JP2000268603A - Lighting system and lighting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lighting system and a lighting method capable of reducing speckles with a compact and simple structure. SOLUTION: This lighting system 20 is equipped with a semiconductor laser 11 as a coherent light source to emit coherent light, a lens array 13 having plural element lenses 22 placed side by side to divide the coherent light emitted from the coherent light source into plural pieces of light flux equivalent to the number of element lenses, and light convergence lenses 14, 15 to converge a plurality of light flux so as to illuminate the almost same area 16 to be irradiated at their focus positions. Thereby, speckles can be reduced in the area to be irradiated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an illuminating device and an illuminating method, and more particularly to an illuminating device and an illuminating method for reducing spatial coherence or speckle noise of coherent light emitted from a coherent light source.

[0002]

2. Description of the Related Art As one form of an image display device, there is an optical projector which illuminates a liquid crystal panel and projects its reflected light or transmitted light on a screen. In an optical projector, a lamp such as a metal halide, halogen, or xenon is generally used as a light source.

[0003] However, this lamp light source has the following several drawbacks, which hinder its usefulness. First, the life of a lamp is short, for example, a metal halide lamp having a long life is about several thousand hours. For this reason, it is necessary to devise a configuration such that the cartridge can be exchanged by being housed in a detachable cartridge. Furthermore, since the three primary colors of white light emitted from the lamp are usually separated to form a color, the optical system for that occupies a large volume, and the overall volume of the optical projector becomes large. In addition, the color reproduction area is limited, and the light use efficiency is reduced.

In order to solve these problems, attempts have been made to use an optical semiconductor device such as a light emitting diode or a semiconductor laser as a light source. For example, a light emitting diode has an excellent lifetime of generally 10,000 hours or more. However, in general, light emitting diodes have low effective light extraction or utilization efficiency, and it is technically difficult and difficult to improve them.

[0005] In this respect, the semiconductor laser can use the emitted light efficiently due to its excellent directivity. A semiconductor laser has a sufficiently long life as a practical light source, and generally has a higher energy utilization efficiency than a light emitting diode. Further, a semiconductor laser can have a large color reproduction area due to its monochromaticity. However, when a semiconductor laser is used as a light source of the above-mentioned optical projector or the like, there is a problem of speckle noise as described below.

In general, when a laser light source such as a semiconductor laser is used as a light source, for example, for illumination of an image display device, on an image plane, for example, on the retina of an observer, an object plane, for example, each point of a screen, from each region, It can be considered that the contributions are aggregated to form an image. At this time, it is natural that the surface of the object has irregularities of about the wavelength or more.
If the light beams having a complicated phase relationship overlap each other and the light beams interfere with each other, the interference results in a complex light and dark pattern. Speckle noise is a phenomenon in which when a rough surface or an inhomogeneous medium is irradiated with highly coherent light such as laser light and the scattered light is observed, the light intensity distribution becomes random and the surface or medium becomes granular. In the case of a display device, the quality of a displayed image is significantly impaired. Semiconductor lasers also have a problem of coherence sufficient to cause speckle noise in general.

[0007] Speckle noise is a common problem not only in semiconductor lasers but also in laser light sources having high coherence, and various attempts have been made to solve them. One of the typical methods is a method using a rotating diffusion plate. This is to insert a diffusion plate having a random uneven surface such as frosted glass between the illumination source from the illumination light source, and by rotating it, the speckle pattern generated on the image plane is fluctuated with time, This is a method of averaging patterns by the integration effect within the response speed of the light receiving system. For example,
In the case of the human eye, the response speed is said to be about 30 msec. By rotating the diffuser at a sufficient speed so that the pattern fluctuates multiple times within that time, the speckle pattern for the human eye can be obtained. It can be almost unrecognizable.

[0008] However, since the rotating diffusion plate originally has a function of diverging light, loss of incident light occurs when inserted in an optical system. In addition, the rotating diffusion plate rotated by the motor is bulky, consumes energy, generates driving noise, and is not preferably applied to a consumer image display device.

Another conventional method for reducing speckle noise is to divide a coherent light beam having a certain coherence length into a plurality of luminous fluxes, give an optical path difference larger than the coherence length to each other, and then merge or arrange them again. Is the way. This reduction method does not cause interference between the light beams. Therefore, as the number of light beams to be split is larger, the spatial coherence degree of the combined or arranged coherent light can be reduced.

[0010] Specific known examples of this method include:
Fiber bundles are known. In this method, a plurality of fibers are bundled, and the length of each fiber is given an optical path difference longer than the coherence length of the incident light source.
When both ends of the fiber bundle are aligned and light enters from one end, the light emitted from each fiber at the other end is
There is no interference with each other and overall spatial coherence is reduced. Therefore, when this is used as a light source for illumination or the like, speckle noise on the irradiated surface can be reduced.

However, the method using the fiber bundle has the following problems. For example, when 51 optical fibers are bundled and the difference in length between them is 1 cm, the difference in length between the shortest optical fiber and the longest optical fiber is 50 cm. And align both ends,
For example, a large volume is required to be accommodated in the image display device, which is a hindrance in downsizing the image display device. Also, the aperture ratio at the input end of the fiber bundle is 1
Because of the following, a loss occurs when the incident coherent light is coupled into the fiber bundle. Further, at the emission end, light is emitted from each fiber as a light beam, that is, the emitted light is configured as a light beam diverging from each point of the emission port having a fixed area, and the light is emitted by the subsequent optical system. It may cause loss. In addition, mass production of equipment such as fiber bundles is basically
It is economically difficult and is also unsuitable for illumination of consumer image display devices.

By the way, the coherence length of coherent light emitted from a coherent light source having a single-mode power spectrum is generally sufficiently long, so that even if a means for producing a difference in optical path length is used, spatial coherence is not required. There are limits to reducing For example, when a single mode semiconductor laser is used as a light source, a typical spectrum width is 100 MHz.
Therefore, the coherence length is about 3 m. An optical system in which such a long optical path difference is generated in multiple stages has a considerable volume and is bulky, and is a major hindrance when used in a consumer image display device.

[0013]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a lighting device and a lighting method which can reduce speckle noise with a small and simple structure.

[0014]

In order to achieve the above object, an illumination apparatus according to the present invention comprises a coherent light source which emits coherent light arranged on an optical axis and a coherent light source on a plane intersecting the optical axis. A plurality of element lenses arranged side by side, a lens array that divides coherent light emitted from a coherent light source into a plurality of light fluxes substantially equal to the number of element lenses, and along a plane intersecting the optical axis And a condenser lens that is disposed and condenses a plurality of divided light fluxes so as to illuminate substantially the same irradiated area at the focal position.

The coherent light source used in the present invention is not particularly limited as long as it emits coherent light. For example, a laser such as a semiconductor laser, a solid-state laser, or a gas laser can be suitably used.

Preferably, the degree of coherence of the light emitted from the coherence light source is expressed as a function of distance, and the distance until the degree of coherence first becomes １ ／ of the value of the degree of coherence at a distance of 0 is determined. When Lc, the lens array has a numerical aperture NA, is composed of a plurality of element lenses arranged at intervals d, and satisfies the relationship dNA> Lc. As a result, speckle noise on the irradiated surface can be further reduced.

In a preferred embodiment of the present invention, the coherent light source is constituted by a laser which emits laser beams having a plurality of oscillation wavelengths different from each other. Further, preferably, the coherent light source is composed of a plurality of lasers independent of each other. The illumination device according to the present invention can be applied without limitation to those requiring illumination light, and is preferably, for example, an illumination device for illumination light of an image display device, a measurement device, a microscope, or an exposure device. .

The illumination method according to the present invention, which achieves the above object, emits coherent light from a coherent light source disposed on an optical axis, and arranges a plurality of element lenses on a plane intersecting the optical axis. Through the lens array, the coherent light emitted from the coherent light source is passed through and divided into a plurality of light fluxes substantially equal to the number of element lenses, and then the plurality of split light fluxes are condensed by a condenser lens, It is characterized in that substantially the same irradiation area located at the focal position of the condenser lens is illuminated.

[0019]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Embodiment 1 of Illumination Apparatus This embodiment is an example of an embodiment of an illumination apparatus according to the present invention. FIG. 1 is a schematic diagram showing a configuration of an illumination apparatus according to the embodiment, and FIG. 2 is a semiconductor laser. And FIG. 3 is a graph showing the coherence degree of the coherent light emitted from the semiconductor laser. As shown in FIG. 1, the illumination device 20 according to the present embodiment includes a semiconductor laser light source 11 that is sequentially arranged on an optical axis 19 and provided as a coherent light source.
A collimating lens 12 that is emitted from a semiconductor laser light source 11 to convert coherent light into parallel rays, a lens array 13, and combined condenser lenses 14 and 15, and an irradiated surface 16 orthogonal to an optical axis 19. It is a device that irradiates light to the light.

As shown in FIG. 2, the semiconductor laser 11 provided as a coherent light source is a so-called vertical multi-mode semiconductor laser that oscillates at a plurality of frequencies at regular intervals. Semiconductor laser 1
The degree of coherence of the output light from No. 1 is obtained by Fourier transform of the power spectrum, and generally has a periodic maximum as shown in FIG. The coherence degree is expressed in units of distance, and the half width of the first local maximum waveform, that is, the distance until the coherence degree first becomes 1/2 of the value of the coherence degree at the distance 0 is expressed as Lc. . Therefore, the full width at half maximum of the first local maximum waveform is 2Lc. The collimating lens 12 is arranged along a plane orthogonal to the optical axis 19, converts the light beam emitted from the semiconductor laser light source 11 into a parallel light beam, and guides the light beam to a lens array 13 arranged at a subsequent stage.

The lens array 13 has the same numerical aperture NA =
When a plurality of element lenses 22 having Sin θ have a period d
Are lens arrays arranged side by side. Lens array 1
In the subsequent stage of 3, the lens 14 and the lens 15 are arranged orthogonal to the optical axis 19, respectively. Lens 14
And the lens 15 function as a condenser lens,
The light is condensed on the irradiated surface 16 arranged at the focal position on the rear side of the lens.

The light beams divided by the number equal to the number of lenses 22 by the lens array 13 are overlapped by the condenser lenses 14 and 15 so as to irradiate the same area on the irradiated surface 16. Conversely, the light flux reaching one point on the irradiated surface 16 is transmitted to each element lens 22 of the lens array 13.
Are formed by superimposing the light beams transmitted through. The light flux reaching one point on the irradiated surface 13 has the same angular spectrum component of the light transmitted through each element lens 22. This means that the irradiated surface 16
And the rear focal plane of the lens 15, that is, the Fourier transform plane.

Here, of the light flux reaching the end point 17 of the irradiated area, one element lens 2 in the lens array 13 is used.
Focusing on the light beam transmitted through the element lens 22a and the adjacent element lens 22b, a difference in the optical path length of dNA occurs between the two light beams. Therefore, when the optical path length difference is larger than Lc, the light fluxes reaching the end point 17 on the irradiated surface can be incoherent with each other, and speckle noise can be reduced.

At a point closer to the optical axis 19 in the region to be irradiated, each element lens 2
If the optical path length difference from 2 is larger than Lc, speckle noise can be reduced. Therefore, it is desirable that the numerical aperture NA of the element lens is as large as possible.

Although there is no optical path length difference at least between the light fluxes reaching the point 18 on the optical axis 19, even if speckle noise occurs in a limited portion near the optical axis in the irradiated area, The contrast of speckle noise can be reduced in the entire irradiated region. Also in this case, it is possible to more effectively reduce speckle noise by combining with another illumination method for reducing speckle noise. Furthermore, as shown in Embodiment 2 below, the speckle noise can be more effectively reduced by configuring the coherent light source from a plurality of lasers independent of each other.

For example, by using the illumination device 20 of this embodiment as an illumination light source of an image display device, speckle noise can be reduced and image quality can be improved.

Embodiment 2 of Illumination Apparatus This embodiment is another example of the embodiment of the illumination apparatus according to the present invention, and FIG. 4 is a schematic diagram showing the configuration of the illumination apparatus of the embodiment. is there. The illumination device 50 according to the present embodiment includes a coherent light source 40 including a plurality (three in FIG. 4) of semiconductor laser light sources 41 which are sequentially arranged on the optical axis 49 and are independent of each other. A first lens array 42 for collimating light emitted from the
This is an apparatus that irradiates light to an irradiation surface 46 orthogonal to the optical axis 49, with the lens array 43 of FIG. The second lens array 43 has the same configuration as the lens array 13 of the first embodiment and has the same function. The lenses 44 and 45 are the same as the lenses 14 and 15 of the first embodiment.
Have basically the same configuration and the same action.

The coherent light source 40 comprises a plurality of semiconductor laser light sources 41, and each semiconductor laser 41
As shown in FIG. 2, it oscillates at a plurality of wavelengths. The luminous flux emitted from each semiconductor laser 41 is incident on a first lens array 42 composed of almost the same number of element lenses 52 as the number of semiconductor lasers 41 constituting the coherent light source 40, and is collimated. The light flux further reaches the second lens array 43, and the number of the element lenses 52 constituting the second lens array 43 is larger than the number of the semiconductor laser light sources 41. Therefore,
In the second lens array 43, light emitted from one semiconductor laser light source passes through the plurality of element lenses 52 and reaches the irradiated surface 46. The process of reaching the irradiated surface 46 through the second lens array 43 is the same as in the first embodiment.

With the above configuration, in the second embodiment, similarly to the first embodiment, the light beams emitted from the semiconductor laser 41 and transmitted through the different element lenses 52 of the second lens array 43 are mutually incoherent. I do. As a result, speckle noise on the irradiated surface 16 can be reduced.

Further, in the present embodiment, the irradiated surface 46
Since the light beams reaching the point 48 on the optical axis 49 are incoherent with each other, speckle noise can be reduced over the entire irradiated surface 46. That is, if the number of laser light sources constituting the coherent light source is N, the speckle noise contrast on the optical axis is 1 /
(N) ^0.5 . Furthermore, if dNA> Lc is satisfied at the point deviating from the optical axis 49, the speckle noise is further reduced as described in the first embodiment.

In FIG. 4, in order to explain the basic operation, the first lens array 42 and the second lens
Although the number of element lenses constituting the lens array 43 is set to 3, 7 respectively, it is needless to say that the present invention is not limited to this. Generally, the semiconductor laser and the first and second lens arrays each have two-dimensionally arranged element lenses. Also in this case, speckle noise can be reduced in accordance with the same effect as described above. In general, the larger the number of semiconductor lasers constituting a coherent light source, the more advantageous in reducing speckle noise.

In the first embodiment, the lens array 1
As a condenser lens at the subsequent stage of 3, a combination lens of two lenses 14 and 15 is used.
As a condenser lens at the subsequent stage of the second lens array 43, a combination lens of two lenses 44 and 45 is used. However, the present invention is not limited to this. More than one combination lens may be used. Further, the element lenses constituting the lens arrays 13 and 43 do not necessarily need to be periodically arranged at regular intervals.

In the first and second embodiments, a multi-mode semiconductor laser oscillating at a plurality of frequencies has been described as an example of a light source. However, a multi-mode semiconductor laser originally oscillates at a plurality of oscillation wavelengths. Alternatively, light of a plurality of wavelengths can be obtained by superimposing a high-frequency signal on an injection current of a semiconductor laser oscillating at a single wavelength. Such a multi-mode semiconductor laser generally has a small Lc, and the configuration for obtaining the above-described effect of reducing speckle noise is relatively easy. However, the coherent light source is not limited to a multi-mode semiconductor laser, and it is needless to say that the same principle can be applied to a laser having a finite coherence length.

Embodiment of Illumination Method The illumination method according to the present invention can be easily implemented by using the illumination device of the first embodiment or the second embodiment. For example, by using the illumination device 20 of Embodiment 1, coherent light is emitted from the coherent light source 11 composed of a semiconductor laser, and a lens array in which a plurality of element lenses 22 are juxtaposed to the emitted coherent light. The light passes through the light path 13 and is divided into a plurality of light beams equal to the number of element lenses. Next, the plurality of split light beams are condensed by the condensing lenses 14 and 15 and illuminate substantially the same irradiated region 16 located at the focal position of the condensing lenses 14 and 15. Thus, speckle noise on the irradiated surface 16 can be reduced.

[0035]

As described above, according to the present invention, a coherent light source for emitting coherent light, and a plurality of element lenses arranged side by side, and the coherent light emitted from the coherent light source is substantially equal to the number of element lenses. By providing a lens array that divides the light into a plurality of light fluxes equal to each other and a condenser lens that collects a plurality of light fluxes so as to illuminate substantially the same irradiated area at the focal position, the specification of the irradiated surface is improved. Lighting device that reduces noise. Furthermore,
Specifically, (1) the light beam from the coherent light source reaches one point on the irradiated surface with different optical paths and optical path lengths, and overlaps incoherently, thereby reducing speckle noise on the irradiated surface. it can. (2) If the coherent light source is a multi-mode laser that oscillates at a plurality of wavelengths, the optical path difference to be generated is relatively small. Therefore, even if a lens array composed of element lenses having the same numerical aperture is used, the irradiation area Speckle noise can be reduced in a wider area above. (3) When the coherent light source is composed of a plurality of lasers, speckle noise can be more effectively reduced in front of the irradiation area. (4) Further, if an image display device or an exposure device is configured by using the illumination device according to the present invention, a high-quality image or exposure with reduced speckle noise can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a configuration of a lighting device according to a first embodiment.

FIG. 2 is a diagram illustrating a spectrum of coherent light emitted from a semiconductor laser.

FIG. 3 is a graph showing the degree of coherence of coherent light emitted from a semiconductor laser.

FIG. 4 is a schematic diagram illustrating a configuration of a lighting device according to a second embodiment.

[Explanation of symbols]

11 ... Semiconductor laser, 12 ... Collimate lens, 13 ... Lens array, 14, 15 ... Combination condenser lens, 16 ... Irradiated surface, 19 ... Optical axis, 20
... The illumination device of the first embodiment, 22 ... an element lens, 40 ... a coherent light source, 41 ... a semiconductor laser, 42 ... a first lens array, 43 ... a second lens array, 44, 45 ... … Combination condenser lens, 46
... A surface to be illuminated, 49 an optical axis, 50 an illumination device of the embodiment, 52 an element lens.

Claims (9)

[Claims] 1. A coherent light source which emits coherent light arranged on an optical axis, and a plurality of element lenses arranged side by side on a plane intersecting the optical axis, and coherent light emitted from the coherent light source And a lens array that divides the light into a plurality of light fluxes substantially equal to the number of element lenses, and is arranged along a plane intersecting the optical axis, and is divided so as to illuminate substantially the same irradiation area at the focal position. And a condensing lens for condensing a plurality of light beams. 2. Expressing the degree of coherence of the light emitted from the coherence light source as a function of distance, wherein the degree of coherence is initially 1 / the value of the degree of coherence at distance 0.
Assuming that the distance to 2 is Lc, the lens array has a numerical aperture NA, is composed of a plurality of element lenses arranged at an interval d, and satisfies the relationship dNA> Lc. The lighting device according to claim 1. 3. The illumination device according to claim 1, wherein the coherent light source is constituted by a laser that emits laser beams having a plurality of oscillation wavelengths different from each other. 4. The lighting device according to claim 1, wherein the coherent light source is composed of a plurality of lasers independent of each other. 5. The illumination device according to claim 1, wherein the illumination device is an illumination device for illumination light of an image display device, a measurement device, a microscope, or an exposure device. 6. Coherent light is emitted from a coherent light source arranged on the optical axis, and emitted from the coherent light source to a lens array in which a plurality of element lenses are juxtaposed on a plane intersecting the optical axis. The coherent light is passed to divide the light into a plurality of light fluxes substantially equal to the number of element lenses. An illumination method characterized by illuminating an irradiation area. 7. The degree of coherence of the light emitted from the coherence light source as a function of distance, wherein the degree of coherence is initially 1 / the value of the degree of coherence at distance 0.
Assuming that the distance to 2 is Lc, the lens array has a numerical aperture NA, is composed of a plurality of element lenses arranged at an interval d, and satisfies the relationship dNA> Lc. The lighting method according to claim 6, wherein 8. The illumination method according to claim 6, wherein a laser that emits laser light having a plurality of mutually different oscillation wavelengths is used as the coherent light source. 9. The illumination method according to claim 6, wherein a plurality of mutually independent lasers are used as the coherent light source.