Abstract:
A method and an apparatus for measuring viewing-angle dependent luminance (luminance characteristic) of an LCD panel by condensing radiation from LCD pixels by means of a condensing device which includes a mirror, and by forming a real image of the pixels on an CCD imaging device. Correct luminance characteristic of the LCD panel is calculated by making a correction of the data thus obtained using a predetermined condensing function. The correction removes blurring of data due to condensing effect of the condensing device, thereby preventing degradation of the resolution of the angular luminance measurement apparatus. The condensing device and the CCD imaging device are moved depending on the viewing-angle of the pixels. When the intensity of light entering the condensing device decreased by the move, the correction is performed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to an LCD displays or a semimanufactured goods therefor for use with terminal devices such as notebook type personal computers (PCs) and cellular phones, and more particularly to the methods and apparatuses of measuring viewing angle characteristics and positional characteristics of luminance of such products.  
           [0003]    2. Description of the Prior Art  
           [0004]    A typical prior art apparatus for measuring angular distribution of luminance (also referred to as luminance distribution measurement apparatus) is disclosed in FIGS. 15, 16, and 17 of Japanese Patent Early Publication 6-94515. The apparatus shown in FIG. 15 includes a radiant object in the form of liquid crystal display (LCD) panel 41a having radiant areas (pixels), a light source 42, and a photometer 43 for measuring the intensity of light. The apparatus shown in FIG. 16 includes a mounting table 44, a lens 45, and an array of charge coupled device (CCD) 46. The apparatus shown in FIG. 17 includes a CCD array 47. In what follows like reference numerals refer to like or corresponding parts throughout the specification.  
           [0005]    in the prior art apparatus shown in FIG. 15, angular distribution of luminance or viewing angle characteristic of luminance is obtained by measuring the luminance of a pixel 41a, i.e. intensity of radiation from the pixel pixels 41a, of the CCD panel 41 which is illuminated by the light source 42. The measurement is done by rotating the photometer 43 about the pixel 41a in the tangential direction perpendicular to the radiation and measure the intensity of the radiation by the photometer 43.  
           [0006]    In the prior art apparatus shown in FIG. 16, the beams of light radiated from the pixel 41 a are collimated into parallel beams of light by a lens 45 and supplied to the CCD array 46. The directional characteristic of a radiant pixel 41 a is obtained in terms of positional characteristic of the information collected on the CCD array 43.  
           [0007]    The apparatuses shown in FIGS. 15 and 16 are designed for the measurement of a characteristic angular distribution of a pixel. Apparatuses such as shown in FIG. 17 are widely used is widely used in LCD factories as means for evaluating linear defects and uneven luminance of LCD panels. The apparatus shown in FIG. 17 uses a lens to condense the light that emerges from a pixel 41 a onto a CCD array 46.  
           [0008]    In the conventional apparatus shown in FIG. 15, it is necessary to make the aperture of the lens sufficiently small in order to attain a high resolution, since the angular resolution of a lens of photometer 43 is determined by the aperture of the lens. The apparatus shown in FIG. 16 is handy in that the CCD array 46 need not be moved for the measurement. However, if the pixel density is increased in an attempt to raise the resolution, the average intensity of light per one pixel is disadvantageously decreased. Furthermore, a large aperture lens is needed to obtain a better viewing-angle characteristic, which makes the apparatus costly.  
           [0009]    The prior art apparatus of FIG. 17 has a further disadvantage that accurate information of pixels cannot be obtained due to the fact that the information, obtained in the form of image on the CCD array 47, includes angular characteristics of the pixels. Unevenness of luminance of a LCD panel due to angular luminance characteristic of each pixel is best evaluated by comparing the parallel radiation beams emitted from the comparing pixels in the same direction. However, in the apparatus of FIG. 17, the information carried by the light from the central region of the LCD panel onto the CCD array is greatly affected by the information carried by the light from peripheral regions of the LCD panel. (This can be seen from the fact that the vertical angle ψ1 of the beam starting from a peripheral region is greater than the corresponding angle ψ0 from the central region, ψ1&gt;ψ0). As a result, if the angular distribution of luminance is the same in the central and the peripheral regions of the LCD panel, the apparatus of FIG. 17 can provide different CCD outputs for the two regions. Hence, the apparatus of FIG. 17 has a drawback that must be overcome if it is usable as an apparatus for evaluating unevenness of pixels.  
         SUMMARY OF THE INVENTION  
         [0010]    It is therefore an object of the invention to provide an improved apparatus attaining a desired resolution in the measurement of angular luminance distribution by increasing the intensity of light falling on each CCD pixel as much as possible, thereby preventing the loss of sensitivity of the CCD device caused by increasing CCD density on the CCD device and preventing loss of the accuracy of the measurement due to condensation effect of the condensing lens.  
           [0011]    It is another object of the invention to provide an improved method of measuring the angular distribution of luminance and an apparatus therefor, without loosing the resolution in the measurement.  
           [0012]    It is a further object of the invention to provide a method of obtaining accurate data and an apparatus therefor through a correction of the relative position of a condensing lens or a mirror condensing the light from radiant areas when the relative position is changed.  
           [0013]    It is a still further object of the invention to provide a measurement apparatus equipped with a light receiving element such as a condensing lens or mirror and a mechanism for moving an imaging device, making expendable conventional large aperture condensing lenses and reducing the number of imaging elements.  
           [0014]    It is a still further object of the invention to provide a high-precision apparatus capable, through an appropriate data processing, of obtaining information on the angular dependency of luminance of a radiant area of a panel as viewed from an arbitrary direction by taking account of the radiation not only from particular pixels but from all the pixels of the panel, so that the invention enables inspection of the panel for defects and unevenness of luminance as viewed from an arbitrary direction.  
           [0015]    To the accomplishment of the above objects of the invention, there is provided an apparatus as defined in claim  1  for measuring viewing-angle characteristic of luminance and positional characteristic (2-dimensional characteristic) of a radiant area of an object under measurement, by moving a condensing device and an imaging device relative to said object, said condensing device condensing light from different areas (hereinafter referred to as radiant area) of said radiant object and said imaging device forming thereon a real image of said area, said apparatus comprising:  
           [0016]    a first mechanism for moving the light receiving elements of said condensing device while keeping constant the solid angle subtended by said light receiving element as viewed from the radiant area;  
           [0017]    a second mechanism for moving said imaging device in association with said condensing device, while maintaining thereon said image of said radiant area;  
           [0018]    a memory for storing a predetermined condensing function of said condensing device;  
           [0019]    a circuit for evaluating the characteristic viewing angle dependent and potion dependent luminance (hereinafter referred to as luminance characteristic) of said radiant area based on the data of said measurement and condensing function; and  
           [0020]    a display device for displaying characteristics of said object and said luminance characteristic of said area evaluated.  
           [0021]    In the apparatus of the invention as defined in claims  2  and  3 , the condensing device may have light receiving elements in the form of a lens and/or a flat/concave mirror such that the lens condenses the light reflected by the mirror.  
           [0022]    The apparatuses defined in claims  4 ,  5 , and  6  may further includes a circuit for making a correction of the measurement, when the solid angle subtended by of the light receiving element (lens or mirror) of the condensing device as viewed from the radiant area is changed by the movement of the condensing device, wherein the correction is given by the output of the imaging device measuring the luminance at a solid angle, divided by the output of the imaging device measuring the luminance at a reference solid angle.  
           [0023]    A method of measuring viewing -angle characteristic and positional characteristic of a radiant object of the invention, as defined by claim  7 , comprises steps of moving a condensing device and an imaging device relative to said radiant object so as to construct a real image of said radiant object on said imaging device, said method further comprising steps of:  
           [0024]    moving the light receiving elements of said condensing device while keeping constant the solid angle subtended by said light receiving element as viewed from the radiant area;  
           [0025]    moving said imaging device in association with said condensing device so as to form thereon the real image of said radiant area to measure the luminance thereof; and  
           [0026]    evaluating the data of said viewing angle-dependent luminance using said condensing function.  
           [0027]    The method of the invention as defined in claim  8  further includes a step of making a correction when the solid angle subtended by the light receiving element (lens or mirror) of the condensing device as viewed from the radiant area of a radiant object is altered by the movement of the condensing device, wherein the output of the imaging device representing the directional luminance characteristic of the pixel in that moved direction as viewed from the area towards the center of the light receiving element is dived by the output obtained for the radiation emitted from the object in a specified direction.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIG. 1A is a perspective view showing an arrangement of the invention;  
         [0029]    [0029]FIGS. 1B and 1C show operation of an LCD panel.  
         [0030]    [0030]FIG. 2 shows the principle of the invention.  
         [0031]    [0031]FIG. 3 graphically illustrates derivation of a condensing function used in the invention.  
         [0032]    [0032]FIGS. 4A, 4B, and  4 C shows derivation of an exemplary condensing functions, with FIG. 4A showing the output characteristic of a laser; FIG. 4B showing luminance characteristic of a laser as measured by a CCD device; and FIG. 4C showing a condensing function derived from the data shown in FIGS. 4A and 4B.  
         [0033]    [0033]FIGS. 5A, 5B,  5 C, and  5 D together illustrate the process of calculating a condensing function based on the principle of the invention, with FIGS. 5A specifically showing luminance distribution of a light emitting area (pixels), FIG. 5B showing a condensing function of a condensing device, FIG. 5C showing measured luminance distribution as measured by a CCD device, and FIG. 5D showing a luminance distribution obtained through calculations.  
         [0034]    [0034]FIG. 6 is a schematic view of a measurement apparatus according to the invention.  
         [0035]    [0035]FIG. 7 is a flowchart illustrating the concept of the methods of measurement and evaluation according to the invention.  
         [0036]    [0036]FIG. 8 shows another embodiment of the invention.  
         [0037]    [0037]FIG. 9 shows a condensing device of the invention having a mirror and lenses,  
         [0038]    [0038]FIG. 10 shows another condensing device having a mirror and lenses.  
         [0039]    [0039]FIG. 11 shows still another condensing device having a lens and a mirror.  
         [0040]    [0040]FIG. 12 shows a condensing device having a concave mirror.  
         [0041]    [0041]FIG. 13 shows another condensing device having a concave mirror.  
         [0042]    [0042]FIG. 14 shows an example of a moving mechanism for use with a condensing device and an imaging device according to the invention;  
         [0043]    [0043]FIG. 15 shows a first conventional apparatus.  
         [0044]    [0044]FIG. 16 shows a second conventional apparatus.  
         [0045]    [0045]FIG. 16 shows a third conventional apparatus. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]    Referring to FIGS. 2 through 5, the principle of the invention will be described prior to detailed description of the embodiments of the invention. For simplicity, attention will be restricted to one dimensional case. Referring now to FIG. 2, the direction that makes an angle θ with the normal line N to the LCD panel  1  will be referred to as θ-direction. When a real image of a radiant area  2  (pixels) having an angle-dependent radiation intensity g(θ), is formed on a CCD device  5  by the lens  4  oriented in θ direction and having a condensing function |(θ), the output power r(θ) of the CCD device  5  is given by Eq. (1) below.  
           r (θ)= g (θ)*|(θ)  (1)  
         [0047]    where the symbol * stands for convolution integration, or simply convolution. Definition and determination of the condensing function |(θ) through experiment will be described in detail later.  
         [0048]    For further details of the convolution integration g(θ)*|(θ)., refer to “Mathematical Formula II (Series and Fourier Analysis)” by Moriguchi Sigeichi et al. , (ed), published from Iwanami Shorten (1957). Also refer to S. G. Chamberlain et al., IEEE Transaction, ED-25, pp. 145-154(1978) for more information on the convolution integration applied to the light that has passed a lens having a given width function (which corresponds to the condensing function).  
         [0049]    Here, “condensing function” is defined as follows. When a real image of an radiant area is formed by an optical system (e.g. lenses) at its imaging position, component of radiant energy in a direction of θ with respect to a referential direction (which is the direction radiation under measurement) is defined to be the relative intensity of light condensed onto the image. For example, assuming that all the beams of light within an angular interval (δ) are condensed to forma an image, the intensity function l(θ) is given by  
                     I        (   θ   )       =     1        (       -   δ     ≦   θ   ≦   δ     )                   =     0        (       θ   &gt;   δ     ,     θ   &lt;     -   δ         )                     (   2   )                               
 
         [0050]    It is noted that if some of the light impinging on the peripheral region of the lens is lost due to for example reflection, equation l(θ) may be approximated by the following formula.  
           l (θ)= exp[−θ   2 /Σ 2 ](−δ≦−θ≦6) =0  (3)  
         [0051]    where Σ is a constant.  
         [0052]    By taking Fourier transforms of both sides of Eq. (1), the following relationship is obtained.  
           F[r (θ)]= F[g (θ)]× F[l (θ)]  (4)  
         [0053]    where F[r(θ)] stands for the Fourier transform of r(θ). In the above calculation, use is made of an important theorem (called convolution theorem) that the Fourier transform of a convolution or convolution integration is given by the product of the Fourier transforms of each integrand (formula No. 229, Moriguchi et al. , op. cit. ) Hence,  
           F[g (θ)]= F[r (θ)]/ F[l (θ)]  (5)  
         [0054]    Since g(θ) is the inverse Fourier transform of F[g(θ)], it can be obtained from F[g(θ)] by taking the inverse Fourier transform of Eq. (5), which yields  
           F[g (θ)]= F   −1   [F[r (θ)]/ F[l (θ)]]  (6)  
         [0055]    where F −1  [. . . ] represents the inverse Fourier transform of the quantity “. . . ” in the bracket [ ]. Using Eq. (6), the luminance distribution function g(θ) of the lens with the condensing effect of the lens remove is obtained from the measured value r(O) and the condensing function l(θ).  
         [0056]    Derivation of the condensing function l(θ) will now be described below. FIG. 3 shows that the real image of a laser light source  9  formed by a lens  4  on a planar array of photodiodes  6 . By inclining the laser light source  9 , the direction of the laser beam  10  can be changed through an angle θ with respect to the optical axis of the lens  4 . Since the condensing function l(θ) of the lens  4  is given by the transmittance of lens, i.e. the intensity of light exiting the lens divided by the intensity of light entering the lens at angle of incidence θ, angular characteristic of the laser can be obtained by measuring the output power of the CCD device.  
         [0057]    Mathematically, the luminance distribution function g(θ) can be approximated by Dirac&#39;s delta function. Since the Fourier transform of delta function is a constant (See page 273, Moriguchi et al. , op. cit. ), one obtains the following result.  
           F[r (θ)]=Constant× F[l (θ)].   (7)  
         [0058]    Hence, measured luminance function r(θ) is obtained from the condensing function l(θ) of Eq. (8).  
           r (θ)=Constant×l(θ).  (8)  
         [0059]    Further, the condensing function l(θ) is determined by measuring the absolute value of the transmittance of the lens  4  for the light impinging on the lens at right angle of incidence.  
         [0060]    FIGS.  4 A- 4 C together show how the condensing function of a lens is obtained. For a beam of light emerging from the laser  9  of FIG. 3 in a very narrow solid angle subtending (from the laser  9 ) the lens  4  and entering the CCD device through the lens  4 , the output r(θ) of the CCD device has an angular distribution as shown in FIG. 4B.  
         [0061]    It is seen that the condensing function l(θ) of the lens has the same angular distribution as the measured output of the CCD device shown in FIG. 4B.  
         [0062]    FIGS.  5 A- 5 D together show steps of calculating the luminance distribution of a given radiant object using a known condensing function l(θ) of a lens. As an example, when the luminance distribution g(θ) depends on θ as shown in FIG. 5A, and the condensing function l(θ) of the lens depends on θ as shown in FIG. 5B, the output of the CCD device will have a luminance distribution as shown in FIG. 5C. This is due to the fact that the lens collects light from different areas having different angles of incidence. However, if the condensing function l(θ) of the lens is known as shown in FIG. 5B, the distribution of luminance g′(θ) as shown in FIG. 5D can be obtained from Eq. (6). It is noted that this distribution is the same as the luminance distribution shown in FIG. 5A. Therefore, it is seen that the luminance distribution can be accurately obtained without being influenced by the condensing effects of the lens, through Eq. (6).  
         [0063]    [0063]FIG. 6 illustrates schematically the measurement apparatus of the invention. The apparatus includes: a condensing device 4LM consisting of such optical elements as lens or mirror; an imaging device translation mechanism  11  for translating the CCD device  5 ; condensing de vice translation mechanism  12  for translating the condensing device  4 LM; a stage translation mechanism  13 ; a light source controller  14 ; a drive controller  15 ; an A/D converter  16 ; an input controller  17 ; a memory  18 ; a display device  19 ; and a data processing device  20 .  
         [0064]    The data processing device  20  has a circuit for performing Fourier transform of F[r(θ)] and F[l(θ)]; a division circuit 22 for performing division F[r(θ)] /F[I(θ)]; an inverse Fourier transform circuit  23  for performing inverse Fourier transform of F[r(θ)]/F[I(θ)]; and an evaluation circuit  24  for evaluating the values of the luminance distribution function g(θ) obtained. The memory  18  stores the condensing function l(θ) and its incident flux correction Ω(θ)/Ω(θ), along with measured luminance data r(x, y, θ) obtained by the imaging device, calculated values of the luminance distribution g(θ), and the result of the evaluation thereof. The display device  19  displays data stored in the memory  18  to facilitate relevant inspections and examinations of the characteristics of the LCD panel  1  during the manufacture thereof. A CPU may be used to facilitate data processing.  
         [0065]    [0065]FIG. 7 is a flowchart showing a procedure of the measurements according to the invention. Referring now to FIGS. 6 and 7, operations of the apparatus will be described below. The drive controller  15  provides the stage translation mechanism  13  with x-, y-, and φ- instructions indicating (x, y) coordinate of the radiant area (e.g. LCD pixel(s) on the stage  3  and rotational angle φ with respect to the coordinate (x, y), respectively. The stage translation mechanism  13  moves, rotates and positions the stage  3  at the position of the object. Following the positioning, the drive controller  15  instructs the condensing device translation mechanism  12  of the viewing angle θ at which measurement of luminance is made. The condensing device translation mechanism  12  then moves the condensing device 4LM to the position where the viewing angle is θ relative to the normal to the radiant area of the LCD panel. Similarly, upon receipt of the θ instruction, the imaging device translation mechanism  11  moves the CCD device  5  to a position where the image of the radiant area is formed.  
         [0066]    The CCD device  5  receives the light propagating in the direction of θ and condensed by the condensing device, and transform it into electric signals, which is converted into digital signals r(x, y, θ) = by the A/D converter  16 . The input control device  17  sends the digital data r(x, y, θ) to the memory  18 . (Step S 1 ).  
         [0067]    Next, the condensing device 4LM and the CCD device  5  are moved to the next angular position, while the stage  3  is moved to a new x- and y-coordinate position and θ- direction for the measurement of the next radiant area by the  5 . (Step S 3 ).  
         [0068]    When the luminance measurements are done for all the radiant areas (Step S 2 ), calculations of the luminance distribution is started.  
         [0069]    Using the data r(θ) obtained in the measurement and the condensing function l(θ) of the condensing device, the Fourier transforms of the measured r(θ) and the condensing function l(θ) (Step S 4 ) are calculated by the Fourier transform circuit  21 , from which F[r(θ)]/F[l(θ)] is calculated in the division circuit  22  (Step S 5 ), from which the inverse Fourier transform F −1 [F[r(θ)]/F[l(θ)]] is calculated by the Inverse Fourier transform circuit  23  to obtain the luminance distribution function g(θ) (Step S 6 ).  
         [0070]    This procedure is repeated for each of the data points (x, y, θ). (Steps S 7  and S 8 ).  
         [0071]    In a case where the intensity of light entering the condensing device varies with the position of the condensing device 4LM, measured value r(θ) is multiplied by the incident flux correction factor Ω(θ)/Ω(θ) before the data is Fouier transformed. This factor will be further described in detail later.  
         [0072]    The luminance distribution function g(θ) of the radiant object thus obtained by the calculations based on the actual imaging information on the imaging device is displayed on the display device  19  as representing the viewing angle characteristic of the object. (Steps S 9 ).  
         [0073]    The calculated function g(θ) is representing the viewing angle luminance characteristic of a radiant area of an object in the direction of the viewing angle is compared with a corresponding reference threshold value, to thereby determine if unevenness exists in the luminance at the position of that object. (Steps S 10 ).  
         [0074]    Embodiment 1  
         [0075]    A first embodiment of the inventive apparatus will now be described.  
         [0076]    Referring to FIG. 1A, there is shown a bird-eye view of a first luminance distribution measurement apparatus of the invention. FIG. 1B shows a cross sectional view of the stage section of the apparatus shown in FIG. 1A. As shown in FIG. 1A, the apparatus includes an LCD panel  1  which is the radiant object under measurement, a radiant area  2  (which is an LCD pixel in this example), radiating beams of light  7  from the radiant area (LCD pixel)  2 , a stage  3  for translating the LCD panel, a lens  4  for receiving and condensing the light, a CCD serving as an imaging device, and photodiodes  6  of the CCD device. Shown in FIG. 1B by a reference numeral  8  is a light source of the LCD panel  1 .  
         [0077]    In the EMBODIMENT 1 shown in FIG. 1A, the CCD device  5  is positioned such that the beams of light emerging from the pixel  2  at the coordinate (x, y) of the LCD panel  1  forms its real image at the coordinate (x′, y′) on the plane of the photodiodes  6   ln  the EMBODIMENT 1, the lens  4  is moved by the condensing device translation mechanism  12  in a direction L parallel to the plane of the stage  3 , to the angular position which corresponds to the direction of θ at which luminance measurement is performed. At the same time the CCD device  5  is also moved in a direction S in parallel to the stage to a position where the image of the pixel  2  may be formed on an area of the plane of photodiodes  6  through the lens  4 . Because of this arrangement, accurate luminance information can be obtained by reading the information gathered by the regional photodiodes  6 , should a minor misalignment occur in the optical geometry.  
         [0078]    It should be understood, however, that a necessary condition for the invention is to move the lens (condensing device) in harmony with the CCD device so that the CCD device  5  maintains the image of the pixel formed thereon and that the CCD device  5  and the lens  4  need not be moved in parallel with each other, as will be apparent from other embodiments described below.  
         [0079]    The luminance distribution function g(θ) can be obtained from Eq. (6). When the solid angle subtended by the lens as viewed from the pixel under measurement is small for the movement as described above, the value of the condensing function l(θ) may be safely approximated by the value of the function at a given reference direction, l(0) say.  
         [0080]    When the measurable domain of the angle θ for the measurement apparatus (lens  4 ) is limited within a small finite range −β&lt;θ&lt;+β for example because of the geometry of the measurement apparatus used, the values of r(θ) cannot be actually measured outside the region. However, the Iminance can be obtained accurately through Fourier transformation for the region −β+a&lt;θ&lt;+β−α irrespective of the values of r(θ) in the range , where a is the angle corresponding to the radius of the lens as shown in FIGS. 2 and 3.  
         [0081]    It is noted that in the embodiment shown in FIG. 1A, the CCD device  5  is moved so as to maintain the image formed on the CCD panel during the movement of the lens  4 . Thus, the information on the pixels at a given point (x, y) of the LCD panel, collected at the point (x′, y′) of the CCD device, also contains the information on the pixels at different points of the LCD panel. Accordingly, all the information regarding the luminance distribution of any LCD pixel can be obtained from the limited measurements made by the lens  4  within the limited domain.  
         [0082]    The measurement apparatus of the invention has a further advantage that the translation mechanisms for the CCD device  5  and for the lens  4  require only one degree of freedom of motion, provided that the stage  3  can be rotated by the stage translation mechanism  13  through an arbitrary = angle φ.  
         [0083]    Embodiment 2  
         [0084]    [0084]FIG. 8 is a bird-eye view of a second apparatus for measuring the luminance distribution of an LCD panel according to the invention. In the arrangement shown in FIG. 8, the lens  4  serving as the condensing element is rotated by the condensing device translation mechanism  12  about the radiant area  2  in the direction L, keeping constant the solid angle subtended by the lens  4  as viewed from the radiant area  2 . At the same time the CCD device is also rotated by the imaging device translation mechanism  11  about the radiant area  2  in the direction S as shown, keeping thereon the image of the radiant area  2  formed. The luminance distribution function g(θ) of the LCD panel  1  can be obtained from Eq. (6) as described in connection with EMBODIMENT 1. In this case the condensing function l(θ) is constant, since the solid angle subtended by of the lens  4  as viewed from the radiant area  2  is constant at any angle θ, so that the accuracy of the data obtained in the measurement is high.  
         [0085]    If, in addition, the stage  3  can be rotated by the stage translation mechanism  13  (through an angle Φ), the translation mechanisms for the CCD device  5  and the lens  4  may advantageously have only one degree of freedom of motion in a specified direction.  
         [0086]    Embodiment 3  
         [0087]    It is noted that in the embodiment 1 the angle subtending the lens  4  from the radiant area  2  varies with the position of the lens  4 . In this instance also, highly accurate evaluation of the luminance may be obtained by making a correction of the luminance as described below.  
         [0088]    Assuming in FIG. 1A that θ is the angle subtended the lens  4  from an radiant area  2  of the LCD panel with respect to the normal line N at the radiant area; P 0  is the output power of the CCD device and Ω0 is the solid angle (steradian) subtended the lens when the lens  4  is located at the angle θa; P 1  is the output of the CCD device; and Ω1 is the solid angle subtended the lens  4  when the lens  4  is positioned at angle θb, then the output power P 1 ′ of the CCD device after the correction of P 1  with respect to the lens position θa is given by the following formula.  
           P 1 ′=P 1×(Ω0/Ω1)  (9)  
         [0089]    A decrease in flux of incident light to the lens  4  due to a decrease in solid angle of the lens  4 , a correction is made to improve the accuracy. of the measurement, using Eq. (9). For details of solid angle, see for example, a book by Cinch Treason, pp. 108-111, entitled as “A compendium of Mathematics for Natural Scientists”, published from Iwanami Shorten (1983).  
         [0090]    This correction is not limited to the embodiment shown in FIG. 1A, but is applicable to all cases when more than one radiant element (e.g. pixels) are included in the converging device 4LM and/or when the angle subtending the lens  4  changes during the measurement.  
         [0091]    Embodiment 4  
         [0092]    [0092]FIG. 9 is a bird-eye view of a fourth apparatus for measuring the luminance distribution of a radiant object. Shown in FIG. 9 by a reference numeral  25  is a planar or concave mirror. Thus, the condensing device of FIG. 9 comprises a lens  4  along with the planar/concave mirror  25 . In the example shown herein the mirror  25  works as the optical condensing element. The CCD device  5  is arranged in position so that the image of the radiant area  2  located at the pixel coordinate (x, y) on the LCD panel I is formed at the position (x′, y′) on the photodiodes by the mirror  25  and the lens  4 . This example has an arrangement where lens  4  and the CCD device  5  need not be moved if the mirror  25  is moved in the direction M by the condensing device translation mechanism  12  from an angular position of θa, measured with respect to the normal line N, to the angular position of θb.  
         [0093]    The luminance distribution function g(θ) of the LCD panel can be obtained from Eq. (6) as in the first embodiment. It is noted that in the example shown herein the condensing function l(θ) represents the angle dependent characteristic (θ-characteristic) of the radiant area at (x, y), defined in cases where the product of the reflectivity of the mirror  25  and the transmissivity of the lens  4  changes with the angular deviation θ made by a line connecting the center of the mirror  25  and the point (x, y). However, the reflectivity of the mirror and the transmissivity of the lens can be safely approximated by 1, so that the functions l(θ) can be replaced by a rectangular function as defined by Eq. (2). The measurement is performed using a laser, as shown in FIG. 3.  
         [0094]    Embodiment 5  
         [0095]    [0095]FIG. 10 is a bird-eye view of a fifth apparatus for measuring the luminance distribution of a radiant object according to the invention. In contrast to the CCD device of the fourth apparatus which is not movable, the CCD device  5  of the fifth embodiment can be linearly rotated (in the direction S) by the imaging device translation mechanism  11  in association with the rotation of the mirror  25  (in the direction M) through an angle θa and θb with respect to the normal line N. Thus, the fifth embodiment advantageously has a higher degree of freedom in the design of optical elements such as lens  4 .  
         [0096]    Embodiment 6  
         [0097]    [0097]FIG. 11 is a bird-eye view of a sixth apparatus for measuring luminance of a radiant object according to the invention. It can be seen in the fifth embodiment that if the flat or concave mirror  25  is located at and near the highest position directly above the LCD panel, where the angle θ is small, it is difficult to form the image of the radiant area (pixel)  2  by the lens  4  within a given area of photodiodes  6 . Therefore, in the sixth embodiment, when the flat or concave mirror  25  is in such small angle region, this difficulty is overcome by slightly rotating the mirror  25  through an angle γ with respect to the normal line N so that the reflected light is directed to the photodiodes  6  to form a real image of the radiant area (pixel)  2  within a desired section of the photodiodes  6 . Similarly, in the fourth embodiment shown in FIG. 9, accurate measurement of luminance distribution can be carried out in such a small θ region by inclining the flat or concave mirror.  
         [0098]    Embodiment 7  
         [0099]    [0099]FIG. 12 is a bird-eye view of a seventh apparatus for measuring luminance distribution of a radiant area using a concave mirror  26 . This is an example which has a concave mirror  26  working as an light receiving/condensing element of the condensing device, instead of the flat mirror  25  of the sixth embodiment. In this example, because the concave mirror  26  condenses light, a condensing lens is unnecessary. The condensing function l(θ) of this optical system may be given by the reflectivity of the concave mirror  26  inclined at angle θ.  
         [0100]    Embodiment 8  
         [0101]    [0101]FIG. 13 is a bird-eye view of a eighth apparatus for measuring luminance distribution of a radiant body according to the invention. When the concave mirror  26  approaches the uppermost position above the LCD panel  1  through rotation in the direction M, the CCD device  5  is also rotated in the S direction closer to the LCD panel  1 . By the rotations of the concave mirror  26  and the CCD device  5 , the light reflected by the mirror is advantageously projected onto the intended section of the photodiodes of the CCD device  5  even for small measurement angle θ. This approach can be applied to the fourth embodiment shown in FIG. 9 to bring both the lens and the CCD panel closer to the LCD panel.  
         [0102]    Referring to FIG. 14, mechanisms for moving the condensing device and the imaging device will be described below. The mechanism shown in FIG. 14 for moving the condensing device and the imaging device is similar to the one shown in FIG. 1. This mechanism includes a rail  31  for translating the lens; a guide  32  for guiding the lens  4 ; a lens holder  33 ; a lens arm  34 ; a rail  35  for translating the CCD device; a guide  36  for guiding the CCD device  5 ; an arm  37  for supporting the CCD device; and a controller  39  for controlling the movements of the lens  4  and the CCD device  5 . In the example shown herein, the controller  39  is adapted to receive the output of the CCD device  5  and provide its output to the input controller. The guides  32  and  36  each move on the respective rails  31  and  35  as instructed by the controller  39 . At the same time the guides  32  and  36  rotate the respective arms  34  and  37  as instructed by the controller  39 .  
         [0103]    It will be understood that the mechanism of FIG. 14 can be applied equally well to the examples shown in FIGS.  8  thorough  13 , so that the lens of the condensing device and/or the mirror as well as the imaging device is(are) moved to the prescribed position(s) specified by the measurement angle θ.  
         [0104]    It should be appreciated that in the inventive method of measuring a luminance distribution, and an apparatus therefor, the incident-angle characteristic (i.e. condensing function) of a condensing lens of the condensing device and/or the incident-angle characteristic of a mirror can be measured beforehand, so that the data obtained in the manners as described above using the condensing device can be corrected for sufficiently accurate and precise luminance and the angular distribution thereof of a radiant object. It should be also appreciated that, if the relative position of the condensing lens or a condensing mirror to the radiant area changes during the measurement, a change in intensity of light due to this positional change can be corrected by an advance calculation thereof, to thereby obtain accurate luminance and the angular distribution thereof of the radiant object.  
         [0105]    The invention makes unnecessary a large aperture condensing lens and minimizes the number of imaging elements of the imaging device by providing mechanisms for moving the condensing lens and/or the mirror.  
         [0106]    It should be appreciated that the invention allows acquisition of the luminance and the angular distribution thereof of only a localized radiant area (e.g. a pixel) on a given (LCD) display but also the entire display panel. The entirety of such information permits accurate inspection/examination of the display panel for defects and uneven luminance of the panel. In addition, the invention displays the results of such inspection on a display device for easy evaluation of the inspection/examination.