Patent Publication Number: US-2009219501-A1

Title: Optical unit using optical attenuator  and printing apparatus provided therewith

Description:
BACK GROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electro photographic printing apparatus, and in particular, to an electro photographic printing apparatus that uses diffusive light sources. 
     2. Description of the Related Art 
     There are several types of electronic printing devices, such as, for example, wire dot printers, electro photographic printers, and inkjet printers. Currently, electro photography and inkjet are two leading electronic printing systems for use in office, home, small office-home office (SOHO) or industrial environments. Electro photographic printing devices are of the relatively faster printing speed type and are capable of massive print jobs, while inkjet printers are generally used for relatively slower and smaller print jobs while providing a high print quality. 
     Electro photography is a method of printing electronic information using a series of basic steps: exposure, development, and image transfer, just like photography. A laser printer is a typical commercial machine making use of electro photography. 
     Recently, Organic Light Emitting Diode (OLED) light sources have been employed for applications in next generation printing systems, because they have a small footprint and the cost of fabricating them is low. Therefore, by using OLED light sources, it is possible to manufacture compact electro photographic printers at low cost. 
     However, there are some key requirements for using OLED light sources. OLED light sources require a high optical coupling efficiency so that an OLED element can operate at a low current, which extends the life of the OLED element. Second, a high modulation is necessary (e.g., Modulation Transfer Function (MTF) should be close to 100%) because modulation determines the resolution of the printed image. Modulation can be assessed by MTF, which is defined as a measure of how images on an OPC (Organic Photo Conductive) drum from two light sources set apart at a certain distance on a light source array are distinguishable (see  FIG. 13 ) and is expressed in a formula as: 
         MTF =( I max− I min)/( I max+ I min) 
     where Imax and Imin are a maximum intensity and a minimum intensity, respectively, and P 1  and P 2  are positions of two separate light sources with a separation |P 2 -P 1 |. 
     OLED light sources have a very large divergence angle (i.e., it can be described as a Lambertian light source), making it difficult to achieve a high coupling efficiency and good modulation. 
     It has been attempted to address this issue in prior devices by improving the extraction efficiency of light power from EL sources, where individual micro ball lenses are placed mostly contacting with each of the EL sources to extract as much light as possible and refract it towards an image plane (i.e., an OPC drum surface in the case of electro photography).  FIG. 14  depicts an arrangement employing micro ball lenses. In this arrangement, the ball lens captures more light from EL light sources, but it is difficult to get a good image of ELs on a target (OPC drum). This leads to a problem of modulation. In addition, another problem is that the short focus is not able to keep the required working distance between the lens and OPC drum. 
       FIGS. 15A and 15B  illustrate an alternative approach utilizing a GRIN lens used for the coupling device for a good image, good modulation and enough working distance between the lens and the OPC drum. A GRIN (Gradient Index) lens focuses light through a precisely controlled radial variation of the lens material&#39;s index of refraction from the optional axis to the edge of the lens. However, one critical issue of using a GRIN lens is that its coupling efficiency is less than 7%, since it is very difficult to make SELFOCUS lenses with a high Numerical Aperture (NA; small cone angle). For a low efficiency lens system, the OLED light sources have to be operated at a high current to get enough emitted energy (intensity). This reduces the lifetime of the OLED light sources. Therefore, people are looking for an alternative optical device for use in an electro photographic printing apparatus that employs an OLED light source. 
       FIG. 16  illustrates an array of diffusive light sources, a first lens disposed to receive light emitted from two or more light sources of the array, an aperture plate disposed to receive light from the first lens, and a second lens disposed to receive the light after passing through the plurality of apertures and to focus the light onto an image plane, such as a surface of an OPC drum. This construction provides a high coupling efficiency, by using a high NA lens, and a high MTF that is achieved by employing a pin-hole-array aperture in the aperture plate. However, this construction still exhibits limitations, in particular with regards to uniformity, as follows:
     1. At the target plane, such as a surface of the OPC drum, the intensity of light from the OLED light sources is not uniform, depending on each pixel of the image, because the first lens collects more light emitted from the OLEDs located close to the center of the lens;   2. Although adjusting individual circular aperture sizes may compensate the intensity variation, varying circular aperture size results in a change of image size on a target, which is not desired;   3. There is very little tolerance in aperture alignment error, so alignment becomes a big challenge; and   4. The plurality of apertures constrain the beam in both the horizontal and vertical directions, which results in an unnecessary energy loss, considering the fact that the MFT is only a concern in the horizontal direction.   
     Thus, further improvement of the optical device for a printing apparatus using an OLED light source is required. 
     SUMMARY OF THE INVENTION 
     It is thus an object of this invention to overcome the above-mentioned problems of a conventional printing apparatus that uses diffusive light sources and, more particularly, to provide a printing apparatus that makes efficient use of diffusive light sources while maintaining a uniform light intensity and energy distribution and maintaining a high MTF. 
     According to an aspect of the present invention, an optical unit for a printing apparatus includes a plurality of light emitting elements, a lens that collects light from the plurality of light emitting elements, and a light filter provided in a light path of the light from the plurality of light emitting elements to compensate for an intensity of the light passed through the lens. 
     According to another object of the present invention, an optical unit for a printing apparatus comprises a first light emitting element that emits a first light, a second light emitting element that emits a second light, a lens provided in light paths of the first light and the second light, the first light and the second light passing through the lens, and a light filter that compensates an intensity of the first and second light which passes through the lens. 
     According to a feature of the invention, the light filter has a first portion to transmit the first light from the lens and a second portion to transmit the second light from the lens. A light transparency of the first portion is different from a light transparency of the second portion. 
     Further, the light filter comprises a transparent substrate and a light absorbing layer provided on the transparent substrate, while a second lens transmits the first and second light transmitted from the light filter. The first and second light emitting elements may comprise an organic light emitting diode. 
     Accordingly to another object of the invention, a printing apparatus comprises a photosensitive member, a charger that charges the photosensitive member, an optical unit that irradiates the photosensitive member with light to form an electrostatic latent image, a developer that adheres toner to the photosensitive member to form a toner image of the electrostatic latent image, and a transferor that transfers the toner image onto a recording medium. The optical unit of the printing apparatus comprises a plurality of light emitting elements, a first lens that collects light from the plurality of light emitting elements, a second lens that transmits the light from the first lens to the photosensitive member, and a light filter provided between the first lens and the second. The light filter is positioned in light paths of the light from the plurality of light emitting elements and compensates an intensity of the light which passed through the lens. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, described in brief below. 
         FIG. 1  shows an exemplary configuration of a printer for a first embodiment of the present invention. 
         FIG. 2  illustrates an exemplary optical unit in the printer of  FIG. 1 . 
         FIG. 3  illustrates a subsystem of the optical unit in  FIG. 2 . 
         FIGS. 4A and 4B  show functions of a light attenuator of a first embodiment. 
         FIGS. 5A ,  5 B and  6  illustrate a first example of the light attenuator of the first embodiment. 
         FIG. 7  illustrates a second example of the light attenuator of the first embodiment. 
         FIGS. 8A and 8B  illustrate a third example of the light attenuator of the first embodiment. 
         FIGS. 9A ,  9 B and  10  illustrate a fourth example of the light attenuator of the first embodiment. 
         FIGS. 11A and 11B  illustrate a fifth example of the light attenuator of the first embodiment. 
         FIGS. 12A ,  12 B and  12 C show a non-sequential ray-tracing simulation result using a commercial optical design software called ZEMAX™. 
         FIG. 13  shows the intensity from two adjacent light sources. 
         FIGS. 14 ,  15 A,  15 B and  16  illustrate conventional optical units. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     According to a first embodiment of the present invention, a printer  1  comprises an optical unit  2 , an organic photo conductive drum (OPC drum)  3 , an electric charger  4 , a developer  5 , a transcribing roller  6  and a transferring roller  7 , as shown in  FIG. 1 . 
     Optical unit  2  comprises Organic Light Emitting Diode (OLED) light sources to illuminate light onto the OPC drum  3  in order to form an electrostatic latent image according to original image data to be printed on a recording medium  8 . The OPC drum  3  is electrically charged by the electric charger  4  located at an up-rotation position before the light from the optical unit  2  is illuminated onto the surface of the OPC drum  3 . Upon illumination of the light on the surface of the OPC drum  3 , the illuminated portion changes to be neutralized due to a mechanism of organic photo conduction, wherein an electric current is created by a photo-conducting effect and an electrostatic latent image according to the original image is formed on the OPC drum  3 . Developer  5  adheres toners in a toner tank  5 - 1  to the surface of the OPC drum  3  by developing roller  5 - 2 . Then, a toner image is formed on the surface of the OPC drum  3  according to the original image data. The transcribing roller  6  nips a recording medium  8  with the OPC drum  3  and transcribes the toner image onto the recording medium  8 . The transferring roller  7  transfers the recording medium  8 , such as a paper, in a direction described by arrow A in  FIG. 1 . 
     In this first embodiment, printer  1  comprises a monochromatic printer having a single printing engine including optical unit  2 , OPC drum  3 , developer  5  and so on, but printer  1  can comprise a full color printer having several optical units for yellow, magenta, cyan and black. An example of a full color printer is disclosed in U.S. Pat. No. 7,116,345, which was assigned to Matsushita Electric Industrial CO., Ltd., and such U.S. Pat. No. 7,116,345 is hereby incorporated by reference in its entirety as though fully and completely set forth herein. 
     Further, while the present invention is described with reference to plural OLED light sources, it is understood that the invention is equally applicable to a single OLED light source. 
       FIG. 2  illustrates an exemplary optical unit of the printer  1  of  FIG. 1  and  FIG. 3  illustrates a subsystem of the optical unit in  FIG. 2 . 
     Optical unit  2  includes OLED array  11  as the light source, first lens array  12 , second lens array  13  and light attenuator unit  14 . OLED array  11  comprises a plane substrate  11 - 1 , on which, in the disclosed embodiment, about 10,000 pieces of organic electroluminescence (EL) elements (S 1 , S 2 , S 3 , S 4 , S 5 , . . . ) are aligned in one line, which is parallel with an axis of rotation of the OPC drum  3 . However, it is understood that the actual number of EL elements is not critical to the present invention, and may be varied without departing from the spirit and/or scope of the invention. In this first embodiment, the EL elements are grouped by five adjacent elements, such as EL elements S 1 , S 2 , S 3 , S 4  and S 5  and each grouped elements are mounted on the plain substrate  11 - 1  having a predetermined distance, such as the distance between EL element S 5  and S 6 . In the case that a resolution of the printer  1  is 600 dpi, a distance between adjacent EL elements, such as EL element S 1  and S 2 , is preferably 42.3 μm, but it is not limited to this size. In this embodiment, EL elements are used for light sources, but a laser diode (LD) could be used without departing from the scope and/or spirit of the instant invention. 
     First lens array  12  comprises a plurality of first lens FL 1 , FL 2 , FL 3 , . . . , each of which covers each group of EL elements. For example, first lens FL 1  covers the group of EL elements S 1 , S 2 , S 3 , S 4  and S 5 . Second lens array  13  is located between the first lens array  12  and the OPC drum  3 , in parallel with the first lens array  12 , and comprises second lens SL 1 , SL 2 , SL 3 , . . . , each of which corresponds to the first lens FL 1 , FL 2 , FL 3 , . . . , respectively. One example of the first lens and second lens is an even-aspheric lens with a diameter of approximately 1˜2 mm, which is made of transparent glass or plastic at a visible wavelength. Light attenuator unit  14  is located between first lens array  12  and second lens array  13 . Light attenuator unit  14  includes a plurality of light attenuators OA 1 , OA 2 , OA 3  . . . , each of which corresponds to the first lens FL 1 , FL 2 , FL 3 , . . . , respectively. In this embodiment, each of the first lens array  12 , the second lens array  14  and the light attenuator  13  is formed as a single unit, respectively, but each element (the first lens FL 1 , FL 2 , FL 3 , . . . , the second lens SL 1 , SL 2 , SL 3 , . . . , and attenuator LA 1 , LA 2 , LA 3 , . . . ) can be provided individually. 
       FIG. 3  shows the system magnification and optical configuration for a subsystem of the optical unit in  FIG. 2 . In  FIG. 3 , first lens FL 1  collects light from EL elements S 1 , S 2 , S 3 , S 4  and S 5 , and is preferably placed very close to EL elements S 1 , S 2 , S 3 , S 4  and S 5 , for instance, with approximately 50 microns separation. In this way, the viewing angle from EL elements S 1 , S 2 , S 3 , S 4  and S 5  against the first lens FL 1  increases so that the first lens FL 1  collects more light. Second lens SL 1  is placed after the first lens FL 1  to deliver light to the OPC drum  3  that is positioned a certain distance away from the second lens SL 1 . Second lens SL 1  delivers the light to the OPC drum  3  with a desired distance. It should be noted that the first lens FL 1  could give an inverted image of the EL elements S 1 , S 2 , S 3 , S 4  and S 5  on light attenuator OA 1  and the second lens SL 1 . Then, the second lens SL 1  could invert the image from the light attenuator OA 1 . Accordingly, each pixel from the EL elements S 1 , S 2 , S 3 , S 4  and S 5  could be delivered onto image I 1 , I 2 , I 3 , I 4  and I 5  on the OPC drum  3 , respectively. 
     As shown in  FIG. 3 , the image spacing on the OPC drum  3  can be different from that of the EL elements S 1 , S 2 , S 3 , S 4  and S 5 . This follows from the result that the magnification of this lens system is not equal to one. In the case of an electro photographic printer, there is typically a need to deliver a lot of EL light and also a need to deliver the light far away. As pointed out above, these objectives tend to be at cross purposes to one another. To achieve these competing objectives, the first lens FL 1  is positioned close to the EL elements S 1 , S 2 , S 3 , S 4  and S 5 ; in other words, the first lens FL 1  is placed so that the viewing angle, more technically, numerical aperture (NA), becomes large. On the other hand, in order to deliver the light far away, the second lens SL 1  is provided with a larger focal length, i.e., it has a small NA. The array of first lenses, second lenses and light attenuators may be a linear array or a multi-dimensional array. 
     Light attenuator OA 1  is positioned between the first lens FL 1  and the second lens SL 1 , preferably proximate a focal point of the light emitted from the first lens FL 1 . Attenuator OA 1  functions as a ND (Neutral Density) filter to compensate for a variable intensity and energy distribution of the light emitted from the first lens FL 1 , to be described later. 
     The function of the light attenuator OA 1  is described with reference to  FIG. 4A . Light attenuator OA 1  has a light-absorbing portion, the light transparency of which changes according to a distance from the center of light attenuator OA 1 , as illustrated in  FIG. 4B . For example, the light transparency at the center of light attenuator OA 1  is lower than the light transparency at a peripheral portion thereof. Attenuation of light L 3 , which is emitted from EL element S 3  and passed through the center part of the first lens FL 1  and light attenuator OA 1 , is large compared to attenuation of light L 1  which is emitted from EL element S 1  and passed through the peripheral portion of the light attenuator OA 1 . In other words, light is emitted from EL elements S 1 , S 2 , S 3 , S 4  and S 5  almost uniformly (see Profile I in  FIG. 4A ), and its intensity and energy distribution becomes non-uniform (see Profile II in  FIG. 4A ), and then, it is compensated by the light attenuator OA 1  to become uniform (see profile III in  FIG. 4A ). Therefore, the intensity and energy distribution is compensated by the light attenuator OA 1 , and the light, having a substantially uniform intensity and energy distribution, is delivered to the OPC drum  3 . 
     It is well-known that light transmittance (1-absorbance (A)) through a material is determined by material absorption co-efficiency (α) and material thickness (L) according to Lamba-Beer law: A=exp(α·L). 
     Thus, for a design of this type of light attenuator, varying the absorption efficiency or varying the thickness changes the optical transmission. In principle, a change of absorption efficiency can be achieved by darkening materials (polymers, glasses, etc.) via dying, ion implantation, ion doping and light irradiation methods or bleaching materials via laser irradiation. A change of thickness can be achieved using convenient photo-lithography technology. 
       FIG. 5A  is a cross-sectional view of a first example of the light attenuator OA 1 , based on changing a thickness of an absorbing layer, but maintaining a same absorption co-efficiency.  FIG. 5B  is a perspective view of the light alternator OA 1  shown in  FIG. 5A . In the disclosed embodiment, the light attenuator OA 1  comprises a transparent glass substrate  21 , which is preferably 0.7 mm in thickness, and a light absorbing layer  22 , which is preferably 1-5 um in thickness, provided on the glass substrate  21 . In this first embodiment, BK7, produced by Corning Co., which is based on SiO 2 , is used for the glass substrate  21 . The glass substrate  21  is stable and insensitive to heat. Instead of BK7, Acrylic Plastic can be used to replace the glass substrate  21 , which reduces the cost of manufacture. 
     Light absorbing layer  22  comprises a light-absorbing material, such as, but not limited to, doped glasses, and polymers, and has areas  22 - 1 ,  22 - 2 ,  22 - 3 ,  22 - 4  and  22 - 5 , each of which has a different thickness relative to each other. For example, area  22 - 3 , located at a center portion of the light is thicker than areas  22 - 2  and  22 - 4  located adjacent to area  22 - 3 , and areas  22 - 2  and  22 - 4  are thicker than areas  22 - 1  and  22 - 5  located at a peripheral portion of the light attenuator OA 1 . Therefore, each of areas  22 - 1 ,  22 - 2 ,  22 - 3 ,  22 - 4  and  22 - 5  exhibits a different light transparency. For example, the light transparency of areas  22 - 2  and  22 - 4  are larger than area  22 - 3 , and is smaller than areas  22 - 1  and  22 - 5 . In this example, the light transparency of areas  22 - 1 ,  22 - 2 ,  22 - 3 ,  22 - 4  and  22 - 5  is 100%, 90%, 80%, 90% and 100%, respectively, if the total non-uniformity of the beam after the first lens FL 1  is approximately 20%. 
     As shown in  FIG. 5A , light L 1 , L 2 , L 3 , L 4  and L 5 , which are emitted by OLED elements S 1 , S 2 , S 3 , S 4  and S 5  and collected by the first lens FL 1 , pass through areas  22 - 5 ,  22 - 4 ,  22 - 3 ,  22 - 2  and  22 - 1 , respectively. As explained above, the energy and intensity distribution of the light passed through the first lens FL 1  is not uniform. In other words, lights L 1  and L 5  after the first lens FL 1  have a small intensity and energy compared to light L 3 . However, by providing attenuator OA 1  between the first lens FL 1  and the second lens SL 1 , the lights L 1 , L 2 , L 3 , L 4  and L 5  delivered to the OPC drum  3  have an almost uniform intensity and energy so that a latent image developed on the OPC drum  3  is uniform. 
       FIG. 6  illuminates a process for manufacturing the attenuator OA 1 . 
     In the first step, a photo resist  23 , which is deposited on the light absorbing layer  22 , is exposed with UV light so that a pattern of area  22 - 3  is transferred to the photo resist  23  ( FIG. 6A ). Parts of the photo resist  23  excluding area  22 - 3  is removed by a developer solution ( FIG. 6B ). Then, area  22 - 3  is formed at the center of the light absorbing layer using an ion etching process or equivalent process ( FIG. 6C ). After washing the remaining photo resist  23  away with a strong alkali solution, photo resist  25  for areas  22 - 2 ,  22 - 4  and  22 - 3  are formed on the light absorbing layer  22  in the same manner as the above ( FIG. 6D ). Then, areas  22 - 1 ,  22 - 2 ,  22 - 3 ,  22 - 4  and  22 - 5  are formed onto light absorbing layer  22  by an ion etching process ( FIG. 6E ) and the remaining photo resists  23 - 2  and  24 - 4  are washed away ( FIG. 6F ). 
     In this first embodiment, an ion etching process is used to form the light absorbing layer, but other processes, such as, but not limited to, for example, spattering and the like, can be used without departing from the scope and/or spirit of the invention. 
       FIG. 7  is a perspective view of a second example of a light attenuator OA 1 , based on changing a thickness of the absorbing layer, but maintaining the same absorption co-efficiency. 
     In the second example, light attenuator OA 1  comprises a SiO 2  based glass transparent substrate  31 A hog-backed light absorbing layer  32  is deposited on the transparent substrate  31 . In the disclosed embodiment, light absorbing layer  32  is made by polymers. However, other materials may be used without departing from the scope and/or spirit of the invention. As shown in  FIG. 7 , a thickness of the light absorbing layer  32  varies proportionally to a distance from its center portion, so that center portion  32 - 3  is thicker than peripheral portions  32 - 1  and  32 - 5 . Therefore, light transparency at center portion  32 - 3  is low compared to peripheral portions  32 - 1  and  32 - 5 . The fabrication of this kind of hog-backed profile can be done using the same method discussed with respect to  FIG. 6 . However, more processing steps are required. 
       FIG. 8A  and  FIG. 8B  are a cross sectional view and a plain view, respectively, of a third example of a light attenuator OA 1  of the present invention, based on a change of absorption co-efficiency of an absorbing layer, in which the thickness of the absorbing layer is maintained constant. 
     Light attenuator OA 1  of the third example comprises a SiO 2  based glass transparent substrate  41  and dielectric coating layer  42  that is deposed on the transparent substrate  41 , which is made by either metal-like films or dielectric films (such as, but not limited to SiO 2 , AL 2 O 3 , TiO 2  and the like). Coating layer  42  has an almost uniform thickness over the transparent substrate  41 , but comprises several areas  42 - 1 ,  42 - 2 ,  42 - 3 ,  42 - 4  and  42 - 5  having different light transparencies to each other. Different patterns (pixels) of dither matrix are formed on each surface of areas  42 - 1 ,  42 - 2 ,  42 - 3 ,  42 - 4  and  42 - 5  to cause different absorptions among areas  42 - 1 ,  42 - 2 ,  42 - 3 ,  42 - 4 ,  42 - 5 , respectively. Because the density of the dither matrix formed on area  42 - 3  is high compared to areas  42 - 1 ,  42 - 2 ,  42 - 4 , and the density of the dither matrix formed on areas  42 - 1  and  42 - 5  is low compared to areas  42 - 2  and  42 - 4 , light transparency T 1 , T 2 , T 3 , T 4  and T 5  of areas  42 - 1 ,  42 - 2 ,  42 - 3 ,  42 - 4  and  42 - 5  of the dielectric coating layer  42  have a relationship as follows: T 3 &lt;T 2  (=T 4 )&lt;T 1  (=T 5 ). Thus, non-uniformity of a light intensity and energy distribution after the first lens FL 1  is compensated. 
       FIG. 9A  and  FIG. 9B  are a cross sectional view and a plain view, respectively, of a fourth example of a light attenuator OA 1 , based on a change of absorption co-efficiency of an absorbing layer, while maintaining a uniform thickness of the absorbing layer. 
     Light attenuator OA 1  of the fourth example comprises a transparent substrate  51  and photo-sensitive coating layer  52 , which is deposited on transparent substrate  51 , comparable to the substrate  41  of the third example. In the fourth example, however, photo-sensitive coating layer  52  has several areas  52 - 1 ,  52 - 2 ,  52 - 3 ,  52 - 4  and  52 - 5  that have different light transparencies to each other with a real gray scale (photo-sensitive polymers or dielectric materials have the properties of increasing light-induced absorption to adjust the transparency). Photosensitive coating layer  52  is formed on the transparent substrate  51  by, for example, a vacuum deposition or the like. The polymer films are formed on the substrate by, for example, a spin coating method. Dielectric films are formed by physical deposition methods. Lasers having a wavelength from near infrared (IR) to ultraviolet (UV) dielectric coating layer  52  via gray scale mask  53  change the transparency (see  FIG. 10 ), and then, areas  51 - 1 ,  51 - 2 ,  51 - 3 ,  51 - 4  and  51 - 5  are formed on dielectric coating layer  52 . However, it is understood that the present invention is not limited to a light attenuator as described in the fourth example, this being merely an exemplary example. 
     Gray scale mask  53  has several areas  53 - 1 ,  53 - 2 ,  53 - 3 ,  53 - 4  and  53 - 5  that correspond to areas  52 - 1 ,  52 - 2 ,  52 - 3 ,  52 - 4  and  52 - 5 , respectively. The light transparency of areas  53 - 1 ,  53 - 2 ,  53 - 3 ,  53 - 4  and  53 - 5  is different relative to each other with a dither matrix as follows: (light transparency of area  53 - 3 )&gt;(light transparency of areas  53 - 2  and  53 - 4 )&gt;(light transparency of areas  53 - 1  and  53 - 5 ). In other words, area  52 - 3  of dielectric coating layer  52  is exposed to a more intense laser light than areas  52 - 1 ,  52 - 2 ,  52 - 4  and  52 - 5 . Because photo-sensitive coating layer  52  comprises photo-sensitizers or color centers and has a specific characteristic that its light transparency of portions exposed with laser light will decrease according to the intensity of the laser light, the light transparency of area  52 - 3  will be lower than the light transparency of areas  52 - 2  and  52 - 4 , which will be lower than the transparency of areas  52 - 1  and  52 - 5 . Therefore, non-uniformity of light intensity and energy distribution after the first lens FL 1  is compensated for by the light attenuator OA 1  comprising dielectric coating layer  52 . 
       FIGS. 11A and 11B  illustrate a cross sectional view and a plain view, respectively, of a fifth example of the light attenuator OA 1 . Light attenuator OA 1  of the fifth example comprises a thermal sensitive glass, such as, but not limited to, “laser direct write (LDW) glasses”. The glass has several areas  11 - 1 ,  11 - 2 ,  11 - 3 ,  11 - 4  and  11 - 5  having different light transparencies to each other with a real gray scale which is generated by a high power laser that direct exposes the glass with different intensities for each area, respectively. For this kind of glass, increasing the laser intensity increases the light transparency thereof. This is because this kind of glass exhibits properties of light induced absorption decrease (bleaching). Thus, a high power laser source having wavelengths from near infrared (IR) to ultraviolet (JV) can be imposed onto the thermal sensitive glass and generate heat to change its transmission. Since the laser exposure intensity on area  11 - 3  is less than the laser exposure intensity on areas  11 - 2  and  11 - 4 , which is less than a laser exposure intensity on areas  11 - 1  and  11 - 5 , the light transparency of area  11 - 3  will be lower than the light transparency of areas  11 - 2  and  11 - 4 , which will be lower than the transparency of areas  11 - 1  and  11 - 5 . 
       FIGS. 12A-12C  show a non-sequential ray-tracing simulation result using a commercial optical design software, called ZEMAX™, in terms of light intensity distribution before and after an optical attenuator, respectively.  FIG. 12A  depicts a ⅔D solid model layout as a result of the non-sequential ray-tracing simulation for a model having five EL elements, a single first lens and an optical attenuator.  FIGS. 12B and 12C  show a cross-sectional plot of an intensity distribution before and after the optical attenuator of the model of  FIG. 12A , respectively. It clearly shows that the optical attenuator improves the intensity uniformity of the light after the first lens by reducing the light intensity emitted from the center of the five EL elements without affecting the image geometry. 
     Although preferred embodiments and aspects of the present invention have been described and disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutions are possible, without departing from the scope and spirit of the invention as set forth in the accompanying claims.