Light regulation device

A light regulation element regulates both the travelling direction and diffusion state of a light emitted from a surface light source device. Lights from a cylindrical light source 2 such as a cold cathode tube is introduced in a light guide 1. The light guide 1 has an emission surface 1a on the side opposite to a reflection element 4, and a light regulation element 3 is arranged in opposition to the emission surface 1a. Protrusions 3a are formed on one surface or both surfaces of the light regulation element 3. Coated on the surface on the light emission side of the light regulation element 3 is a light diffusion layer 5. The turbidity of the light diffusion layer 5 falls within a range from 1900 cm.sup.-1 to 2400 cm.sup.-1 . The light emitted from the emission surface 1a of the light guide penetrates the light regulation element 3, thereby causing the light travelling direction to be regulated. Then, the light whose travelling direction has been regulated penetrates the light diffusion layer 5, thereby causing a light diffusion. The light diffusion layer 5 is formed, for example, by mixing fine particles of a glass having a light penetrability or of a synthetic resin such as acryl and silicone, or fine hollow particles of glass, acryl resin or the like, or air bubbles in a coating material having a light penetrability.

BACKGROUND OF THE INVENTION
 1. Technical Field
 The present invention relates to a light regulation device for regulating
 the travelling direction of light. and more particularly to a light
 regulation element for regulating the travelling direction of the light
 emitted from a surface light source device.
 2. Description of Related Art
 Where a surface light source device is applied to the backlight of a liquid
 crystal display, a range in which the display contents of the liquid
 crystal display can be clearly and visually recognized, that is, a view
 angle, and a brightness observed when the display screen is watched are
 affected by the travelling direction and spread of the light emitted from
 the surface light source device.
 The more diffused the light emitted from the surface light source device,
 the wider the view angle becomes, but the darker the display becomes.
 On the contrary, the less diffused the emitted light, the narrower the
 range of view angle becomes, but the larger the quantity of the light
 travelling in a specific direction becomes. When the display is watched in
 a direction corresponding to the light travelling direction, the display
 is observed to be bright.
 On the contrary, when the direction in which the display is watched is
 largely deviated from the light travelling direction, the contrast of the
 display is lowered to cause the display contents not to be visually
 recognized.
 Thus, where a surface light source device is applied to the backlight of a
 liquid crystal display, it has been necessary to change the diffusion
 state and travelling direction of the light emitted from the surface light
 source device in such a manner that the view angle and the display
 required for the liquid crystal display are adapted to the watching
 direction.
 Also, where a surface light source device is used for a local illumination
 for illuminating a specific area, the more diffused the light, the wider
 area the device can illuminate, but the lower the illuminance becomes. On
 the contrary, the less diffused the light, the narrow area the device
 illuminates, but the higher the illuminance becomes.
 Of course, also for the local illumination, the regulation of the light
 travelling direction is important, so that the light is required not to
 proceed in an undesirable direction.
 According a conventional a method of changing the diffusion state and
 travelling direction of the light emitted from a surface light source
 device, both a diffusion element are arranged and a light regulation
 element are arranged on the emission surface of a surface light source
 device.
 FIG. 5 illustrates an example of the configuration of a conventional
 surface light source device. Referring to FIG. 5, a cylindrical light
 source 2 is arranged along an incident surface 1b of a light guide 1.
 Arranged on an emission surface 1a of the light guide 1 are a light
 regulation element 3 and a diffusion element 6 in a state in which they
 are overlapped in a manner not to completely contact to each other.
 Arranged on the surface on the side opposite to the emission surface 1a of
 the light guide 1 is a reflection element 4.
 The light emitted from the light source 2 enters the light guide 1 from the
 incident surface 1b of the light guide, and part of the light while
 travelling in the light guide 1 is emitted directly from the emission
 surface 1a. And another part of the light is reflected by a rough surface
 (not shown), or an ink print portion or the like formed on the light guide
 1, and emitted form the emission surface 1a.
 The light emitted from the emission surface 1a penetrates the light
 regulation element 3. The light travelling direction is bent by the action
 of many protrusions 3a provided on one surface of the light regulation
 element 3, whereby the light travelling direction is regulated in a
 desirable direction. Then, the light penetrates both a very narrow gap 10
 (shown exaggeratedly in FIG. 5 ) formed between the light regulation
 element 3 and the diffusion element 6 and then the diffusion element 6,
 whereby the diffusion state of the light can be changed to a desired one.
 However, for the surface light source device shown in FIG. 5, it is
 necessary to arrange both the light regulation element 3 and the diffusion
 element 6 on the emission surface 1a of the light guide.
 If foreign matters such as dirt and dust adhere onto optical parts, or a
 flaw occurs thereon, the performance thereof will be remarkably
 deteriorated. Particularly for a surface light source device as shown in
 FIG. 5, there has frequently occurred a problem in that foreign matters
 are caught between the light regulation element 3 and the diffusion
 element 6 in the manufacturing process thereof.
 Also where the surface light source device is assembled, individual parts
 must be checked to make sure there is no flaw or dirt, and then assembled
 with meticulous care. As a result, an increase by only one in the number
 of parts causes the manufacturing cost to be significantly increased.
 Further, means for fixing both the light regulation element 3 and the
 diffusion element 6 are necessary, so that the size of the surface light
 source device tends to be large.
 Further, a fact that the gap 10 consisting of an air layer exists between
 the light regulation element 3 and the diffusion element 6 generally means
 that, a layer having a refractive index much smaller relatively exists
 between the light regulation element 3 and the diffusion element 6.
 Therefore, although the gap 10 has an effect of diffusing the light, the
 gap 10 also causes the light utilization efficiency to be decreased due to
 the reflection on the interface with both the elements 3 and 6.
 OBJECT AND SUMMARY OF THE INVENTION
 An object of the present invention is to provide a light regulation element
 which is easily manufactured. And, another object of the present invention
 is to provide a light regulation element which, while regulating the
 travelling direction of a light emitted from a surface light source
 device, can convert the light to a diffused light which can satisfy a view
 angle required for the liquid crystal display. A still another object of
 the present invention is to provide a light regulation device which can be
 also applied to a surface light source device for local illumination.
 In order to achieve these objects, there is provided a light-penetrable
 light regulation element having at least one light regulation surface. The
 shape of the light regulation surface of the light regulation element
 includes a plurality of concave and convex shape segments (protrusions)
 for bending the travelling direction of the light penetrating the light
 regulation element. And, at least either of the light regulation surface
 or the surface on the side opposite to the light regulation surface is
 covered with a light diffusion layer, and it is desirable that the
 turbidity of the layer falls within a range from 1900 cm.sup.-1 to 2400
 cm.sup.-1. The light diffusion layer may include a portion in which the
 air exists.
 Generally, the regulating of the light travelling direction and the
 allowing of the light to be diffused are opposed to each other. A fact
 that the light becomes diffused means that the light travelling direction
 is disturbed, and the regulating of the light travelling direction means
 the regulating of the light diffusion.
 Therefore, it is necessary to set the light diffusion ability of the light
 diffusion layer provided on the light regulation element so as to obtain a
 desirable balance with the ability to regulate the light travelling
 direction.

DETAIL DESCRIPTION OF PREFERRED EMBODIMENT
 As a method of evaluating the ability of the light diffusion layer coated
 on the light regulation element, the turbidity described below can be
 used. With reference to FIG. 4, a method of measuring the turbidity will
 be explained.
 First, as shown in FIG. 4(a), a He--Ne laser beam 8 is irradiated
 vertically onto a transparent substrate 7 and the intensity of the light
 9, a part of the light penetrating the transparent substrate 7, which
 penetrates in a direction perpendicular to the transparent substrate 7 is
 measured. The value thus obtained is taken as I.sub.0.
 Then, as shown in FIG. 4(b), with the light diffusion layer 5 having
 thickness of d cm coated on the transparent substrate 7, the He--Ne laser
 beam 8 is irradiated vertically onto the transparent substrate 7 in the
 same manner as in FIG. 4(a), and the light intensity of light 9', a part
 of the light penetrating the transparent substrate 7 and light diffusion
 layer 8, which penetrates in a direction perpendicular to the transparent
 substrate 7 is measured. The value thus obtained is taken as I.
 The above-mentioned values I, I.sub.0 and d are substituted in the
 following equation;
EQU E cm.sup.-1 =-(1/d) 1n (I/I.sub.0)
 The value of the E thus obtained is taken as the turbidity. The value can
 be used to evaluate the light diffusion layer coated on the transparent
 substrate. The larger the value of the turbidity, the stronger the light
 diffusion effect becomes, while the smaller the value of the turbidity,
 the weaker the light diffusion effect becomes.
 Therefore, if the value of the turbidity of the light diffusion layer
 provided on the light regulation element is excessively large, the light
 regulation ability which the light regulation element has will be
 deteriorated. On the contrary, if the value of the turbidity is too small,
 the effect of diffusing the light will become weak. Where the light
 regulation element of the present invention is applied to an ordinary
 surface light source device, for the turbidity of the light diffusion
 layer lower than 1900 cm.sup.-1, the action to diffuse the light is too
 weak. For example, when the element is used as the backlight of a liquid
 crystal display, usually, the view angle required generally for the liquid
 crustal display is not realized sufficiently and becomes narrow.
 On the contrary, if a light diffusion layer whose turbidity exceeds 2400
 cm.sup.-1 is provided on the light regulation element, the light
 regulation effect will be cancelled. As a result, when the element is
 applied to the backlight of a liquid crystal display, the quantity of the
 light oriented to outside the range of a view angle required is increased,
 causing a problem with respect to light utilization efficiency.
 For that reason, where the element is used in the surface light source
 device as the backlight of a liquid crystal display, a suitable range of
 the turbidity of the light diffusion layer is from 1900 cm.sup.-1 to 2400
 cm.sup.-1. It is also preferable that the range of the turbidity of the
 light diffusion layer (1900 cm.sup.-1 to 2400 cm.sup.-1) is applied to
 most of local illuminations usually used.
 FIG. 1 shows an example of a state in which a light regulation element of
 the present invention is arranged practically for a surface light source
 device. Reference code 1 indicates a light guide, and reference code 2
 indicates a cylindrical light source such as a cold cathode tube. A light
 regulation element indicated by 3 has a light regulation surface, on which
 protrusions 3a having regularly the same shape are formed. Reference code
 4 indicates a reflection element, and reference code 5 indicates a light
 diffusion layer coated on the surface on the side opposite to the surface
 on which the protrusions 3a are formed. The turbidity of the light
 diffusion layer 5 is within a range from 1900 cm.sup.-1 to 2400 cm.sup.-1.
 The light diffusion layer 5 functions the same action as a light diffusion
 element 6 shown in FIG. 5. After a light emitted from the light source 2
 and introduced into the light guide 1 is emitted from an emission surface
 1a, first the light penetrates the light regulation element 3, thereby
 causing the light travelling direction to be regulated. Then, the light
 whose travelling direction has been regulated penetrates the light
 diffusion layer 5, thereby causing a light diffusion.
 The light diffusion layer 5 is formed, for example, by mixing fine
 particles 5a of a glass having a light penetrability or of a synthetic
 resin such as acryl and silicone, or fine hollow particles 5a of glass,
 acryl resin or the like, or air bubbles 5a in a coating material having a
 light penetrability. Fine particles having various shapes such as
 spherical shape, rectangular shape and bar shape may be used. Generally,
 if the light diffusion ability of a light diffusion layer is high, the
 thickness of the light diffusion layer may be thin.
 Generally, the air may often be much lower in refractive index than a
 coating material having a light penetrability, so that mixing the
 above-mentioned fine hollow particles or air bubbles 5a in the coating
 material having a light penetrability allows a high light diffusion effect
 to be obtained even when the light diffusion layer 5 is similarly thin.
 With consideration to the easiness of manufacture, the size of the hollow
 part of the fine hollow particles or of air bubbles 5a, taking spherical
 shape for example, falls preferably within a range from about 0.1 .mu.m to
 10 .mu.m in diameter.
 FIG. 2 illustrates a surface light source device employing the light
 regulation element 3 in which the shape of protrusions 3a on the central
 portion is different from that on the peripheral portion. As clearly shown
 in FIG. 2, each of the protrusions has a depth, the depth of the
 protrusions being greater toward the peripheral portions of the light
 regulation element then at the central portion of the light regulation
 element. Referring to FIG. 2, a substantially spherical-shape light source
 2' such as an electrical lamp is arranged in a curved the light regulation
 element 3 shown in FIG. 3, a light with a different diffusion state in a
 lateral direction and longitudinal direction can be produced.
 Also in a case of FIG. 3, when the element is used for the surface light
 source device, the light diffusion layer 5 is coated on the surface of the
 protrusions 3b arranged in a manner to face in a direction opposite to the
 emission surface 1a of the light guide, so that the light having
 penetrated the light regulation element 3 is diffused by the light
 diffusion layer 5.
 The light regulation element 3 is provided with another group of
 protrusions 3c which are formed on the other face directed to the emission
 surface 1a. The latter protrusions 3c run approximately at the right angle
 with respect to the former protrusions 3b so that output light after
 passing the light regulation element 3 is diffused differently depending
 on a longitudinal direction and a lateral direction.
 Although in the above-mentioned embodiment, only an example in which the
 light diffusion layer 5 is coated on one side of the light regulation
 element 3, depending on applications, the light diffusion layer 5 may be
 coated on both sides of the light regulation element 3, as shown in FIG.
 6.
 In the manner described above, by the coating of a light diffusion layer on
 a light regulation element used usually, the light regulation element of
 the present invention can by itself convert the light emitted from a
 surface light source device to a diffused light while regulating the light
 travelling direction. The turbidity of the light diffusion layer is set at
 a range from 1900 cm.sup.-1 to 2400 cm.sup.-1, whereby the light can be
 converted to a diffused light which can satisfy a view angle required for
 a liquid crystal display.
 Also there can be eliminated a loss due to light reflection developed by a
 fact that a gap can occur between the light regulation element and the
 light diffusion element. Where the light regulation element of the present
 invention is applied to an ordinary surface light source device, the
 decrease in light loss is about 5 to 10% compared with prior art.
 Further, the light regulation element of the present invention is
 integrally formed, so that the gap having existed conventionally between
 light regulation element and the light diffusion element is removed,
 thereby not causing a problem such as a foreign matter being caught in the
 manufacturing process. In addition, the number of optical parts is
 reduced, whereby the cost of manufacture and quality control is
 significantly lowered.
 Although in the embodiments shown in FIGS. 1, 2 and 3, the protrusions
 provided on the light regulation element is triangular shape in section,
 the surface shape for the light regulation of the light regulation element
 of the present invention is not to be limited thereto. For example, the
 shape of the protrusions may be set at a spherical or corrugated shape to
 allow a lens-like action to be performed, whereby the light regulation
 element be used to change the light travelling direction. Also, a
 trapezoidal or multi-pyramid shape may be employed, whereby the light
 regulation element be used to change irregularly the light travelling
 direction. Generally, the concave or convex shape of the surface for the
 light regulation of a light regulation element is optionally set.