JPH0253905B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a projection type cathode ray tube, and more particularly to a projection type cathode ray tube that projects an image from the cathode ray tube onto a projection screen to obtain an enlarged projected image.

In general, a projection type cathode ray tube is placed at a position a required distance from the projection screen, and the reproduced image on the face plate of the cathode ray tube is projected onto the projection screen to obtain a projected image that is enlarged from the reproduced image. Widely used in projection devices.

FIG. 1 is a side view of essential parts showing an example of a video projection device using a conventional projection type cathode ray tube. In the figure, reference numeral 1 denotes a projection type cathode ray tube, and a projection screen 2 is disposed on the central axis of the projection type cathode ray tube 1, facing the face plate of the cathode ray tube 1 and spaced apart from it by a certain distance. There is. Further, on the front side of the face plate of the cathode ray tube 1, a projection lens system 3 is disposed on the same line as the central axis thereof, and the image reproduced on the face plate is condensed and expanded and projected onto the projection screen 2. I'm letting you do it. In addition, in the case of a color image projection device, as shown in a side view of the main parts in FIG.
A cathode ray tube 4 for a red image, a cathode ray tube 5 for a green image, and a cathode ray tube 6 for a blue image are respectively disposed at the central axis and symmetrical positions with respect to the central axis,
Furthermore, projection lens systems 7, 8, and 9 are arranged on the front side of these projection type cathode ray tubes 4, 5, and 6, respectively. And the cathode ray tubes 4, 5,
The monochromatic image reproduced on the face plate 6 is condensed and expanded by the projection lens systems 7, 8, and 9 and projected onto the projection screen 2, and the three colors are combined and superimposed on each other. An image is obtained.

Here, the relationship between the screen brightness B _O of the cathode ray tube 1 and the brightness B on the projection screen 2 explained in FIG. 1 is determined by the following equation.

B=B _O ×T×G/4f ² (M+1) ² ………(1) However, T: Transmittance of projection lens system 3 G: Gain f of projection screen 2: f number M of projection lens system 3: Enlargement ratio of reproduced image Here, as an example, T=0.8, G=10, f=
2.0, M = 6 into equation (1), the brightness B on the projection screen 2 = 0.01B _O , and the cathode ray tube 1
The screen brightness B _O on the projection screen 2 drops to 1/100 of that on the projection screen 2. In addition, in the case of FIG. 2, the brightness on the projection screen 2 can be approximately three times that in the case of FIG. I couldn't escape it.

Therefore, in order to obtain a practically enlarged reproduced image in this type of image projection apparatus, the luminance output on the face plate (screen) of the cathode ray tube must be significantly increased. For this purpose, there is a method of improving the luminous efficiency of the fluorescent surface on the inner surface of the cathode ray tube, or a method of increasing the energy of the electron beam that excites the fluorescent surface. In this case, there is a limit to improving the luminous efficiency of the former fluorescent surface due to restrictions on the color of the luminescent surface. On the other hand, in order to increase the energy of the latter cathode ray beam, the anode voltage of the cathode ray tube is increased, the beam current is increased, or both the anode voltage and the beam current are increased. However, increasing the anode potential poses problems such as withstand voltage characteristics between the electrodes of the cathode ray tube and X-ray radiation from the cathode ray tube, and there is a limit to increasing the anode voltage. Therefore, the current situation is to generally increase the beam current to increase the brightness output from the cathode ray tube. However, in a projection type cathode ray tube having a conventional configuration, the converging lens system may cause various serious problems. This will be explained below.

FIG. 3 is a sectional view of a main part showing an example of a conventional projection cathode ray tube, particularly an electrostatic focusing type projection cathode ray tube. In the figure, inside the neck portion 10a of the glass bulb 10 is an electron gun consisting of a cathode 12 that emits an electron beam 11, a control electrode 13 that controls the amount of the electron beam 11, and an acceleration electrode 14 that accelerates the electron beam 11. 15 and a focusing electrode 16 that focuses the electron beam 11 emitted from the electron gun 15 are housed. Further, the funnel part 10b and the neck part 1 of this glass bulb 10 are
There is an anode electrode 1 inside the electrode 0a.
7 is arranged, and the focusing electrode 16 and this anode electrode 17 constitute an electrostatic focusing lens system 18 that focuses the electron beam 11. Further, a fluorescent surface 19 is formed on the inner surface of the panel portion 10c, that is, the front side of the glass bulb 10, thereby forming a projection type cathode ray tube.

In the electrostatic focusing type projection cathode ray tube configured as described above, the electron beam 11 emitted from the electron gun 15 must be focused as a minute spot on the fluorescent surface 19 by the electrostatic focusing lens system 18. However, as described above, when the beam current is increased to improve the luminance of light emitted from the fluorescent surface 19, this electrostatic focusing lens system 1
The diameter of the electron beam 11 increases when passing through the electron beam 8, and is greatly affected by the spherical aberration of the electrostatic focusing lens system 18. In other words, since the influence of spherical aberration is proportional to the cube of the beam diameter, the spot diameter of the beam on the fluorescent surface 19 becomes significantly thicker, as shown in the characteristics of FIG.
This method has the disadvantage that the quality of the reproduced image formed on 9 is significantly degraded.

In addition, FIG. 5 is a cross-sectional view of a main part showing an example of a conventional projection cathode ray tube, particularly an electromagnetic focusing type projection cathode ray tube, and the same symbols as in FIG. 3 are the same elements, so a description thereof will be omitted. . In the figure, a neck portion 10a and a funnel portion 10 of a glass bulb 10 are shown.
The anode electrode 1 is located inside the b.
A focusing coil 20 is arranged on the outer periphery of the glass bulb 10 corresponding to this anode electrode 17.
are arranged to constitute the electromagnetic focusing lens 21.

In the electromagnetic focusing type projection cathode ray tube configured as described above, the electron beam 11 emitted from the electron gun 15 is focused by the focusing coil 20 and focused on the fluorescent surface 19 as a minute spot. become. In this case, as described above, when the beam current is increased to improve the luminance of the fluorescent surface 19, the amount of the electron beam 11 increases and the diameter of the electron beam 11 becomes larger within the electromagnetic focusing lens system 21. . However, the focusing coil 20 is disposed on the outer periphery of the neck portion 10a.
Since the diameter of the focusing coil 20 can be increased without being limited to the diameter of the electron beam 11, even when the amount of the electron beam 11 increases, the inner diameter of the focusing coil 20 can be made sufficiently large compared to the diameter of the electron beam 11. This makes it possible to suppress the influence of spherical aberration as much as possible. Therefore, as shown in the characteristics in FIG. 4, the spot diameter of the electron beam 11 on the fluorescent surface 19 can be maintained at approximately the required thickness.

Here, if we compare the two types of electrostatic focusing type and electromagnetic focusing type mentioned above in Figure 4, we can see that the electrostatic focusing type (characteristics) and the electromagnetic focusing type (characteristics) have different characteristics for the electron beam relative to the beam current. As is clear from this figure, the electromagnetic focusing type is most suitable as a projection cathode ray tube that operates with a large current.

Normally, in a projection video device, the brightness of the image to be reproduced on the panel face plate changes from moment to moment, and the beam current also changes accordingly. It is extremely difficult to make such high voltages completely constant, and as shown in the example shown in Figure 6, it is inevitable that the anode voltage will fluctuate by about 10% as the beam current fluctuates. . Therefore, in the electrostatic focusing type cathode ray tube shown in FIG. 3 as described above, the voltages applied to the anode electrode 17 and the focusing electrode 16 are supplied from the same high voltage power supply via the bleeder circuit. If the voltage of the anode electrode 17 changes by a certain value, the voltage of the focusing electrode 16 also changes proportionally, and the ratio of focusing electrode voltage/anode electrode voltage=R _O is always kept constant. Therefore, even if the anode electrode voltage changes due to changes in the beam current, the focusing electrode voltage also changes accordingly, so based on the principle of an electrostatic focusing lens, the focus state of an electrostatic focusing type cathode ray tube does not change at all. . On the other hand, in the case of the electromagnetic focusing type cathode ray tube shown in FIG. Since they are unrelated, if the beam amount changes and the voltage of the anode electrode 17 fluctuates, a significant defocus state will occur, and the panel section 1
This method has the disadvantage that the image reproduced on the 0c face plate is significantly degraded.

Therefore, an object of the present invention has been made with attention to the above points, and is to provide a projection type cathode ray tube which obtains stable focus characteristics against fluctuations in anode voltage and also obtains a small beam spot diameter. Our goal is to provide the following.

A projection type cathode ray tube according to the present invention for achieving the above object is a combination of an electrostatic focusing lens system and an electromagnetic focusing lens system. The projection type cathode ray tube according to the present invention will be explained in detail below with reference to the drawings.

FIG. 7 is a sectional view of a main part showing an embodiment of a projection type cathode ray tube according to the present invention, and since the same symbols as in FIGS. 3 and 5 represent the same elements, a description thereof will be omitted.
In the figure, a neck portion 10 of a glass bulb 10 is shown.
A focusing coil 20 is disposed on the outer periphery of an electrostatic focusing lens system 18 (not shown) consisting of a focusing electrode 16 and an anode electrode 17 formed in a. A projection cathode ray tube is constructed by forming a composite lens 22 which is a combination of a coil 20 and an electromagnetic focusing lens system 21 (not shown).

In the projection type cathode ray tube configured as described above, first, the focusing coil 20 is not supplied with current and the electromagnetic focusing lens system 21 (not shown) is not operated, that is, the projection type is operated only by pure electrostatic focusing operation. The cathode ray tube is operated to emit the electron beam 11 from the electron gun 15, and the focusing electrode 16
and a cathode according to the present invention designed so that the beam 11 becomes the smallest spot on the fluorescent surface 19 when a voltage such that the voltage ratio of the focusing electrode voltage to the anode electrode voltage R _O =0.2 is applied to the anode electrode 17 and the anode electrode 17, respectively. In the tube measurement example, the fluorescent surface 19 at this time
The spot diameter of the upper electron beam 11 is approximately 1.5 (arbitrary unit) as shown at point a in FIG.
Next, when the ratio of the focusing electrode voltage to the anode electrode voltage, that is, R _P , is successively varied within the range of 0.2 or more and less than 1.0, it becomes clear that the focusing effect of the electrostatic focusing lens system 18 (not shown) alone cannot produce a fluorescent surface. Since the electron beam 11 cannot be focused in the best condition on the focusing coil 20, a predetermined current is supplied to the focusing coil 20.
By energizing the focusing effect of the electromagnetic focusing lens system 21 (not shown) by 0 and adjusting it to obtain the best focus condition, the spot diameter on the fluorescent surface 19 can be adjusted to approximately 1.5 as shown in FIG. It can be decreased sequentially within the range of ~0.95 (arbitrary unit). Next, when the voltage ratio R _P is further increased to 1.0, the focusing electrode 16 and the anode electrode 17 have the same potential, so the effect of the electrostatic focusing lens system 18 is completely eliminated, and the electromagnetic focusing lens system by the focusing coil 20 There is only a focusing effect of the system 21, that is, a purely electromagnetic focusing effect, and the spot diameter becomes 1.0 (arbitrary unit) as shown at point b in FIG. Therefore, as is clear from the data in FIG. 8, the spot diameter of the electron beam 11 on the fluorescent surface 19 becomes pure as it moves from pure electrostatic focusing to pure electromagnetic focusing. This is improved and becomes smaller than during electrostatic focusing operation.
In other words, data was obtained in which the focus characteristics were improved and the best condition was obtained especially when the voltage ratio R _P was around 0.8.

Next, FIG. 9 shows changes in the spot diameter of the electron beam 11 on the fluorescent surface 19 when the voltage of the anode electrode changes. In the same figure, this uses the focusing electrode voltage/anode electrode voltage ratio (R _P ) as a parameter, and the current of the focusing coil 20 is set to the value when the fluctuation of the anode electrode voltage is zero for each voltage ratio (R _P ). The figure shows the change in beam spot diameter as a ratio when only the anode electrode voltage and focusing electrode voltage are varied by ±10% while being fixed. Here, the characteristics are for pure electrostatic focusing, voltage ratio (R _P ) =
0.2, characteristics are R _P = 0.4, characteristics are R _P = 0.6, characteristics are R _P = 0.8, characteristics are R _P = for pure electromagnetic focusing.
It is 1.0. Therefore, as can be seen from FIG. 9, the change in the spot of the electron beam 11 with respect to the voltage fluctuation of the anode electrode 19 is due to the focusing electrode voltage/anode electrode voltage ratio (R _P ) being 0.2 to 1.0, that is, the pure static It was confirmed that the condition worsened as one went from electric focusing operation to pure electromagnetic focusing operation.

Therefore, from the experimental results shown in Figures 8 and 9, the focusing electrode voltage/anode electrode voltage ratio R _P is 0.4.
By setting the value to ~0.6, the diameter of the focus spot on the fluorescent surface 19 can be made sufficiently small, and the change in the diameter of the electron beam spot on the fluorescent surface 19 due to fluctuations in the anode voltage is also within the allowable range. This allows stable focus characteristics to be obtained. Also, here the above voltage ratio (R _P )
Regarding the setting of operating conditions, the voltage ratio (R _P ) is selected appropriately depending on whether emphasis is placed on the absolute value of the beam spot diameter or on the fluctuation of the electron beam spot diameter due to fluctuations in the anode electrode voltage. can do. Here, the characteristic curve in FIG. 10 obtained from FIGS. 8 and 9 represents the spot diameter of the electron beam 11 on the fluorescent surface 19 when a predetermined anode voltage is applied. The maximum value of the above spot diameter is shown when the anode voltage fluctuates by 10% from a predetermined value while the current is fixed. In the same figure,
In the case of R _P =0.25, both the minimum spot diameter and the spot diameter when the anode voltage fluctuates by 10% are improved by about 10% compared to the case of R _P =0.2 without the lens action of the focusing coil 20. Spot diameter is not affected by anode voltage fluctuations. Therefore, in applications where variations in beam spot diameter due to variations in anode voltage caused by changes in beam current are undesirable, R _P =
It is effective to set the anode voltage and focusing voltage so that the voltage is about 0.25. In the cathode ray tube in this measurement example, when the focusing electrode voltage/anode electrode voltage ratio R _O during pure electrostatic operation is set to 0.2 as described above, the spot diameter of the electron beam 11 on the fluorescent surface 19 is focused to the minimum. Therefore, R _P under the above conditions corresponds to R _P =1.25R _O. Furthermore, the above ratio
As R _P increases, both the spot diameter when a given anode voltage is applied and the spot diameter when the anode voltage fluctuates by 10% decrease, but the difference between the two tends to increase, but the amount of change in beam current is small. Alternatively, for applications in which the beam current changes infrequently, applications that require a minute spot, or applications in which fluctuations in beam spot diameter are not a major problem, the voltage ratio R _P can be set to a position where the spot diameter is extremely small.
It is effective to set it to a value close to 0.8, and this voltage ratio generally exists approximately in the range R _P =0.8 to 0.9. Therefore, the above voltage ratio R _P should be adjusted so that the spot diameter when a predetermined anode voltage is applied and the spot diameter when the anode voltage is varied satisfy the application.
The gist of the present invention can be achieved by selecting within the range of 1.25R _O <R _P <0.9. In the above explanation
Although the case of R _O =0.2 has been explained as an example, a bipotential lens consisting of the focusing electrode 16 and the anode electrode 17 generally shows the same tendency and the above conditions can be applied. In other words, in a lens system that uses both an electrostatic focusing lens system and an electromagnetic focusing lens system, the voltage ratio R _O of the focusing electrode voltage to the anode electrode voltage is as follows: This is the voltage ratio of the focusing electrode voltage to the anode electrode voltage when the beam spot is best focused on the top. This ratio R _O is mainly dependent on the length of each electrode,
Once the diameter, gap between electrodes, etc. are determined, they are determined almost uniquely.Since the present invention uses both an electrostatic focusing lens system and an electromagnetic focusing lens system, the electrostatic focusing lens system is intentionally selected. By setting a certain amount of defocus operation and using an electromagnetic focusing lens in conjunction with this, it is possible to eliminate this amount of defocus and obtain a minute beam spot on the phosphor screen. is also a cathode tube configuration that can obtain stable focus characteristics. Therefore, the above 1.25R _O < R _P < 0.9 defines the amount of defocus required for the electrostatic focusing lens system. The amount is defined as the amount of deviation ΔE _fpp from the focusing electrode voltage E _fpp at which the best focus is obtained when only the electrostatic focusing lens is operated, and the above 1.25 R _O < R _P < 0.9 means: The amount of defocus Δr _fpp required for an electrostatic focusing lens system is an underfocus of 25% or more in terms of the focusing electrode voltage, that is, E _fpp + ΔE _fpp =
R _P・E _b = (R _O + ΔR _O ) E _b , where R _O + ΔR _O >
1.25R _O In addition, in order to ensure the effectiveness of the electrostatic focusing lens system to suppress fluctuations in the beam spot diameter due to anode voltage fluctuations, the voltage ratio R _O of the focusing electrode voltage to the anode electrode voltage must be less than 0.9. This means that there is.

Therefore, for example, an underfocus amount of 25% or more in terms of the focusing electrode voltage is expressed as the amount of deviation from this R _O , regardless of the absolute value _of R _O.
It is clear that it can be defined by the ratio to the value of .

In short, in the present invention, in order to obtain stable focus characteristics and a minute beam spot, when an electrostatic focusing lens system and an electromagnetic focusing lens system are used together, the focusing electrode voltage of the electrostatic focusing lens system is
From the focusing electrode voltage that gives the best focus with this electrostatic focusing lens system alone, the focusing electrode voltage is calculated as 25
% or more, and by using an electromagnetic focusing lens system to eliminate the underfocus of 25% or more in terms of the focusing electrode voltage, not only can a minute spot be obtained, but also the fluctuation of the anode voltage can be suppressed. Stable focus characteristics can be obtained even for

In the above embodiment, the case where the electrostatic focusing lens system and the electromagnetic focusing lens system are aligned on the neck portion 10a has been described, but the present invention is not limited to this. Even when the focusing coil 20 is disposed at position A or B on the neck portion 10a, for example, at the focusing coil 20a or 20b, as shown in a cross-sectional view of the principal plane in FIG. 12, exactly the same effect as described above can be obtained. Further, in the above embodiment, a bipotential lens consisting of a focusing electrode 16 and an anode electrode 17 was explained as an example.
As shown in FIG. 13, which is a cross-sectional view of the main part, a cathode ray tube using a unipotential lens in which a focusing electrode 23 is provided between an anode electrode 17 or any other electrostatic focusing lens system is completely different from the above. It is clear that similar effects can be obtained. In addition, although the case where the focusing coil 20 is used as the electromagnetic focusing lens system has been explained,
It goes without saying that the present invention is not limited to this, and the same effects as described above can be obtained even when an electromagnetic focusing lens system using a permanent magnet is used. Here, when a unipotential lens is used, its characteristic curve is
As shown in the figure. In the figure, the characteristic curve represents the spot diameter of the electron beam 11 on the fluorescent surface 19 when a predetermined anode voltage is applied, and the characteristic curve represents the spot diameter of the electron beam 11 on the fluorescent surface 19 when a predetermined anode voltage is applied. The maximum value of the above spot diameter is shown when the anode voltage varies by 10% from the predetermined value while remaining fixed at the adjusted value.
The cathode ray tube in this measurement example corresponds to the example in Figure 13, and the focusing electrode voltage /
When the anode electrode voltage ratio R _O is set to 0.002,
The spot diameter of the electron beam 11 on the fluorescent surface 19 is designed to be focused to a minimum. Spot diameter and anode voltage fluctuate by 10% from the predetermined values when a predetermined anode voltage is applied when the focusing electrode/anode electrode voltage ratio R _O =0.002 during pure electrostatic operation without using the lens action of the focusing coil 20. _{For the spot} diameter _when 0.6
The above spot diameter has an extreme value around ~0.8. However, the difference between the spot diameter when a predetermined anode voltage is applied and the spot diameter when the anode voltage varies by 10% from the predetermined value increases as the voltage ratio R _P increases, so the cathode ray tube using a unipotential lens When applying the gist of the present invention to a cathode ray tube using a bipotential lens, the above voltage ratio R _P should be adjusted to 2R _O < R _P depending on the application, as described in the section that explains the cathode ray tube using a bipotential lens as an example. <0.8 (R _O is the focusing electrode voltage/anode voltage ratio during pure electrostatic operation)
If the selection is made within a range that satisfies the above, the gist of the present invention can be effectively utilized. Here, the same discussion applies to the unipotential electrostatic focusing lens system as to the bipotential electrostatic focusing lens system. In other words, in the case of a unipotential electrostatic focusing lens system, the ratio of the focusing electrode voltage to the anode electrode voltage is smaller than in the case of a bipotential electrostatic focusing lens, and the change in the focusing electrode voltage is
Since the beam spot diameter does not change as sensitively as in the case of a bipotential electrostatic focusing lens system, the amount of defocus required for an electrostatic focusing lens system is 100% or more in terms of the focusing electrode voltage, and the beam spot diameter does not change as sensitively as in the case of a bipotential electrostatic focusing lens system. The inequality 2R _O < R _P shows that the voltage ratio R _O of the focusing electrode voltage to the anode electrode voltage must be less than 0.8 in order to ensure the action of the electrostatic focusing lens system to suppress the diameter variation within an acceptable range. <0.8 is specified. In this way, by eliminating the amount of defocus occurring in the electrostatic focusing lens system with the electromagnetic focusing lens system, not only a minute spot can be obtained, but also a focus characteristic that is sufficiently stable against fluctuations in the anode electrode voltage can be obtained.

In this unipotential electrostatic focusing lens, as in the bipotential electrostatic focusing lens system described above, a defocus amount of 100% or more _in terms of the focusing electrode voltage can be achieved regardless of the absolute value of R _O. It is clear that the amount of deviation from R O can be defined as a ratio to the value of R _O .

In this way, in the present invention, by setting R _P to satisfy the inequality defined in the present invention,
The beam spot diameter can be reduced compared to the case of pure electrostatic operation, and even if there is an anode voltage fluctuation, the increase in the beam spot diameter can be suppressed to a range that does not pose a practical problem.

As explained above, according to the projection type cathode ray tube according to the present invention, stable focus characteristics and a small beam spot diameter can be obtained extremely easily, so that the luminance of light emitted from the fluorescent surface and the image quality are greatly improved. It has excellent effects such as being able to significantly improve its quality, performance, etc.

[Brief explanation of the drawing]

Figures 1 and 2 are side views of essential parts showing an example of an image projection device using a projection cathode ray tube, and Figure 3 is a sectional view of essential parts showing an example of a conventional electrostatic focusing type projection cathode ray tube. , Fig. 4 is a characteristic diagram showing the spot diameter ratio with respect to beam current, Fig. 5 is a cross-sectional view of a main part showing an example of a conventional electromagnetic focusing type projection cathode ray tube, and Fig. 6 is a variation of anode voltage with respect to beam current. FIG. 7 is a sectional view of a main part showing an embodiment of a projection type cathode ray tube according to the present invention, FIG. 8 is a characteristic diagram showing the spot diameter ratio with respect to the voltage ratio (R _P ), and FIG. 9 is an anode voltage Figure 10 is a characteristic curve showing the spot diameter ratio versus voltage ratio (R _P ) of a bipotential lens, and Figure 11 is a characteristic curve showing the spot diameter ratio versus voltage ratio (R _P ) of a unipotential lens. A characteristic curve showing the spot diameter ratio, FIG. 12 is a sectional view of a main part showing another embodiment of the projection cathode ray tube according to the invention, and FIG. 13 shows still another embodiment of the projection cathode ray tube according to the invention. It is a sectional view of the main part. 1... Projection type cathode ray tube, 2... Projection screen, 3... Projection lens system, 4, 5, 6... Cathode ray tube, 7, 8, 9... Projection lens system, 10... Glass bulb, 10a... ...Network part, 10b...Funnel part, 10c...Panel part, 11...Electron beam, 12...Cathode, 13...Control electrode, 1
4... Accelerating electrode, 15... Electron gun, 16... Focusing electrode, 17... Anode electrode, 18... Electrostatic focusing lens system, 19... Fluorescent surface, 20, 20a, 20
b... Focusing coil, 21... Electromagnetic focusing lens system,
22... Compound lens, 23... Focusing electrode.

Claims (1)

[Claims] 1 A glass bulb consisting of a panel part, a funnel part and a neck part, a phosphor screen formed on the inner surface of the panel part, an anode electrode formed on the inner surface of the funnel part, and a glass bulb housed in the neck part and connected to the phosphor screen. An electron gun that emits an electron beam, a focusing electrode disposed within the network, an electrostatic focusing lens system consisting of a part of the anode electrode, and an electromagnetic focusing lens system disposed around the outer periphery of the network. The focusing electrode voltage/anode electrode voltage ratio for focusing the electron beam onto the fluorescent screen using only the electrostatic focusing lens system is set to R _O , and the effect of the electromagnetic focusing lens system is superimposed. When the focusing electrode voltage/anode electrode voltage ratio is R _P , the relationship is 1.25R _O <R _P <0.9 for a bipotential electrostatic focusing lens, and 2R _O <R _P <0.8 for a unipotential electrostatic focusing lens. A projection type cathode ray tube, characterized in that the electron beam is focused on the phosphor screen by adjusting the electrostatic focusing lens system and the electromagnetic focusing lens system so as to satisfy the following conditions.