FIG. 5  that shows the green gray scale of the color filter module  40  in the first preferred embodiment. In  FIG. 5 , a green gray scale section  66  with high brightness is presented by the first color filter  44 ; in other words, the proportion of the first color filter  44  is larger than that of the second color filter  48 . A green gray scale section  68  with middle brightness is presented by the first color filter  44  and the second color filter  48 ; in other words, the proportion of the first color filter  44  is similar to that of the second color filter  48 . A green gray scale section  62  with low brightness is mainly presented by the second color filter  48 ; in other words, the proportion of the first color filter  44  is smaller than that of the second color filter  48 . 
   In the first preferred embodiment, the band-pass of the first spectrum relation  51  is larger than that of the second spectrum relation  57 , which means that the hue of the second color filter  48  is purer than that of the first color filter  44 . Thus, as the first color filter  44  and the second color filter  48  are used for generating the green gray scale section  62  with low brightness, the green gray scale section  62  with low brightness is smoother because of the larger proportion of the second color filter  48 . 
   Please refer to  FIG. 6  that shows the color filter module  70  in the second preferred embodiment. The color filter module  70  includes two red color filters  72  with high transmittance, two green color filters with high transmittance, two blue color filters  74  with high transmittance and a green color filter  76  with low transmittance. The two green color filters with high transmittance are defined as a first color filter  78  and a third color filter  80 . The green color filter  76  with low transmittance is defined as a second color filter  76 . Each of the two red color filters  72  with high transmittance, each of the two green color filters with high transmittance, each of the two blue color filters  74  with high transmittance and the green color filter  76  with low transmittance are sector plates with a same radius, and the central angles thereof are 54°, 52°, 59° and 30° respectively. 
   Please refer to  FIG. 7  that shows the spectrogram of the first color filter  78  and the third color filter  80  in the second preferred embodiment. The first color filter  78  in the second preferred embodiment is the same as the first color filter  44  in the first preferred embodiment except the central angle of the sector plate. The third color filter  80  has a third transmittance  82  corresponding to a third spectrum relation  83  about the transmittance and the light wavelength. In the third spectrum relation  83 , 50% of the third transmittance  82  is corresponding to a fifth light wavelength  84  and a sixth light wavelength  86  that is larger than the fifth light wavelength  84 . The hue of the third color filter  80  is similar to that of the first color filter  78 , while a second condition and a third condition must be conformed, wherein the second condition is that the absolute value of the difference between the fifth light wavelength  84  and the first light wavelength  52  is smaller or equal to 4 nm while the absolute value of the difference between the sixth light wavelength  86  and the second light wavelength  54  is smaller or equal to 4 nm, and the third condition is that the absolute value of the difference between the third transmittance  82  and the first transmittance  50  is smaller or equal to 4%. 
   As experiment shows, if the first color filter  78  and the third color filter  80  are not conformed to the second condition and the third condition, dither noise and the non-smooth gray scale will occur when the first color filter  78  and the third color filter  80  are used for generating the green gray scale. 
   Please refer to  FIG. 4  again. The second color filter  76  in the second preferred embodiment is the same as the second color filter  48  in the first preferred embodiment. Thus, the second color filter  76  in the second preferred embodiment must be conformed to the first condition in the first preferred embodiment too. 
   In another preferred embodiment (not shown in the figure), if the dither noise, variation in color and non-smooth gray scale of the red gray scale are to be solved, one can use the principle of the first or the second preferred embodiment without the third color filter  80  and only needs to define the red color filter  42  with high transmittance as the first color filter  44  and substitute the green color filter  48  with low transmittance with a red color filter with low transmittance defined as the second color filter  48  and conformed to the first condition. Further, if a third color filter  80  is included, the two red color filters  72  are defined as the first color filter  78  and the third color filter  80  while the green color filter  76  with low transmittance is substituted by a red color filter with low transmittance defined as the second color filter  76  and conformed to the first, the second and the third conditions. 
   Using the same principal in still another preferred embodiment (not shown in the figures), if the dither noise, variation in color and non-smooth gray scale of the blue gray scale are to be solved without the third color filter  80 , one only needs to define the blue color filter  46  with high transmittance as the first color filter  44  and substitute the green color filter  48  with low transmittance with a blue color filter with low transmittance defined as the second color filter  48  and conformed to the first condition. Further, if a third color filter  80  is included, the two blue color filters  74  are defined as the first color filter  78  and the third color filter  80  while the green color filter  76  with low transmittance is substituted by a blue color filter with low transmittance defined as the second color filter  76  and conformed to the first, the second and the third conditions. 
   In the present invention, the first transmittance  50  of the first color filters  44  and  78  is larger than the second transmittance  56  of the second color filters  48  and  76 . The first color filters  44  and  78  and the second color filters  48  and  76  are conformed to the first condition that the third light wavelength  58  is larger than the first light wavelength while the second light wavelength  54  is larger than the fourth light wavelength  60 . The first color filters  44 ,  78  and the third color filter  80  are conformed to the second condition and the third condition, wherein the second condition is that the absolute value of the difference between the fifth light wavelength  84  and the first light wavelength  52  is smaller or equal to 4 nm while the absolute value of the difference between the sixth light wavelength  86  and the second light wavelength  54  is smaller or equal to 4 nm, and the third condition is that the absolute value of the difference between the third transmittance  82  and the first transmittance  50  is smaller or equal to 4%. The color filter modules  40  and  70  in the present invention can solve the problems of dither noise, non-smoothness and variation of gray scale. In practice, the cost is low and the production is easy. 
   Although the invention has been described in detail herein with reference to its preferred embodiments, it is to be understood that this description is by way of example only, and is not to be interpreted in a limiting sense. It is to be further understood that numerous changes in the details of the embodiments of the invention, and additional embodiments of the invention, will be apparent, and may be made by, persons of ordinary skill in the art having reference to this description. It is considered that such changes and additional embodiments are within the spirit and true scope of the invention as claimed below.