Patent Publication Number: US-7581880-B2

Title: Radiant-temperature measurement device, light-source temperature control device, image projection apparatus and radiant-temperature measurement method

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to radiant-temperature measurement technologies, in particular to radiant-temperature measurement devices, light-source temperature control devices, and image projection apparatus, as well as to radiant-temperature measurement methods. 
   2. Description of the Related Art 
   A spectrum according to thermal radiation or light emitted from a body depends on the temperature thereof. There is one of the methods that utilize spectrum change so as to measure the temperature, i.e. a two-color (ratio) or dual-wavelength radiant-temperature measurement method, in which temperature is obtained based on an intensity ratio of thermal radiations in two different wavelength bands; the method enables obtaining the temperature regardless of emissivity of the thermal radiation from a body. 
   However, in addition to the thermal radiation (light) from the body, when other light is superimposed as stray light, an error is imposed on the value of a temperature obtained. For example, in a case for which a discharge lamp is used as a light source, and when the temperature of electrode portions that are light-source&#39;s light-emitting ends is measured, because of strong discharge-light emission existing near by, the thermal radiation (light) from the electrode portions is resultantly measured with the discharge-light emission being superimposed thereon as the stray light. Therefore, in order to carry out the temperature measurement based on a thermal radiation spectrum, it is necessary to remove an influential component (or the amount of influence) caused by the stray light out from the measured value. As one means for removing an influential component owing to the stray light, there exists an invention described in Japanese Patent Application Publication No. JP1996-152360 (1996), Page 5, FIG. 1. In the invention, in order to remove the influential component owing to discharge-light emission, two wavelength bands are selected so as to exclude spectral peaks, of the discharge-light emission, causing the stray light, so that a dual-wavelength radiant-temperature measurement method is applied. 
   By measuring thermal radiation in a wavelength band excluding spectral peaks of the discharge-light emission, it is possible to eliminate an influence caused by the stray light up to a certain extent. However, actual spectra of discharge-light emission accompany continuous spectra over a wide range of wavelengths other than those in line spectra. As an example, spectra of discharge-light emission when an extra-high pressure mercury lamp is used as a discharge lamp are shown in  FIG. 15 . It should be understood that there exits discharge-light emission of near 700 nm wavelength when those peaks of line spectra are excluded. 
   As a purpose of measuring the electrode temperature of a lamp, a procedure is raised in which the characteristics of the lamp are investigated so as to extend its operating life. For this purpose, it is necessary to measure temperature of an electrode&#39;s tip or end, namely, the temperature of an electrode&#39;s portion in the very close proximity of which strong discharge-light emission exists. At such a portion, even when a tow-color (ratio) or dual-wavelength temperature measurement is carried out excluding wavelength bands in which discharge-light peaks exist, a stray-light component caused by discharge-light emission has been too large to achieve an accurate measurement of the portion&#39;s temperature. As described above, spectra of the stray light intended to be removed more or less have continuous components. For this reason, an influence caused by the stray light cannot be completely removed; because remaining influential components cannot be ignored, there has been a problem in that an accurate temperature measurement cannot be made. 
   The present invention has been directed at solving these problems described above, and an object of the invention is to provide a temperature measurement device and its method by removing stray-light components from a measured value of thermal radiation (light) on which stray light is superimposed. 
   In general, because there has not been a means for measuring electrode&#39;s temperature of a discharge lamp in real time, the lamp cannot be driven while the electrode&#39;s temperature is being controlled. 
   In addition, although a discharge lamp is generally used for an image projection apparatus as a light source, because the electrode&#39;s temperature cannot be controlled at a constant level, there has been a problem in that useful life of the discharge lamp is shortened, requiring frequent replacement of the lamp. 
   SUMMARY OF THE INVENTION 
   In one aspect of the present invention, a radiant-temperature measurement device comprises: 
   a first measurement unit for measuring, from among light-source light emitted from a light source having light-emitting ends in which a pattern of variation in light intensity versus time is repeated at predetermined periods each including a first intensity duration and a second intensity duration during which the second intensity differs from the first intensity, intensity of first light originating in the light-source light; 
   a second measurement unit for measuring intensity of second light, in which stray light originating in the light-source light is superimposed onto thermal radiation emitted from one of the light-source&#39;s light-emitting ends, in each of a first wavelength band and a second wavelength band, whose wavelengths differ from each other; 
   a time-based data measurement unit for acquiring, based on the intensity of the first light, a first data value during a first measuring interval in the first intensity duration, and a second data value during a second measuring interval in the second intensity duration, and for acquiring, based on the intensity of the second light, a third data value in the first wavelength band, and a fourth data value in the second wavelength band, during the first measuring interval, and a fifth data value in the first wavelength band, and a sixth data value in the second wavelength band, during the second measuring interval; and 
   a temperature calculation unit for calculating, based on the first, second, third, fourth, fifth, and sixth data values, the temperature of the light source at its light-emitting end. 
   In another aspect of the present invention, a radiant-temperature measurement device comprises: a measurement unit for measuring, from among light-source light emitted from a light source having light-emitting ends in which a pattern of variation in light intensity versus time is repeated at predetermined periods each including a first intensity duration and a second intensity duration during which the second intensity differs from the first intensity, intensity of first light originating in the light-source light, during a predetermined measuring interval, in either of a first wavelength band or a second wavelength band, whose wavelengths differ from each other, and for measuring intensity of second light, in which stray light originating in the light-source light is superimposed onto thermal radiation emitted from one of the light-source&#39;s light-emitting ends, during a measuring interval that differs from the predetermined measuring interval, in each of a first wavelength band and a second wavelength band, whose wavelengths differ from each other; 
   a drive unit for changing either a position or a direction of the measurement unit so that, during the predetermined measuring interval, the measurement unit measures the intensity of the first light, and during the measuring interval that differs from the predetermined measuring interval, the measurement unit measures the intensity of the second light; 
   a time-based data measurement unit for acquiring, based on the intensity of the first light, a first data value in the first intensity duration and during a first measuring interval in which the measurement unit measures the intensity of the first light, and a second data value in the second intensity duration and during a second measuring interval in which the measurement unit measures the intensity of the first light, and for acquiring, based on the intensity of the second light, a third data value in the first wavelength band, and a fourth data value in the second wavelength band, in the first intensity duration and during the third measuring interval in which the measurement unit measures the intensity of the second light, and a fifth data value in the first wavelength band, and a sixth data value in the second wavelength band, in the second intensity duration and during the fourth measuring interval in which the measurement unit measures the intensity of the second light; and 
   a temperature calculation unit for calculating, based on the first, second, third, fourth, fifth, and sixth data values, the temperature of the light source at its light-emitting end. 
   In yet another aspect of the present invention, a light-source temperature control device comprises: a cooling unit for reducing the temperature of light-source&#39;s light-emitting ends; a radiant-temperature measurement device for measuring the temperature of a light-emitting end of the light source; and a temperature control unit, into which is inputted the light-source&#39;s light-emitting-end temperature obtained by the radiant-temperature measurement device, for controlling, based on the temperature of the light-source&#39;s light-emitting end, the operation of the cooling unit. 
   In yet another aspect of the present invention, an image projection apparatus comprises: a light source; an image display element; an image-projection optical system for shining light-source light from the light source onto the image display element, so as to project images on the image display element into a predetermined position; and the light-source temperature control device described above. 
   In yet another aspect of the present invention, a radiant-temperature measurement method comprises: a first step of measuring, from among light-source light emitted from a light source having light-emitting ends in which a pattern of variation in light intensity versus time is repeated at predetermined periods each including a first intensity duration and a second intensity duration during which the second intensity differs from the first intensity, intensity of first light originating in the light-source light; 
   a second step of measuring intensity of second light, in which stray light originating in the light-source light is superimposed onto thermal radiation emitted from one of the light-source&#39;s light-emitting ends, in each of a first wavelength band λ 1  and a second wavelength band λ 2 , whose wavelengths differ from each other; 
   a third step of acquiring, based on the intensity of the first light, a first data value a 0  during a first measuring interval in the first intensity duration, and a second data value b 0  during a second measuring interval in the second intensity duration, and of acquiring, based on the intensity of the second light, a third data value a(λ 1 ) in the first wavelength band λ 1 , and a fourth data value a(λ 2 ) in the second wavelength band λ 2 , during the first measuring interval, and a fifth data value b(λ 1 ) in the first wavelength band λ 1 , and a sixth data value b(λ 2 ) in the second wavelength band λ 2 , during the second measuring interval; and 
   a fourth step of calculating the temperature of the light source at its light-emitting end in such a way that, by substituting the first data value a 0 , the second data value b 0 , the third data value a(λ 1 ), the fourth data value a(λ 2 ), the fifth data value b(λ 1 ), and the sixth data value b(λ 2 ) into Equation (1) and Equation (2), thermal radiation intensity I 1  in the first wavelength band λ 1  and thermal radiation intensity I 2  in the second wavelength band λ 2 , in the thermal radiation emitted from one of the light-source&#39;s light-emitting ends, are separated and evaluated each other, and simultaneously by substituting the thermal radiation intensity I 1  in the first wavelength band λ 1  and the thermal radiation intensity I 2  in the second wavelength band λ 2  into Equation (3), the intensity ratio R is obtained, and retained data showing the relationship between the intensity ratio R and the temperature is referred. 
   
     
       
         
           
             
               
                 
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   According to a radiant-temperature measurement device in the present invention, by utilizing the difference between characteristics of intensity variation versus time of thermal radiation (light) and stray light, an influence caused by stray light mixed in measured values of the light intensity is estimated; thereby, because only the thermal radiation components can be extracted, even in a case in which a large stray-light source exits at light-emitting ends, the temperature of light-emitting ends each can be accurately measured based on a thermal radiation measuring principle. 
   According to a light-source temperature control device in the present invention, by utilizing the difference between characteristics of intensity variation versus time of thermal radiation (light) and stray light, an influence caused by stray light mixed in measured values of the light intensity is estimated; thereby, only the thermal radiation components can be extracted. Based on the thermal radiation measuring principle, because a radiant-temperature measurement device that can accurately measure the temperature of light-emitting ends each is applied, even in a case in which a large stray-light source exits at the light-emitting ends, by applying a feedback control using a measured temperature signal, the temperature of light-emitting ends of a light source can be held within a constant range. By this arrangement, the operating life of the light source can be extended. 
   According to an image projection apparatus in the present invention, a light-source temperature control device that controls the temperature of light-emitting ends of a light source to be held within a constant range is implemented. Therefore, because the operating life of the light source can be extended, replacement frequency of the light source ascribable to a life of the light source can be reduced; because of reduction in maintenance hours, it is possible to expect an increase in usage of the image projection apparatus, and a decrease in maintenance costs. 
   According to a radiant-temperature measurement method in the present invention, by utilizing the difference between characteristics of intensity variation versus time of thermal radiation (light) and stray light, an influence caused by stray light and mixed in measured values of the light intensity is estimated; thereby, because only the thermal radiation components can be extracted, even in a case in which a large stray-light source exits at the light-emitting ends, the temperature of light-emitting ends each can be accurately measured based on the thermal radiation measuring principle. 
   The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
   The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram outlining a configuration of a radiant-temperature measurement device in Embodiment 1 of the present invention; 
       FIG. 2  is a waveform of a current supplied to a light source under an alternating-current (AC) drive without a superimposed pulse in Embodiment 1 of the present invention; 
       FIG. 3  is a waveform of a current supplied to the light source under an AC drive with superimposed pulses in Embodiment 1 of the present invention; 
       FIG. 4  is a waveform showing a variation of discharge-light emission intensity generated under the AC drive with superimposed pulses in Embodiment 1 of the present invention; 
       FIG. 5  is a waveform showing a temperature variation of an electrode portion of a lamp in Embodiment 1 of the present invention; 
       FIG. 6  is a waveform showing a time-based variation of measured light-intensity by using the radiant-temperature measurement device in Embodiment 1 of the present invention; 
       FIG. 7  shows waveforms indicating time-based variations of measured values by using a main detection unit and a subordinate detection unit of the radiant-temperature measurement device, and extraction intervals (time-frames) of the measured values used for temperature calculation; 
       FIG. 8  is a diagram showing a temperature-calibration chart used for the radiant-temperature measurement device in Embodiment 1 of the present invention; 
       FIG. 9  is a diagram showing the lamp that is an object to be measured by the radiant-temperature measurement device in Embodiment 1 of the present invention; 
       FIG. 10  is a diagram showing a measured result by using the radiant-temperature measurement device in Embodiment 1 of the present invention; 
       FIG. 11  is a diagram outlining a configuration of a radiant-temperature measurement device in Embodiment 2 of the present invention; 
       FIG. 12  is a waveform showing a variation of discharge-light emission intensity generated under an AC drive without superimposed pulses in Embodiment 3 of the present invention; 
       FIG. 13  is a diagram outlining a configuration of a light-source temperature control device in Embodiment 5 of the present invention; 
       FIG. 14  is a diagram outlining a configuration of an image projection apparatus in Embodiment 6 of the present invention; and 
       FIG. 15  is a spectrum diagram of discharge-light emission for explaining existence of stray light even when wavelengths at which discharge-light emission peaks are avoided in a conventional example. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereunder, preferred embodiments according to the present invention will be described in detail with reference to the accompanying drawings. 
   Embodiment 1 
   A radiant-temperature measurement device in Embodiment 1 of the present invention is shown  FIG. 1 . In  FIG. 1 , “100” is a radiant-temperature measurement device, and “200” is a light-source unit including a measuring object of which the radiant-temperature measurement device  100  measures temperature. 
   Hereinafter, a discharge lamp is taken into consideration as an example of a light source, and will be explained as follows. The light-source unit  200  is constructed of a discharge lamp  1  that is the light source, light-source drive circuitry  4 , and cables interconnecting the both. In the discharge lamp  1 , electrode portions  2  each and electrode ends  3  each that are protruded end-portions (tips) of the electrode portions  2  are formed as pairs, respectively, and are placed to oppose each other. Between the opposing electrode portions  2  or electrode ends  3 , a voltage is supplied by way of the light-source drive circuitry  4  so that a discharge is produced between the opposing electrode ends  3 , resulting in light emission by the discharge. Therefore, the electrode portions  2  or electrode ends  3  are light-emitting ends of the discharge light used as light-source light. The temperature of the light-emitting ends is thus a subject of measuring interest in the present invention. 
   Hereinafter, the explanation will be proceeded so that the electrode portions  2  are the light-emitting ends of the light-source light that is a temperature-measuring object. However, each of the electrode portions  2  is a conceptual portion that includes belonging one of the electrode ends  3 . 
   The radiant-temperature measurement device  100  is provided to detect spectra of light due to thermal radiation that is radiated from the electrode portions  2  each as a measuring object, and to measure the temperature at the electrode portions  2  each based on the principle of a dual-wavelength radiant-temperature measurement method. The radiant-temperature measurement device  100  is constructed of a detection unit  50  (or a measurement unit in which the numeral is omitted in  FIG. 1 ), a time-based analysis unit  60 , and a temperature calculation unit  70 . The detection unit  50 , that is the measurement unit, is constructed of a main detection unit  51  that is a second measurement unit, and a subordinate detection unit  52  that is a first measurement unit. The main detection unit  51  is provided as a detection system that detects the light from the electrode portions  2  each as a temperature-measuring object, and is constructed of one part of a space analysis unit  30  and a spectrum analysis unit  40 . The subordinate detection unit  52  is provided as another detection system that only detects discharge-light emission as a temperature-measuring object, and is constructed of the other part of the space analysis unit  30  and a following light detector  24 . 
   First, the main detection unit  51  will be explained as follows. The part of the space analysis unit  30  of the main detection unit  51  has a function to limit a representative measuring object to the light from one of the electrode portions  2  of the discharge lamp  1 . The part of the space analysis unit  30  of the main detection unit  51  is constructed of a projection unit  5  that forms the light, as a measuring object, emitted from either of the electrode portions  2  of the discharge lamp  1 , onto a predetermined imaging surface as an optical image, and a screen  6  that has a pinhole  7  located on the imaging surface. By adjusting the pinhole  7  to coincide with the imaging point of light, basically, the light emitted from one of the electrode portions  2  passes through the pinhole  7 , and the passed light is guided into the following spectrum analysis unit  40 . 
   Although the light emitted from the electrode portions  2  is mainly originating in thermal radiation, discharge-light emission generated in the proximity of the electrode portions  2  is mixed as stray light onto the measuring light. Note that, it is possible to confirm that the measurement is carried out for the light emitted from one of the electrode portions  2  by forming an optical image onto the screen  6  before adjusting a position of the pinhole  7  to a light-imaging position, and by directly making visual observations. In addition, it is also possible to confirm by taking an exposure of the screen  6  using a camcorder. Moreover, as a method to adjust a light-imaging position to the pinhole  7  on the screen  6 , it is also possible to confirm by shifting a position of the projection unit  5 ; on the other hand, it is also possible to confirm by changing a position of the pinhole  7  by shifting the screen  6 . 
   The spectrum analysis unit  40  is constructed of a dual-wavelength spectral unit  8  that performs a spectral distribution of the light emitted from one of the electrode portions  2  of the discharge lamp  1 , having been selected as a measuring object by the part of space analysis unit  30 , into wavelength bands represented by two specific wavelengths λ 1  and λ 2  (hereinafter referred to as a “wavelength band λ 1 ” and a “wavelength band λ 2 ” for brevity), and two light detectors, i.e., a light detector  9  (for a channel  1 ) and a light detector  10  (for a channel  2 ) that are able to measure spectrally distributed light in the wavelength bands λ 1  and λ 2  in fast response time, respectively. Signals measured for the light in the wavelength bands λ 1  and λ 2  by using the light detectors  9  and  10  are inputted into a time-based analysis unit  60  via signal lines  11  and  12 , respectively. 
   Next, the subordinate detection unit  52  will be explained as follows. The part of the space analysis unit  30  of the subordinate detection unit  52  is provided for the purpose of measuring the discharge-light emission only. The part of the space analysis unit  30  of the subordinate detection unit  52  is constructed of a projection unit  21  that forms the light from one predetermined point (position), among points in discharge-light emission generated between the opposing electrode ends  3  of the discharge lamp  1 , onto a predetermined imaging surface as an optical image, and a screen  22  that has a pinhole  23  located on the imaging surface. 
   In order to measure the discharge-light emission only, the subordinate detection unit  52  is placed by changing or adjusting either a position or a direction thereof, so that the discharge-light emission is measured at a position distant from the electrode ends  3 . As the confirmation method, as explained above, it is possible to confirm by forming an optical image onto the screen  22  before adjusting a position of the pinhole  23  to a light-imaging position, and by directly making visual observations. In addition, it is also possible to confirm by taking an exposure of the screen  22  using a camcorder. The light detector  24  is able to measure the light passed through the pinhole  23  in fast response time. 
   Signals measured for the light in the wavelength bands λ 1  and λ 2  outputted from the light detectors  9  and  10  of the main detection unit  51 , and a signal measured for the discharge-light emission outputted from the light detector  24  of the subordinate detection unit  52  are inputted into the time-based analysis unit  60 , respectively. The time-based analysis unit  60  has a time-based data measurement unit  14  that measures, having obtained a trigger signal (via its signal line  13 ) outputted from the light-source drive circuitry  4  that is a constituent element of the light-source unit  200 , time-based waveforms based on the light-detector signals that correspond to time-based variation of the light in the wavelength bands λ 1  and λ 2 , and to the discharge-light emission. Time-based waveform signals measured for the light in the wavelength bands λ 1  and λ 2 , and a time-based waveform signal measured for the discharge-light emission, via the time-based data measurement unit  14 , are together inputted into a temperature calculation unit  70 . In the temperature calculation unit  70 , based on these waveform signals, temperature of one of the electrode portions  2  that is a final representative measuring object is measured and evaluated. 
   Hereinafter, an extra-high pressure mercury lamp that is driven in an alternating-current (AC) is taken into consideration as an example of the discharge lamp  1  composing the light-source unit  200 , and the explanation will be proceeded as follows. The extra-high pressure mercury lamp is supplied with a stepwise-changing AC current (signal) from the light-source drive circuitry  4  as its current waveform is shown in  FIG. 2 , and according to the AC current (signal), discharge-light emission is generated. However, in a case using this lamp, because a phenomenon called flicker occurs in that an intensity distribution varies largely in short time, in order to mitigate the flicker, it is a common practice to supply the stepwise-changing AC current (signal) to the lamp with pulses superimposed. Here, the explanation will be given for a temperature measurement device and its method applied to measure temperature at the electrode portions  2  of the discharge lamp  1  that is using superimposed pulses. 
     FIG. 3  shows a time-based waveform of a drive current onto which superimposed pulses are added, and supplied to the discharge lamp  1  from the light-source drive circuitry  4 . In the figure, just before a current polarity reverses, a pulse is successively added with the same polarity before the reversal, so that the increased amount in the current waveform corresponds due to the superimposed pulses each. 
     FIG. 4  shows time-based variation of discharge-light emission intensity (variation versus time) in a case in which the superimposed pulses are added, i.e. the lamp is AC driven under the time-based current as its waveform is shown in  FIG. 3 . From  FIG. 4 , it can be understood that discharge-light emission intensity increases corresponding to the time-duration the superimposed pulses each are added. 
   In this embodiment of the present invention, through the use of time-based variation of light-emission intensity when the superimposed pulses are added, influential components caused by the discharge-light emission that is mixed as stray light onto the measuring-object light are evaluated. And then, from the influential components caused by the discharge-light emission, contributory components in thermal radiation spectra are separated and evaluated; from the result obtained, temperature at the electrode portions is derived. 
   In this embodiment of the present invention, a basic principle of the temperature measurement is described more in detail as follows. 
   When the superimposed pulses are added to the electrode portions  2 , discharge-light emission intensity not only increases, but also does electrode temperature in theory. Although, when the superimposed pulses are added, discharge-light emission intensity increases instantaneously; however, response time of temperature variation at the electrode portions  2  is extremely slow. Therefore, in a case of a drive current waveform in which the superimposed pulses short in width and small in magnitude (low in pulse-height) are added as shown in  FIG. 3 , it can be treated that the temperature does not change in practice. For example, when AC frequency of the current is 150 Hz, as can be understood from  FIG. 4 , one period of time of the light intensity is doubled with respect to the AC frequency; i.e., the frequency of the light intensity is 300 Hz, and its one period is as short as 3.3 ms. Although each of the superimposed pulses is added onto in the same period (i.e., in every 3.3 ms), the duration of each of the pulses is still about a fraction of the period. Therefore, in such a short time or duration, it can be presumed that the temperature at the electrode portions  2  is kept at a constant level. 
     FIG. 5  shows temperature variation versus time at an end of one of the electrodes when the superimposed pulses are added in the same time scale (the horizontal axis) of  FIG. 3  and  FIG. 4 . As can be understood from  FIG. 5 , it can be presumed that the temperature at the electrode portions  2  is kept at a constant level without depending on time in the degree of period of the discharge-light emission. Therefore, it can be presumed that intensity of the thermal radiation and its spectra emitted from the electrode portions  2  are also kept at constant levels without depending on time in the degree of period of the discharge-light emission. 
   By utilizing the characteristics in that, in a predetermined time range, on one hand, intensity of the discharge-light emission varies depending on time, on the other hand, intensity of the thermal radiation does not vary depending on time, influential components caused by the discharge-light emission are evaluated and then removed from the measured values, having been obtained via the light detectors  9  and  10  of the main detection unit  51 , by applying the following principle; thus, components in the signals originating in the thermal radiation are only separated. 
   Because the explanation for the light detectors  9  and  10  is similar, hereinafter, time-based waveform signals relating to the light detector  9  and the light detector  24  will be explained as follows. A subject matter is to remove influential components caused by the discharge-light emission and mixed into the signal outputted from the light detector  9 , and to evaluate a thermal radiation component only. 
   In  FIG. 6 , when the superimposed pulses are added onto the AC drive current supplied to the discharge lamp  1 , and the light emitted from one of the electrode portions  2  of the discharge lamp  1  is detected by the light detector  9  and its outputted signal is measured by the time-based data measurement unit  14 , a time-based waveform of measured light-intensity (variation versus time) at a wavelength band λ 1  is shown. This measured value (light-intensity) is equivalent to the sum of both measured vales, i.e. one originating in the thermal radiation that does not vary in time, and the other originating in the stray light caused by the discharge-light emission that varies in time and having been mixed into the light detector  9 . 
   On the other hand, a time-based waveform of measured light-intensity, when the signal outputted from the light detector  24  of the subordinate detection unit  52  is measured by the time-based data measurement unit  14 , comes out to be a similar time-based waveform to the time-based variation of the discharge-light emission shown in  FIG. 4 . In both the time-based waveforms, the varied amount in the measured light-intensity originating in the superimposed pulses is in either case originated in the discharge-light emission, and other factors are not included. For this reason, when a position that is a measuring object is the same, the varied amount should be the same value. When a measuring-object&#39;s position is different, light intensity is usually different; because of this, the varied amount of the measured values each originating in the superimposed pulses comes out to be proportional to the difference of light intensity. 
   The difference of light intensity according to the difference at the measuring-object&#39;s position has nothing to do with the existence of the superimposed pulses. Therefore, a ratio of the varied amount of the measured values each originating in the superimposed pulses and according to the difference at the measuring-object&#39;s position is directly equal to a ratio of the difference in the measured light-intensity originating in the discharge-light emission and according to the difference at the measuring-object&#39;s position, when the superimposed pulses are not added. 
   By using  FIG. 7 , the above principle will be explained in more specific detail as follows. In  FIG. 7 , when an AC drive current in which the superimposed pulses are added as shown in  FIG. 3  is supplied to the discharge lamp  1 , variations in measured values versus time are shown in which two waveforms are originated in the light from one of the electrode portions  2  of the discharge lamp  1 , and another waveform is originated in the discharge-light emission that is only a measuring object. 
   In  FIG. 7 , “53” and “54” indicate time-based waveforms of measured light-intensity (variations each versus time) in wavelength bands λ 1  and λ 2  having been obtained via the time-based data measurement unit  14  by measuring the signals inputted from the light detectors  9  and  10  of the main detection unit  51 , respectively. “55” indicates a time-based waveform of measured light-intensity (variation versus time) having been obtained via the time-based data measurement unit  14  by measuring the signal inputted from the light detector  24  of the subordinate detection unit  52 . Therefore, “53” and “54” indicate the light from one of the electrode portions  2  of the discharge lamp  1 ; namely, they show the time-based waveforms of light intensity in which stray light caused by the discharge-light emission is mixed with thermal radiation. “55” shows the time-based waveform of the light intensity originated in the discharge-light emission only. Hereinafter, “53” and “54” are referred to as “mixed-light intensity time-based waveforms,” and “55” is referred to as a “discharge-light intensity time-based waveform.” 
   In  FIG. 7 , in the time-based waveforms of light intensity each, the part in which the light intensity is rapidly increased corresponds to a cause originating in the drive current in which the superimposed pulses are added. In the figure, a time duration  56  (double-headed arrow) shows a duration of time (time-frame) in which influences caused by the superimposed pulses and by variation in light intensity derived from polarity reversals in the drive current are not present. A time duration  57  (double-headed arrow) shows a duration of time (time-frame) in which the light intensity is increased by one of the superimposed pulses, and is kept stable at the increased level. 
   Here, as shown in  FIG. 7 , based on the mixed-light intensity time-based waveforms  53  and  54 , and on the discharge-light intensity time-based waveform  55 , measured light-intensity values a 1 , a 2 , and a 0  each are obtained during a measuring interval  1  in the time duration  56 , respectively; and then, measured light-intensity values b 1 , b 2 , and b 0  each are obtained during a measuring interval  2  in the time duration  57 , respectively. 
   Here, “a 1 ” and “b 1 ” are the measured values with respect to the wavelength band λ 1 , and “a 2 ” and “b 2 ” are the measured values with respect to the wavelength band λ 2 ; each of the four values is the measured value via the main detection unit  51 . “a 0 ” and “b 0 ” are the measured values via the subordinate detection unit  52 . 
   In order to obtain the values in the same measuring interval, by selecting a trigger signal generated in every period of time based on the drive current signal inputted from the light-source drive circuitry  4  (via its signal line  13 ) into the time-based data measurement unit  14  as a starting time-point, each of the measured values can be sampled after a predetermined time has elapsed. The predetermined time is set in advance by carrying out a preliminary measurement. 
   Moreover, instead of obtaining these values during each measuring interval or at a time point, they may be a mean value over a predetermined time. In the time durations  56  and  57  each, when there is no factor exits that varies a measured value largely other than a stochastic error, or when it is determined that the degree of variation is not a problem, by adopting a time-based mean value, it is possible to reduce the stochastic error or the like in the measured value. Therefore, it is possible to achieve increasing accuracy of a measured temperature value, as this will be described hereunder. 
   Furthermore, simultaneousness of the values a 1 , a 2 , and a 0 , and simultaneousness of the values b 1 , b 2 , and b 0 , are not necessarily precise. Namely, the measuring interval  1  may be during any time duration the superimposed pulses are not added; and then, the measuring interval  2  may be during any time duration in which the superimposed pulses are added, and the light intensity is increased reaching up to a constant value. 
   In addition, instead of selecting in the period of light intensity variation, but even in different periods, it may be possible to select that the measuring interval  1  is during any time duration the superimposed pulses are not added, and the measuring interval  2  is during any time duration in which the superimposed pulses are added and the light intensity is increased reaching up to a constant value. Namely, the measuring intervals  1  and  2  include the expanded meaning described above (and, e.g. a scope of their intermittent selection). 
   The measured light-intensity values a 1 , a 2 , and a 0 , and also b 1 , b 2 , and b 0 , having been measured in the time-based data measurement unit  14  are inputted into the temperature calculation unit  70 ; there, a temperature is derived based on the following principle. 
   First, because the measured light-intensity values a 1 , a 2 , b 1 , and b 2 , each are measured values in the main detection unit  51 , stray light caused by the discharge-light emission is mixed onto the thermal radiation; thus, each of them can be regarded as a sum of both the thermal radiation and the discharge-light emission. Hereinafter, “a 1 ” is taken as an example for explanatory purposes. A similar analysis can be held for “a 2 ” also. 
   “a 1 ” can be expressed by the following equation.
 
 a 1 =I 1 +d 1   Equation (4)
 
   Here, “I 1 ” denotes a contributory component (contribution) of the thermal radiation, and “d 1 ” denotes an influence caused by the discharge-light emission. 
   Next, in a case of the mixed-light intensity time-based waveform  53 , and also in a case of the discharge-light intensity time-based waveform  55 , the increased amount of light intensity according to the superimposed pulses is, as explained above, due to the discharge-light emission. Therefore, the increased amount of light intensity according to the superimposed pulses obtained from the mixed-light intensity time-based waveform  53 , i.e. “b 1 −a 1 ” is originating in the discharge-light emission. Similarly, the increased amount of light intensity according to the superimposed pulses obtained from the discharge-light intensity time-based waveform  55 , i.e. “b 0 −a 0 ” is originating in the discharge-light emission. The difference between the both is, as a result, caused by the difference of intensity in the discharge-light emission included in the corresponding measuring-object light. 
   In addition, in the time-based waveform  55 , a ratio between the measured light-intensity value b 0  in which the superimposed pulses are added and the measured light-intensity value a 0  in which they are not added, i.e. “b 0 /a 0 ” does not depend on the discharge-light emission intensity, and the ratio should be constant. Therefore, with respect to the influential component “d 1 ” of discharge-light emission included in the mixed-light intensity time-based waveform  53 , the above characteristic holds true of the ratio “b 0 /a 0 ” having been derived based on the discharge-light intensity time-based waveform  55 . Thus, “d 1 ” satisfies the next relationship.
 
(( b 1 −a 1)+ d 1)/ d 1 =b 0 /a 0  Equation (5)
 
   From Equation (5), the next equation can be derived that expresses for “d 1 .”
 
 d 1 =a 0 ×r   Equation (6)
 
 r =( b 1 −a 1)/( b 0 −a 0)  Equation (7)
 
   Here, “r” denotes, when the discharge-light emission intensity measured via the subordinate detection unit  52  is set as a reference, a degree or rate of influence caused by the discharge-light emission mixed as the stray light into the measurement via the main detection unit  51 , namely, an influential rate of discharge-light emission. 
   Thus, “I 1 ” that is a contributory component of the thermal radiation included in the measured value “a 1 ” with respect to the wavelength band λ 1  can be derived by the following equation.
 
 I 1 =a 1 −a 0×( b 1 −a 1)/( b 0 −a 0)  Equation (8)
 
   “I 2 ” that is a contributory component of the thermal radiation included in the measured value “a 2 ” with respect to the wavelength band λ 2  can be similarly derived by the next equation.
 
 I 2 =a 2 −a 0×( b 2 −a 2)/( b 0 −a 0)  Equation (9)
 
   As shown in the next equation, an intensity ratio “R” that is a ratio of both the values “I 1 ” and “I 2 ” is derived.
 
 R=I 1 /I 2  Equation (10)
 
   And now, a spectrum of thermal radiation emitted from a portion depends on temperature. Namely, a ratio of light intensities between the wavelength bands λ 1  and λ 2  depends on a temperature T. Therefore, the intensity ratio R that is a ratio between the measurement-based evaluation vales “I 1 ” and “I 2 ” has also a one-to-one corresponding relationship with respect to the temperature T of the electrode portions  2 . Thus, when calibration data between the intensity ratio R and the temperature T is obtained in advance and retained to be referred, base on the intensity ratio R derived from above Equation (4) through Equation (10), the temperature T of the electrode portions  2  can be obtained. 
   In order to obtain the calibration data between the intensity ratio R and the temperature T, in place of a discharge lamp, a halogen lamp, a standard blackbody furnace, or the like in that stray light except thermal radiation (light) does not exit can be used. For these standard light sources, when the temperature T is measured by other means, and at the same time, the ratio between the measured values a 1  and a 2  with respect to the wavelength bands λ 1  and λ 2  obtained via the main detection unit  51  in the present invention is taken, the intensity ratio R is obtained, so that the calibration data between the intensity ratio R and the temperature T can be obtained. 
   An example of the calibration data is shown in  FIG. 8 .  FIG. 8  shows a relationship between the intensity ratio R and an estimated temperature T. Actual measured examples are shown in  FIG. 9  and  FIG. 10 . P 1  in  FIG. 9  shows the discharge-light emitting portion. P 2  in  FIG. 9  shows the electrode portion. Corresponding to the positions “A” through “S” in  FIG. 9 , the temperatures are plotted in  FIG. 10 . As shown in the figures, by implementing this embodiment of the present invention, even when bright discharge-light emission exists in the very close proximity as stray light, a contributory component of the thermal radiation can be easily separated from the measured light-intensity values, and be evaluated. From the measured light-intensity values having been evaluated by separation, it is thus possible to measure the temperature on the electrodes based on the principle of the dual-wavelength radiant-temperature measurement method. 
   Moreover, by selecting the wavelength bands λ 1  and λ 2  outside a peak wavelength range of discharge-light emission spectrum, an influential component of stray light caused by the discharge-light emission to the measured values can be reduced from the beginning to a certain extent. By applying this embodiment of the present invention, it is possible to measure the temperature of light-emitting ends more accurately. 
   For example, two measuring wavelengths are set near the values such as λ 1 =700 nm, and λ 2 =800 nm. When the two wavelengths are selected, as can be understood from  FIG. 15  described above, because the peak wavelength range of the discharge-light emission spectrum is avoided, stray light caused by the discharge-light emission can be further lowered. As a matter of course, a wavelength band with longer wavelengths may be selected, or a wavelength near 530 nm which corresponds to a ‘valley’ between the discharge-light peaks may be selected. 
   However, it is also necessary to consider a sensitivity-spectrum characteristic of a light detector, and a wavelength band that has a large variation in the thermal radiation spectrum according to the temperature variation in a measuring-object&#39;s temperature range. By considering these points, when a spectrum of the discharge-light emission is similar to the one shown in  FIG. 15 , it will be a best choice when two wavelength bands in near-infrared are selected. 
   As the light detector to be used here, because it is necessary to respond fast in time according to a variation of the discharge-light emission when the superimposed pulses are added, a fast response characteristic is preferable. At the same time, in order to detect very weak light which is spectrally distributed, having high sensitivity is also preferable. To be more specific, it is preferable to use a photoelectron multiplier, an avalanche photodiode, or the like. 
   Embodiment 2 
   In this embodiment, a main detection unit  51 A combines the function of the subordinate detection unit  52  specifically provided for measuring the discharge-light emission intensity in Embodiment 1.  FIG. 11  shows a configuration of a radiant-temperature measurement device in this embodiment. A detection system or unit, that is a measurement unit, is constituted of the main detection unit  51 A only, and a drive mechanism  20  that is a drive unit is equipped for the main detection unit  51 A so that either a position or a direction of a space analysis unit  30 A in the main detection unit  51 A is changeable or adjustable. By providing the drive mechanism  20 , a measuring-object&#39;s position via the projection unit  5  is so arranged that, by time sharing, the position can be changed between one of the electrode portions  2  and a discharge-light emitting portion of the discharge lamp  1 . In  FIG. 11 , the same reference numerals and symbols designate the same items as or items corresponding to those shown in  FIG. 1  in Embodiment 1; thus, their explanation is omitted. 
   Moreover, as an object to adjust either the position or the direction, not only the space analysis unit  30 A, but also the spectrum analysis unit  40  may be together constructed to be changeable or adjustable. In addition, although the main detection unit  51 A has a spectral measurement function, and is provided to measure light intensity in specific wavelength bands, the subordinate detection unit  52  in Embodiment 1 does not have the spectral measurement function. However, this has little effect on the present invention, in particular. This is because a spectrum of the discharge-light emission does not change, and its intensity only changes depending on measuring-object&#39;s position. Namely, a time-based waveform of the discharge-light intensity that is spectrally measured is identical to the one that is not spectrally measured. For this reason, there is no inconvenience in that the main detection unit  51 A substitutes the subordinate detection unit  52 . 
   As a measurement procedure in this case, first, by using the drive mechanism  20 , the space analysis unit  30 A is shifted, so that a measuring-object&#39;s position via the projection unit  5  is set at the discharge-light emitting portion. Then, a time-based waveform of the discharge-light emission that is a source of stray light is measured. Subsequently, by using the drive mechanism  20 , the space analysis unit  30 A is shifted again, so that a measuring-object&#39;s position via the projection unit  5  is set at one of the electrode portions  2  that is a primary measuring object. Then, a time-based waveform of light emission from one of the electrode portions  2  is measured. Moreover, as a method to adjust a light-imaging position to the pinhole  7  on the screen  6 , it is also possible to confirm by shifting a position of the projection unit  5 ; on the other hand, it is also possible to confirm by changing a position of the pinhole  7  by shifting the screen  6 . In either case, this can be carried out via the drive mechanism  20 . 
   In this manner, by providing the drive mechanism  20 , and by applying the time sharing so as to shift the measuring-object&#39;s positions in the main detection unit  51 A, it combines the function of the subordinate detection unit  52 . By this way, an expensive detection system or unit can be eliminated by one channel; thus, it is possible to reduce in expenses, and to save space. 
   Embodiment 3 
   In this embodiment, there are features in that superimposed pulses are not added. There are some cases in which a lamp drive method does not apply the superimposed pulses; however, in such cases, according to the present invention as described in this embodiment, similarly to the manners as set forth in Embodiment 1 or Embodiment 2, components originating in thermal radiation can be separated from measured time-based light-intensity waveforms, and be evaluated. When superimposed pulses are not added, the drive current waveform is shown in  FIG. 2 , as this has already been explained above.  FIG. 12  is a time-based waveform showing discharge-light emission intensity of the discharge lamp  1  when it is driven under such a condition.  FIG. 12  and  FIG. 2  are shown in the same time scale (the horizontal axis); according to  FIG. 12 , it can be understood that, when the drive current (so its positive or negative polarity) is reversed at recurring intervals of time as shown in  FIG. 2 , the discharge-light emission intensity is lowered, correspondingly. In this embodiment, time-based variation in the discharge-light emission intensity is utilized, so that a contributory component in a thermal radiation spectrum of one of the electrode portions is separated from measured values. 
   Although in  FIG. 7 , measured light-intensity results are shown when the superimposed pulses are added, the measured values are simultaneously lowered at the time when each of the superimposed pulses goes down (i.e. on the trailing edge of pulse), namely when the drive current reverses its polarity. From this viewpoint, similar lowering tendencies have been demonstrated when the superimposed pulses are not added (as shown in  FIG. 12 ). Therefore,  FIG. 7  is also used for explaining this embodiment. Namely, as shown in  FIG. 7 , when the drive current reverses its polarity, a “dip” or U-shaped duration  58  (time-frame) appears in each of the measured time-based light-intensity waveforms (measured light-intensity values c 0 , c 1 , and c 2  each are shown in the figure corresponding to the measuring points for each dip). As explained in Embodiment 1, a time-based variation of light intensity is originated in time-based variation of the discharge-light emission, but not due to a variation of the thermal radiation. 
   By also using  FIG. 7 , a measured light-intensity value is denoted as c 0 , c 1 , and c 2 , for each dip. And then, by replacing the values b 0 , b 1 , and b 2  described in Embodiment 1 by these values c 0 , c 1 , and c 2 , respectively, based on the method explained in Embodiment 1, the temperature Tat one of the electrode portions  2  can be measured. 
   Measuring intervals to measure values a 0 , a 1 , and a 2 , and also the values c 0 , c 1 , and c 2  may be taken based on a trigger signal inputted from the light-source drive circuitry  4  (via its signal line  13 ) into the time-based data measurement unit  14  as a starting point, in the same manner as described in Embodiment 1. It should be noted that, in comparison to an accuracy required for measuring intervals in Embodiment 1 of the present invention, the degree of accuracy required for the measuring intervals in this embodiment of the present invention is much higher. This is because, as shown in  FIG. 12  or  FIG. 7 , duration time of each dip is short in comparison to the duration in which the light intensity varies according to each of the superimposed pulses; thus, without measuring exactly by aiming at a measuring interval that corresponds to each dip, an error imposed in the measured value is large. 
   In this manner, according to Embodiment 3, even when superimposed pulses are not added to the drive current of a discharge lamp, by focusing on the variation of measured light-intensity at polarity reversals in the drive current, it is possible to evaluation temperature at the electrode portions  2  with ease. 
   Embodiment 4 
   In Embodiment 1, superimposed pulses are added to the AC drive current in the same polarity as the drive-current polarity; however, in this embodiment of the present invention, the polarity of superimposed pulse current is reversed with respect to the polarity of the AC drive current. Therefore, contrary to the case in Embodiment 1, discharge-light emission intensity decreases in the time duration that corresponds to each of the superimposed pulses added. Even in this case, according to the similar principle in Embodiment 1, Equation (4) through Equation (10) are equally held. Therefore, based on these equations, influences originating in the discharge-light emission can be removed, so that contributory components of the thermal radiation at the electrode portions can be separated; thus, evaluation thereof can be carried out. 
   Embodiment 5 
   In this embodiment of the present invention, by using a radiant-temperature measurement device described in any one of Embodiment 1 through Embodiment 4, while measuring temperature at the electrode portions  2  of the discharge lamp  1 , a light-source temperature control device includes functioning to curb the electrode temperature within a constant temperature range. 
   In  FIG. 13 , a configuration of the light-source temperature control device in this embodiment is shown. The light-source temperature control device is constructed of the radiant-temperature measurement device  100  as explained above, a cooling unit  80  that is a cooling means such as a cooling fan placed near the light-source unit  200  and having a function to regulate the temperature at the electrode portions  2  of a discharge lamp, and a temperature-signal feedback cable  81  that connects between the cooling unit  80  and the temperature calculation unit  70 . 
   The operations of the light-source temperature control device are explained as follows. 
   First, it is presumed that, for the temperature calculation unit  70 , a temperature range is given in advance as a pre-set temperature range under which temperature of the electrode portions  2  should be controlled. And then, the temperature calculation unit  70  measures and evaluate the temperature at the electrode portions  2  by using the radiant-temperature measurement device described in any one of Embodiment 1 through Embodiment 4. In the temperature calculation unit  70 , the temperature at the electrode portions  2 , having been measured and evaluated, is determined whether or not it is within the pre-set temperature range inputted in advance. When the measured and evaluated temperature is within the pre-set temperature range, the temperature calculation unit  70  does not take any action to the cooling unit  80 . Therefore, the cooling unit  80  continues operating in the present state. When the cooling unit  80  is a cooling fan, the amount of air flow by the cooling fan is kept at the present level. 
   When the measured and evaluated temperature is higher than the pre-set temperature range, the temperature calculation unit  70  sends a control signal to the cooling unit  80  via the temperature-signal feedback cable  81 , so as to increase the cooling capacity. When the cooling unit  80  is the cooling fan, the amount of air flow by the cooling fan is increased. 
   When the measured and evaluated temperature is lower than the pre-set temperature range, the temperature calculation unit  70  sends a control signal to the cooling unit  80  via the temperature-signal feedback cable  81 , so as to decrease the cooling capacity. When the cooling unit  80  is the cooling fan, the amount of air flow by the cooling fan is decreased. 
   Although optimum conditions as the temperature range for the electrode portions  2  may differ depending on a first priority taken from among brightness, extended operating life, flicker countermeasures, or the like, there may be cases in which, for example, a temperature range of 3700° C.±200° C. that is a little over the melting point of tungsten electrodes is taken for the control purposes. 
   In this manner, by performing a feedback control using a measured temperature signal, the operations can be kept under optimum conditions in the temperature at electrode portions; thus, it is possible to provide a stable and highly reliable lamp. As a consequence, it is further possible to extend the operating life of the light source. 
   Moreover, although in  FIG. 13 , the radiant-temperature measurement device in Embodiment 2 is shown as the radiant-temperature measurement device in this embodiment, it is also possible to apply the radiant-temperature measurement device described in Embodiment 1 in that the subordinate detection unit is separately provided with the main detection unit; there exist similar effects described above. In addition, although the temperature calculation unit  70  is used as an example and explained in this embodiment as a temperature control means or unit, instead of using the temperature calculation unit  70 , it is also possible to independently provide a temperature control means or unit that can perform the function to control the temperature at the electrode portions  2  by the cooling unit  80 . 
   Embodiment 6 
   In this embodiment of the present invention, an image projection apparatus is provided, as its configuration is shown in  FIG. 14 . The image projection apparatus shown in  FIG. 14  is constructed of the light-source unit  200 , a light-source temperature control device described in Embodiment 5 that measures temperature of electrode portions of a lamp, based on the measured temperature of electrode portions, and controls the temperature of the electrode portions within a predetermined temperature range, and an image-projection optical system  90  that shines light from the light-source unit  200  onto an image display element, and to project images on the image display element into a predetermined position of, for example, a screen. 
   The image-projection optical system  90  may be any one of the systems usually applicable to image projection apparatus. For example, it may be constructed of an image display element  92 , a projection lens  93  that is placed between the discharge lamp  1  and the image display element  92  so as to shine the light from the discharge lamp  1  onto the image display element  92 , and a condenser lens unit  91  that projects the light (images) having been shined onto the image display element  92 , and passing through it or being reflected by it. 
   In the image projection apparatus constructed as above, because the light source is used under the optimum electrode temperature conditions, the operating life of the light source can be extended, and replacement frequency of the light source ascribable to a life of the light source can be reduced; because of reduction in maintenance hours, it is possible to expect an increase in usage of the image projection apparatus, and a decrease in maintenance costs. 
   While the present invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be realized without departing from the scope of the invention.