with a flat illumination face , neither the illumination device with a light guide having a 5 - sided cross - section nor the one with a light guide having a rectangular cross - section in the above prior art examples has provided any special design features on further improving the focusing of the illuminating light output from the light guide , thus improving the coupling between the illuminating light and the imaging system and consequently improving resolution of the image reading . thus , in the present invention , a new geometry light guide is disclosed which is specially designed to significantly improve the focusing of the output illuminating light , yet at the same time to achieve simplicity by having fewer components . further , the accompanying light source is also redesigned with separate light emitting elements having different light emission wavelength ranges . the preferred embodiments of the present invention are now explained in conjunction with the drawings . referring to fig3 there is shown a preferred embodiment of the image reading apparatus of the present invention . fig3 a shows a sectional view of a photo - electric conversion element array of the image reading apparatus as viewed along a main scan direction , and fig3 b shows a sectional view of the photo - electric conversion element array of the image reading apparatus as viewed along a sub - scan direction that is along arrow a in fig3 a . in fig3 a to 3 b , numeral 10 denotes a light transmissive sensor substrate on which a plurality of photo - electric conversion elements formed by using a thin film semiconductor layer such as amorphous silicon or polycrystalline silicon are arranged in one dimension . a protective layer , not shown , is formed on the light transmissive sensor substrate 10 to protect the photoelectric conversion elements , not shown , from damage by movement relative to a document sheet . the light transmissive sensor substrate 10 is packaged on a light transmissive packaging substrate 15 by bonding , and is electrically connected to a drive circuit , not shown , packaged thereon by wire bonding . numeral 100 denotes the read position of the document sheet , also called an original target . numeral 53 denotes a light conductor or light guide made of a light transmissive member having a rod like cross - section shown in fig3 b . numeral 30 denotes a light source whose light emitting elements can have up to three different light emission wavelength ranges . numeral 4 denotes the curved part of the light transmissive member 3 , also called illumination face , through which the light will get out of the light guard 53 and get focused toward the target 100 . in order to reflect the light beam , a scatter and reflection area 5 is formed on a plane facing the illumination face 4 by roughening the surface of the light transmissive member 3 or applying light diffusion and reflective paint . as discussed above , a prior art light guide having a pentagon cross - section was examined , in spite of the significant light loss through the top and reflection on white plastic holder , the width of the illuminating light which output through the illumination face is approximately 4 mm , while only 0 . 5 mm is needed by the imaging reading system . in the present embodiment , as shown in fig3 b , the curved illumination face 4 focuses light rays reflected and scattered from the scatter and reflection area 5 into a narrow stripe when they get out of the light transmissive member 3 though the illumination face 4 . thus the illumination intensity is greatly enhanced without changing light extraction efficiency . the rod like cross - section of the light transmissive member 3 is symmetric along central illumination line l . the symmetric cross - section and the central illumination line l is tilted toward and aimed at the target paper ( at an angle θ with perpendicular direction n ) for a reason that will be explained later . fig3 b shows that the portion of the lights reflected and scattered from the paint stripe 5 , which is directed toward the illumination face 4 , get out of the light transmissive member 3 focused to the target 100 referring now to fig4 the light focusing mechanism according to the invention will be explained in detail . fig4 which presents a closer look of the light guide cross - section , shows that a portion of the light rays that are directed toward the side wall of the light guide , namely light l 1 and l 2 , will be experiencing a total internal reflection , tir in short , and are redirect towards the illumination face 4 . thus , the design provided by the present invention uses a curved illumination face 4 to focus light onto a narrow strip . if the width of the illumination strip can be halved without changing efficiency , then the flux available for the imaging system will be doubled . to focus light onto a smaller strip on the target 100 , the size of the paint stripe 5 should be narrow as possible and constant along the length of the light guide . the extraction efficiency depends on the ratio of the paint stripe &# 39 ; s width to the light guide &# 39 ; s width . thus , the light guide cross - sectional area should be as small as possible to maximize performance . however , it should be noted that the width of the paint stripe 5 corresponds directly to the illumination intensity on the target 100 , the size of the paint strip 5 can not be too narrow to provide insufficient illumination intensity . the aforementioned challenge is resolved by the present invention by forming a “ virtual paint strip ” on the sidewall of tir . when l 1 and l 2 were redirected to the illumination face 4 after reflecting on the sidewall , in effect , they were as if emitted from an imaginary paint stripe point v as shown in fig4 and consequently the virtual width of the paint stripe 5 is larger than its physical width . since the width of the paint stripe 5 relates to the illumination intensity on the target , the present invention uses a narrower paint stripe 5 to achieve the same the illumination intensity on the target 100 . to match the narrower paint stripe width , the light guide cross - sectional area is smaller , reducing building materials substantially . referring to fig5 a to 5 b , the light guide according to the present invention is now discussed . fig5 a is a schematic drawing showing a sectional view of the light guide body viewed from the main scan direction . numeral 30 denotes a light source . numeral 3 denotes the light transmissive member . numeral 5 denotes a paint stripe , which is the scatter and reflection area . numeral 25 denotes two mounting tabs that are built in the light guide . numeral 6 denotes a metal end clip . light emitted from light source 30 enters light transmissive member 3 through an incident plane 9 and bounces using tir until it scatters off the paint stripe 5 out of light transmissive member 3 and is focused to the target as described above , the rest of the light beam remaining in light transmissive member 3 propagates along the length until it reaches the end of the light guide , where the metal end clip 6 reflects light and it heads back through the light guide towards the incident plane and has a second chance to scatter off the paint stripes as described above . only those light rays that survives the second round extraction and reaches the incident plane , which is rare , leak out of the light transmissive member 3 and are lost . for comparisons , the prior art design adopts direct painting of one end of the light guide as shown as the reflection area 6 in fig2 a . direct painting the end of the light guide will cause a spike at the end of the illumination pattern as shown in fig6 a . in fig6 a , the x - axis marks the distance from the incident plane 9 along the light guide , where distance 120 is the end of the light guide . the y - axis marks the relative illumination on target , the solid line represents the relative illumination of red light wavelength range ( center light emission wavelength 650 am , half value 30 am ) at different distance from the incident plane . the two dotted lines represent the relative illuminations of green and blue light wavelength at different distances from the incident plane respectively . the resulting relative illumination over different location along the light guide for the present invention is shown in fig6 b . in fig6 b , again the x - axis marks the distance from the incident plane 9 along the light guide , where distance 120 is the end of the light guide . the y - axis marks the relative illumination at target , the solid line represents the relative illumination of red light wavelength range ( center light emission wavelength 650 am , half value 30 am ) at different distance from the incident plane . the two dotted lines represent the relative illuminations of green and blue light wavelength at different distance from the incident plane respectively . it is clear that the spike between distance 100 to 110 in the prior art fig6 a is eliminated in the present invention shown in fig6 b . the metal end clip 6 also holds the light transmissive member 3 at the end of the light guide but allows space for thermal expansion over the storage temperature range . in addition , the metal end clip 6 in fig5 a can be replaced by a white plastic end clip as well , suffering only an approximately 10 % loss of light in reflection while saving significant cost on material . by comparing fig6 a and fig6 b , improvement in uniformity along the light guide according to the present invention becomes apparent . one important factor that contributes to this improvement in uniformity is the special design of the painting stripe in the present invention . as light emitted from the light source 30 and enters the light transmissive member 3 through incident plane 9 , the light extraction from the light guide tends to decrease along the length of the light guide . as a result , the light extraction and losses along the way will reduce the light flux intensity of the light beam as it travels further away from the incident plane 9 . in the present invention , the paint intensity of the paint stripe 5 increases as distance from the incident plane 9 increases , allowing the uniformity of the light extraction out of light transmissive member 3 through illumination face 4 along the light guide . the mounting tabs 25 in fig5 a are built into the front and end of the light guide , decreasing the number of components and simplifying the structure of the system . as shown in fig5 b , the mounting tabs 25 have a pair of high wings 26 that attach at the bottom of the cross - sectional portion of the light guide . notches 27 are cut out of the light guide near the curved illumination face . the wings 26 at the bottom of the mount prevent an initial spike that arises from too much light from the red led hitting the paint stripe directly . the notch 27 on the top prevents the curved illumination face from focusing light from the green and blue leds onto the paint stripe to eliminate dips and spikes . in addition , with the two mounting tabs at each end of the light guide in place , the light is given a distance to propagate right after it enters the light guide through the incident plane 9 or right after it is reflected on the metal end clip 6 and heading back , this will further eliminate spikes that tend to be generated at both ends of the light guide . the mounting pole on each wing is not optically active and can be altered . the inclusion of the mounting tabs 25 as an integral part of the light guide should further improve uniformity in two ways . first , the led light is given a distance to propagate and this will cause the led patterns to blur and fill up more of the light guide . second , the top and the bottom of the mounting tabs are removed . thus , light from the leds is allowed to spread out , but the tir prevents it from coming too near to the curved lens or paint stripe . the mounting tab design minimizes spikes and dip at the start . the red led may lose some light initially , but this is not a major problem because red leds are usually much brighter than green and blue leds . the employment of the mounting tabs 25 to minimize spikes and dips at the start provides a unique opportunity to construct a light guide with greater flexibility for led placement in relation to the physical location of the led light source without compromising the uniformity of light output from the light guide as will be discussed below . in the prior art illumination device of fig2 b , it is required that centers of the led chips 81 g and 81 r of the respective light emission wavelength ranges of the light source 30 be deviated from a normal line passing through the center of the scatter and reflection area 5 . more particularly , the prior art disfavors and , in fact , prohibits the placement of an led directly on or in any physical proximity to a line normal to the paint stripe . while this arrangement allows the prior art device to eliminate an initial spike at the beginning when retaining illumination uniformity along the light guide , it undesirably limits the physical location and number of the leds that may be placed in the light source 30 . contrary to the prior art , the present invention now describes an illumination device having a red led placed along the line normal to the paint stripe yet achieving and retaining illumination uniformity along the light guide due to the inclusion of the mounting tabs 25 . referring now jointly to fig3 a , fig3 b , fig5 a and fig7 the led placement according to the present invention is discussed . numeral 30 in fig3 a and fig5 a denotes the light source . fig7 is a schematic drawing of the light source 30 as illustrated in fig3 a as viewed along the direction of arrow c in fig3 a . numeral 41 r denotes a led having a red light emission wavelength range , numeral 41 g denotes a led having a green light emission wavelength range , and numeral 41 b denotes a led having a blue light emission wavelength range . note the light source is symmetric to the line connecting the center of 41 r and the top point t of the curve as shown in fig7 and so do 41 g and 41 b . in addition , 41 g and 41 b are also inter - changeable . when this light source is mounted to the light guide , this line that connects the center of 41 r and the top point t of the curve as shown in fig7 should be normal to the paint stripe 5 in fig3 a and 3b . the placement of led 41 r , 41 g and 41 b , as shown in fig6 proves to be the best led placement that matches the present light guide design discussed herein . in addition , the light source and light guide fits best to the rest of the system if they are tilted toward target paper ( at an angle 0 with perpendicular direction n as shown in fig3 b ). as shown in fig5 b , the wings 26 at the bottom of the mount tabs prevent the occurrence of an initial spike that may arise from too much light emitted from the red led hitting the paint stripe directly as the red led is placed on the normal line to the paint stripe . the notch 27 on the top prevents the curved illumination face from focusing light from the green and blue leds onto the paint stripe to eliminate the dip and spike effect . it should be noted that while the mounting tabs minimize spikes and dip at the start , the red led might lose some light initially . however , this is not a major problem because red leds are much brighter than green and blue leds . fig8 a and 8b show the longitudinal illumination patterns performed by the red led and the green and blue leds according to the invention . it is evident that illumination uniformity along the cross - section of the light guide is demonstrated for both the red led and the green and blue leds . there are slight differences in the uniformity pattern with the red led as compared to the blue and green leds . the red led is less efficient , but its light output is still much greater than the prior art illumination device . although the preferred embodiment of the invention has been 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 disclosed in the accompanying claims .