JPH0449217B2 - - Google Patents

Description

[Detailed description of the invention]

This invention aims to improve image contrast by reducing scattered light within a substrate glass in an output fluorescent surface structure for an image tube. As shown in Figure 1, the output phosphor surface structure of a conventional image tube usually has a phosphor layer 2 made of Zncds (Ag), ZnS (Cu or Ag), etc. on a glass substrate 1 with a thickness of around 1 mm. It is formed into a circular shape, and is configured so that a visible light image converted by this fluorescent layer 2 is observed through a glass substrate 1 using, for example, a tandem lens system. Therefore, among the light excited by an electron beam or the like and emitted radially at, for example, point P of the fluorescent layer 2, the light having an incident angle within the critical angle θ _c at the output surface 3 of the substrate glass 1 is output as a signal. However, the light 4 having a temperature of θ _c or more is totally reflected at the output surface 3 and irradiates the fluorescent surface 2 again as 4'. The fluorescent surface 2 diffuses some percentage of this irradiated light, and totally reflects the rest as 4'' again. By repeating this process, a certain distribution is formed as a background. This totally reflected light 4' is reflected by the fluorescent layer. 2
This reduces the contrast of the converted visible light image and deteriorates the output image quality of the image tube. Conventionally, measures to prevent this have included using a dark screen that absorbs some light on the glass substrate 1, or increasing the thickness of the glass substrate, but in the case of only the former, the output light also decreases and the output image In the latter case, the objective lens of the tandem lens system, which is placed opposite to the rear stage of the output fluorescent surface to guide the output image to an optical device such as a TV camera, has the disadvantage of causing a decrease in brightness. It is not practical because it is limited by the back focus (distance from the end surface of the objective lens to the focal plane) and has problems such as not being able to produce the desired effect.
This invention has been made in view of the above-mentioned current situation, and is intended to eliminate the decrease in contrast of visible light images of conventional fluorescent surface structures. This is the _{ratio SBR} (Signal
The following conclusions were reached by theoretically elucidating the interrelationship among the three elements of the background ratio: the radius of the fluorescent surface, the thickness of the glass substrate, and its light transmittance. (1) The signal component L _S is independent of the glass thickness t and is determined by the light transmittance T and refractive index n at that time. (2) The background component L _B is a function of the ratio of the fluorescent surface radius R and the glass thickness t. From this conclusion, L _S /L _B = SBR is used as a kind of contrast ratio as a standard for image evaluation, and SBR
In the region of ≧0.85, the light transmittance T is 60% to 80% at the thickness t determined next, taking into account brightness attenuation.
%, and by combining the conditions for reducing L _B in (2) above, the following optimal relational expression between the glass thickness t and the radius R of the fluorescent surface was obtained. 0.1R≦t<F However, here, 0.1R=0.05D. The above R is the radius of the fluorescent surface, but it can be replaced with the effective field of view dimension D of the fluorescent layer, and if the fluorescent layer is circular, the diameter is
When the shape is rectangular or square, the length of the diagonal is D. Further, F is the back focus of an objective lens of a tandem lens system disposed at the rear of the fluorescent surface structure to guide the output image to an optical device. The present invention will be explained below with reference to the drawings. FIG. 2 is a diagram showing, as a basic experiment, the illuminance distribution appearing on the output surface 3 of the glass substrate 1 when one point P at the center of the fluorescent surface 2 is excited by a point light source. When the luminous flux from P is emitted with a Lambertian distribution, it is brighter near the P point, and as the distance r away from the P point increases, the illuminance decreases, and then the total reflected light 4' shown in Fig. 1 becomes fluorescent light. A halo with a radius of 2r _C incident on surface 2 is generated, and the illuminance distribution is as shown in the _figure . Figure 3 shows the halo L _B ′ caused by the totally reflected light 4′.
In this figure, the horizontal axis r indicates the distance from the center P, and the vertical axis L _B indicates the luminous flux that becomes the background. However, the signal component L _S is not shown in this figure. If the radius of such a fluorescent surface 2 is R, the thickness of the glass 1 is t, the light transmittance at the glass thickness t is T, and the luminous flux at point P is (1 m), then the critical The light flux within the angle, that is, the signal component L _S is theoretically expressed by the following equation (1). However, r is the distance from the light emitting point P, and μ is the light absorption coefficient of the glass. Also, 2r _C in Figure 2 is

It is determined by the formula: n in this formula is the refractive index of the glass 1 with respect to vacuum. Next, the luminous flux L _B which becomes the back ground is determined by the following equation (2). Here, the signal-to-background luminous flux ratio SBR corresponds to a kind of contrast ratio, and can evaluate the quality of the image. In other words, SBR=L _S /L _B (3), and of course the larger the SBR, the better. This SBR and
By examining L _S , it is possible to determine the practical glass thickness t and the transmittance T at that time. Correctly, evaluation should be made not only by the SBR when only one point P in the center of the fluorescent surface 2 is illuminated, but also by the average value of the SBR when any point on the fluorescent surface 2 is illuminated.
There is not much difference, so for the sake of simplicity, we will proceed with the elucidation by illuminating the center. In equations (1) and (2), by setting μt=-nT and solving the integral, equations (4) and (5) are obtained. In particular, when μ=0 (T=1), equations (4) and (5) are as follows.
(6)(7). L _S =1/n ² (lm) ...(6) L _B =1-1/n ² -1/1+(R/2t) ² (lm)...(
7) From equations (4) and (5) above, we can see the following. (1) The signal component L _S is independent of the glass thickness t, and its t
It is determined by the transmittance T and the refractive index n when . ...Thus, for example, a substrate with a transmittance of 70% at 1 mm and a transmittance of 2.6
The luminous flux L _S used as a signal for a substrate with a transmittance of 70% in mm is exactly the same. (2) The background component L _B is a function of the ratio (R/t) between the radius R of the fluorescent surface and the glass thickness t. ...Of course, it is also influenced by the transmittance T and the refractive index n. Simply increasing the glass thickness t does not reduce L _B ;
There is no effect unless the above (R/t) is made small. Therefore, when layering a glass substrate with a constant transmittance T and a large fluorescent surface radius R, the L _B component will increase unless the thickness t is increased proportionally.
SBR gets worse. Also, it is not good to make the fluorescent surface larger than necessary. From the above equation (5),
It can be seen that SBR is also a function of (R/t). Next, for optical glass BK7 with a fluorescent surface radius R = 11.25 mm and a refractive index against vacuum n = 1.514, which is commonly used as a glass substrate, the glass thickness t,
L _S and SBR when changing the transmittance T are (4) ~
Calculated according to (7). Table 1 shows some of them.

【table】 … … … …
… …
5.0 6.347 6.200 6.057
5.192 3.565

Claims (1)

[Claims] 1. In an image tube output phosphor screen structure in which a phosphor layer is formed on a glass substrate, the thickness t of the glass substrate is such that total reflected light from the output surface of the glass substrate is incident on the phosphor layer. 1. An output phosphor screen structure for an image tube, the structure having a thickness set according to the following formula, and having a light transmittance of 60% to 80% at the set thickness. 0.05D≦t<F Here, D is the field of view of the fluorescent layer layered on the glass substrate, and F is the objective of the tandem lens system disposed after the fluorescent surface structure to guide the output image to the optical device. Back focus of the lens.