The present invention relates to a cathode ray tube (hereafter referred to as CRT), and in particular to a CRT having symmetrical anode potential for improving uneven luminance on its screen.
FIG. 1 illustrates a conventional CRT for a color picture as in the U.S. Pat. Nos. 4,528,477 and 4,638,213.
The CRT 10 comprises a faceplate panel 14 on whose inner surface fluorescent material 12 is deposited, a neck 16 positioned opposite the panel 14, a funnel 18 to form a bulb by connecting integrally the faceplate panel 14 with the neck 16, and an electron gun mounted in the neck 16.
The electron gun is of a conventional bipotential type, including a cathode 22 having a heater 20, a plate type control electrode 24, a cup type screen electrode 26, a focus electrode 28, and a high-voltage electrode 30 to which anode voltage is supplied.
An anode button 32 or receptacle is fixed on the outer surface of the funnel 18 and led through the funnel 18 and electrically connected to a conductive coating 34 on the inner surface of the funnel 18.
FIG. 2 illustrates a conventional envelope structure of the anode button 32 mounted on the funnel 18. Button 32 is in electrical contact with coating 34 via metal strip 36 on the inner face of the funnel 18.
The conductive coating 34 is deposited by colloid-type graphite to the extent of electrically contacting the fluorescent material 12 as shown in FIG. 1, a shadow mask 38 adjacent to the fluorescent material, and snubbers 40 which provides an electrical path to the high-voltage electrode 30 of the gun from the conductive coating 34. The strips may also function to dampen vibrations.
Electrodes from the gun are respectively supplied from each of stem leads 42 which extend out the stem of the neck 16. Examples of the supplied voltage are as follows:
cathode ; 200-400 V PA0 control electrode ; 0-1 KV PA0 screen electrode ; 0-1 KV PA0 focus electrode ; 1-10 KV PA0 high-voltage electrode ; 5-35 KV,
wherein the voltage supplied to the high-voltage electrode is the same as the anode voltage.
Thermions emitted from the cathode 22 due to the heater 20 are changed into electron beams through the control electrode 24, screen electrode 26, focus electrode 28, and high-voltage electrode 30. The electron beams then land on the fluorescent material 12, to thereby form a picture.
This picture, as it appears on the faceplate panel, will have sectional differences in luminance.
FIGS. 3 (A) and (B) show the measured examples with regard to the luminance dispersed on the screen of two randomly-selected 29" color CRTs having conventional structure. In the drawings of luminance, the numerals are luminance values with its unit of lux, and the numerals in parentheses are comparative values of surrounded portions with taking the central luminance value of the screen as 100.
In the drawings of landing & strip width, the numerals are strip width values of a fluorescent pattern with its unit of lux, and the A, B, C, and D are grades of mis-landing in surrounded portions compared with the landing condition of central portion as a standard.
As shown in the figures, the luminance dispersion of peripheral portions which are in a symmetrical position with each other is especially uneven.
The luminance of a CRT depends on the material quality and the deposited condition of fluorescent pattern, the voltage dispersion in the CRT, the strip width, and the landing condition of electron beams.
If any fluorescent pattern has the same depositing condition, the voltage dispersion, the strip width, and the landing condition of electron beams are factors having a great effect on the luminance of CRT.
The measured result of the landing condition and stripe width in the CRT does not include any factors which cause the uneven luminance. Accordingly, the subject cause of the uneven luminance on the CRT screen is posited to be voltage dispersion. However since the inner voltage dispersion of CRT is in fact impossible to be measured, there is no direct method of proof.
It is only given that, if the measured luminance of a CRT is charted according to each portion of the screen, the portion to which the anode button 32 is fixed has a higher luminance value than a symmetrically disposed portion, e.g., with respect to a horizontal, vertical or diagonal axis. This gives indirect evidence that the inner voltage dispersion of the CRT has strong effects on luminance.
That is, the conductive coating 34 adjacent to the anode button 32 has denser potential than the conductive coating 34 in the symmetrical position remote from the anode button 32, whereby unevenness of the voltage dispersion occurs and thus effects the luminance.
Such a supposition is evidently supported by the data which is gained from measuring the terrestrial magnetic field effect of the CRT.
FIG. 4 (A) shows the respective varying amount of electron beams according t horizontal magnetic field variations measured at each screen part in the respective cases of directing the CRT in FIG. 3 (A) to east, west, south, or north, changing the direction of the CRT into the west after degaussing it in the east, and changing the direction of the CRT into the south after degaussing it in the north. And, the illustrations of the right side show the differences of varying amount of beams.
According to the result of the measurement, the unevenness appears in the peripheral portions which are symmetrical with each other. Such a state is found in the varying amount of beams according to vertical magnetic field variations due to the terrestrial magnetic field as shown in FIG. 4 (B). These may result from much more effect of the terrestrial magnetic field on the peripheral portions of the anode button 32 because of higher charge density of peripheral portions than that of its symmetrical portion.