the invention will now be described based on the preferred embodiments . this does not intend to limit the scope of the present invention , but exemplify the invention . a general basic structure of an organic el display apparatus which is applied to the present invention is explained . fig1 is a block diagram showing a basic structure of the organic el display apparatus . the organic el display apparatus comprises an image source input unit 20 , an organic el driving circuit 30 , an organic el panel 60 , and a power supply unit 62 which supplies electric power for optical elements comprised in the organic el panel 60 . the organic el driving circuit 30 includes an image adjusting unit 32 which performs various processings described later to digital image signals inputted from the image source input unit 20 , an mpu ( micro processing unit ) 34 which performs calculation when the organic el driving unit 30 performs those processings , a dac unit 42 which converts the image signal processed by the image adjusting unit 32 to an analog signal . the organic el driving circuit 30 further includes a control signal generation unit which generates timing control signals and a memory which functions as a table at the time of performing actual process , which are not shown in fig1 . the image adjusting unit 32 performs an offset control for adjusting a brightness , a gain control for adjusting a contrast of the image , and processing which adjusts the input image signal to which the gamma correction is performed to the voltage - luminance ( v - t ) characteristic of the organic el panel 60 . operation of the above mentioned structure is explained briefly . first , three kinds of rgb of r , g and b digital signals r_in , g_in , and b_in ( hereinafter , these three kinds of digital signals are referred to as “ digital input signals ” as a whole ) are inputted into the image adjusting unit 32 from the image source input unit 20 . next , the offset control and the gain control are performed to the digital input signals by the image adjusting unit 32 . finally , adjusted digital input signals are outputted to the organic el panel 60 as voltages of three kinds of r , g and b analog signals r_out , g_out and b_out ( hereinafter , these three kinds of analog signals are referred to as “ analog output signals ” as a whole ). fig2 shows a general basic circuit of a single pixel comprised in an active - matrix type organic el display apparatus . the single pixel comprises an oled 100 , a selecting transistor tr 1 , a driving transistor tr 2 and a capacitor sc . a data line dl , a scan line sl and a power supply line vdd are also connected to the pixel . when the scan line sl goes high and a period of writing the luminance data has come , the selecting transistor tr 1 goes on and the luminance data applied to the data line dl is set on the driving transistor tr 2 and held in the capacitor sc . next , when a light - emitting period has come , the oled 100 emits light with a luminance corresponding to the luminance data set on the driving transistor tr 2 . fig3 shows a relation between an applied voltage which is the luminance data applied to the data line dl and emission luminance . there is a threshold voltage required to emit light for the oled 100 . here , this threshold voltage is called “ black - side reference voltage vb ”. a voltage set on the oled 100 to emit light with a rated luminance is called “ white - side reference voltage vw ”. in fig3 , the luminance is 50 % of the rated luminance when v 1 is applied . accordingly , the dac unit 42 stores a table which represents the black - side reference voltage vb and the white - side reference voltage vw , which is not shown in fig3 . in terms of hardware , this structure can be realized by a cpu , a memory and other lsis of an arbitrary computer . in terms of software , it is realized by memory - loaded programs or the like , but drawn and described here are functional blocks that are realized in cooperation with those . thus , it is understood by the skilled in the art that these functional blocks can be realized in a variety of forms by hardware only , software only or the combination thereof . embodiments are explained below based on the above basic technology . in a first to fifth embodiment , if an operation of a user to the display apparatus is not performed over a predetermined period of time , and during downloading a file , a processing of darkening a display image is performed . in a first embodiment , when the analog signal is outputted to the organic el panel 60 , the display image is made dark by adjusting the reference voltage used as the basis at the time of converting the digital signal into the analog signal in the dac unit 42 to lower the luminance . that is , the display image is darkened by setting the voltage to lower value than original value when converting the digital signal to the voltage which is the analog signal . fig4 shows a circuit structure of an organic el display apparatus 11 which realizes the first embodiment . the circuit structure of the organic el display apparatus 11 is based on the structure of the organic el display apparatus 10 shown in the basic technology . characteristic structure in the first embodiment is explained hereinafter . the organic el display apparatus 11 comprises an input detecting unit 64 which detects the fact that there is no operation from a user to the display apparatus for a predetermined term , and a processing detecting unit 66 which detects a processing which is performed in the display apparatus . the organic el driving unit 30 comprises a reference voltage adjusting unit 72 and lowers the maximum luminance allowed to set in the optical element by method mentioned below . the input detecting unit 64 and the processing detecting unit 66 are comprised in the display apparatus according to a second to a fifth embodiment commonly . at first , when the input detecting unit 64 detects the fact that there is no operation from the user to the organic el display apparatus 11 for a predetermined term or the processing detecting unit 66 detects the advance of the predetermined processing , the dac unit 42 instructs the reference voltage adjusting unit 72 to change the reference voltage . that is , the width of the signal outputted to the organic el panel 60 is changed by controlling the white - side reference voltage corresponding to the maximum luminance . for example , the maximum luminance of the optical element is normally 100 % of the rating value , but the maximum luminance is changed to 50 % of the rating value for example . that is , the rating voltage corresponding to the maximum luminance is changed from v w to v 1 , in the relation between applied voltage and the luminance shown in fig3 . in a second embodiment , the image to be displayed is made darker than original brightness by performing a gain control or a gamma control to a digital signal inputted in the organic el driving circuit 30 . fig5 shows a circuit structure of the organic el display apparatus 12 of the present embodiment . this circuit structure is also based on the circuit structure shown in the basic technology . the organic el driving circuit 30 comprises a luminance changing unit 74 which is characteristic in the second embodiment . the luminance changing unit 74 includes a gain control unit 76 for adjusting a gain of the signal and a gamma correction unit 78 which performs the gamma correction . these gain control and the gamma correction are performed in collaboration with the mpu 34 . here , the image adjusting unit 32 performs the gain control and the gamma correction to make the display image proper . on the other hand , the gain control performed by the luminance changing unit 74 and the gamma correction performed by the gamma correction unit 78 is performed to make the display image darker than original . in a third embodiment , a processing to lower a tone of a display image is performed to a digital signal inputted to an organic el driving circuit 30 to make the display image darker than original brightness . fig6 shows an organic el display apparatus 13 according to the third embodiment , which is also based on the circuit structure shown in the basic technology . the organic el driving circuit 30 comprises a tone changing unit 80 which is characteristic in the third embodiment . the tone changing unit 80 reduces the number of gradations of a display image to generate a summary image . the tone changing unit 80 changes the number of the gradation of the display image originally having 16 gradation levels into 8 gradation levels , which is half of the number of the original gradation levels , for example , and changes the maximum luminance level into half in accordance with the change of the number of the gradation levels . by carrying out like this , the display image can be made dark , and the advance of the degradation of the optical element can be suppressed . the processing of the tone changing unit 80 is also performed in collaboration with the mpu 34 . in a fourth embodiment , the display image is made darker than original brightness by controlling a voltage of a power supply unit 62 which supplies electric power to the organic el panel 60 . one of the method of controlling the voltage is lowering the voltage level supplied to the organic el panel 60 , for example , from 10v as original to 5v . another method is setting up the non - emitting period to the optical element . that is , the period where the voltage supplied from the power supply unit 62 to the optical element is made zero or the voltage by which the optical element does not emit is set up in the period where the optical element emits originally . these two methods may be combined . fig7 shows a circuit structure of the organic el display apparatus 14 according to the fourth embodiment . this is also based on the structure of the organic el display apparatus shown in the basic technology . the organic el driving circuit 30 comprises a power supply control unit 82 , which is characteristic in the fourth embodiment , and controls the voltage of the power supply unit 62 in collaboration with the mpu 34 . in a fifth embodiment , text data contained in a image to be displayed is highlighted , and the other display area is relatively made dark . in this case , text data may be extracted by arbitrary method . fig8 shows a circuit structure of an organic el display apparatus 15 according to the fifth embodiment . this is also based on the structure of the organic el display apparatus 10 shown in the basic technology . the organic el display apparatus 15 comprises a text extracting unit 84 , which is characteristic in the fifth embodiment . the text extracting unit 84 extracts the text data included in the display apparatus as mentioned above , and the text data is displayed in the predetermined luminance , and the luminance of the other area is set as the luminance darker than the text data . this processing is also performed in collaboration with the mpu 34 . as mentioned above , according to the first to fifth embodiment , current supplied to the oled which is the optical element comprised in the organic el panel 60 can be reduced without any trouble for a user , and thereby , the advance of the deterioration of the oled can be reduced . the technical field of the sixth embodiment relates to a mobile terminal comprising an organic el display and a luminance control method in the mobile terminal comprising the organic el display . there are a passive type which has simple matrix structure and an active type which uses tft in an organic el display . fig9 shows a basic pixel structure of the active type organic el display . a circuit of the single pixel of the active type organic el display is comprised with a switching tft 301 , a condenser 302 , a driving tft 303 and an organic el element 304 . a drain electrode of the switching tft 301 is applied with a display signal data ( vin ) via a display signal line 311 . a select signal scan is applied for a base electrode of the driving tft 301 via a select signal line 312 . a source electrode of the switching tft 301 is connected to the base electrode of the driving tft 303 and is grounded via the condenser 302 . a drain electrode of the driving tft 303 is applied with a driving power voltage vdd . a source electrode of the driving tft 303 is connected to an anode of the organic el element 304 . a cathode of the organic el element 304 is grounded . on and off of the switching tft 301 is controlled by the select signal scan . the condenser 302 is charged by the display signal data ( vin ) supplied via the switching tft 301 when the switching tft 301 is on , and holds the charged voltage when the switching tft 301 is off . the driving tft 303 supplies current according to the voltage which the condenser 302 holds and applied to the base electrode thereof to the organic el element 304 . fig1 shows a relation between a display signal data ( vin ) and emitting luminance ( driving current ) of the organic el element 304 in the basic pixel structure shown in fig9 . in fig1 , “ refw ” shows a white - side reference voltage for defining the emitting luminance corresponding to a white level of the input signal , and “ refb ” shows a black - side reference voltage for defining the emitting luminance corresponding to a black level of the input signal , respectively . by the way , in the active type organic el displays as mentioned above , big current flows through the organic el element 304 when displaying the bright image as a whole screen ( image with high luminosity ). if the big current flows through the organic el element 304 , power consumption will increase . moreover , if the big current continuously flows through the organic el element 304 , the so - called phosphor burn - in phenomenon will occur , or degradation of the performance thereof will be accelerated . the object of the sixth embodiment is to provide a mobile terminal comprising an organic el display and a luminance control method in the mobile terminal , which can prevent a generation of a phosphor burn - in and can reduce a degradation of performance of the organic el element . an aspect of the present embodiment is a mobile terminal comprising : an organic el display ; a distinction means which distinguishes a type of a content displayed on said organic el display ; a control means which controls display luminance of an image to be displayed on said organic el display in accordance with the type of the content distinguished by said distinction means . in the mobile terminal , the control means may include : a da converter which converts a digital image signal corresponding to the content to be displayed on said organic el display to an analog image signal , based on an input / output characteristic defined by a given reference voltage , and supplies the analog image signal to said organic el display ; and a reference voltage adjusting circuit which controls the reference voltage to be supplied to said da converter based on the type of the content distinguished by said distinction means . in the mobile terminal , there may be a black - side reference voltage for defining an emitting luminance corresponding to a black level of an input signal and a white - side reference voltage for defining an emitting luminance corresponding to a white level of the input signal as reference voltages supplied to said da converter , and the reference voltage adjusting circuit may control the white - side reference voltage based on the kind of the content distinguished by the distinction means . another aspect of the present embodiment is a luminance control method in a mobile terminal comprising an organic el display , the method including : a first step of distinguishing a type of a content to be displayed on the organic el display ; and a second step of controlling a display luminance of an image to be displayed on the organic el display in accordance with the type of the content distinguished by the first step . in the luminance control method in the mobile terminal , the mobile terminal may comprise a da converter which converts a digital image signal corresponding to the content to be displayed on the organic el display to an analog image signal , based on an input / output characteristic defined by a given reference voltage , and supplies the analog image signal to the organic el display ; and the second step may control an amplitude of an image input signal by controlling a reference voltage supplied to the da converter based on the type of the content distinguished by said first step . in the luminance control method in the mobile terminal , there may be a black - side reference voltage for defining an emitting luminance corresponding to a black level of an input signal and a white - side reference voltage for defining an emitting luminance corresponding to a white level of the input signal as reference voltages supplied to the da converter , and the second step may control the white - side reference voltage based on the kind of the content distinguished by the distinction means . optional combinations of the aforementioned constituting elements , and implementations of the invention in the form of methods , apparatuses and systems may also be practiced as additional aspects of the present invention . hereinafter , an example where the present invention is adopted to a cellular phone will be explained with reference to fig1 to 14 . fig1 shows a schematic structure of the cellular phone . an mpu 109 controls a cellular phone as a whole . an antenna 101 transmits and receives an electric wave . a transmitter - receiver unit 102 receives the electric wave and transmits the received content to the mpu 109 . the transmitter - receiver unit 102 transmits a transmission signal outputted from the mpu 109 with the electric wave . a microphone 103 communicates an audio signal to the mpu 109 . a speaker 104 outputs an audio signal outputted from the mpu 109 as a sound . a camera 105 is attached in the front of a main body of the cellular phone in which the organic el display 114 is formed , and transmits shot image to the mpu 109 . at a shooting mode , an image shot by the camera 105 is displayed on the organic el display 114 instead of a display image at a normal mode . an operating unit 108 is comprised in the main body of the cellular phone , includes various buttons and various switches . in a flash memory 110 , data which should be saved when a power turns off are stored . in a graphics memory 112 , image data to be displayed on the display is stored . image data is written at the predetermined address in the graphics memory 112 based on the image data and a write control signal outputted from the mpu 109 . pixel data of the pixel corresponding with a display cycle of the organic el display 114 is outputted from the graphics memory 112 according to a scan timing . a timing control ic 113 supplies the image data and a driving signal to the organic el display 114 , and displays an image on the organic el display 114 . in this embodiment , the mpu 109 has a function for controlling a luminance of a whole screen in accordance with a kind of a content displayed on the organic el display 114 . the kind of the content displayed on the organic el display 14 may be : ( 1 ) an animation ( a screen displayed at a camera mode or the like ), ( 2 ) a mail screen ( a screen displayed at a mail mode ), ( 3 ) a game screen ( a screen displayed at a game mode ), ( 4 ) a calling screen ( a screen displayed at a calling mode ( during speaking )), ( 5 ) an idle screen ( a screen displayed at an idle mode ). a gain for controlling the luminance of the whole screen for every kinds of contents is set to the mpu 109 beforehand . the mpu 109 distinguishes the kind of the content displayed on the organic el display 114 based on the present operating mode , and outputs the gain corresponding thereto to a total luminance control circuit in the timing control ic 113 . the total luminance control circuit is described later . the smaller the gain is , the lower the total luminance becomes , as described later . the relation between the total luminance ( gain ) and each content is set as following : ( 2 ) a mail screen : the total luminance is lowered because the visibility is maintained even if the luminance is lowered because of high contrast . ( middle gain ) ( 3 ) a game screen : the total luminance is lowered in the range of maintaining the visibility because a possibility of being continuously displayed over a long time is high , and a possibility of generating the phosphor burn - in phenomenon is high . ( middle gain ) ( 4 ) a calling screen : the total luminance is lowered because only information which is not much important such as a call time is displayed . ( small gain ) ( 5 ) an idle screen : the total luminance is lowered because a possibility of being continuously displayed over a long time is high , and a possibility of generating the phosphor burn - in phenomenon is high . ( small gain ) explanation of the total luminance control circuit in the timing control ic 113 fig1 shows a structure of the total luminance control circuit 200 comprised in the timing control ic 113 . the total luminance control circuit 200 comprises a reference voltage control circuit 1 and a dac 2 . the digital image input signals r_in , g_in , and b_in are transmitted to the dac 2 and converted to the analog image output signals r_out , g_out , and b_out . then , the analog image output signals are supplied to the organic el display 114 . the reference voltage control circuit 1 controls a reference voltage supplied to the dac 2 . there are a black - side reference voltages r_refb , g_refb , and b_refb ( these are referred to as the “ refb ” as a whole ) for each r , g , and b , and a white - side reference voltages r_refw , g_refw , and b_refw ( these are referred to as the “ refw ” as a whole ) for each r , g , and b . the black - side reference voltage refb is a basic voltage to define an emitting luminance corresponding to a black level of an input signal . the black - side reference voltage is fixed in this embodiment . the white - side reference voltage refw is a basic voltage to define an emitting luminance corresponding to a white level of an input signal . the white - side reference voltage is controlled by the reference voltage control circuit 1 in this embodiment . the dac 2 converts the digital image input signals r_in , g_in , and b_in to the analog image output signals r_out , g_out , and b_out based on the input / output characteristic defined by the black - side reference voltage refb and the white - side reference voltage refw ′. the analog image output signals r_out , g_out , and b_out are supplied to the organic el display 114 . these analog image output signals r_out , g_out , and b_out correspond to the display signal data ( vin ) shown in fig9 . the reference voltage control circuit 1 comprises a reference voltage adjusting circuit 11 and a plurality of dacs 221 to 226 . the reference voltage adjusting circuit 11 generates adjusted white - side reference voltages r_refw ′, g_refw ′, and b_refw ′ for each r , g , and b , based on the black - side reference voltages r_refb , g_refb , and b_refb set for each r , g , and b beforehand , and the white - side reference voltages r_refw , g_refw , and b_refw set for each r , g , and b beforehand , and the gain supplied by the mpu 109 . each basic black - side reference voltage r_refb , g_refb , b_refb and each basic white - side reference voltage r_refw , g_refw , b_refw are supplied as digital signals . the reference voltage adjusting circuit 11 includes the reference voltage adjusting circuit for each r , g , and b . here , the reference voltage adjusting circuit for r is explained because structure of reference voltage adjusting circuit for each r , g , and b are the same . fig1 shows the reference voltage adjusting circuit for r . this reference voltage adjusting circuit comprises a subtracter 231 , a multiplier 232 , and a subtracter 233 . the subtracter 231 calculates a difference between the basic black - side reference voltage r_refb and the basic white - side reference voltage r_refw ( r_refb − r_refw ). the multiplier 232 multiplies the output of the subtracter 231 ( r_refb − r_refw ) by the gain . the subtracter 233 calculates the adjusted white - side reference voltage r_refw ′ by subtracting the output of the multiplier 232 ( gain *( r_refb − r_refw )) from the basic black - side reference voltage r_refb . if the gain is 1 . 00 , the adjusted white - side reference voltage r_refw ′ is equal to the basic white - side reference voltage r_refw ( refw shown in fig1 ). the smaller the gain becomes , the larger the adjusted white - side reference voltage r_refw ′ becomes , and approaches the basic black - side reference voltage r_refb ( refb shown in fig1 ). that is , the smaller the gain becomes , the lower the emitting luminance ( driving current ) of the organic el element corresponding to the white level of the input signal becomes . each black - side reference voltage r_refb , g_refb , b_refb is converted to the analog signal by the dacs 221 , 222 , and 223 , respectively , and supplied to the dac 2 . each adjusted white - side reference voltage r_refw ′, g_refw ′, b_refw ′ is converted to the analog signal by the dacs 224 , 225 , 226 , respectively , and supplied to the dac 2 . fig1 shows a input / output characteristic of the dac 2 . in fig1 , refw ′ 1 shows the white - side reference voltage (= basic white - side reference voltage refw ) supplied to the dac 2 when the gain is large ( gain = 1 ). refw ′ 3 shows the white - side reference voltage supplied to the dac 2 when the gain is small . refw ′ 2 shows the white - side reference voltage supplied to the dac 2 when the gain is middle . when the white - side reference voltage supplied to the dac 2 is refw ′ 1 , the input / output characteristic of the dac 2 is the characteristic shown by the straight line l 1 . in this case , if the input signals changing from the black level to the white level are inputted to the dac 2 periodically , the output wave shown by the curve s 1 is obtained . accordingly , the total luminance becomes high . when the white - side reference voltage supplied to the dac 2 is refw ′ 3 , the input / output characteristic of the dac 2 is the characteristic shown by the straight line l 3 . in this case , if the input signals changing from the black level to the white level are inputted to the dac 2 periodically , the output wave shown by the curve s 3 is obtained . accordingly , the total luminance becomes low . when the white - side reference voltage supplied to the dac 2 is refw ′ 2 , the input / output characteristic of the dac 2 is the characteristic shown by the straight line l 2 . in this case , if the input signals changing from the black level to the white level are inputted to the dac 2 periodically , the output wave shown by the curve s 2 is obtained . accordingly , the total luminance becomes middle of the total luminance in the case where the white - side reference voltage supplied to the dac 2 is refw ′ 1 and the total luminance in the case where the white - side reference voltage is refw ′ 3 . that is , the total luminance is controlled by controlling the gain given to the total luminance control circuit 200 according to the kind of the content displayed on the organic el display 114 . according to the sixth embodiment , generation of the phosphor burn - in phenomenon can be prevented in the cellular phone comprising the organic el display and the degradation of performance of the organic el element can be reduced . although the present invention has been described by way of exemplary embodiments , it should be understood that many changes and substitutions may be made by those skilled in the art without departing from the spirit and the scope of the present invention which is defined only by the appended claims .