Wavelength separation device and 3-dimensional image acquisition apparatus including the same

Provided are a wavelength separation device and a 3-dimensional (3D) image acquisition apparatus including the same. The wavelength separation device includes a first prism having an inclined surface and a second prism bonded to a first region of the inclined surface of the first prism. A wavelength separation coating may be disposed at a junction between the first portion of the inclined surface of the first prism and the second prism, and a second portion of the inclined surface of the first prism, different from the first portion, is a total reflection surface that totally internally reflects light.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2013-0149996, filed on Dec. 4, 2013, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

Apparatuses consistent with exemplary embodiments relate to wavelength separation devices and 3-dimensional (3D) image acquisition apparatuses including the same, and more particularly, to wavelength separation devices capable of making uniform the light intensity distribution of separated reflected light, and 3D image acquisition apparatuses including the same.

2. Description of the Related Art

Recently, 3-dimensional (3D) content has become more significant along with the development and increasing demand for 3D display apparatuses capable of displaying an image having a sense of depth. Accordingly, research is being conducted into various 3D image acquisition apparatuses, such as 3D cameras, that enable users to personally produce 3D content. The 3D cameras have to be able to obtain depth information together with conventional 2-dimensional (2D) color image information through a single photographing operation.

A binocular stereo vision method using two cameras or a triangulation method using structured light and a camera may be used to obtain depth information about the distances between a 3D camera and the surfaces of an object. However, in these methods, it is difficult to obtain accurate depth information because the accuracy of depth information depends on the surface state of an object and degrades rapidly as the distance to the object increases.

In order to solve this problem, a time-of-flight (TOF) method has been introduced. TOF technology is a method of measuring the flight time of light between when illumination light is direction to an object until the light reflected from the object is received by a light-receiving unit. According to the TOF technology, an illumination optical system, including a light-emitting diode (LED) or a laser diode (LD), is used to project light of a certain wavelength (e.g., 850 nm near-infrared light) onto an object, light of the same wavelength reflected from the object is received by a light-receiving unit, and then a series of processing operations, such as modulation of the received light by a modulator having a known gain waveform, are performed to extract depth information. According to such a series of optical processing operations, various TOF technologies have been introduced.

In general, a 3D camera employing a TOF technology includes an illumination optical system for emitting illumination light of an infrared wavelength band, and an imaging optical system for acquiring an image of an object in order to obtain depth information. The imaging optical system generates a typical color image by sensing a visible light reflected from an object, and simultaneously generates a depth image having only depth information by sensing an illumination light of an infrared wavelength band reflected from the object. To this end, the imaging optical system may include a color image sensor and a separate depth image sensor. In this structure, various methods are proposed to allow a color image and a depth image to have the same viewing angle. For example, a beam splitter may be used to separate a visible light and an illumination light, direct the visible light to the color image sensor, and direct the illumination light to the depth image sensor.

SUMMARY

One or more exemplary embodiments may provide wavelength separation devices capable of uniformizing the light intensity distribution of separated reflected lights and 3-dimensional (3D) image acquisition apparatuses including the same.

According to an aspect of an exemplary embodiment, a wavelength separation device includes: a first prism comprising a light entrance surface, a light exit surface, and an inclined surface between the light entrance surface and the light exit surface; a second prism that is smaller than the first prism and is bonded to a first portion of the inclined surface of the first prism; and a wavelength separation coating that is disposed at a junction region between the first portion of the inclined surface of the first prism and the second prism to reflect light in a first wavelength band and transmit light in a second wavelength band, wherein a second portion of the inclined surface of the first prism, different from the first portion and adjacent to the light entrance surface is a total reflection surface that totally internally reflects incident light.

A width of the second prism in a first direction parallel to the light entrance surface of the first prism may be smaller than a width of the first prism in the first direction, and the first prism and the second prism may be disposed such that a central axis of the second prism and a central axis of the first prism are aligned.

The first prism may have a shape that is formed by cutting and removing a third portion of the inclined surface of the first prism, different from the first portion and the second portion and adjacent to the light exit surface.

The second prism may include a light exit surface and an inclined surface that is adjacent to the light exit surface.

The inclined surface of the second prism may be bonded to the first portion of the inclined surface of the first prism.

The wavelength separation device may further include an anti-reflection coating that is disposed on each of the light entrance surface of the first prism, the light exit surface of the first prism, and the light exit surface of the second prism.

The wavelength separation device may be configured such that light in the first wavelength band and light in the second wavelength band are incident on the light entrance surface of the first prism; light in the first wavelength band, which is reflected by the wavelength separation coating, and light in the first wavelength band and light in the second wavelength band, which are totally reflected by the total reflection surface of the first prism, exit through the light exit surface of the first prism; and light in the second wavelength band, which is transmitted through the wavelength separation coating, exits through the light exit surface of the second prism.

The second wavelength band may be shorter than the first wavelength band.

For example, the first wavelength band may be an infrared wavelength band, and the second wavelength band may be a visible wavelength band.

According to an aspect of another exemplary embodiment, a 3D image acquisition apparatus includes: a light source that generates light in a first wavelength band; a first imaging unit that provides a first image signal by using light in the first wavelength band that is reflected from an external object; a second imaging unit that provides a second image signal by using light in a second wavelength band that is reflected from the external object; an image signal processing unit that generates a 3D image by using the first image signal and the second image signal; and a wavelength separation device that has the above structure to separate the light in the first wavelength band and the light in the second wavelength band and provide the light in the first wavelength band and the light in the second wavelength band to the first imaging unit and the second imaging unit, respectively.

The light source may be disposed to be adjacent to a side surface of the second prism and to face the total reflection surface of the first prism.

The 3D image acquisition apparatus may further include a light diffusion device that is disposed between the light source and the total reflection surface of the first prism.

The light diffusion device may include a light exit surface on which a light diffusion surface is formed.

The light diffusion surface of the light diffusion device may be disposed to face the total reflection surface of the first prism.

Also, the light diffusion device may be spaced apart from the first prism.

Also, the first imaging unit may include: an objective lens that focuses light in the first wavelength band; a band-pass filter that transmits only light in the first wavelength band; an optical shutter that amplitude-modulates light in the first wavelength band; and an image sensor that senses a modulated illumination light to generate the first image signal.

DETAILED DESCRIPTION

Hereinafter, wavelength separation devices and 3-dimensional image acquisition apparatuses including the same will be described in detail with reference to the accompanying drawings. In the drawings, like reference numerals denote like elements, and the sizes of components may be exaggerated for clarity. The embodiments described hereinafter are merely exemplary, and various changes and modifications may be made therein. It will be understood that when a layer is referred to as being “on” another layer, it may be directly on the other layer, or one or more intervening layers may also be present.

FIG. 1is a schematic conceptual diagram illustrating an exemplary configuration of a 3D image acquisition apparatus100according to an exemplary embodiment. Referring toFIG. 1, the 3D image acquisition apparatus100may include a light source101that generates illumination light of a predetermined wavelength band, a first imaging unit120that provides a depth image signal by using illumination light that is reflected from an external object (not illustrated), a second imaging unit130that provides a color image signal by using visible light that is reflected from the external object, an image signal processing unit105that generates a 3D image by using the depth image signal and the color image signal, and a wavelength separation device110that separates the illumination light and the visible light and provides the illumination light and the visible light to the first imaging unit120and the second imaging unit130, respectively.

The first imaging unit120may include an objective lens121that focuses illumination light; a band-pass filter122that transmits only light having a wavelength corresponding to a wavelength of the illumination light from light incident thereon, an optical shutter123that amplitude-modulates illumination light according to a time-of-flight (TOF) method, and an image sensor124that senses modulated illumination light to generate a depth image signal. The optical shutter123may be a modulator that has a driving rate of several tens of MHz to several hundreds of MHz in order to obtain accurate depth information. For example, the optical shutter123may include an image intensifier tube having a multi-channel plate (MCP), a GaAs-based semiconductor modulator device, and a thin-film modulator device using an electro-optic material.

Although not illustrated inFIG. 1, the second imaging unit130may include an objective lens that focuses visible light, a band-pass filter that transmits only a wavelength component of visible light, and an image sensor that senses visible light to generate a color image signal. The image sensor124of the first imaging unit120and the image sensor of the second imaging unit130may be semiconductor imaging devices such as charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) devices. The image sensor124of the first imaging unit120may be a black-and-white image sensor that may sense only the brightness and darkness of light, and the image sensor of the second imaging unit130may be a color image sensor that may sense colors.

The 3D image acquisition apparatus100may further include a control unit103that is configured to control an operation of the light source101, an operation of the optical shutter123, an operation of the first imaging unit120, an operation of the second imaging unit130, and an operation of the image signal processing unit105, a light source driving unit102that drives the light source101under the control of the control unit103, and an optical shutter driving unit104that drives the optical shutter123under the control of the control unit103.

For example, the light source101may include a light emitting diode (LED) or a laser diode (LD) that may emit illumination light having about an 850 nm near-infrared (NIR) wavelength that is invisible to the human eye to protect it. However, this is merely exemplary, and illumination light of a different suitable wavelength band and a different type of light source may be used according to design. Also, the light source101may emit illumination light having a specially-defined waveform, such as a sine wave, a ramp wave, and a square wave, according to a control signal received from the light source driving unit102.

Hereinafter, an operation of the 3D image acquisition apparatus100will be described. First, under the control of the control unit103, the light source driving unit102provides a driving signal having a predetermined period and wavelength to the light source101. Then, the light source101projects illumination light, of an NIR wavelength band having the same period and wavelength as the driving signal, onto an object. For example, the light source101may sequentially irradiate illumination light at least three times, the different times of illumination light having the same period and different phases, onto the object according to the TOF method. Thereafter, incident light reflected from the object is incident onto the wavelength separation device110. At this time, visible light reflected from the object may be simultaneously incident onto the wavelength separation device110. The wavelength separation device110may separate incident light according to wavelength and provide the reflected illumination light to the first imaging unit120, while providing visible light to the second imaging unit130.

For example, visible light may be provided to the second imaging unit130through the wavelength separation device110, and illumination light of an NIR wavelength band may be reflected by the wavelength separation device110and provided to the first imaging unit120. The second imaging unit130may receive the visible light, which has passed the wavelength separation device110, generate a color image signal, and provide the color image signal to the image signal processing unit105. The first imaging unit120may modulate the illumination light, which has been reflected from the wavelength separation device110, by the optical shutter123, sense the illumination light to generate a depth image signal using the image sensor124, and provide the depth image signal to the image signal processing unit105. The image signal processing unit105may extract depth information of the object from the depth image signal according to the TOF method, and generate a 3D image by combining the depth information with a color image.

The 3D image acquisition apparatus100according to the present embodiment has no parallax between the depth image and the color image because it provides the illumination light and the visible light, which have been incident onto the wavelength separation device110, to the first imaging unit120and the second imaging unit130respectively. Thus, since a viewing angle of the depth image and a viewing angle of the color image are aligned with each other, via the wavelength separation device, it is not necessary to perform a separate image processing operation for aligning the viewing angle of the depth image and the viewing angle of the color image with each other. Also, since the objective lens121of the first imaging unit120and the objective lens of the second imaging unit130are independently used, the objective lenses may be designed to be optimized for the color image and the depth image respectively.

FIG. 1illustrates that the wavelength separation device110transmits the visible light and reflects the illumination light. However, the wavelength separation device110may alternately be configured to reflect the visible light and transmit the illumination light. As illustrated inFIG. 1, when the wavelength separation device110is configured to transmit the visible light and reflect the illumination light, the thickness of the 3D image acquisition apparatus may be made to be smaller than a case in which the wavelength separation device is configured to transmit the illumination light and reflect the visible light. In general, the objective lens121of the first imaging unit120may be a bright lens having a small F number for the accuracy of the depth information. Thus, the objective lens21of the first imaging unit120is generally larger and longer than the objective lens of the second imaging unit130. Thus, when the objective lens121of the first imaging unit120is disposed to be substantially perpendicular to the incident direction of the incident light, the thickness of the 3D image acquisition apparatus100may be smaller than a case in which the wavelength separation device is configured to transmit the illumination light and reflect the visible light. Thus, the 3D image acquisition apparatus100according to the present embodiment may be easily applied to a television (TV), a game, or a 3D camera that has a 3D motion recognition function. In the following description, it is assumed that the wavelength separation device110transmits the visible light and reflects the illumination light.

In general, the wavelength separation device110may use a flat panel-type dichroic minor or a cube-type beam splitter that is formed by joining two prisms having the same size. However, when the wavelength separation device110uses a flat panel-type dichroic minor, it may be difficult to reduce the thickness of the 3D image acquisition apparatus100since there is a large amount of unused space. Also, in the case of a cube-type beam splitter, a wavelength separation coating is disposed between two prisms to reflect the illumination light and transmit the visible light. However, it may be very difficult to design the wavelength separation coating to have the same reflectance with respect to incident lights of all angles. For example, the reflection efficiency of the wavelength separation coating may be low with respect to illumination light of an infrared wavelength band that is incident at a high incident angle (i.e., that is incident at an angle). Accordingly, the illumination light may be partially lost, and the light intensity distribution of the reflected illumination light may be non-uniform.

The 3D image acquisition apparatus100according to the present embodiment may include the wavelength separation device110that may make uniform the light intensity distribution of the reflected illumination light while reducing the size thereof.FIG. 2is a cross-sectional view illustrating a detailed configuration of the wavelength separation device110of the 3D image acquisition apparatus100, andFIG. 3is a perspective view of the wavelength separation device110.

Referring toFIGS. 2 and 3, the wavelength separation device110according to the present embodiment may include two prisms111and112that have different sizes. The first prism111includes a light entrance surface111ionto which both visible light and illumination light are incident, a light exit surface111ethrough which reflected illumination light exits, and an inclined surface between the light entrance surface111iand the light exit surface111e. Also, the second prism112includes a light exit surface112ethrough which transmitted visible light exits, and an inclined surface that is adjacent to the light exit surface112e. The first prism111is larger than the second prism112, and the inclined surface of the second prism112is bonded to a portion of the inclined surface of the first prism111. A wavelength separation coating114is disposed at a junction region between the first prism111and the second prism112to transmit visible light having a short wavelength and reflect infrared illumination light having a long wavelength. Also, an anti-reflection coating113may be disposed at the light entrance surface111iand the light exit surface111eof the first prism111and the light exit surface112eof the second prism112.

As illustrated inFIGS. 2 and 3, since the second prism112is smaller than the first prism111, the inclined surface of the second prism112is bonded to only a portion of the inclined surface of the first prism111. As a result, the other portion of the inclined surface of the first prism111contacts air that has a low refractive index. Thus, the other portion of the inclined surface of the first prism111will totally internally reflect light that is incident on the inclined surface of the first prism111at a high incident angle. For example, a first surface111aof the inclined surface of the first prism111, which is adjacent to the light entrance surface111iand is not bonded to the second prism112, and a second surface111b, which is not bonded to the second prism112, may be total internal reflection surfaces at which total internal reflection occurs.

However, the light totally internally reflected by the second surface111bis not incident onto the first imaging unit120or the second imaging unit130and is discarded. Thus, since the second surface111bis optically unavailable, a vertex region of the first prism111including the second surface111bmay be cut and removed as illustrated in a cross-sectional view ofFIG. 4Aand a perspective view ofFIG. 4B. Thus, the thickness and weight of the 3D image acquisition apparatus100may be further reduced by reducing the volume and weight of the wavelength separation device110.

Also, since visible light that is transmitted through both edges111cof the inclined surface of the first prism111is not used in the second imaging unit130, the width of the second prism112may be made to be smaller than the width of the first prism111, and the first prism111and the second prism112may be disposed such that a central axis of the second prism112and a central axis of the first prism111are aligned with each other, as illustrated inFIGS. 3 and 4B. Thus, the volume and weight of the wavelength separation device110may be further reduced by reducing the width of the second prism112.

FIG. 5Aschematically illustrates a path of light that is transmitted through the wavelength separation device110illustrated inFIGS. 4A and 4B. Referring toFIG. 5A, light is incident from the outside of the 3D image acquisition apparatus100onto the light entrance surface111iof the first prism111at various angles. Visible light and illumination light that are reflected from the object may make up the light incident on the light entrance surface111i.FIG. 5Aillustrates only light components having one of three different incident angles among all angles of light incident on the light entrance surface111i. That is,FIG. 5Aillustrates a first light component L1that is incident at an angle normal to the light entrance surface111i, a second light component L2that is incident at a negative (−) angle with respect to the first light component, and a third light component L3that is incident at a positive (+) angle with respect to the first light component.

The first, second and third light components L1, L2and L3may be incident onto the light entrance surface111iof the first prism111, travel through the first prism111, and then be incident onto the wavelength separation coating114that is disposed at the junction region between the first prism111and the second prism112. As described above, the wavelength separation coating114may be configured to transmit the visible light and reflect the infrared light. Thus, the visible light among the first, second and third light components L1, L2and L3may pass through the wavelength separation coating114and travel through the second prism112. Thereafter, the visible light may exit through the light exit surface112eof the second prism112and be incident onto the second imaging unit130. The second imaging unit130may sense the visible light to generate a color image signal.

FIG. 5Bschematically illustrates a path of light that is reflected by the wavelength separation device110illustrated inFIGS. 4A and 4B.FIG. 5Billustrates a fourth light component L4that is incident at an angle normal to the light entrance surface111i, a fifth light component L5that is incident at a negative (−) angle with respect to the fourth light component, and a sixth light component L6that is incident at a positive (+) angle with respect to the fourth light component. Referring toFIG. 5B, the fourth and sixth light components L4and L6, among the fourth, fifth and sixth light components L4, L5and L6, are be incident onto the light entrance surface111iof the first prism111, travel through the first prism111, and then are incident on the wavelength separation coating114that is disposed at the junction region between the first prism111and the second prism112. Infrared illumination light in the fourth and sixth light components L4and L6may be reflected by the wavelength separation coating114, exit through the light exit surface111eof the first prism111, and then be incident onto the first imaging unit120.

The fifth light component L5is incident onto the light entrance surface111iof the first prism111, travels through the first prism111, and then is incident onto the first surface111aof the first prism111. Herein, the fifth light component L5is totally internally reflected by the first surface111a. Thus, regardless of the wavelengths of light in the fifth light component, all of the fifth light component L5exits through the light exit surface111eof the first prism111. However, only the illumination light among the fifth light component L5exiting through the light exit surface111eof the first prism111passes through the band-pass filter122ofFIG. 1and is incident onto the first imaging unit120. Thus, the first imaging unit120may sense the infrared illumination light in the fourth, fifth and sixth light components to generate a depth image signal. Since the visible light among the fifth light component L5does not contribute to generating the color image signal of the second imaging unit130, it does not affect the performance of the second imaging unit130even when disappearing after being totally internally reflected by the first surface111aof the first prism111.

FIG. 6is a graph illustrating a light intensity distribution profile of light that is reflected by the wavelength separation device110illustrated inFIGS. 4A and 4B. For example, the solid line ofFIG. 6represents a light intensity distribution profile of illumination light reflected by the wavelength separation device110according to the present embodiment; the dotted line represents a light intensity distribution profile of illumination light reflected by the cube-type beam splitter in which the wavelength separation coating114is formed throughout the junction region between two prisms having the same size. Also, inFIG. 6, a horizontal axis represents a cross-sectional direction of a reflected light, and a vertical axis represents the intensity of the reflected light. As described above, the reflection efficiency of the wavelength separation coating114is low with respect to the infrared illumination light that is incident at an angle. Thus, in the case of the cube-type beam splitter, since the illumination light in the fifth light component L5illustrated inFIG. 5Bis lost, a non-uniform light intensity distribution profile is formed as represented by a dotted-line graph ofFIG. 6. However, in the case of the wavelength separation device110according to the present embodiment, since the fifth light component L5is totally reflected, the illumination light that is incident at an angle is not lost. Thus, since a uniform light intensity distribution profile may be formed as represented by a solid-line graph ofFIG. 6, the accuracy of the depth information may be improved.

FIG. 7is a schematic conceptual diagram illustrating an exemplary configuration of a 3D image acquisition apparatus200according to another embodiment. In the case of the wavelength separation device110according to the present embodiment, since the second prism112smaller than the first prism111is bonded to the first prism111, an empty space may be formed on the first surface111aof the inclined surface of the first prism111, which is adjacent to the light entrance surface111iand is not bonded to the second prism112. Thus, when the light source101is disposed in the empty space of the wavelength separation device110, the size of the 3D image acquisition apparatus200may be further reduced.

To this end, as illustrated inFIG. 7, the light source101may be disposed to be adjacent to a side surface of the second prism112to face the first surface111aof the first prism111, and a prism-type light diffusion device106may be further disposed between the light source101and the first surface111aof the first prism111. For example, a light diffusion surface107may be formed at a light exit surface that is an inclined surface of the prism-type light diffusion device106. The light diffusion surface107of the light diffusion device106may be disposed to face the first surface111aof the first prism111. In this configuration, illumination light emitted from the light source101may be diffused by the light diffusion device106, and the diffused light may be projected through the first prism111to the outside of the 3D image acquisition apparatus200. A gap108may be provided between the light diffusion device106and the first surface111aof the first prism111so that the light incident into the first prism111may be totally internally reflected by the first surface111a. That is, the light diffusion device106may be spaced apart from the first prism111by the gap108without being bonded to the first prism111.