The present invention relates in general to a color picture tube with a shadow mask, and in particular to a color picture tube having a shadow mask which is provided with a plurality of electron beam transmissive aperture rows extending perpendicularly to the scanning lines and each comprising a plurality of the individual rectangular apertures for transmission of the electron beams arrayed in line with a predetermined pitch.
Lately, with an effort to simplify the structure of deflection system and at the same time to enhance the visual sharpness of the produced image of the color picture tube with the shadow mask such as color cathode ray tube, Braun tube or the like (hereinafter referred to also as CPT), there have been developed and increasingly employed CPT's of the shadow mask type in which the shadow mask is provided with aperture rows extending orthogonally to the scanning lines and each comprising a plurality of the electron beam transmissive rectangular apertures (hereinafter also referred to as apertures) arrayed vertically in line with a predetermined vertical pitch and in which three electron guns are arrayed in line, in place of the heretofore known color picture with shadow mask in which circular phosphor dots are arrayed in a form of equilateral triangle. However, the color picture tube of the above type suffers from the drawbacks that a strip or fringe pattern, that is, moire of a great pitch is produced as a result of the interaction between shades of the bridge portions between the rectangular apertures formed vertically in a repeated pattern with the predetermined pitch and the bright-dark portions of the scanning lines, thereby to deteriorate the picture quality of the produced image.
Many and various attempts have been hitherto proposed for reducing the moire phenomenon. According to a known method, the apertures of the horizontally adjacent aperture rows are deviated from one another in respect of the vertical position for a distance of 1/αPy where α is an integer and Py is the vertical pitch of the aperture row. This method starts from two observations. Namely, on one hand, the moire is determined by the scanning lines and the deviation, since the moire pitch becomes greater as the difference between the pitch of the scanning line and the vertical pitch of the apertures in the rows is selected smaller and since the deviation causes horizontal fringe whose pitch is Py /α. In other words, the deviation in the vertical position between the horizontally adjacent rows will bring about a shade patern in the substantially horizontal direction and the moire will become more imperceptible as the magnitude of the deviation is selected smaller because the ratio between the pitch of the scanning line and the pitch of the shade pattern will then become large. On the other hand, in accordance with the other observation, the horizontal pattern of shade, i.e. interlaced dark and bright portions will not be produced if the integrated value of the electron transmissivity or through rate of the apertures remains the same for each of the sanning lines. Accordingly, the moire can be suppressed by adjusting the deviation and the width of the bridge portions between the vertically aligned aperture in a row. However, the inventors have found after repeated experiments that the hitherto proposed method as described above can not make the moire in oblique directions imperceptible although the method is certainly effective in suppressing the moire appearing in a form of bright and dark pattern in the vertical position.
It has also been proposed to array the apertures in a random pattern. However, this solution meets with difficulties in the manufacturing thereof.
Accordingly, a main object of the invention is to provide a color picture tube of the shadow mask type in which the moire is made imperceptible.
Another object of the invention is to realize a shadow mask for a color picture tube which is effective in suppressing the occurrence of the moire.
Still another object of the invention is to reduce undesirable influences of moire due to the harmonics of the luminance distribution pattern of the scanning lines and of the transmissivity or through rate pattern of the vertically arrayed apertures and the poor linearity of the vertical distribution pattern of the scanning lines.
A further object of the invention is to provide a color picture tube which can be employed commonly in NTSC (National Television System Commitee), PAL (Phase Alternation By Line) and SECAM (Sequential a Memoire) color television systems without any appreciable moire.
With the above objects in view, the present invention contemplates to determine the array of the apertures so that the pitch and phase of beat components, ie moires, produced by the mutual product of the vertical transmissivity or through rate distribution pattern of the aperture row and the vertical luminance distribution pattern of the scanning lines will take predetermined values.
More specifically, the shadow mask according to the present invention is so constructed as to comprise at least two different types of the aperture rows of different deviations which fulfill the following condition: ##EQU2## wherein Py represents the pitch of apertures in the vertical aperture row, Δy represents the vertical deviation between the apertures in the horizontally adjacent aperture rows, n is a positive interger of 1 to 5 and k is an odd number smaller than 2n.
The shadow mask according to the invention provides significant advantages particularly when the luminance distribution pattern of the scanning line cannot be approximated by a sine wave or when the face plate of the color picture tube is remarkably curved at peripheral portions.