3D camera and method of measuring transmittance using the same

Provided are a three-dimensional (3D) camera including a wavelength-variable light source for directly measuring transmittance and a method of measuring the transmittance. The 3D camera includes, as well as a light source, a transmission type shutter, and an image sensor, and a wavelength-variable light source capable of irradiating a light with a variable wavelength without being thermally affected by the light source, the image sensor, and the transmission type shutter. The wavelength-variable light source may directly measure a change in transmittance by irradiating light toward the transmission type shutter while the 3D camera operates.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No. 10-2015-0147538, filed on Oct. 22, 2015, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The exemplary embodiments disclosed herein relate to three-dimensional (3D) cameras including a wavelength-variable light source and methods of measuring transmittance using the same.

2. Description of the Related Art

In addition to a general function of photographing images, a 3D camera has a function of measuring distances from a plurality of dots on an object surface to the 3D camera. Various algorithms for measuring a distance between an object and a 3D camera, such as a time-of-flight (TOF) method, have been used. In the TOF method, a flight time of an illumination light emitted toward an object, reflected from the object, and received at a light receiving unit is measured. The flight time of the illumination light may be obtained by measuring a phase delay of the illumination light, i.e., a phase difference between light emitted to a subject and light reflected from the subject. A high-speed optical modulator may be used to accurately measure the phase delay.

A transmission type shutter having superior electricity-optical response characteristics has been used in order to obtain a 3D image with high distance precision. Recently, a transmission type shutter having a PIN diode structure has been used. The transmission type shutter is used to correct an effect of temperature on transmittance since a transmittance spectrum may vary with temperature as well as an applied voltage. Recently, a method of estimating a transmittance spectrum by measuring the internal temperature of a 3D camera, which is an indirect method, has been used as a transmittance correction method. However, according to the above method, it is impossible to directly measure the internal temperature of a transmission type shutter and errors may occur while estimating transmittance via the internal temperature.

SUMMARY

Exemplary embodiments disclosed herein may provide three-dimensional (3D) cameras which include a wavelength-variable light source and which are capable of directly measuring transmittance and methods of measuring transmittance using the same.

According to an aspect of an exemplary embodiment, there is provided a three-dimensional (3D) camera including: a first light source configured to emit first light toward an object, the first light being reflected from the object; a transmission type shutter configured to modulate the reflected first light reflected to generate modulated light; an image sensor configured to sense the modulated light that passes through the transmission type shutter; and a second light source that is spaced apart from the first light source, the image sensor, and the transmission type shutter in order to be thermally unaffected by the first light source, the image sensor, and the transmission type shutter, and is configured to emit second light toward the object, and to vary a wavelength of the second light, the varied wavelength of the second light being used by the 3D camera to measure transmittance of the transmission type shutter.

The 3D camera may further include a light blocker provided between the first light source and the object and configured to block the first light when the second light source is emitting the second light toward the object.

The second light source may be further configured to emit uniform light having uniform intensity.

The second light source may further include: a light emitter configured to emit light; an optical fiber configured to transmit the light emitted by the light emitter, wherein one end of the optical fiber is connected to the light emitter; a light controller configured to adjust an intensity of the light emitted by the light emitter; and a photodiode configured to feed back a part of the light that passed through the optical fiber to the light controller to thereby generate the uniform light.

The 3D camera may further include: a first controller configured to control the first light source, the image sensor, and the transmission type shutter; and a second controller configured to control the second light source.

The second controller may be further configured to control a wavelength and an intensity of the second light emitted by the second light source according to a signal transmitted from the first controller.

The 3D camera may further include a spectrometer configured to measure an intensity of the second light with respect to a wavelength of the second light source and transmit information about the intensity of the second light to the first controller.

The 3D camera may further include a beam splitter configured to align the second light to reach a same position on the object as a position reached by the first light.

The 3D camera may further include a beam expander configured to expand and radiate the second light of the second light source.

The 3D camera may further include a spectrometer configured to measure a wavelength of the first light.

The transmission type shutter may be one from among a PIN diode type shutter, an electro-optical type shutter configured to use the Pockel effect, or an electro-optical type shutter configured to use the Kerr effect.

The first light source may be one from among an edge-emitting laser, a vertical-cavity surface emitting laser, or a distributed feedback laser.

According to an aspect of another exemplary embodiment, there is provided a transmittance-measuring device of a three-dimensional (3D) camera including a transmission type shutter and a light source, the transmittance-measuring device including: a wavelength-variable light source configured to emit light toward an object, and to vary a wavelength of the light, the light being reflected from the object toward the transmission type shutter of the 3D camera, and the wavelength-variable light source being spaced apart from the transmission type shutter in order to be thermally unaffected by the transmission type shutter; and a light blocker configured to block light emitted by the light source of the 3D camera.

The device may further include an optical device configured to adjust a path of the light emitted by the wavelength-variable light source.

The optical device may include: an optical fiber configured to transmit the light emitted by the wavelength-variable light source; and a beam expander located at one end of the optical fiber and configured to expand the emitted light.

According to an aspect of another exemplary embodiment, there is provided a method of measuring transmittance of a three-dimensional (3D) camera system including a first light source configured to emit first light toward an object; a transmission type shutter configured to modulate the first light reflected from the object to generate modulated light; an image sensor configured to sense the modulated light that passes through the transmission type shutter; and a second light source that is spaced apart from the first light source, the image sensor, and the transmission type shutter in order to be thermally unaffected by the first light source, the image sensor, and the transmission type shutter, and is configured to emit second light toward the object and to vary a wavelength of the second light, the method including: operating the first light source and the transmission type shutter at the same frequency; blocking the first light emitted by the first light source; emitting the second light to the object so that the second light is reflected from the object to the transmission type shutter; and measuring an intensity of the second light that passes through the transmission type shutter and reaches the image sensor.

The method may further include varying a wavelength of the second light; and obtaining a center wavelength of the transmission type shutter when the intensity of the second light that passes through the transmission type shutter and reaches the image sensor is a maximum.

The method may further include correcting a difference between the center wavelength of the transmission type shutter and a wavelength of the first light.

The correcting the difference may include adjusting an operating current of the first light source and matching the wavelength of the first light with the center wavelength of the transmission type shutter.

The correcting the difference may include adjusting a driving voltage of the transmission type shutter and matching the center wavelength of the transmission type shutter with the wavelength of the first light.

The second light may reach a plurality of parts of the image sensor, and the method may further include comparing the intensity of the second light that reaches the plurality of parts of the image sensor and measuring uniformity corresponding to a position of the transmission type shutter, from among a plurality of image points of the object, based on the comparing.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. Constituent elements having the same name may be formed of the same material. In the drawings, the sizes of layers and regions may be exaggerated for clarity. The exemplary embodiments described below are only examples, and thus, it should be understood that the exemplary embodiments may be modified in ways and to be embodied in various forms. For example, when an element is referred to as being “on the front side of” or “in front of” another element, the element may be directly on the other element, or intervening elements may also be present. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list.

FIGS. 1A, 1B, 1C, 1D, 1E and 1Fare views illustrating a structure and characteristics of a three-dimensional (3D) camera according to an exemplary embodiment.FIG. 1Ais a schematic view illustrating a structure and characteristics of a 3D camera100according to an exemplary embodiment. Referring toFIG. 1A, the 3D camera100according to an exemplary embodiment may include a first light source110for emitting (radiating) a first light111toward an object180, a transmission type shutter130for modulating light reflected from the object180, and an image sensor140for sensing a light passing through the transmission type shutter130. The 3D camera100may further include a second light source120that is not thermally affected by a first light source110, an image sensor140, and a transmission type shutter130by being spaced apart from the first light source110, the image sensor140, and the transmission type shutter130. The second light source120is capable of emitting second light121toward the object180, wherein a wavelength of the second light121may be changed.

The 3D camera100may measure a distance to the object180. The 3D camera100may use a time-of-flight (TOF) method. In the TOF method, a flight time of the first light111emitted toward the object180, reflected from the object180, and received at the image sensor140is measured. The measuring is performed based on a phase delay, and thus, the transmission type shutter130, which performs modulation at high speed, is used. The transmission type shutter130is one type of electro-optical device among various types of electro-optical devices which are configured such that transmittance of light changes with a reverse bias voltage. A wavelength of light maximally transmitted by the transmission type shutter130is a center wavelength, and this wavelength may change according to a reverse bias voltage, and furthermore, according to a temperature of the transmission type shutter130. In order for the 3D camera100to operate based on the TOF method, the first light source110and the transmission type shutter130may be modulated to have an identical frequency. This feature will be described below in detail. A process of deriving distance information based on an intensity of light measured by the image sensor140is referred to as demodulation, and maximum demodulation efficiency may be obtained when a wavelength of the first light111and the center wavelength of the transmission type shutter130match each other.

The first light source110may be a light source device emitting the first light111toward the object180. For example, the first light source110may emit the first light111in an infrared region (range). The first light source110may prevent light in an infrared region from mixing with natural light in a visible light region such as sunlight. However, the first light source110may emit light in a variety of wavelength regions as well as light in an infrared region. In this case, correction for removing information of mixed natural light may be required. The first light source110may be a laser light source. For example, the first light source110may be one of an edge-emitting laser, a vertical-cavity surface emitting laser (VCSEL), and a distributed feedback laser.

Furthermore, the first light source110may further include an optical device. For example, the object180may include a diffuser for evenly emitting light. The first light111from the first light source110may be reflected from the object180and received at the image sensor140via the transmission type shutter130. The first light111does not need to reach the transmission type shutter130while transmittance of the transmission type shutter130is measured via the second light source120which will be described below. Therefore, as illustrated inFIG. 1, an optical path of the first light111may not reach the transmission type shutter130or an additional device capable of blocking the first light111may be used. The measuring of transmittance may be performed under a heating condition due to operations of the first light source110and the transmission type shutter130, and therefore, the first light source110may be in an operating state.

The second light source120may be a light source device emitting the second light121to the transmission type shutter130. The second light source120may not be thermally affected by the first light source110, the transmission type shutter130, and the image sensor140. In other words, temperatures of the second light source120may not be changed due to the operation of the first light source110, the image sensor140, and the transmission type shutter. To achieve this feature, the second light source120may be spaced apart from the first light source110, the image sensor140, and the transmission type shutter130by at least a predetermined distance, or may further include a thermal barrier190thermally separating the first light source110, the image sensor140, and the transmission type shutter130from the second light source120. For example, the first light source110, the image sensor140, and the transmission type shutter130may be included in the thermal barrier190, and the second light source120may be located outside the thermal barrier190. For example, the thermal barrier190may be a case of the 3D camera100.

The second light source120may change the wavelength of the second light121. The second light source120may measure transmittance of the transmission type shutter130with a change of the wavelength. The second light121from the second light source120may be emitted toward the object180or the transmission type shutter130. The second light source120may be a laser device, for example, a laser diode device. A wavelength variable region of the second light source120may include a center wavelength region of the transmission type shutter130to be measured. For example, when the 3D camera100uses the first light source110and the transmission type shutter130in an infrared region (e.g., 940 nm), a width of the wavelength variable region of the second light source120may be, for example, 890 nm to 990 nm including the infrared region. It is understood that exemplary embodiments are not limited to a range of 890 nm to 990 nm.

The second light source120may emit uniform light even if a wavelength of the second light121changes. The term “uniform” with respect to the phrase “uniform light” may refer to a feature that the intensity of the light with respect to time is constant. An intensity of light according to a wavelength of the second light source120may be constant. The transmittance of the transmission type shutter130may be defined as (an intensity of light reaching the image sensor140)/(an intensity of light emitted by the second light source120). For example, if the intensity of light emitted by the second light source120corresponding to a 900 nm wavelength is 100 and the intensity of the second light121reaching the image sensor140after passing through the transmission type shutter is 40, the transmittance of the transmission type shutter130corresponding to the 900 nm wavelength is 40/100=0.4. If the intensity of the second light121from the second light source120is constant regardless of a wavelength of the second light121, a denominator of the above transmittance formula may be constant. Therefore, a relative value of the transmittance of the transmission type shutter130according to a wavelength may be calculated based only on the intensity of light received at the image sensor140. Therefore, the operation of measuring the intensity of the second light121may be omitted during the operation of measuring of a center wavelength of the transmission type shutter130, and thus, the overall measuring operation may be simplified. However, even if the intensity of light according to a wavelength of the second light source120is not constant, it is possible to measure the intensity of light according to a wavelength of the second light source120and generate a transmittance diagram thereof by a separate spectrometer. Therefore, the intensity of the second light is not required to be constant all the time.

The second light source120may be used for directly measuring the transmittance of the transmission type shutter130. A thermometer may be included in the 3D camera100to be used in an indirect measuring method. However, when using a thermometer, a measured temperature value may be different from temperatures of the first light source110and the transmission type shutter130, and thus, errors may occur when the transmittance is calculated. When the second light source120is used, the second light source120may directly emit the second light121to the transmission type shutter130without being thermally affected by the first light source110, the transmission type shutter130, and the image sensor140.

The transmission type shutter130may be an electro-optical device modulating light that passes through the transmission type shutter130. The transmission type shutter130and the first light source110may be modulated to have an identical frequency ω. For example, the first light source110may be represented as A sin(ωt) when the first light111emitted from the first light source110is modulated at frequency ω. When the first light111bounces off the object180and is reflected therefrom, the first light111may be represented as B+C sin(ωt+φ), that is, an intensity and phase of the first light111are changed. As the transmittance of the transmission type shutter130is also modulated at sin(ωt) while the first light111passes through the transmission type shutter130, an intensity of the first light111that passed through the transmission type shutter130may be represented as B sin(ωt)+C sin(ωt) sin(ωt+φ). The first light111may be received at the image sensor140, and an intensity of light measured in the image sensor140during one period may be represented as a periodic value of B sin(ωt)+C sin(ωt) sin(ωt+φ) integrated over time. B sin(ωt) disappears and only the term φ in C sin(ωt) sin(ωt+φ) remains after the integration. The period may be determined based on the frequency ω during a modulating operation. Since information, such as velocity, position, etc., of the light related to φ is superimposed during every 2π unit, it is possible to derive an accurate distance from φ by adding a fixed phase delay during modulation of the transmission type shutter130. For example, it is possible to derive a deviation by performing a photographing operation with the transmission type shutter130after delaying the light by a phase of about π/2, π, and 3π/2 and comparing respective light intensity information. In order to measure the deviation, the transmission type shutter130may have electro-optical characteristics whereby transmittance of the light through the transmission type shutter130changes with an applied voltage. As illustrated inFIG. 1B, the transmission type shutter130may be an electro-optical device having a PIN diode structure. For example, the transmission type shutter130may be an electro-optical device using a multiple quantum well (MQW) method and may be formed on a GaAs substrate. Furthermore, the transmission type shutter130may be an electro-optical device using the Pockel effect or the Kerr effect. The PIN diode structure may include a P-type region131, an N-type region133, an intrinsic layer132provided between the P-type region131and the N-type region133, an anode134, a cathode135, and a power source136, which may be modified in various ways, as would be appreciated by an artisan having ordinary skill in the art.

Transmittance of the transmission type shutter130may vary with an applied reverse bias voltage. Referring toFIG. 1C, an x-axis indicates a reverse bias voltage (−V) applied to the transmission type shutter130, and a y-axis indicates transmittance (%) of the transmission type shutter130. A region illustrated with dotted lines is an operating region due to an AC voltage and a DC voltage applied to the transmission type shutter130. Regarding the operating region, the DC voltage corresponds to a center position of the operating region, and operates the transmission type shutter130, which only transmits the light having a certain wavelength corresponding to the operating region. Regarding the operating region, the AC voltage corresponds to a swing component changing the transmittance into a sin, cos function type. For example, if the magnitude of the DC voltage applied to the transmission type shutter130is Vdc, and the amplitude of the AC voltage applied to the transmission type shutter130is Vac, then, the transmission type shutter operates from Vdc−Vacto Vdc+Vac. An amplitude of the AC voltage may be freely selected, but preferably, may be less than the magnitude of the DC voltage because, if the amplitude of the AC voltage is greater than the magnitude of the DC voltage, a voltage may be 0 V or lower in some parts during a swing process, and thus, the transmission type shutter130may not be operated. The magnitude of the DC voltage may also be freely selected, but preferably, may be in a range in which the transmittance linearly changes according to a change in a voltage. As an inclination of the linear section becomes steeper, a transmittance difference may become larger according to a change of a voltage. Therefore, distance precision of the 3D camera100may become higher by reducing noise. The inclination indicates a demodulation contrast and may be changed according to types and materials of the transmission type shutter130. A graph ofFIG. 1Cis an exemplary graph of a GaAs type MQW transmission type shutter and may vary with a type of the transmission type shutter130. In the graph, a DC voltage of a reverse bias voltage is about −4.8V, and transmittance of the transmission type shutter130is about 40%. As an AC voltage is 0.8V based on the DC voltage, the reverse bias voltage may change from −4V to −5.6V. Therefore, the transmittance may change from 20.2% to 58.5%. The DC voltage and the AC voltage may be set such that the transmittance linearly changes. Since a point in time of when the reverse bias voltage is −5.8V in the graph corresponds to a last point of a linear part in the transmittance change, an operating efficiency of the transmission type shutter130may be reduced when the reverse bias voltage exceeds −5.8V. If it is required or desired to operate the transmission type shutter130in a region of −6V or higher, the magnitude of the DC voltage and the AC voltage may be determined so that the transmission type shutter130may operate in a region where a right inclination of the graph becomes a positive number.

Referring toFIG. 1F, the transmission type shutter130has a transmittance of a peak value corresponding to a specific wavelength of an incident light and does not have uniform transmittance corresponding to every wavelength of the incident light. The peak value corresponds to a center wavelength of the transmission type shutter130. The center wavelength of the transmission type shutter130is a function of a voltage and temperature, and may change according to a reverse bias voltage applied to the transmission type shutter130and a peripheral temperature. When the transmission type shutter130is operated via the AC voltage, heat according to the operation thereof is generated to be proportional to a square of the AC voltage, and thus, the center wavelength of the transmission type shutter130may change due to a temperature change thereby. However, if the AC voltage is too low, the above demodulation contrast may become lower and cause deterioration of a function of the transmission type shutter130. On the other hand, if the AC voltage is too high, a voltage becomes 0V or lower as a voltage swing becomes too large, and the transmission type shutter130may stop operating. Therefore, the center wavelength of the transmission type shutter130may be adjusted by adjusting the AC voltage within an operating range of the transmission type shutter130.

Furthermore, the center wavelength of the transmission type shutter130may also change according to the temperature change. Heat caused by the first light source110and the image sensor140and heat caused by the transmission type shutter130may raise an internal temperature of the 3D camera100. The temperature rise may keep changing the temperature of the transmission type shutter130. Accordingly, the center wavelength of the transmission type shutter130may also change. Referring toFIG. 1F, the center wavelength of the transmission type shutter130and a wavelength of the first light source110may be spaced apart from each other. Referring toFIG. 1D, the center wavelength of the transmission type shutter130moves so that the center wavelength increases as the temperature increases.

The wavelength of the first light111and the center wavelength of the transmission type shutter130may match each other during the transmittance measuring operation described above. However, the wavelength of the first light111and the center wavelength of the transmission type shutter130may continuously change due to heat according to operations of thereof. When the center wavelength of the transmission type shutter130changes due to a temperature change, an intensity of light received at the image sensor140may decrease due to the change of the center wavelength as well the reduction in transmittance, and therefore, the demodulation efficiency may be lowered. Therefore, it is possible to increase the demodulation efficiency by matching the center wavelength of the transmission type shutter130according to temperature with the wavelength of the first light111by adjusting the amplitude of the AC voltage. Referring toFIG. 1E, the wavelength of the first light source110may increase as the temperature rises. As the wavelength of the first light source110may vary with a magnitude of an operating current applied to the first light source110, the magnitude of the operating current may be adjusted in order to match the wavelength of the first light source110with the center wavelength of the transmission type shutter130.

The image sensor140may be a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS), which are types of image sensors frequently used in a typical two-dimensional (2D) camera imaging system. Thus, cost may be reduced since a separate image sensor for the 3D camera100is not required.

FIG. 2is a schematic view of a 3D camera200according to another exemplary embodiment.

Referring toFIG. 2, a light blocking unit250(e.g., light blocker) for blocking a first light211may be provided in front of a light source210. The light blocking unit250may prevent the first light211from being incident on a transmission type shutter230as the first light source210should be in an operating state for measuring transmittance of the transmission type shutter230. The light blocking unit250may be an impermeable film through which light cannot pass. Alternatively, the light blocking unit250may be a polarizer that changes directions of light so that the first light211may not be incident on the transmission type shutter230. When the first light source210includes a diffuser and emits the first light211toward an object280in a wide range of angles, it may be useful to implement the light blocking unit250as an impermeable film capable of blocking the first light211. The light blocking unit250may be required for operating a second light source220to measure the transmittance of the transmission type shutter230, and may be controlled by a first controller701(ofFIG. 7A) or a second controller702(ofFIG. 7A) to block the first light source210only when the second light source220is in operation. The first controller701and the second controller702will be described below. The image sensor240and thermal barrier290may be the same as the image sensor140and thermal barrier190, respectively.

FIG. 3is a schematic view of the second light source220having a constant intensity of the light with respect to varying a wavelength of the light, according to an exemplary embodiment. Referring toFIG. 3, the second light source220may further include a light controller222which adjusts an intensity of light, a light emitter223which emits light, an optical fiber224that is flexible and may transmit light as one end thereof is connected to the light emitter223, a photodiode225which feeds back some of the light that passed through the optical fiber224to the light controller222, and a cable226which transmits a signal fed back from the photodiode225to the light controller222. Accordingly, it may be easier to generate a transmittance diagram when the intensity of the second light221of the second light source220is constant. Therefore, the second light source220may act as a feedback system and maintain the intensity of the second light221. The light emitter223may be, for example, a laser diode. The optical fiber224is not limited to any particular type of material as long as the optical fiber224is able to transmit the second light221(ofFIG. 2) through total internal reflection. If the optical fiber224is flexible, it is easy to set an incident angle of the second light221. The photodiode225located in a part of the optical fiber224may feed back light in a certain ratio with respect to the emitted second light221to the light controller222. The light controller222may adjust the intensity of the second light221to be constant based on an intensity of the fed back light, in which the fed back light is continuously transmitted from the photodiode225to the light controller222.

FIG. 4is a schematic view of a 3D camera400according to another exemplary embodiment. Referring toFIG. 4, the 3D camera400according to an exemplary embodiment may further include a spectrometer460for measuring a wavelength of a first light411. As described above, the wavelength of the first light411may change according to a magnitude of an operating current applied to a first light source410and a temperature of the first light source410. Therefore, the wavelength of the first light411may also be measured to measure a difference between a wavelength of the first light source410and a center wavelength of a transmission type shutter430. The wavelength of the first light411may be measured via the spectrometer460and be used to change a wavelength difference. The second light source420may emit the second light421in substantially the same fashion as the second light source120emits the second light121. Additionally, the image sensor440, light blocking unit450, object480and thermal barrier490may be substantially the same as the image sensor140, light blocking unit250, object180and thermal barrier190, respectively.

FIGS. 5A and 5Bare schematic views of a 3D camera500according to another exemplary embodiment. Referring toFIG. 5A, an optical device capable of changing an optical path may be provided to change a path of second light521emitted from a second light source520toward an object580. The optical device may be, for example, a beam splitter571. When the second light source520is used as a laser light source, the second light521may be emitted in a very narrow range without a separate optical device like a beam expander. The second light521, which is emitted toward the object580at a certain angle through the beam splitter571, may be emitted toward a transmission type shutter530after being reflected from the object580. The transmission type shutter530may have a 2D structure with a certain area, and the second light521may be emitted toward a part of the 2D structure of the transmission type shutter530, which is disposed near the image sensor540. Referring toFIG. 5B, the part of the transmission type shutter530toward which the second light521is emitted may include a center portion C, un upper portion U, a left portion L, a right portion R, and a down portion D. Thus, it is possible to examine transmission uniformity of the transmission type shutter530by examining and comparing transmittances of each portion of the transmission type shutter530based on the second light521. The 3D camera500according to an exemplary embodiment is able to measure the transmission uniformity of the transmission type shutter530by using the second light source520without requiring a user of the 3D camera500to disassemble the transmission type shutter530, and thus, may be used as a performance inspection device. Furthermore, a beam splitter571may match incident positions of a first light511emitted from a first light source510and the second light521emitted from the second light source520so that the first light511and the second light521are emitted toward the object580at the same position. Therefore, it is easy to use the beam splitter571when measuring transmittance and transmission uniformity of the transmission type shutter530and further measuring a distance between the object and the 3D camera500because it is easy to use the 3D camera500after measuring the transmittance, as there is no need to additionally adjust a position of the 3D camera500. In this case, the user only removes a light blocking unit between the beam splitter571and the first light source510and stops an operation of the second light source520. The thermal barrier590may be substantially the same as the thermal barrier190.

FIG. 6is a schematic view of a 3D camera600according to another exemplary embodiment. A beam expander672is provided on the front side of the second light source620and second light621may be widely emitted toward an object680. The second light621reflected from the object680may be incident to the entire area of the transmission type shutter630. The 3D camera600according to an exemplary embodiment may obtain a transmittance diagram by measuring an average value of light, which passes through the transmission type shutter630and is sensed by an image sensor640. The image sensor640may measure an intensity of light sensed in unit periods as described above. Furthermore, information processing for obtaining a transmittance diagram may be performed by a first controller701(ofFIG. 7A) which will be described below. The first light source610may emit first light611in substantially the same fashion as the first light source110emits first light111. Additionally, the beam splitter671and thermal barrier690may be substantially the same as the beam splitter571and thermal barrier190, respectively.

FIGS. 7A and 7Bare schematic views of a 3D camera700according to another exemplary embodiment. Referring toFIG. 7A, the 3D camera700according to an exemplary embodiment may include a first light source710, a first controller701controlling an image sensor740and a transmission type shutter730, and a second controller702controlling a wavelength and an intensity of the second light721emitted by a second light source720.

The first controller701may demodulate the transmission type shutter730and the first light source710which emits first light711at the same frequency ω towards the object780. The first controller701may apply a reverse bias voltage to the transmission type shutter730and may apply an operating current to the first light source710. The first controller701may receive information about a measured intensity of light from the image sensor740. Furthermore, the first controller701may receive information about a wavelength and the intensity of the second light721from the second controller702. The first controller701may generate a transmittance diagram of the transmission type shutter730by combining the information about a wavelength and an intensity of the second light received from the second controller702with the information about a measured intensity of light received from the image sensor740. The transmittance diagram may be processed in the 3D camera700or may be transmitted to the outside. For example, the 3D camera700may transceive a signal by being connected to an electronic processor like a personal computer (PC). In detail, the first controller701and the electronic processor may be connected to each other. The second controller702may be connected to the first controller701indirectly via the electronic processor. The connection may be of a wire type or a wireless type. The first controller701may include a separate memory device for processing the obtained transmittance diagram. The memory device may be a nonvolatile memory device such as an electrically erasable programmable read-only memory (EEPROM).

The second controller702may control a wavelength and a light emitting operation of the second light source720that emits second light721and may be electrically connected to the first controller701. The second controller702may adjust the wavelength and the light emitting operation of the second light source720according to an electrical signal of the first controller701. Since the second controller702is connected to the first controller701, wavelength difference information according to a temperature state of the transmission type shutter730may be continuously measured by the first controller701, and thus, a demodulation efficiency of the 3D camera may be increased by adjusting the wavelength and the light emitting operation of the second light source720. Furthermore, when the light emitting operation of the second light source720is not adjusted, a separate spectrometer760may be used for measuring the intensity of the second light721as described above. Referring toFIG. 7A, the wavelength and the intensity of the second light721may be accurately measured as a part of the second light721is incident to the spectrometer760. The first controller701may correct the transmittance diagram by receiving information about the measured wavelength and intensity from the spectrometer760.FIG. 7Bis a graph illustrating an intensity of a current according to a wavelength when the second light source720operates in a fixed light-emitting mode. Referring to the graph ofFIG. 7B, it can be seen that the intensity of a current according to a wavelength changes although the second light source720is being operated in a constant mode which generates the second light721having constant light intensity regardless of a wavelength of the second light721. Therefore, an error in the transmittance diagram may be reduced by providing the separate spectrometer760and measuring an actual intensity of light per wavelength of the second light source720thereby. However, in the case of the second light source220(ofFIG. 3), the separate spectrometer760may not be required as the intensity of light may be constant via an intensity of light feedback system that is separately provided.

The second light source720, a light blocking unit750, and the second controller702may form a separate transmittance-measuring device for measuring transmittance of many types of 3D cameras, and not merely for the 3D camera700. The transmittance-measuring device may be used to perform a performance test of many types of 3D cameras in a manufactured state. For example, the second controller702may control, via the measuring device, the second light source720and the light blocking unit750in order to obtain a transmittance diagram of the 3D camera in a manufactured state. The second controller702may operate the second light source720to emit light toward a transmission type shutter of a corresponding 3D camera while a wavelength of the emitted light is varying and may control the light blocking unit750to block a light source of the corresponding 3D camera. For example, the second light source720may emit the light directly toward the transmission type shutter of the corresponding 3D camera or emit the light indirectly, the light being reflected from the object toward the transmission type shutter of the 3D camera. Thus, it is possible to simplify the performance test since there is no need to disassemble the 3D camera700. Furthermore, it is possible to emit the second light source720to only a part of the transmission type shutter of the 3D camera700and measure transmittance of the part. Accordingly, transmission uniformity of the transmission type shutter may also be measured by determining whether transmittance of each part of the transmission type shutter is constant. The thermal barrier790may be substantially the same as the thermal barrier190.

FIG. 8is a schematic block diagram illustrating a method of measuring a wavelength difference between a transmission type shutter and a first light source, according to another exemplary embodiment. Referring toFIG. 8, a center wavelength of the transmission type shutter via a second light source and a wavelength of the first light source are respectively measured, and the wavelength difference may be determined.

In operation S810, the first light source and the transmission type shutter may be operated to be modulated at the same frequency. In other words, a frequency component of an operating current applied to the first light source and a frequency component in an AC component of a reverse bias voltage applied to the transmission type shutter may match each other. The modulation at the same frequency of the first light source and the transmission type shutter may correspond to a typical operating state of a 3D camera. Therefore, phase information may be extracted from an intensity of light measured by an image sensor when the modulation at the same frequency is performed. Since the first light source and the transmission type shutter are in an operating state, temperature of the first light source and the transmission type shutter change as heat is generated therein. Therefore, the wavelength of the first light source and the center wavelength of the transmission type shutter may vary. Respective phase components of the transmission type shutter and the first light source do not necessarily match each other.

In operation S811, in order not to emit a first light from the first light source, the first light may be blocked by using a light blocking unit in front of the first light source or an optical path of the first light may be changed.

In operation S812, a wavelength variation range of the second light source may be set. As described above, the wavelength variation range of the second light source may be set to include a range of the center wavelength of the transmission type shutter. Therefore, several testing operations may be required. Alternatively, the wavelength variation range of the second light source may be set based on the range of the center wavelength of the transmission type shutter which is already known. The wavelength variation range may be set from λminwhich is a minimum wavelength to λmaxwhich is a maximum wavelength. Furthermore, the wavelength variation range may be set to Δλ which is a numerical value according to wavelength variation. Alternatively, the second light source may have a present Δλ.

In operation S813, second light may be emitted toward the transmission type shutter by the second light source. The second light may be emitted toward the transmission type shutter after being emitted and reflected to or from an object, or may be directly emitted toward the transmission type shutter.

In operation S814, an intensity of the second light may be measured by the image sensor after the second light passes through the transmission type shutter. As the intensity of light measured by the image sensor may change according to a transmittance of the transmission type shutter, a transmittance diagram may be obtained.

In operation S815, a wavelength A of the second light is compared with the maximum wavelength λmaxset in advance. If the wavelength A of the second light is greater than the maximum wavelength λmax, a transmittance diagram of the transmission type shutter is obtained in operation S817). Otherwise, the wavelength is increased by Δλ and operations S812, S813and S814may be repeated. Δλ may be determined by a user or may be determined in various other ways (e.g., automatically).

In operation S830, if the intensity of the second light emitted by the second light source according to a wavelength of the second light is not constant, it is possible to correct the intensity of the second light based on the transmittance diagram of the transmission type shutter. The correction may be performed by a first controller.

In operation S818, the center wavelength of the transmission type shutter may be measured based on the obtained transmittance diagram.

In operation S820, a wavelength of the first light source is measured through a spectrometer separately from the process of obtaining the transmittance diagram. Furthermore, in operation S819, a wavelength difference may be determined between the wavelength of the first light source and the center wavelength of the transmission type shutter.

FIGS. 9A to 9Care schematic views illustrating a method of correcting a wavelength difference, according to another exemplary embodiment. According to a wavelength obtained in the process ofFIG. 8, the wavelength difference may be corrected by adjusting a center wavelength of a transmission type shutter or a wavelength of a first light source. As described above, demodulation efficiency increases as the wavelength difference decreases. Therefore, it is possible to further increase the demodulation efficiency of the 3D camera by minimizing the wavelength difference.

Referring toFIG. 9A, it is possible to change the wavelength difference by adjusting an operating current of the first light source. In operation S910, it is determined whether the center wavelength of the transmission type shutter and the wavelength of the first light source match each other. If the wavelengths match, the maximum demodulation efficiency may be obtained without further changing of the wavelength difference. Otherwise, in operation S911, it is determined whether the center wavelength of the transmission type shutter is greater than the wavelength of the first light source. In operation S912, if the center wavelength is greater than the wavelength of the first light source, the operating current of the first light source may be increased. Accordingly, in operation S913, the wavelength of the first light source may be increased. Therefore, it is possible to reduce the wavelength difference between the center wavelength of the transmission type shutter and the wavelength of the first light source. If the operating current of the first light source increases, operations S910to S913may be repeated until the center wavelength of the transmission type shutter and the wavelength of the first light source match each other. On the other hand, if the center wavelength is less than the wavelength of the first light source, the operating current of the first light source may be reduced in operation S914. Accordingly, it is possible to reduce the wavelength difference as the wavelength of the first light source becomes smaller in operation S915. Similarly, after returning to operation S910, all of the operations may be repeated until the wavelength difference becomes the minimum. A first controller may control the processes.

Referring toFIG. 9B, it is possible to change the wavelength difference by adjusting an operating current of the first light source. In operation S920, it is determined whether the center wavelength of the transmission type shutter and the wavelength of the first light source match each other. If the wavelengths match, maximum demodulation efficiency may be obtained without further changing of the wavelength difference. Otherwise, in operation S921, it is determined whether the center wavelength of the transmission type shutter is greater than the wavelength of the first light source. The term “match” may include not only identically numerical matching but also matching within a certain range of a numerical value. For example, in operation S921, an allowable value with respect to a range in which the center wavelength of the transmission type shutter and the wavelength of the first light source match each other, may be set. In operation S922, if the center wavelength is greater than the wavelength of the first light source, a driving voltage of the transmission type shutter may be lowered. Accordingly, in operation S923, the center wavelength of the transmission type shutter may be reduced. Therefore, it is possible to reduce the wavelength difference between the center wavelength of the transmission type shutter and the wavelength of the first light source. If the driving voltage of the transmission type shutter is lowered, operations S920to S923may be repeated until the center wavelength of the transmission type shutter and the wavelength of the first light source match each other. On the other hand, if the center wavelength is less than the wavelength of the first light source, the driving voltage of the transmission type shutter may be increased in operation S924. Accordingly, it is possible to reduce the wavelength difference as the wavelength of the first light source increases in operation S925. Similarly, after returning to operation S920, all of the operations may be repeated until the wavelength difference becomes the minimum.

FIGS. 9A and 9Bare described separately with respect to the above operations, however, the operations may be combined. In addition,FIGS. 9A and 9Bare not limited to the above-described examples. However, as described above, in the operations described with reference toFIG. 9B, an AC (ofFIG. 1C) driving voltage of the transmission type shutter may be maintained in a range which does not influence an operation of the transmission type shutter.

Referring toFIG. 9C, a change in wavelengths of the transmission type shutter and the first light source according to temperature may be determined. The maximum demodulation efficiency may be obtained when the wavelengths of the first light source and the transmission type shutter match each other. Otherwise, a wavelength difference may be adjusted by adjusting at least one of an operating current of the first light source and the AC driving voltage of the transmission type shutter. Wavelengths of the transmission type shutter and the first light source match each other and have a value of 942 nm at 30° C. Therefore, maximum demodulation efficiency may be obtained when the first light source and the transmission type shutter start operating. However, as the temperature of the first light source and the transmission type shutter rise due to heating according to operations thereof, a change in a center wavelength of the transmission type shutter according to temperature is 0.135 nm/° C., and a change in a wavelength of the first light source according to temperature is 0.31 nm/° C. Accordingly, the demodulation efficiency may be reduced as a wavelength difference occurs. Therefore, the wavelength differences may be reduced by adjusting an operating current (or a driving voltage) of at least one of the first light source and the transmission type shutter.

A 3D camera according to exemplary embodiments may include a second light source that is capable of adjusting a wavelength of emitted light. The second light source may be spaced apart from a first light source and a transmission type shutter in order not to be thermally affected by the same. Transmittance of the transmission type shutter may be directly measured via a separate second light source without changing a temperature condition of the first light source and the transmission type shutter in an operating state. Furthermore, distance precision of the 3D camera may be increased by obtaining a center wavelength of the transmission type shutter for which the transmittance of the transmission type shutter may be a maximum and correcting the wavelength difference between the center wavelength of the transmission type shutter and a wavelength of the first light source.

Furthermore, a transmittance-measuring device according to exemplary embodiments may include a first controller controlling the transmission type shutter and the first light source, and a second controller controlling the second light source. As described above, the measuring of the transmittance of the transmission type shutter and the correction of the wavelength difference may be continuously performed by the first and second controllers. Thus, it is possible to continuously correct a wavelength difference which continuously changes according to an operating state, and therefore, demodulation efficiency of the 3D camera may be improved.

It should be understood that the exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of the features or aspects within each exemplary embodiment should typically be considered as being available for other similar features or aspects in other exemplary embodiments.