the following description is of the best - contemplated mode of carrying out the invention . this description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense . the scope of the invention is best determined by reference to the appended claims . fig1 is a diagram showing a gray level formed by modulating a light modulator . in cases where pulse width modulation is used to control the switching of a light modulator to produce a gray level , the conventional method is shown in fig1 . the horizontal axis represents time wherein the number means time sequence in which the light modulator is switched , and the vertical axis represents brightness . in fig1 the brightness during any unit period ( the unit period is a period in which the light modulator performs a switching , so it is also called a switching period ) is fixed to the maximum value in an exemplary manner , but the light modulator can selectively decide whether or not to output light of the light source by switching on or off the light modulator . therefore , a gray level “ 0 ” or a gray level “ 1 ” can be output during each unit period . in fig1 , an 8 - bit gray level signal is input to the light modulator . the light modulator has to produce any one gray level of 256 (= 2 8 ) gray levels during a predetermined display period . a gray level is produced from a combination of the number of times that the light modulator is switched on and the number of times that the light modulator is switched off . a gray level “ 1 ” and a gray level “ 0 ” is output by switching the light modulator on and off once respectively , so any one of gray levels 0 - 255 can be output after 255 times that the light modulator is switched on or off . for example , the light modulator is switched on 255 times successively to output a gray level “ 255 ” and the light modulator is switched off 255 times successively to output a gray level “ 0 ”. namely , to display a gray level “ n ” the light modulator should be switched on n times . in this gray level forming method , the maximum number of gray levels is equal to the number of times the light modulator is switched . therefore , if the projector desires a larger maximum of the number of gray levels ( for example , 1024 gray levels ) or the projector desires more images ( for example , more than 2 images ), the light modulator has to bear the more number of times to switch within a limited period . however , the switching frequency of the light modulator has physical limitations . fig2 is a diagram showing a gray level formed by modulating a light modulator as well as a light source . in cases where light modulator modulation and light source brightness modulation are both used to produce a gray level , the conventional method is shown in fig2 . the horizontal axis represents time wherein the numerals mean a time sequence in which the light modulator is switched , and the vertical axis represents brightness , wherein the light source brightness varies in accordance with the time sequence in which the light modulator is switched . here , in response to 8 times that the light modulator is switched , the light source outputs 8 brightness with a ratio of 128 : 64 : 32 : 16 : 8 : 4 : 2 : 1 during the 8 unit periods , respectively . according to the switching of the light modulator , the light modulator decides whether or not to output light from the light source . therefore , a gray level “ 0 ” or a gray level “ 128 ” can be output during the first unit period , a gray level “ 0 ” or a gray level “ 64 ” can be output during the second unit period , and so on . in fig2 , the number labeled above the rectangle during a unit period means a gray level value except a gray level “ 0 ” which can be output during that unit period . in fig2 , any gray level in 256 gray levels can still be displayed by light modulator modulation and light source brightness modulation . for example , a gray level “ 137 ” is output by switching on the light modulator during the 1 st , 5 th , and 8 th unit periods and switching off the light modulator during the 2 nd , 3 rd , 4 th , 6 th and 7 th unit periods . therefore , a gray level “ 128 ”, a gray level “ 8 ”, and a gray level “ 1 ” are output and added up to a total gray level “ 137 ”. in comparison with the gray level forming method shown in fig1 , the method shown in fig2 only need 8 times that the light modulator is switched to output the same number of gray levels . therefore , the data amount for the light modulator outputting a gray level is reduced to 1 / 32 (= 8 / 256 ) of that by using the method shown in fig1 . the limited bandwidth of the light modulator is utilized effectively . however , in terms of a light source , it only outputs the maximum brightness during the 1 st unit period and decreases its output brightness gradually during the following unit periods . thus , the overall brightness of the maximum gray level is reduced to about 25 %(=( 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 )/ 128 * 8 ) of the maximum of the overall brightness the light source can output . a substantial decrease of brightness is a main shortcoming while adopting this gray level forming method . moreover , the light source brightness modulation can be achieved by pulse width modulation ( pwm ) or pulse amplitude modulation ( pam ). if the light source is also a pwm light source , it has to be switched on or off at least 128 times during a unit period to generate light of which the ratio of the maximum brightness and the minimum brightness is 128 : 1 . namely , the switching frequency of the light source must be at least 128 times the switching frequency of the light modulator . however , a rapid - switching light source has physical limitations , which makes it difficult to implement . fig3 is a diagram showing a gray level is formed by modulating a light modulator as well as a light source in accordance with embodiment 1 of the present invention . in this embodiment , the brightness of the light source is also varied in accordance with the time sequence in which the light modulator is switched . here , in response to 19 times that the light modulator is switched , the light source outputs 19 brightness with a ratio of 16 : 16 : 16 : 16 : . . . : 16 : 16 : 16 : 8 : 4 : 2 : 1 during the 19 unit periods , respectively . namely , the gray level which can be output during a unit period is shown as the number labeled above the rectangle during that unit period . during each of the 1 st to 15 th unit periods , a gray level “ 0 ” or a gray level “ 16 ” can be output . during the 16 th unit period , a gray level “ 0 ” or a gray level “ 8 ” can be output . during the 17 th unit period , a gray level “ 0 ” or a gray level “ 4 ” can be output . during the 18 th unit period , a gray level “ 0 ” or a gray level “ 2 ” can be output . during the 19 th unit period , a gray level “ 0 ” or a gray level “ 1 ” can be output . in fig3 , any gray level in 256 gray levels can still be displayed by light modulator modulation and light source brightness modulation . for example , a gray level “ 137 ” is output by switching on the light modulator during any 8 unit periods of the 1 st to 15 th unit periods , switching on the light modulator during the 16 th and 19 th unit periods , and switching off the light modulator during the remaining unit periods . therefore , 8 gray levels “ 16 ”, a gray level “ 8 ”, and a gray level “ 1 ” are output and added up to a total gray level “ 137 ”. the number of times that the light modulator is switched in this embodiment is 19 , which is a minor increase in comparison to 8 times shown in fig2 , but it &# 39 ; s is a huge decrease in comparison to 255 times shown in fig1 . in addition , the light source outputs the maximum brightness during the 1 st to 15 th unit periods and lowers its output brightness gradually during only the last 4 unit periods . thus , the overall brightness of the maximum gray level is merely reduced to about 84 %(=( 16 * 15 + 8 + 4 + 2 + 1 )/ 16 * 19 ) of the maximum of the overall brightness the light source can output . in comparison with the gray level forming method shown in fig2 , the brightness of a gray level is substantially increased . moreover , if the light source is a pwm light source , it has to be switched on or off at least 16 times during a unit period to generate light of which the ratio of the maximum brightness and the minimum brightness is 16 : 1 . namely , the switching frequency of the light source must be at least 16 times the switching frequency of the light modulator . in comparison with the gray level forming method shown in fig2 , the switching frequency of the light source is also decreased substantially , which makes the implementation easier . however , the combination of the aforementioned number of times that the light modulator is switched and the corresponding light source brightness is merely an example . there are more possible combinations in practical terms . the concept of the invention is increasing the number of the unit periods where the light source outputs the maximum brightness . for example , it is preferred that the number of the unit periods where the light source outputs the maximum brightness is at least 60 % of the total number of the unit periods . in this way , a balance between lowering the data bandwidth of the light modulator and raising the overall brightness is achieved . another possible gray level forming method in accordance with embodiment 2 of the invention is described below . as the response time of modulating brightness of a light source depends on the characteristics of the light source itself and the light source driver , the switching speed of the light source may not be faster than the switching speed of the light modulator . if the response time of the light source driven by a switching ( a pulse ) of the light source driver is longer than a unit period of the light modulator , there is not enough time for the light source to generate a desired brightness during each unit period . therefore , the gray level forming method in accordance with embodiment 1 is unable to be utilized . fig4 is a diagram showing a gray level is formed by modulating a light modulator corresponding to a waveform of a light source in accordance with embodiment 2 of the present invention . in fig4 the curve similar to a trapezoid represents a brightness waveform generated from a light source driven by a switching of the light source driver . in the period of the brightness waveform , the brightness is increased gradually from 0 to a maximum value and then is maintained at the maximum value for a while , and after that the brightness is decreased gradually to 0 . in this embodiment , this period of brightness waveform of the light source is used as a total period for integrating gray levels . here , the total period is divided into a plurality of the aforementioned unit periods , and gray levels are distributed to each unit period according to the waveform of the brightness . as an example shown in fig4 , a period of brightness waveform of the light source has 21 unit periods . a gray level value except for the gray level “ 0 ” to be output during a unit period corresponds to integration of the brightness during that unit period . therefore , gray levels “ 2 ”, “ 8 ”, “ 13 ”, “ 16 ”, “ 16 ”, “ 16 ”, . . . , “ 16 ”, “ 16 ”, “ 14 ”, “ 10 ”, “ 8 ”, “ 4 ”, “ 1 ” are output sequentially . in the other words , during the 4 th to 16 th unit periods , the light source outputs the maximum brightness , and during the 1 st to 3 rd and 17 th to 21 st unit periods , the light source outputs a lower brightness . in this gray level forming method of the embodiment , by no matter a pwm light source or a pam light source , the switching frequency that the light source requires is decreased to 1 / 21 of the switching frequency of the light modulator . it is more applicable than requesting for a rapid switching frequency of a light source . certainly , the brightness waveform of the light source and the total number of unit periods for the light modulator to modulate a total gray level are an example . there are countless practical combinations . note that the shape of the brightness waveform of the light source is not invariable , the brightness waveform may change because of factors such as manufacturing , and environment . therefore , a concept of the invention is the length of the total period for the light modulator to modulate a total gray level and the distribution of the gray level output during each unit period can be adjusted according to the new brightness waveform . on the contrary , the invention also can set the length of the total period for integrating a gray level and the distribution of the gray level output during each unit period , and then adjust the brightness waveform of the light source by pwm or pam to fit the setting . following , a system for implementing the gray level control method in accordance with embodiments 1 and 2 is described . fig5 is a structure diagram showing an optical projection system in accordance with embodiment 3 of the present invention . as shown in fig5 , the basic elements of an optical projection system include : a light source 51 , a light source driver 52 , a light modulator 53 , a lens group 54 , a controller 55 , and a sensor 56 . the light source 51 is pwm light source or a pam light source , which is driven by the light source driver 52 and changes its output brightness . the light modulator 53 selectively switches whether to output the light from the light source 51 . when the light modulator 53 is switched on , the light from the light source 51 can pass the light modulator 53 and be incident to the lens group 54 for imaging . when the light modulator 53 is switched off , the light came from the light source 51 is incident to the sensor 56 . the controller 55 is used to control the light modulator 53 and the light source driver 52 to make each unit period of the light modulator 53 synchronize with the period of a brightness variation of the light source 51 . if the optical projection system implements the gray level control method in accordance with embodiment 1 , the sensor 56 can be omitted . if the optical projection system implements the gray level control method in accordance with embodiment 2 , the sensor 56 is indispensable in consideration that the brightness waveform of the light source 51 may change . that is to say , the sensor 56 is used to sense the brightness waveform of the light source 51 and transmit the waveform information back to the controller 55 . then , as described above , the controller 55 controls the length of the total period for integrating a gray level and the distribution of the gray level output during each unit period according to the brightness waveform of the light source 51 . otherwise , the controller 55 presets the length of the total period for integrating a gray level and the distribution of the gray level output during each unit period and adjusts the pulse output from the light source driver 52 to adjust the brightness waveform of the light source 51 . according to the gray level control methods or the optical projection system described in the above embodiments , the invention can utilize the bandwidth of the light modulator effectively so as to be applicable in naked - eye 3d projectors that require more multi - view images , raise the brightness corresponding to a gray level , and reduce the switching frequency of the light source . while the invention has been described by way of example and in terms of the preferred embodiments , it is to be understood that the invention is not limited to the disclosed embodiments . on the contrary , it is intended to cover various modifications and similar arrangements ( as would be apparent to those skilled in the art ). therefore , the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements .