Abstract:
A light-transmitting element ( 10 ) includes a substrate ( 12 ) made of polymethyl methacrylate, and at least one coating film ( 14 ). The substrate has a first surface ( 122 ), and a second surface ( 124 ) opposite to the first surface. The coating film is deposited on at least one of the surfaces of the substrate by electron beam evaporation. The coating film is selected from the group consisting of a single layer and a plurality of layers, and comprises a material selected from the group consisting of tantalum pentoxide, magnesium fluoride, silicon oxide, and any mixture or combination thereof. The light-transmitting element provides improved light transmittance for an imaging system. A method for making the light-transmitting element is also provided.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to passive light-transmitting elements and methods for making the same, and particularly to a light-transmitting element for an imaging system and a method for making the light-transmitting element.  
         [0003]     2. Related Art  
         [0004]     With the ongoing development of optical technology, light-transmitting elements are now in widespread use in a variety of applications. Polymethyl methacrylate (PMMA) is a transparent thermoplastic resin which has a visible light transmittance higher than that of glass, excellent optical properties, and low birefringence. Therefore PMMA has long been used as a material for a wide variety of optical products such as optical lenses and optical discs.  
         [0005]     In recent years, there has been an increasing demand for PMMA to be used as a light-transmitting element for the plastic lens of imaging systems. The light-transmitting element for the lens functions to propagate and diffuse light that enters from a certain direction, such that the light exits in the direction of imaging.  
         [0006]     In a typical imaging system, the light-transmitting element is a light-transmitting plate. If the distance traveled by light through the light-transmitting plate is relatively long, the amount of light lost in the light-transmitting plate is correspondingly high. For preventing or minimizing loss of light, the material of the light-transmitting plate is required to have a high light transmittance. Thus PMMA has been routinely employed for use in light-transmitting plates.  
         [0007]     However, a light-transmitting element made of PMMA still has relatively high light reflection at interfaces thereof. This reduces the overall light transmittance of the light-transmitting element. Even when a light-transmitting element is configured to be optically optimized, the light transmittance is generally only in a range up to 92 percent. That is, at least 8 percent of light is reflected. Thus the resolution of the image obtained in the imaging system is decreased, and the quality of the obtained image may not be satisfactory.  
         [0008]     Therefore, a light-transmitting element and a method for making the light-transmitting element which overcome the above-described problems are desired.  
       SUMMARY OF THE INVENTION  
       [0009]     An object of the present invention is to provide a light-transmitting element for an imaging system which has a high light transmittance.  
         [0010]     Another object of the present invention is to provide a method for making a light-transmitting element for an imaging system which has a high light transmittance.  
         [0011]     To achieve the first of the above objects, a light-transmitting element for imaging system includes a substrate made of polymethyl methacrylate, and at least one coating film. The substrate has a first surface, and a second surface opposite to the first surface. The coating is formed on at least one surface of the substrate. The coating film is selected from the group consisting of a single layer and a plurality of layers, and comprises a material selected from the group consisting of tantalum pentoxide, magnesium fluoride, silicon oxide, and any mixture or combination thereof.  
         [0012]     To achieve the second of the above objects, a method for forming a light-transmitting element comprises the steps of: providing a substrate made of polymethyl methacrylate, the substrate having a first surface and a second surface opposite to the first surface; and depositing at least one coating film on at least one surface of the substrate. The coating film is selected from the group consisting of a single layer and a plurality of layers, and comprises a material selected from the group consisting of tantalum pentoxide, magnesium fluoride, silicon oxide, and any mixture or combination thereof.  
         [0013]     A main advantage of the invention is that the light transmittance of the light-transmitting element is improved. Accordingly, the quality of images obtained by the imaging system is enhanced.  
         [0014]     Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a schematic, side cross-sectional view of part of a light-transmitting element in accordance with a first preferred embodiment of the present invention;  
         [0016]      FIG. 2  is a schematic, side cross-sectional view of a light-transmitting element in accordance with a second preferred embodiment of the present invention; and  
         [0017]      FIG. 3 a  schematic, side cross-sectional view of a light-transmitting element in accordance with a third preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]      FIG. 1  shows a light-transmitting element  10  according to the first preferred embodiment of the present invention. The light-transmitting element  10  is used in an imaging system, and may for example function as a plastic lens. The light-transmitting element  10  comprises a substrate  12  and a coating film  14 . The substrate  12  has a first surface  122 , and a second surface  124  opposite to the first surface  122 . The coating film  14  is deposited on the first surface  122  of the substrate  12 .  
         [0019]     The substrate  12  is made of polymethyl methacrylate (PMMA) and has a thickness of 0.85 mm. The coating film  14  is made of silicon oxide (SiO 2 ), and has a thickness of 67.22 nm.  
         [0020]     A method for making the light-transmitting element  10  comprises the steps of: providing a substrate  12  made of PMMA having a first surface  122  and a second surface  124  opposite to the first surface  122 ; and depositing a coating film  14  made of SiO 2  on the first surface  122  of the substrate  12  by electron beam evaporation.  
         [0021]     The coating film  14  can also be deposited on the substrate  12  in any conventional manner, such as by way of (but not limited to) magnetron sputter vapor deposition (MSVD), chemical vapor deposition (CVD), spray pyrolysis (i.e., pyrolytic deposition), atmospheric pressure CVD (APCVD), low-pressure CVD (LPCVD), plasma-enhanced CVD (PEVCD), plasma assisted CVD (PACVD), thermal or electron-beam evaporation, cathodic arc deposition, plasma spray deposition, and wet chemical deposition (e.g., sol-gel, mirror silvering etc.). It is noted that sputter deposited coatings are perceived by some to be less mechanically durable than coatings deposited by spray pyrolysis or CVD-type coating methods. Examples of suitable CVD coating apparatuses and methods are found, for example (but not limiting the present invention to), in U.S. Pat. Nos. 3,652,246, 4,351,861, 4,719,126, 4,853,257, 5,356,718 and 5,776,236.  
         [0022]     When external light enters the coating film  14  of the light-transmitting element  10 , travels through the substrate  12 , and exits from the second surface  124 , the light transmittance of the light-transmitting element  10  is increased. The average light transmittance of the light-transmitting element  10  at light wavelengths of 800 nm, 750 nm, and 350 nm can be seen from the following table 1:  
                           TABLE 1                                   Light wavelength (nm)   Average light transmittance %                           800   93.05           750   93.08           550   93.18           350   92.94                      
 
         [0023]      FIG. 2  shows a light-transmitting element  20  according to the second preferred embodiment of the present invention. The light-transmitting element  20  comprises a substrate  12  made of PMMA, a coating film  22  deposited on a first surface  122  of the substrate  22 , and a coating film  24  deposited on a second surface  124  of the substrate  12 . The substrate  12  has a thickness of 0.85 mm. The coating films  22 ,  24  are made of SiO 2 , and each has a thickness of 59.44 nm. Deposition of the coating films  22 ,  24  can be performed in the same manner as described above in relation to the coating film  14  of the first embodiment.  
         [0024]     When external light enters the coating film  22  of the light-transmitting element  20 , travels through the substrate  12 , and exits from the second surface  124  in the direction of the coating film  24 , the light transmittance of the light-transmitting element  20  is increased. The average light transmittance of the light-transmitting element  20  at light wavelengths of 800 nm, 750 nm, 550 nm and 350 nm can be seen from the following table 2:  
                           TABLE 2                                   Light wavelength (nm)   Average light transmittance %                           800   93.37           750   93.43           550   93.65           350   93.38                      
 
         [0025]     In alternative embodiments, a material with a special refractive index and/or a thickness of the coating film  22  and/or the coating film  24  can be varied according to particular requirements. The average light transmittance of various different embodiments of the light-transmitting element  10  at light wavelengths of 800 nm, 750 nm, 550 nm and 350 nm can be seen from the following tables 3 through 6:  
                                         TABLE 3                           Film material/thickness (nm)   Light wavelength   Average light            Film 22   Film 24   (nm)   transmittance %               MgF 2 /88.33   MgF 2 /88.29   800   95.52       MgF 2 /88.33   MgF 2 /88.29   750   95.79       MgF 2 /88.33   MgF 2 /88.29   550   96.91       MgF 2 /88.33   MgF 2 /88.29   350   95.50                  
 
         [0026]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                   
               
               
                 Film material/thickness (nm) 
                 Light wavelength 
                 Average light 
               
             
          
           
               
                 Film 22 
                 Film 24 
                 (nm) 
                 transmittance % 
               
               
                   
               
               
                 MgF 2 /62.67 
                 MgF 2 /67.52 
                 800 
                 94.39 
               
               
                 MgF 2 /62.67 
                 MgF 2 /67.52 
                 750 
                 94.60 
               
               
                 MgF 2 /62.67 
                 MgF 2 /67.52 
                 550 
                 95.80 
               
               
                 MgF 2 /62.67 
                 MgF 2 /67.52 
                 350 
                 97.04 
               
               
                   
               
             
          
         
       
     
         [0027]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                   
               
               
                 Film material/thickness (nm) 
                 Light wavelength 
                 Average light 
               
             
          
           
               
                 Film 22 
                 Film 24 
                 (nm) 
                 transmittance % 
               
               
                   
               
               
                 SiO 2 /63.65 
                 MgF 2 /67.52 
                 800 
                 93.99 
               
               
                 SiO 2 /63.65 
                 MgF 2 /67.52 
                 750 
                 94.13 
               
               
                 SiO 2 /63.65 
                 MgF 2 /67.52 
                 550 
                 94.84 
               
               
                 SiO 2 /63.65 
                 MgF 2 /67.52 
                 350 
                 95.15 
               
               
                   
               
             
          
         
       
     
         [0028]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                   
               
               
                 Film material/thickness (nm) 
                 Light wavelength 
                 Average light 
               
             
          
           
               
                 Film 22 
                 Film 24 
                 (nm) 
                 transmittance % 
               
               
                   
               
               
                 SiO 2 /59.40 
                 MgF 2 /67.52 
                 800 
                 93.94 
               
               
                 SiO 2 /59.40 
                 MgF 2 /67.52 
                 750 
                 94.08 
               
               
                 SiO 2 /59.40 
                 MgF 2 /67.52 
                 550 
                 94.79 
               
               
                 SiO 2 /59.40 
                 MgF 2 /67.52 
                 350 
                 95.16 
               
               
                   
               
             
          
         
       
     
         [0029]      FIG. 3  shows a light-transmitting element  30  according to the third preferred embodiment of the present invention. The light-transmitting element  30  comprises a substrate  12  made of PMMA, a first hybrid coating film  32  deposited on a first surface  122  of the substrate  12 , and a second hybrid coating film  34  deposited on a second surface  124  of the substrate  12 . The substrate  12  has a thickness of 0.85 mm. The first hybrid coating film  32  comprises a first outer layer  322  made of tantalum pentoxide (Ta 2 O 5 ), and a first inner layer  324  made of magnesium fluoride (MgF 2 ). The first outer layer  322  has a thickness of 4.16 nm. The first inner layer  324  has a thickness of 94.60 nm. The second hybrid coating film  34  comprises a second inner layer  342  made of SiO 2 , and a second outer layer  344  made of MgF 2 . The second inner layer  342  has a thickness of 83.83 nm. The second outer layer  344  has a thickness of 77.36 nm.  
         [0030]     A method for making the light-transmitting element  30  comprises the steps of: providing the substrate  12  made of PMMA having the first surface  122  and the second surface  124  opposite to the first surface  122 ; depositing the first inner layer  324  on the first surface  122  of the substrate  12 ; depositing the first outer layer  322  on the first inner layer  324  of the substrate  12  by electron beam evaporation; depositing the second inner layer  342  on the second surface  124  of the substrate  12  by electron beam evaporation; and depositing the second outer layer  344  on the second inner layer  342  of the substrate  12  by electron beam evaporation.  
         [0031]     When external light enters the first hybrid coating film  32  of the light-transmitting element  30 , travels through the substrate  12 , and exits from the second surface  124  in the direction of the second hybrid coating film  34 , the light transmittance of the light-transmitting element  30  is increased. The average light transmittance of the light-transmitting element  30  at light wavelengths of 800 nm, 750 nm, 550 nm and 350 nm can be seen from the following table 7:  
                           TABLE 7                                   Light wavelength (nm)   Average light transmittance %                           800   95.32           750   95.46           550   96.44           350   97.22                      
 
         [0032]     In alternative embodiments, a material and/or a thickness of the first hybrid coating film  32  and/or the second hybrid coating film  34  can be varied according to particular requirements. For instance, the first outer layer  322  is made of SiO 2 , and has a thickness of 8.52 nm. The first inner layer  324  is made of MgF 2 , and has a thickness of 69.56 nm. The second inner layer  342  is made of SiO 2 , and has a thickness of 8.55 nm. The second outer layer  344  is made of MgF 2 , and has a thickness of 69.19 nm. The average light transmittance of the above-described alternative embodiment of the light-transmitting element  30  at light wavelengths of 800 nm, 750 nm, 550 nm and 350 nm can be seen from the following table 8:  
                           TABLE 8                                   Light wavelength (nm)   Average light transmittance %                           800   94.79           750   95.02           550   96.23           350   96.88                      
 
         [0033]     In a further alternative embodiment, the first outer layer  322  is made of Ta 2 O 5 , and has a thickness of 5.59 nm. The first inner layer  324  is made of MgF 2 , and has a thickness of 90.46 nm. The second inner layer  342  is made of SiO 2 , and has a thickness of 57.69 nm. The second outer layer  344  is made of MgF 2 , and has a thickness of 91.36 nm. The average light transmittance of the above-described further alternative embodiment of the light-transmitting element  30  at light wavelengths of 800 nm, 750 nm, 550 nm and 350 nm can be seen from the following table 9:  
                           TABLE 9                                   Light wavelength (nm)   Average light transmittance %                           800   95.06           750   95.23           550   96.21           350   97.12                      
 
         [0034]     In a still further alternative embodiment, the first outer layer  322  is made of SiO 2 , and has a thickness of 53.08 nm. The first inner layer  324  is made of Ta 2 O 5 , and has a thickness of 4.14 nm. The second inner layer  342  is made of SiO 2 , and has a thickness of 37.73 nm. The second outer layer  344  is made of MgF 2 , and has a thickness of 72.31 nm. The average light transmittance of the above-described still further alternative embodiment of the light-transmitting element  30  at light wavelengths of 800 nm, 750 nm, 550 nm and 350 nm can be seen from the following table 10:  
                           TABLE 10                                   Light wavelength (nm)   Average light transmittance %                           800   95.34           750   95.50           550   96.29           350   97.30                      
 
         [0035]     In a yet further alternative embodiment, the first outer layer  322  is made of SiO 2 , and has a thickness of 51.00 nm. The first inner layer  324  is made of Ta 2 O 5 , and has a thickness of 3.20 nm. The second inner layer  342  is made of Ta 2 O 5 , and has a thickness of 3.21 nm. The second outer layer  344  is made of MgF 2 , and has a thickness of 97.14 nm. In addition, the first hybrid coating film  32  further includes an innermost layer, which is made of MgF 2  and has a thickness of 56.19 nm. The second hybrid coating film  34  further includes an innermost layer, which is made of SiO 2  and has a thickness of 50.95 nm. The average light transmittance of the above-described yet further alternative embodiment of the light-transmitting element  30  at light wavelengths of 800 nm, 750 nm, 550 nm and 350 nm can be seen from the following  
                           TABLE 11                                   Light wavelength (nm)   Average light transmittance %                           800   95.52           750   95.63           550   96.27           350   97.53                      
 
         [0036]     It is can be seen that a material and/or a thickness of the substrate  12  can be varied according to a particular requirements. Also, a thickness of the coating films  22 ,  24 ,  32 ,  34  can be varied according to particular requirements.  
         [0037]     It is believed that the present invention and its advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.