reference will now be made in detail to the preferred embodiments of the present invention , examples of which are illustrated in the accompanying drawings . fig2 illustrates a triple - panel lighting optical system according to a first embodiment of the present invention . as shown in fig1 the lighting optical system includes a lamp 210 , a polarization converting system 220 , a color - separating prism group 230 ( a first , a second and a third dichroic prism 231 , 232 , 233 ), a first , a second and a third lcd 240 , 250 , 260 , and a color combining prism group 270 ( a first , a second , a third and a fourth color combining prism 271 , 272 a , 272 b , 273 . the polarization converting system 22 — includes a first and a second fly - eye lens 220 a , 220 b , and a pbs array 220 c . the first and second fly - eye lenses 220 a , 220 b split white light emitted from the lamp 210 into micro lens cell units , thereby resulting in a uniform light distribution . the pbs array 220 c separates an incident light to linearly polarized light having one optical axis , namely p - polarization and s - polarization . here , the pbs array 220 c emits an s - wave as it is , but converts a p - wave to an s - wave by means of a ½ wave plate ( not shown ) before emitting . in this manner , the resulting polarization states are conformed . the color separating prism 230 separates a polarized light inputted through the polarization converting system 220 into red , green and blue lights , and output the lights to different paths from each other . to this end , the color separating prism group 230 has an approximately ‘ l ’ shape . the color separating prism group 230 includes the first and second color separating prisms 231 , 232 in the parallelogram shape , and the third color separating prism 233 in the triangle shape . the first color separating prism 231 forms a first dichroic coating 230 b on one inclined surfaces , and a total reflection coating 230 c on the other inclined surface . the second color separating prism 232 forms a second dichroic coating 230 b on one inclined surface . here , the first dichroic coating 230 a and the second dichroic coating 230 b are disposed in parallel with the x - axis direction . and the first dichroic coating 230 a and the total reflection coating 230 c are disposed in parallel with the y - axis direction . the inclined surface of the third color separating prism 233 is attached to the second dichroic coating 230 b of the second color separating prism 232 in such a manner that the first , second and third color separating prisms 231 , 232 , 233 are not exposed to air . the first and second dichroic coatings 230 a , 230 b and the total reflection coating 230 c are disposed in angles so that the emission direction of the light reflected therefrom is at right angles to the direction of incidence . therefore , the light reflected from the first and second dichroic coatings 230 a , 230 b is emitted in the y - axis direction that makes a right angle with the x - axis direction . the light , which was reflected from the first dichroic coating 230 a and traveled along the y - axis direction , is then reflected by the total reflection coating 230 c and travels in the x - axis direction . more specifically speaking , the first dichroic coating 230 a reflects a desired wavelength band , the red light for example , towards the total reflection coating 230 c , and transmits the cyan ( g , b ) light towards the second dichroic coating 230 b . the total reflection coating 230 c reflects again the red light that has been reflected from the first dichroic coating 230 a , and emits it outside through a first plane of emission s 1 . the second dichroic coating 230 b reflects the green light for example out of the transmitted cyan light , and emits it outside through a second plane of emission s 2 . the residual blue light is emitted outside through a third plane of emission s 3 . the first , second and third lcds 240 , 250 , 260 are disposed , respectively , on the light paths of rob lights corresponding to the first through third planes of emission s 1 , s 2 , s 3 . in other words , when rgb lights are emitted from the first , second and third planes of emission s 1 , s 2 , s 3 , the first , second and third lcds 240 , 250 , 260 installed on opposite sides of those planes provide rgb images , respectively . the color separating prism group 230 is designed in such a manner that the traveling distance of each of the rgb lights from the lamp ( the light source ) 210 to the first , second and third lcds 240 , 250 , 260 is the same ( please refer to fig8 ). in other words , the distances from the plane of incidence of the color separating prism group 230 to the first lcd 240 ( a 1 + a 2 ), the second lcd 250 ( b 1 + b 2 ) and the third lcd 260 ( c 1 + c 2 ) are all the same . since the traveling distances of the incident light from the lamp 210 to the first , second and third lcds 240 , 250 , 260 via the color separating prism 230 are the same , a relay system is no longer needed . the color combining prism group 270 combines rgb lights emitted from the first , second and third lcds 240 , 250 , 260 , and outputs white light ( r + g + b ) to outside . the color combining prism group 270 is similar to the 180 degree - rotated color separating prism group . the color combining prism group 270 includes a first color combining prism 271 in the parallelogram shape , and a second , a third and a fourth color combining prism 272 a , 272 b , 272 in the triangle shape , each of the color combining prisms being made out of the same material . here , an inclined surface of the second color combining prism 272 a forms a third dichroic coating 270 a . similarly , an inclined surface of the second color combining prism 272 b forms a fourth dichroic coating 270 b . also , a total reflection coating 270 c is formed on one inclined surface of the first color combining prism 271 in the parallelogram shape . the third dichroic coating 270 a and the fourth dichroic coating 270 b are disposed in parallel with the x - axis direction . the fourth dichroic coating 270 b and the total reflection coating 270 c are disposed in parallel with the y - axis direction . later the parallelogram - shaped prism 271 and the three triangle - shaped prisms 272 a , 272 b , 273 are attached to each other not to be exposed to air . the fourth color combining prism 273 is attached in such a manner that it is not exposed to air especially between the second color combining prism 272 a and the first lcd 240 . the third dichroic coating 270 a transmits a light element , the red light for example , entering through the first plane of incidence ti , and reflects in the x - axis direction a light element , the green light for example , entering through a second plane of incidence ti , whereby a yellow light synthesized with the red and green lights is outputted towards the fourth dichroic coating 270 b . the total reflection coating 270 c reflects a light element , the blue light for example , which enters through the third plane of incidence t 3 , in the direction of the fourth dichroic coating 270 b . the fourth dichroic coating 270 b transmits the blue light from the total reflection coating 270 c and reflects the yellow light ( r , g ) from the third dichroic coating 270 a , whereby a white light synthesized with the blue and yellow lights is outputted to outside . fig3 illustrates a triple - panel lighting optical system according to a second embodiment of the present invention . the constitution of the triple - panel lighting optical system according to the second embodiment of the present invention is basically similar to that of the first embodiment of the present invention , except for one difference in the following . the fourth dichroic coating 270 b is not formed on the inclined surface of the third color combining prism 272 b , but on the inclined surface of the first color combining prism 271 that is in parallel with the total reflection coating 270 c . as a result , the yellow light from the third dichroic coating 270 a is transmitted through the second color combining prism 272 b , and the blue light from the total reflection coating 270 a is reflected in the x - axis direction by the fourth dichroic coating 270 b . more specifically , the synthesized light with yellow and blue is outputted outside along the x - axis . since the rest of constitution and functions are identical with those of the first embodiment , they will not be discussed here . fig4 illustrates a triple - panel lighting optical system according to a third embodiment of the present invention . the triple - panel lighting optical system according to the third embodiment of the present invention includes a color separating prism group 230 consisting of five triangle - shaped prisms , and a color combining prism group 270 consisting of five triangle - shaped prism . to make the color separating prism group 230 , five triangle - shaped prisms made out of the same material are prepared . first and second dichroic coatings 230 a , 230 b are formed on the inclined surfaces of two triangle - shaped prisms 231 b , 232 b , respectively , and a total reflection coating 230 c is formed on the inclined surface of another triangle - shaped prism 231 a . these five triangle - shaped prisms are then attached to each other in such a manner that the first and second dichroic coatings 230 a , 230 b are disposed in parallel with the x - axis direction , and the first dichroic coating 230 a and the total reflection coating 230 c are disposed in parallel with the y - axis direction . in like manner , five triangle - shaped prisms ( a first to a fifth color combining prism ) 271 a , 271 b , 272 a , 272 b , 273 made out of the same material are prepared to make the color combining prism 270 . then third and fourth dichroic coatings 270 a , 270 b are formed on the inclined surfaces of the third and the fourth color combining prisms 272 a , 272 b , respectively , and a total reflection coating 270 c is formed on the inclined surface of the second color combining prism 272 b . these five triangle - shaped prisms are attached to each other in such a manner that the third and fourth dichroic coatings 270 a , 270 b are disposed in parallel with the x - axis direction , and the third dichroic coating 270 a and the total reflection coating 270 c are disposed in parallel with the y - axis direction . the fourth dichroic coating 270 b transmits the blue light from the total reflection coating 270 c , and reflects in the y - axis direction the yellow light ( r , g ) from the third dichroic coating 270 a , whereby a white light synthesized with the blue and yellow lights is outputted outside along the y - axis direction . fig5 illustrates a triple - panel lighting optical system according to a fourth embodiment of the present invention . the constitution of the triple - panel lighting optical system according to the fourth embodiment of the present invention is basically similar to that of the third embodiment of the present invention , except for one difference in the following . the fourth dichroic coating 270 b in the fourth embodiment is formed on an inclined surface of the first color combining prism 271 a . in other words , unlike in the third embodiment of the present invention , the fourth dichroic coating 270 b transmits in the x - axis direction the yellow light from the third dichroic coating 270 a , and also reflects in the x - axis direction the blue light from the total reflection coating 270 c . as a result , a white light synthesized with the blue and yellow lights is outputted outside along the x - axis direction . fig6 illustrates a triple - panel lighting optical system according to a fifth embodiment of the present invention . the constitution of the triple - panel lighting optical system according to the fifth embodiment of the present invention is basically similar to that of the first embodiment of the present invention , except for one difference in the following . instead of using two triangle - shaped color combining prisms 272 a , 272 b as illustrated in fig2 , in the fifth embodiment of the present invention a parallelogram - shaped color combining prism 272 ( a second color combining prism ) is employed . the third dichroic coating 270 a is formed on one inclined surface of the second color combining prism 272 , and the fourth dichroic coating 270 b is formed on another inclined surface of the second color combining prism 272 . since the rest of constitution and functions are identical with those of the first embodiment , they will not be discussed here . fig7 illustrates a triple - panel lighting optical system according to a sixth embodiment of the present invention . the constitution of the triple - panel lighting optical system according to the sixth embodiment of the present invention is basically similar to that of the second embodiment of the present invention shown in fig3 , except for one difference in the following . instead of using two triangle - shaped color combining prisms 272 a , 272 b as illustrated in fig3 , in the sixth embodiment of the present invention a parallelogram - shaped color combining prism 272 ( a second color combining prism ) is employed . the third dichroic coating 270 a is formed on one inclined surface of the second color combining prism 272 , and the fourth dichroic coating 270 b is formed on one of inclined surfaces of the first color combining prism 271 in parallel with the total reflection coating 270 c . in other words , unlike in the fifth ( second ?) embodiment of the present invention , the fourth dichroic coating 270 b transmits in the x - axis direction the yellow light from the third dichroic coating 270 a , and also reflects in the x - axis direction the blue light from the total reflection coating 270 c . as a result , a white light synthesized with the blue and yellow lights is outputted outside along the x - axis direction . since the rest of constitution and functions are identical with those of the first embodiment , they will not be discussed here . fig8 is a diagram to explain traveling distances of three color lights from a color separating prism to first , second and third lcds , respectively , in a triple - panel lighting optical system according to the present invention . basically the color separating prism group 230 is designed in such a manner that rgb lights entering the first , second and third lcds 240 , 250 , 260 through the color separating prism group 230 travel the same amount of distance from the lamp ( the light source ) 120 . in other words , the distances from the plane of incidence of the color separating prism group 230 to the first lcd 240 ( a 1 + a 2 ), the second lcd 250 ( b 1 + b 2 ) and the third lcd 260 ( c 1 + c 2 ) are all the same . since the traveling distances of the incident light from the lamp 210 to the first , second and third lcds 240 , 250 , 260 via the color separating prism 230 are the same , a relay system is no longer needed . although not illustrated in the drawing , white light outputted from the color combining prism 270 is projected onto the screen through a projection lens and displaced as an image . in conclusion , the lighting optical system of the present invention has the following advantages . first , unlike in the related art , the traveling distance of each of the rgb lights from the lcds to the projection lens are the same so the relay system is not required . this spontaneously fixes problems ( such as an increase in the optical engine size , an increase in the optical elements , and a necessity for an optical axis array ) that are often caused by installing the relay system in the optical system . second , the lighting optical system of the present invention has such a simple constitution that it includes prisms having dichroic coatings and / or total a reflection coating and lcds only . third , instead of the x - prism as in the related art , the present invention utilizes dichroic coatings to synthesize light . this makes it possible to conform the states of polarization of incident lights and emitted lights according to color , and the contrast of the optical system can be corrected more easily . fourth , the lighting optical system of the present invention is smaller and cost - effective , and provides superior optical performance . fifth , the color separating prisms and the color combining prisms are tightly attached so that none of them is exposed to air , whereby any transformation of the light caused by air during the color separation / combination can be minimized . the forgoing embodiments are merely exemplary and are not to be construed as limiting the present invention . the present teachings can be readily applied to other types of apparatuses . the description of the present invention is intended to be illustrative , and not to limit the scope of the claims . many alternatives , modifications , and variations will be apparent to those skilled in the art .