JPH05197022A - Fresnel lens screen - Google Patents

Abstract

PURPOSE:To shorten the focus of the screen by installing a member having a specific refractive index to either or both of the Fresnel surface or base of a Fresnel lens. CONSTITUTION:The member 1 having the refractive index n=1.2 to 1.45 is installed to either or both of the Fresnel surface (front surface) or the base. One surface of the Fresnel lens may not be formed as a flat surface but be formed as a lenticular surface in such a manner that the orientation characteristic can be independently controlled in the horizontal direction and vertical direction. This Fresnel lens is constituted by forming a transparent substrate 2 consisting of polycarbonate (PC) having a refractive index n=1.58, forming peaks 3 of Fresnel consisting of a UV curing type resin having a refractive index n=1.55 on this transparent substrate 2 and forming an alpha-fluoroacrylate polymer having a prescribed structure and a refractive index n=1.40 on the front surfaces of the peaks 3 of Fresnel and the base of the transparent substrate 2 at several mum thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Fresnel lens screen used for a rear screen of a rear projector.

[0002]

2. Description of the Related Art Conventionally, a rear projector in which a projection optical system is housed in a housing is commercially available.

In this rear projector, a screen composed of a Fresnel lens is arranged on the front surface of a housing, and a cathode ray from a CRT (light source) is projected on the rear surface of this screen.
An image is projected on the screen surface.

[0004]

By the way, in recent years, there has been a demand for reducing the weight of rear projectors, and the trend toward thinner casings has been accelerating.

The fact that the housing becomes thin means that the focal length of the Fresnel lens must be shortened (shortening of the focal point).

As a means for shortening the focal length of the Fresnel lens, it is conceivable to make the angle change (height change) of the Fresnel peaks of the Fresnel lens large (high), but in the case of this means the following problem There is a point. Increasing (increasing) the angle change of the Fresnel mountain means CR
The so-called hot spot phenomenon occurs in which the refraction angle of the T light becomes large, surface reflection occurs, the transmittance of the Fresnel lens inevitably decreases, and the brightness difference between the screen center portion and the screen corner portion occurs.

Further, as another means for shortening the focal length of the Fresnel lens, the refractive index of the material forming the Fresnel peak is increased so that the angle change (height change) of the Fresnel peak is reduced. A means for achieving a short focal length while suppressing it is conceivable, but in this case, if the refractive index of the constituent material of the Fresnel mountain is increased, the surface reflection inevitably becomes large and the loss of light, that is, the CRT light. Occurs (the transmittance decreases), and the screen image becomes dark.

SUMMARY OF THE INVENTION It is a technical object of the present invention to provide a Fresnel lens screen which achieves a short focus of the screen without deteriorating the optical characteristics of the screen.

[0009]

The gist of the present invention will be described with reference to the accompanying drawings.

The present invention relates to a Fresnel lens screen in which a member 1 having a refractive index n of 1.2 to 1.45 is attached to either or both of the Fresnel surface and the bottom surface of the Fresnel lens.

[0011]

In general, in order to shorten the focal length of the Fresnel lens, it is necessary to increase the angle change (height change) of the Fresnel crest (of course, as described in the section of the conventional example, the Fresnel crest angle change). (There is also a way to increase the refractive index of the material that makes up the Fresnel crest while suppressing (height change).) If the angle change (height change) of the Fresnel peak becomes large, the refraction angle of the light from the light source becomes large, and surface reflection occurs, which inevitably reduces the transmittance of the lens.
Since a member having a refractive index n = 1.2-1.45 is attached to either or both of the Fresnel surface (front surface) and the bottom surface, surface reflection can be suppressed, and therefore the transmittance of the lens does not decrease.

[0012]

EXAMPLE FIG. 1 shows a Fresnel lens according to this example. Although the Fresnel lens of FIG. 1 illustrates a case where one surface is a flat surface, it may be a lenticular surface so that the orientation characteristics can be controlled independently in the horizontal and vertical directions, instead of the flat surface. Further, a lenticular lens may be superposed on the Fresnel lens shown in FIG. 1 to form a double rear screen.

The Fresnel lens shown in FIG. 1 has a refractive index n = 1.58.
The transparent substrate 2 is formed by the polycarbonate (PC), and the Fresnel peaks 3 are formed on the transparent substrate 2 by the ultraviolet curable resin having the refractive index n = 1.55, and the surface of the Fresnel peaks 3 and the transparent substrate 2 are formed. Refractive index n of the following structure on the bottom
= 1.40 α-fluoroacrylic acid ester polymer having a thickness of several μm to several μm (the surface of the Fresnel peak 3 is 10 μm or less, and the bottom surface of the transparent substrate 2 is 10 to 200 μ in the case of a film).
m).

[0014]

[Chemical 1]

The Fresnel lens shown in FIG. 1 has a refractive index n =
The 1.55 member forms the Fresnel part. The height of the Fresnel peak is naturally determined by the refractive index of the constituent material of the Fresnel portion, as will be apparent from the equation described later.

The .alpha.-fluoroacrylic acid ester polymer is not limited to coating and formation, and a film may be formed and attached to the surface of the Fresnel crest 3 and the bottom surface of the transparent substrate 2.

Here, the optical characteristics of the screen according to this embodiment will be described.

First, regarding reflection and refraction at the boundary surface, when considering the boundary surface between two dielectric media having refractive indices n ₁ and n ₂ in FIG. 2, from Snell's law,

[0019]

[Equation 1]

The following holds.

Now, in the case of a 50 inch size screen,
The projection distance is, for example, a normal 1000 mm and a short focal length 70
The optical system is as shown in FIGS.

In the case of FIG. 5, from the expression of Snell's law,

[0023]

[Equation 2]

Further, in the case of FIG.

[0025]

[Equation 3]

[Mathematical formula-see original document] The following equations (11) can be summarized as follows (in this case, n ₁ is the refractive index of air, and n ₁ =
It is 1. ).

Here, the configurations (1) to (1) shown in FIG.
The transmittances at the corner (A) and the center (B) shown in FIGS. 3 and 4, respectively, were calculated for the types having different refractive indexes according to (6).

Here, the transmittance is calculated by the following equation.

In FIG. 2, the electric vector of the light wave can be decomposed into an S component having an oscillating surface perpendicular to the incident surface and a P component having an oscillating surface parallel to the incident surface. Coefficient ts: S component amplitude transmission coefficient rp: P component amplitude reflection coefficient tp: P component amplitude transmission coefficient

[0030]

[Equation 4]

It becomes

Considering the energy intensity, the reflectance Rp and the transmittance Tp for the P component are

[0033]

[Equation 5]

The reflectance Rs and the transmittance Ts for the S component are

[0035]

[Equation 6]

From the energy conservation law,

[0037]

## EQU00007 ## Since Rp + Tp = 1 and Rs + Ts = 1, the TOTAL transmittance is

[0038]

## EQU8 ## T = ((1-Rp) (1-Rs)) / 2 ..18 where .theta. = 0, that is, in the case of vertical incidence

[0039]

[Equation 9]

It becomes

Therefore, the transmittance can be calculated from the above-mentioned expression and the expression in which the formulas to 11 are summarized and the 18 expression, and the calculation result is shown in FIG. From FIG. 8, when one or both of the Fresnel surface and the bottom surface (configuration (5) in FIG. 8) or both (configuration (6) in FIG. 8) is coated with the α-fluoroacrylic acid ester polymer, high transmission is obtained. It has a high brightness as a percentage, and further,
It was confirmed that the rear screen had excellent optical characteristics with a small luminance ratio in the center and corners.

Theoretically, as described in the bottom column of FIG.
By setting = 1.251, a rear screen with extremely high transmittance can be obtained.

The above is the case where one or both of the Fresnel surface and the bottom surface are coated with the α-fluoroacrylic acid ester polymer having the above structure. The point is that a material having a low refractive index (for example, α-fluoroacrylic acid having the above structure is used. Fluoroacrylic ester polymer in which CH ₂ CF ₃ is replaced with CH ₂ CH ₂ CF ₃ , methacrylic acid ester polymer, CF ₂ ═CF ₂ and C
It is sufficient to coat the Fresnel surface and the bottom surface with a copolymer of F ₂ = CF (CF ₃ ) or polyfluorotrimethyethylmethacrylate, etc. According to experiments, similar effects can be obtained with a fluorine resin or a silicon resin. Was confirmed.

[0044]

As described above, the present invention provides a Fresnel lens screen having a short focal length, high transmittance and high brightness.

[Brief description of drawings]

FIG. 1 is an enlarged explanatory diagram of the present embodiment.

FIG. 2 is an explanatory diagram of the present invention.

FIG. 3 is an explanatory diagram of the present invention.

FIG. 4 is an explanatory diagram of the present invention.

FIG. 5 is an explanatory diagram of the present invention.

FIG. 6 is an explanatory diagram of the present invention.

FIG. 7 is an explanatory diagram of an optical characteristic experiment of this example.

FIG. 8 is a table showing the experimental results of the above.

[Explanation of symbols]

 1 member

Claims (1)

[Claims] 1. A Fresnel lens screen, wherein a member having a refractive index n = 1.2-1.45 is attached to either or both of the Fresnel surface and the bottom surface of the Fresnel lens.