JPH06109904A - Fresnel lens sheet - Google Patents

Abstract

PURPOSE:To prevent a stripe pattern from being observed at the position of a changing point in the change of the number of steps in the case of forming a stairs-like notch on a non-lens surface as to a Fresnel lens sheet. CONSTITUTION:The stripe pattern caused by the change of the number of steps is prevented from being observed by performing graduation processing to graduation ranges B1 to B9 before and after the changing points S01 to S89 of the terraced notches formed on the non-lens surface of the Fresnel lens sheet 10 by a gradation pattern method, a dither method and an error diffusion method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Fresnel lens sheet suitable for use in a screen of a rear projection type projection television or the like.

[0002]

2. Description of the Related Art A rear projection type screen, called a rear screen, has a Fresnel lens portion for collimating the image light toward the observation side and the image light collimated by the Fresnel lens portion to be diffused in the horizontal direction. There are a single screen type that is composed of a lenticular part and is formed on one sheet, and a double screen type that is formed on two sheets.

The Fresnel lens portion of the lens sheet has a cross section composed of a lens surface formed as a surface for refracting the projected light to form parallel light or focused light, and a non-lens surface that does not essentially act optically. In the case of a double-screen type rear projection type screen, in which a plurality of angle-shaped prisms are formed in concentric circles, the Fresnel lens sheet usually has a Fresnel lens portion because the loss of projected light is small. The surface is used as an emission surface for projected light, and the other surface (for example, a smooth surface) is arranged as an incident surface.

When the image light incident from the incident-side smooth surface of the lens sheet is refracted by the lens surface of the Fresnel lens forming surface (emission surface), a part of the image light is reflected and reflected again on the incident surface, The light is emitted from the non-lens surface of the emission surface.

This light has a different emission direction from the light rays refracted by the lens surface, and in addition to this, the image projection light source is usually a horizontal monochromatic light source of red, green and blue. Since the lights of the respective colors are incident on the Fresnel lens sheet at different incident angles, the internally reflected light is also emitted from the different places of the respective colors at different angles.

For this reason, there is a problem in that unnecessary light other than the image light looks like a rainbow (rainbow eye or rainbow) to an observer or causes a phenomenon such as a ghost, resulting in deterioration of the image.

In order to solve this problem, conventionally, for example, as disclosed in Japanese Utility Model Laid-Open No. 63-187139, the non-lens surface of the Fresnel lens is roughened so that strong unnecessary light is emitted in one direction. There are some things I decided not to do. or,
In order to further enhance the effect, as disclosed in Japanese Patent Application Laid-Open No. 4-127101, there is one in which unevenness of 1 μm Rmax is formed on the non-lens surface.

Here, the mold for molding the Fresnel lens sheet as described above uses a saw-toothed diamond tool and rotates a plate material such as copper or brass while corresponding to each lens (prism) unit. The bite angle is changed for each groove to be cut, and the grooves are cut one by one. At this time, by devising the feeding sequence of the bite or by using an uneven bite, The lens surface is made uneven.

[0009]

In the Fresnel lens, since the lens angle is changed in each lens unit, the lens height is naturally changed sequentially. Therefore, when forming a step-like cut (unevenness) on the non-lens surface of the Fresnel lens, the length of the non-lens surface differs depending on the radial position of the Fresnel lens, and the number of steps differs. As described above, when the number of cut steps changes, a change in the number of steps is observed as a striped pattern at the change point, which is not preferable on the image.

The present invention has been made in view of the above-mentioned problems of the prior art, and a striped pattern does not occur at a change point where the number of steps of stepwise cuts formed on the non-lens surface of the Fresnel lens is different. An object of the present invention is to provide a Fresnel lens sheet having such a structure.

[0011]

According to the present invention, there is provided a Fresnel lens sheet for a rear projection type screen, comprising a plurality of concentric circular prisms each having a mountain-shaped cross section, and having step-like cuts formed on a non-lens surface of each prism. In the vicinity of a change point where the number of steps of the notch should be changed in the radial direction from the center of the Fresnel, in the radial constant region, the appearance frequencies of the prisms with a large number of steps and the prisms with a small number of steps are dispersed to perform blurring processing. Is achieved.

In the blurring process, a plurality of prisms in the fixed area are combined into one virtual prism,
One virtual prism is composed of a pattern having 0 or more prisms each having the number of stages before and after the change point of each number of stages,
Before and after the change point, at least one virtual prism of each pattern is arranged in order, from a virtual prism having a larger number of prisms before the change to a virtual prism having a larger number of prisms after the change. May be.

Further, in the blurring process, a boundary is provided for each change point of the number of steps, and within the boundary, numerical values of a one-dimensional array defined below are arranged periodically, and the value is changed for each change point. Any one of the prism characteristic values for determining the number of steps of the notch, including the height of the prism and the length of the non-lens surface in the Fresnel radial direction with the product of the sum of the set offset values and the scale parameter as a threshold value. When the prism characteristic value is larger than the threshold value, the larger step number is used, and when the prism characteristic value is smaller than the threshold value, the smaller step number is used as the cut step number of the prism. In the one-dimensional array, n is a natural number and U (n) is n.
An array in which all the elements are 1 and the initial value of the array is D
(2) = (0,1), and an array having more elements than the initial value is D (2n) = (2D (n),
It may be defined by a recurrence formula represented by 2D (n) + U (n).

Further, in the blurring process, a boundary is provided for each change point of the number of cutting steps, a fixed threshold value is set within this boundary, and the height of the prism and the length of the non-lens surface in the Fresnel radius direction are set. Including one of the prism characteristic values for determining the number of steps of the notch and the threshold value, and when the prism characteristic value is smaller than the threshold value, the smaller step number is the step number of the prism, and The prism characteristic value is added to the prism characteristic value of the next prism, and when the prism characteristic value is larger than the threshold value, the larger number of steps is taken as the number of steps of that prism, and the maximum prism characteristic value within the boundary is calculated from the prism characteristic value. The subtracted value is added to the prism characteristic value of the next prism, and the number of cutting steps is determined while sequentially adding excess and deficiency of the prism characteristic value to the next prism. It may be.

[0015]

According to the present invention, the gradation pattern method, dither method and error diffusion method used for gradation conversion of a two-dimensional image are
It is based on the finding that good results can be obtained by modifying and applying it for dimensions and dispersing the appearance frequency of prisms with many steps and prisms with few steps before and after the change point of step numbers and performing blurring processing. And divide the pseudo contour of the boundary part including the change point of the number of cutting steps into continuous and discontinuous,
The striped pattern at the change point of the number of steps can be blurred to make it less noticeable.

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

First, as shown in FIG. 1, the screen size is 40 inches, the refractive index is 1.49, and the projection tube distance is 800.
Consider a Fresnel lens sheet 10 having a mm, a focusing distance of 10000 mm and a Fresnel pitch of 0.1 mm.

The Fresnel lens (prism) 12 in the Fresnel lens sheet 10 has a lens surface 14 whose angle is changed according to the distance from the center of the Fresnel lens, and the Fresnel lens (prism) 12 as shown in FIG. 15 ° to the plane orthogonal to the incident side smooth surface 10A of the sheet 10.
And a non-lens surface 16 having a taper angle of. The non-lens surface 16 of the Fresnel lens is formed with a cut 18 in a step shape.

Due to the relationship with the Fresnel pitch, the prism height in the Fresnel lens 12 changes from 0 to 0.127 mm, and the length of the non-lens surface 16 also changes correspondingly. Assuming that the number of cuts 18 from the Fresnel center to the outermost Fresnel lens 12 is 0 to 9 and the cuts are linearly arranged in the prism height, as shown in FIG. 3, there are 9 places in the Fresnel lens sheet 10 as a whole. Thus, there are step number change points S _{01 to} S ₀₉ .

In this embodiment, for example, the number of stages is 0 and 1.
First blurring range B for applying a scar₁F = 0 mm
Le Center F₀And the number of notches 18 is 0 and 1
Change point with the range of
S)₀₁From the midpoint of₀₁And 1st and 2nd
Change point S₁₂And the number of steps is 1 and 2
Blur range B₂Is the change point S₀₁And S₁₂Midpoint with
Change point S₁₂And S_{twenty three}Between the midpoint and
In the same way, for the one with a larger number of steps, the blur range B ₃~
B₉To set. As a result of the blurring process, the change point S₀₁
~ S₈₉Disappear, so these are virtual points in FIG.
(Line).

Next, a large number of prisms exist in each of the set blurring ranges. In each prism, (prism height-minimum prism height in the corresponding blurring range) / (in the corresponding blurring range) Maximum prism height-minimum prism height within the corresponding blurring range) (1) is calculated, and a preferable value is applied to the gradation pattern method, dither method, and error diffusion method of each of the following embodiments. Multiply by and the product is the displacement.

Here, the number of prism steps within the range of the blurring process is determined from the smaller prism height to the larger prism height, that is, from the Fresnel center toward the outside.

First, an embodiment using the gradation pattern method will be described.

For example, suppose that a plurality of prisms in each blurring range are grouped into seven prisms to form a pseudo prism,
A prism with a low number of stages and a prism with a high number of stages are distributed in each pseudo prism.

At this time, when a prism having a low step number is represented by 0 and a prism having a high step number is represented by 1, the following eight kinds of prisms from a prism in which all seven are 0 to a prism in which all seven are 1 Pseudo prisms are conceivable, and these eight types of pseudo prisms are appropriately combined and arranged before and after the change point.

0000000 0001000 0010100 0101010 1010101 1101011 1110111 1111111

The above equation (1) has a value of 0 to 1, and in the gradation pattern method, this value itself is taken as a displacement and 0 to 1.
0.1, 0.1-0.25, 0.25-0.4, 0.4
~ 0.5, 0.5-0.6, 0.6-0.75, 0.7
The above virtual prisms are applied to 5 to 0.9 and 0.9 to 1, respectively.

That is, when the value of the equation (1) is 0 to 0.1, a virtual prism of 0000000 is used and 0.1 to 0.1 is used.
When 0.25, 0001000 virtual prisms are used, and when 0.9 to 1, 1111111 virtual prisms are used.

The number of steps actually engraved on the prism is determined by the following
To do so. First, Fresnel center F ₀From each prism
Numbering from 0 to screen size 40
Pitch, prism pitch 0.1 mm, 0-5080
Numbered. For the number given to each prism,
If we take the remainder system of 7, we get 0 for each prism.
Numbers of ~ 6 are assigned.

For a certain prism, the number of steps to be engraved in the prism is determined from the range of the value of the above-mentioned formula (1) of the prism and the number obtained from the remainder system. For example, if the value of the expression (1) is in the range of 0.4 to 0.5, it is a pseudo prism of 0101010.
The prisms with the numbers 0, 2, 4, and 6 have a small number of steps, and the prisms with the numbers 1, 3, and 5 have a large number of steps.

As a result, the distribution of the small number of stages and the large number of stages within the same blurring range is as shown in FIG. 4 (B). Further, in the case of the conventional simple division, that is, when the small number of stages and the large number of stages are not appropriately dispersed before and after the change point, the result is as shown in FIG. Here, a small number of steps is shown in white and a large number of steps is shown in black.

In the above embodiment, it is assumed that seven prisms are combined into one pseudo prism, but the present invention is not limited to this, and if necessary, a blurring effect can be surely obtained. And within the range where die cutting is possible,
The number is not limited.

For example, if three prisms are combined into one pseudo prism, a sufficient blurring effect cannot be obtained. On the contrary, if too much, Fresnel lens sheet 10
It becomes difficult to cut the mold for forming the. Due to the restrictions imposed by both of them, it is appropriate that the number of prisms constituting one pseudo prism is 5 to (the number of prisms in the region) / 20.

Next, the dither method of the second embodiment will be described.

For example, consider a dither array D (32) which is periodically arrayed in one blur range with 32 prisms as one cycle.

D (32) = {0,16,8,24,4,20,12,28,2,18,10,26,6,22,14,30,1,17,9,25,5 , 21, 13, 29, 3, 19, 11, 11, 27, 7, 23, 15, 31}

With the offset value set to 0, this dither array is fitted to each Fresnel prism 32 pitch in the range in which the blurring process is performed, and is used as a threshold value.

The numerical value of the equation (1) is multiplied by 32 to match the width of change with the dither array, and the product thereof is taken as the displacement. The displacement based on the height of each prism is compared with the threshold value obtained from the dither array, and if the displacement is large, the number of steps is large, and if the displacement is small, the number of steps is small. As a result, the number of small steps and the number of large steps in the same blurring range are as shown in FIG.

In the dither method, the dither array is D (2).
In that case, the effect cannot be expected, and if the dither array has only a few cycles in one blur range, the number of arrays becomes too large. Due to the nature of the recurrence formula, it does not make sense to increase the number of sequences, so D (4), D (8), D (1
6), D (32), and dither arrays for D (64) may be selected.

Next, an embodiment of the error diffusion method will be described.

In this error diffusion method, the value of the equation (1) in each prism is used as a displacement, and a fixed value of 0.5 is set as a threshold value for determining the number of steps. The sum of the displacement and the error (the initial value is 0) in the equation (1) is compared with the threshold value, and if the sum of the displacement and the error is smaller than the threshold value, the smaller number of steps is adopted for the prism, and at the same time, Let the sum of the displacement and the error be the error for the next prism. When the sum of the displacement and the error is larger than the threshold value, the larger number of steps is adopted for the prism, and the maximum value 1 of the displacement is subtracted from the sum of the displacement and the error.
The resulting negative number is the error for the next prism. In this way, the number of prism steps in each blur range is sequentially determined from the side closer to the Fresnel center F ⁰ .

The distribution of the number of steps by this error diffusion method is as shown in FIG. 4D within the same blurring range.

In each of the above embodiments, the blurring range is set between the midpoints of the change points, but the present invention is not limited to this, and the blurring process is performed between the change points. A region that is not protected may be provided.

[Brief description of drawings]

FIG. 1 is a sectional view showing a Fresnel lens sheet according to an embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view showing a lens surface and a non-lens surface of a prism in this embodiment.

FIG. 3 is a plan view showing a change point in the number of steps of a stepwise cut formed on a non-lens surface in this embodiment.

FIG. 4 is a diagram showing a comparison between the conventional case and the distribution of the number of stages according to each embodiment of the present invention.

[Explanation of symbols]

10 ... the Fresnel lens sheet 12 ... the Fresnel lens 14 ... lens surface 16 ... non-lens surface 18 ... cut S ₀₁ to S ₈₉ ... change point B ₁ .about.B ₉ ... blur radius F ₀ ... fresnel center

Claims (4)

[Claims] 1. A Fresnel lens sheet for a rear projection type screen, comprising a plurality of concentric prisms each having a chevron cross section, and stepwise cuts formed on the non-lens surface of each prism. A Fresnel lens sheet obtained by performing a blurring process by dispersing the appearance frequencies of a prism having a large number of steps and a prism having a small number of steps in a constant area in the radial direction near a change point to be changed in the radial direction. 2. The blurring process according to claim 1, wherein a plurality of prisms in the fixed region are grouped into one virtual prism, and each virtual prism is the number of steps before and after the change point of each step. Of prisms each having 0 or more prisms, each of which has at least one virtual prism of each pattern before and after the change point and has more prisms of the number of stages before the change to prisms of the number of stages after the change. A Fresnel lens sheet characterized in that it is sequentially arranged in a virtual prism having a larger number of. 3. The blurring process according to claim 1, wherein a boundary is provided at each change point of the number of steps, and within the boundary, numerical values of a one-dimensional array defined below are arranged periodically, and the value thereof is set. The prism for determining the number of steps of the notch, including the height of the prism and the length of the non-lens surface in the Fresnel radial direction, with the product of the sum of the offset value and the scale parameter set for each change point as a threshold value. Compared with any one of the characteristic values, when the prism characteristic value is larger than the threshold value, the larger number of steps is used, and when it is smaller than the threshold value,
A Fresnel lens sheet, wherein the smaller number of steps is the number of cut steps of the prism. Let n be a natural number and U (n) be an array in which all n elements are 1, and define D (2) = (0,1) as the initial value of the array,
An array with more elements for the initial value is D
It is defined by a recurrence formula represented by (2n) = (2D (n), 2D (n) + U (n)). 4. The blurring process according to claim 1, wherein a boundary is provided for each change point of the number of cutting steps, a fixed threshold value is set within the boundary, and the height of the prism and the Fresnel of the non-lens surface are set. The threshold value is compared with any one of the prism characteristic values for determining the step number of the cut including the length in the radial direction, and when the prism characteristic value is smaller than the threshold value, the smaller step number is set as the step number of the prism. And, the prism characteristic value is added to the prism characteristic value of the next prism, and when the prism characteristic value is larger than a threshold value, the larger number of steps is taken as the number of steps of the prism, and the maximum value within the boundary from the prism characteristic value. The value obtained by subtracting the prism characteristic value is
A Fresnel lens sheet characterized in that the number of cuts is determined by sequentially adding excess and deficiency of the prism characteristic value to the next prism so as to be added to the prism characteristic value of the next prism.