This invention relates to an apparatus for optically measuring the distance between two grating-like periodic structures and/or the size of the pattern elements of one of the grating-like structures. By way of example, the two grating-like structures are a shadow mask and a faceplate of a color picture tube.
A color picture tube comprises a panel section having a phosphor screen formed on the inner surface of a faceplate and a shadow mask mounted by panel pins at a predetermined distance from the phosphor screen, and a funnel section having a three-electron gun assembly at one end thereof. The panel section has its shadow mask mounted thereon after the formation of the phosphor screen on the inner surface of the faceplate and is then sealed to the funnel section. The phosphor screens of present-day color picture tubes can be classified into a tri-color phosphor dot type and a tri-color phosphor stripe type. To enhance the quality of a reproduced picture, two types of phosphor screens have recently been developed: a black matrix screen in which a black, nonfluorescent material is buried between phosphor dots and a black stripe screen in which a black, nonfluorescent material is buried between vertical phosphor stripes. As the shadow mask, a circular-aperture mask is used for phosphor dot screen and a slotted mask is used for the black stripe screen.
For the black stripe type color picture tube, the width of the respective phosphor stripes is determined by the interval between the black stripes. Thus, the respective phosphor stripes need not be formed with high accuracy in width because they are formed after the black stripes have been formed. Thus, for the black stripe tube, a better white uniformity is obtained without the need of forming the respective phosphor stripes with high precision. In the case of the black stripe tube, however it is indispensable to check whether or not the black stripes are accurately formed. Since a distance between the shadow mask and the inner surface of the faceplate also imparts an influence to the reproduced picture quality, it is important to precisely control the distance to a prescribed value. Therefore, a picture tube production line must be equipped with an automated apparatus which can detect the width/spacing of the black stripes (the pattern measurement) and/or can measure distance between the shadow mask and the phosphor screen.
A method has been known which measures a narrow clearance (distance) by using an air micrometer. According to this method, the air micrometer is inserted into a clearance to be measured and the clearance is measured by supplying air to the micrometer and measuring the back pressure of the air. Since, with this method, it is necessary to insert the micrometer into the clearance, if it is used in a color picture tube measurement, both the shadow mask and faceplate are likely to be damaged. This is a serious drawback for this method to be used in an automated manufacturing process.
A Japanese Patent laid-open specification No. 55-33679 discloses an apparatus to measure a spatial distance by moire fringes resulting from an interference between a projection grating and a reference grating. Since the apparatus measures the distance through the variation of an angle or a pitch of the moire fringes, a detector is required to be of a two-dimensional type like an image pick-up tube. This complicates the apparatus as well as the signal processing method.
A pattern measuring apparatus is disclosed in a Japanese Patent laid-open specification No. 55-27321. The apparatus is adapted to direct a laser beam to a pattern to be measured, subject a transmitted or reflected beam to a Fourier transformation and measure the pattern width on the basis of the intensity of a predetermined frequency component in the Fourier transform pattern. The apparatus has advantages in that it does not need to focus the laser beam onto the pattern to be measured and that it is possible to readily find an average value of pattern within the diameter of the laser beam. However, this measuring method is not applicable to measuring a pattern which is printed on an optically inhomogeneous diffusion plate such as ground glass or an opaque medium such as paper, since the S/N ratio of the Fourier transform pattern is low.
For the measurement of a black stripe pattern of a color picture tube, the Journal of the Institute of Television Engineers of Japan 34, No. 2, PP 141 to 146, (1980) discloses an apparatus adapted to scan the black strip pattern with a fine laser beam and measure the width of the black stripe on the basis of a time interval of portions of an electric signal corresponding to the black stripes which is obtained by detecting the transmitted light. In this apparatus, it is difficult to narrow the laser beam substantially negligible to the black stripe width. Furthermore, an error is liable to occur during the pattern width information extracting process from the electric signal. For the laser beam to be uniformly focused on the pattern to be measured, it is necessary to accurately set the scanning center of the laser beam to be positioned at the curvature center of the faceplate of the color picture tube.