(a) Field of the Invention
The present invention relates to an electron gun assembly for a cathode ray tube (CRT) and, more particularly, to an electron gun well adapted for a monochrome CRT to be mounted within a projection display device to realize a monochrome image.
(b) Description of the Related Art
Generally, a CRT-based projection display device mainly has three monochrome CRTs for realizing red (R), green (G) and blue (B) monochrome images, and an optical system for amplifying the monochrome images made at the three CRTs and projecting the amplified images to a projection screen to produce color images.
As the monochrome CRT scans the display screen with one stream of electron beams, and the respective monochrome CRT screen images are projected to the projection screen while being amplified by about ten times, the brightness of the display screen related to the monochrome CRT is lower than that of the display screen related to the usual CRT. Therefore, compared to the usual CRT, relatively high electric currents need to be applied to the electron gun for the monochrome CRT to heighten the brightness of the display screen.
Usually, the electron gun for the monochrome CRT emits electron beams with the application of electric currents ranging from 0.5 mA to 3 mA, which are two or three times more than those applied to the electron gun for a usual color CRT ranging from 0.2 mA to 1 mA. A high unipotential focus (Hi-UPF) type exhibiting an excellent focus characteristic in the range of higher currents is commonly used for the monochrome CRT electron gun.
With the Hi-UPF type electron gun, the second electrode receives the screen voltage, and the fourth electrode receives the focus voltage. A third electrode is placed between the second electrode and the fourth electrode to receive a high anode voltage (roughly, 32 kV). A strong pre-focus lens is formed between the second and the third electrodes due to the high potential difference between the second and the third electrodes, and reduces the spot size of electron beams in the range of higher electric currents.
Further, the monochrome CRT electron gun serves to make formation of monochrome images practically under the application of electric currents of 2 mA or less. With the available electric current range of 0.5-3 mA, the Hi-UPF type electron gun exhibits an excellent focus characteristic in the higher current range of more than 2 mA. By contrast, in the relatively lower current range of 2 mA or less, it turns out that the spot size of electron beams is increased.
The increase in the beam spot size occurs because when the electron
The increase in the beam spot size occurs because when the electron beam current is lowered, the crossover point of the electron beams formed at the triode portion moves from the second electrode to the third electrode, and the emission power of the electron beams incident upon the pre-focus lens is weakened. Consequently, with the electric current range of 2 mA or less, the spot size of the electron beams is increased while deteriorating the resolution, resulting in unclear display images.
In order to reduce the beam spot size with the current range of 2 mA or less, it has been proposed that the size of the beam-guide hole formed at the first electrode be reduced. However, this reduction makes the area of electron emission for the cathode so small that the life span of the electron gun with the cathode is reduced.
U.S. Pat. No. 4,271,374 discloses a CRT electron gun with a structure where the equivalent diameter of the main-focus lens (formed between the fourth electrode receiving the focus voltage and the fifth electrode receiving the anode voltage) is enlarged to increase the capacity thereof.
However, with the above structure, as the CRT neck portion mounting the electron gun thereon is limited in its diameter, there is a limit in mechanically enlarging the opening diameter of the fourth and the fifth electrodes forming the main-focus lens. Such a limit is made because the electron gun formation electrodes need to be spaced apart from the inner surface of the neck portion by a predetermined distance to grant the withstand voltage characteristic to the CRT. Accordingly, as the opening diameter of the fourth and the fifth electrodes is established in a predetermined manner, it is difficult to achieve the desired electrode capacity in an effective manner.