JP5478902B2 - Optical distance sensor - Google Patents

Optical distance sensor Download PDF

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JP5478902B2
JP5478902B2 JP2009009597A JP2009009597A JP5478902B2 JP 5478902 B2 JP5478902 B2 JP 5478902B2 JP 2009009597 A JP2009009597 A JP 2009009597A JP 2009009597 A JP2009009597 A JP 2009009597A JP 5478902 B2 JP5478902 B2 JP 5478902B2
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JP2010169405A (en
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康雄 板津
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スタンレー電気株式会社
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  The present invention relates to a so-called time-of-flight optical distance sensor.

Conventionally, the operation principle of a so-called time-of-flight (hereinafter referred to as TOF) optical distance sensor will be described with reference to FIG.
In other words, in FIG. 12, the optical distance sensor 1 includes a light emitting unit 2, a light receiving unit 3, and a control unit 4.
The light emitting unit 2 is, for example, an infrared LED, emits near infrared light as blinking light, and irradiates the distance measuring target 5.
The light receiving unit 3 receives near-infrared light emitted from the light emitting unit 2 and reflected by the distance measurement target 5.

The control unit 4 generates a light emission signal S1 and drives the light emission unit 2 to emit light based on the light emission signal.
Further, the control unit 4 performs, for example, A / D conversion on the light reception signal from the light reception unit 3 to obtain the light reception signal S2, and, as shown in FIG. 13, the waveforms of the light emission signal S1 and the light reception signal S2 (in the case of illustration, a pulse waveform). ), That is, the phase difference φ is measured.
And the control part 4 calculates | requires the distance D from the optical distance sensor 1 to the distance measurement target 5 by calculation based on the phase difference (phi) measured in this way.

By the way, since the optical distance sensor 1 described above has a very high speed of light, the phase difference is actually measured by modulating light at a certain frequency.
That is, the optical distance sensor 1 is more specifically configured as shown in FIG.
The light emitting unit 2 actually irradiates intermittent light based on the light emission signal S1 at intermittent timing so that it can be visually recognized as continuous light emission by human eyes.
In response to this, the light receiving unit 3 receives the reflected light from the distance measurement target 5, and similarly generates a light reception signal S2 delayed by the phase difference φ at intermittent timing.

  Then, as shown in FIG. 15, the control unit 4 measures the magnitude of the light reception signal S2 at the four locations A0, A1, A2, A3 of the sine waveform of the light reception signal S2 with respect to the light emission signal S1 of the sine wave signal. To do. Then, the phase difference φ is calculated by the following formula 1.

Next, the distance D to the distance measurement target 5 is obtained by the following expression 2.

Further, as shown in FIG. 16, the control unit 4 switches between the two types of internal signals A and B whose phases are shifted, and compares these internal signals A and B with the light emission signal S1 and the light reception signal S2. .
In FIG. 16, when there is a phase difference φ between the light emission signal S1 and the light reception signal S2, the signal A90 shifted by 90 degrees from the signal A0 having the same phase as the light emission signal S1 and the signal B0 having the opposite phase to the light emission signal S1. Take signal B90.
Thereby, the phase difference φ is given by the following Expression 3, and the distance L to the distance measurement target 5 is obtained based on the phase difference φ.

In this way, in the TOF optical distance sensor 1, the light emitted from the light emitting unit 2 is reflected by the distance measurement target 5, and the returning light is received by the light receiving unit 3. Then, the control unit 4 calculates the distance from the optical distance sensor 1 to the distance measurement target by measuring the phase difference φ between the light emission signal S1 and the light reception signal S2.
Here, since the speed of light is high, it is impossible to measure the phase difference as it is. Therefore, the light is modulated at a certain frequency, and the phase difference of the modulation frequency is measured intermittently with the distance. ing.
Thus, for example, when light is emitted at a modulation frequency of 10 MHz, the period is 1 × 10 −7 seconds and the speed of light is about 3 × 10 8 m / second. −7 seconds) × (3 × 10 8 m / sec) = 30 m.
Therefore, when the modulation frequency is 10 MHz, distance measurement up to 30 m is possible. In addition, when calculating by Formula 3, the measurable distance will be halved.

  On the other hand, Patent Document 1 includes a display element including a spatial light modulation element, rotates a color wheel having segments color-coded for each color, irradiates the segment with white light, and transmits light. A sensor integration control signal generating means for generating a sensor integration control signal synchronized with the rotation of the color wheel, and a phase difference sensor A projection device comprising: distance measuring means for integrating the sensor output signal by the sensor integration control signal and calculating a distance to a distance measuring point on the screen by the integrated sensor output signal. It is disclosed.

Further, in Patent Document 2, at least a pair is formed and provided to be electrically excitable at an angle so as to face each other, and one of the pair is sent to be crossable with the first target region, and the other of the pair Is a light source for generating a light beam having a common chromaticity sent so as to be able to intersect with a second target area disposed at a predetermined distance farther from the light source than the first target area, and a combined reflected light of the pair of light sources In order to receive the diffusely reflected light from the object disposed between the first and second target regions between the pair of light sources so as to be detected and generate an electrical signal corresponding to the sum of the reflected light There is disclosed a photoelectric color sensor comprising photoelectric detection means and means for exciting the light source simultaneously and intermittently.
JP 2008-020196 A JP 05-113370 A

By the way, the TOF optical distance sensor 1 generally uses, for example, near infrared light having a wavelength of 850 nm as measurement light so that the measurement light is not visually recognized. On the other hand, the light receiving element used in the light receiving unit has low sensitivity in the near-infrared light region.
For example, a silicon light-receiving element such as a photodiode has a peak wavelength vs. sensitivity curve, and the sensitivity is low in the near infrared region. For this reason, in the near-infrared region, it is necessary to increase the emission intensity, and if it is looked at for a long time, although it is invisible light, it may adversely affect the eyes.

  On the other hand, in special applications such as industrial applications, visible monochromatic light, for example, red light, may be used as measurement light of the TOF optical distance sensor. In such a case, the intensity of the reflected light is significantly reduced for the distance measurement target having a low reflectance with respect to the wavelength of the monochromatic light. Therefore, the phase difference φ cannot be measured, and therefore it becomes impossible to measure the distance substantially.

  Furthermore, it is effective to use near-infrared light as measurement light in dark places such as nighttime, or outdoors or semi-outdoors that are affected by sunlight, but in places where lighting such as indoors is used. The near infrared light or monochromatic light becomes harmful light or interference light for human eyes.

On the other hand, according to the projection apparatus according to Patent Document 1, in the projection apparatus using the spatial light modulator, a sensor integration control signal synchronized with the rotation of the color wheel is generated, and the sensor output signal of the phase difference sensor is generated. Is integrated by the sensor integration control signal, and the distance to the distance measuring point on the screen is calculated by the integrated sensor output signal.
In this projection device, a stable integrated sensor output signal is obtained, distance measurement is performed with high accuracy, and the color of each segment of the color wheel, for example, three monochromatic lights of red, green and blue are used as measurement light. Is done.
For this reason, even a distance measurement target having a low reflectance with respect to a specific wavelength can perform distance measurement, but a wheel having segments color-coded for each color is required. In the projection apparatus according to Patent Document 1, such a color wheel is included as a component of the projection apparatus. However, when configuring a distance sensor, it is necessary to prepare such a color wheel separately. Therefore, the number of parts increases and the configuration becomes complicated, resulting in an increase in manufacturing cost.

Further, according to the photoelectric color sensor disclosed in Patent Document 2, the light from the LED is irradiated from the photoelectric detection means toward a position at a different distance with respect to the target, and passes through a distance within a predetermined range from the photoelectric detection means. It is possible to detect a change in the color of a target product such as a product.
Therefore, although the influence of the variation in the distance to the target can be greatly reduced and a change in the color of the target can be reliably detected, the distance to the target cannot be measured. The purpose, composition and effect are different.

  In view of the above, the present invention provides an optical device capable of accurately measuring a distance even with a distance measurement target having high sensitivity and wavelength selectivity with respect to reflection by a simple configuration. It aims to provide a distance sensor.

According to the present invention, the object is to provide a light emitting unit, a light receiving unit that receives light reflected from the distance measurement target by irradiating light from the light emitting unit, a light emission signal from the light emitting unit, and a light receiving unit. A time-of-flight optical distance sensor including a control unit that calculates a distance to a distance measurement target based on a phase difference of a received light signal, wherein the light-emitting unit includes red light and green light that are the three primary colors of light. The blue light and the blue light are emitted as discontinuous intermittent light at different timings, and the light of each color is mixed and emitted at a timing that can be visually recognized as continuous white light. The reflected light from the target is received as a continuous light reception signal without being dispersed for each color, and the control unit calculates a phase difference for each color from the light reception signal, and a distance measurement target based on the phase difference. Distance to things Characterized by calculating a by optical distance sensor is achieved.

In this first aspect, the light emitting unit irradiates the distance measurement target with light of a plurality of colors, and receives the reflected light from the distance measurement target for each color light as it is, and integrates it. Thus, the control unit determines the distance to the distance measurement target based on the phase difference between the light emission signal and the light reception signal based on the light reception signal for each color from the light reception unit, corresponding to the light emission timing for each color. Calculate.
At this time, at least one color having a level as high as possible is selected according to the level of the light reception signal. Thus, a distance with less error is obtained by calculating the distance from the phase difference between the light reception signal and the light emission signal for the color with high reflectance on the surface of the distance measurement target.

In this case, since visible light is used as the measurement light, when the measurement light is incident on the human eye, the human can visually recognize the measurement light. I will never see it. Therefore, it does not adversely affect human eyes.
In addition, it is possible to perform highly accurate distance measurement by using light of a wavelength band in which light receiving sensitivity of the light receiving unit is high as light of each color.

As described above, according to the optical distance sensor of the present invention, it is possible to perform distance measurement with higher accuracy using visible light having a wavelength with high sensitivity of the light receiving unit.
In addition, by using light of a plurality of colors having different wavelengths as measurement light, even if the distance measurement target has wavelength selectivity with respect to reflection, a color with high reflectance on the surface of the distance measurement target can be obtained. By selecting light, the distance can be calculated with high accuracy from the phase difference between the light emission signal and the light reception signal.
Furthermore, by using a combination of colors that can be mixed with each other to produce white light, the measurement light does not become harmful light or interference light even if it enters the human eye. It can also be used as light.

  As described above, according to the present invention, it is possible to accurately measure a distance even with a distance measurement target having high sensitivity and wavelength selectivity with respect to reflection by a simple configuration. An optical distance sensor could be provided.

It is the schematic which shows the structure of 1st embodiment of the optical distance sensor by this invention. It is a block diagram which shows the electrical structure of the optical distance sensor of FIG. It is the schematic which shows the light emission of each light emission part of the optical distance sensor of FIG. 1, and the light reception state of a light-receiving part. It is a time chart which shows the light emission signal of each color of the optical distance sensor of FIG. It is a time chart which shows the light reception signal by various conditions of the optical distance sensor of FIG. It is a time chart which shows the light reception signal by the reflection state of the distance measurement target of the optical distance sensor of FIG. It is a graph which shows an example of the spectral sensitivity characteristic of the CCD camera used by the light-receiving part of the optical distance sensor of FIG. It is a graph which shows the other example of the spectral sensitivity characteristic of the CCD camera used with the light-receiving part of the optical distance sensor of FIG. It is a figure which shows the reflectance with respect to each light of red, blue, green, and infrared in various distance measurement targets. It is the schematic which shows the structure of 2nd embodiment of the optical distance sensor by this invention. It is the schematic which shows the structure of 3rd embodiment of the optical distance sensor by this invention. It is a block diagram which shows the structure of an example of the conventional optical distance sensor. It is a time chart which shows the phase difference of the light emission signal and light reception signal in the optical distance sensor of FIG. It is the schematic which shows the specific structure of the conventional optical distance sensor. It is a time chart which shows the light reception signal measurement for the distance calculation in the optical distance sensor of FIG. It is a time chart which shows the relationship between the light emission signal for the distance calculation in the optical distance sensor of FIG. 14, a light reception signal, and two internal signals.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to FIGS. 1 to 11.
The embodiments described below are preferable specific examples of the present invention, and thus various technically preferable limitations are given. However, the scope of the present invention particularly limits the present invention in the following description. As long as there is no description of the effect, it is not restricted to these aspects.

FIG. 1 shows a configuration of a first embodiment of an optical distance sensor according to the present invention.
In FIG. 1, the optical distance sensor 10 includes a light emitting unit 11, a light receiving unit 12, and a control unit 13.
The light emitting unit 11 includes three light emitting units, that is, a red light emitting unit 14, a green light emitting unit 15, and a blue light emitting unit 16 in the illustrated case.
As shown in FIG. 2, the light emitting units 14, 15, and 16 for each color are configured by light emitting circuits 14 a, 15 a, and 16 a and LEDs 14 b, 15 b, and 16 b, respectively.

Here, the LED 14b is a red LED, the LED 15b is a green LED, and the LED 16b is a blue LED.
The LEDs 14b, 15b, and 16b are driven by the light emitting circuits 14a, 15a, and 16a, respectively, and generate red light, green light, and blue light, respectively.

The light receiving unit 12 is not provided for each color, and includes a single light receiving unit 12a such as a CCD camera, and an A / D conversion unit 12b that performs A / D conversion on an image pickup signal from the light receiving unit 12a. ing.
Thereby, the light receiving unit 12 receives the reflected light from the distance measurement target 17 as it is by the light receiving unit 12a, integrates it, converts it to a digital signal by the A / D conversion unit 12b, and outputs it as a light receiving signal.

The control unit 13 generates light emission signals S1r, S1g, and S1b for each color, and sends these light emission signals to the light emission circuits 14a, 15a, and 16a, respectively. Thereby, each light emission circuit 14a, 15a, 16a drives LED14b, 15b, 16b based on light emission signal S1r, S1g, S1b, respectively, and makes it light-emit.
In the illustrated case, the control unit 13 includes a display unit 13a for performing various displays.
Thereby, each light emission part 14,15,16 irradiates red light, green light, and blue light toward the distance measurement target 17, respectively, as shown in FIG. Then, the light receiving unit 12 receives the reflected light of each color from the distance measurement target 17 as it is (without splitting each color).

Here, the control unit 13 sends light emission signals S1r, S1g, and S1b (see FIG. 4) that are discontinuously intermittent to the light emitting circuits 14a, 15a, and 16a. Thereby, each LED14b, 15b, and 16b of each light emission part 14,15,16 light-emits red light, green light, and blue light mutually shifting timing.
More specifically, as shown in FIG. 4, the control unit 13 causes each color to emit light as one frame sequentially with 4 pulses, and after repeating such a frame six times, takes a rest period t0. .

  Thus, the light receiving unit 12 receives the light reflected by the distance measurement target 17 for each color as it is without color-coding. That is, as shown in FIG. 5A, the light reception signal S2 of each color is obtained as a continuous pulse. In this case, the reflectance of the distance measurement target 17 for each color is the same, and the pulses for each color are at the same level.

  On the other hand, in the case where a pause period t1 is provided after emitting two pulses for each color in the same cycle, the light reception signal S2 is similarly transmitted at the end of each frame as shown in FIG. The rest period is t1.

  Furthermore, for example, when the reflectance of the distance measurement target 17 is low for blue, the level of blue reflected light is low. Accordingly, as shown in FIG. 5C, the light reception signal S2 of the light receiving unit 12 varies in the level of the light reception signal pulse of each color.

The optical distance sensor 10 according to the embodiment of the present invention is configured as described above and operates as follows.
That is, the control unit 13 drives and controls the light emitting unit 11 to irradiate the distance measurement target 17 with light of each color, and the light receiving unit 12 receives reflected light from the distance measurement target 17. As a result, the light receiving unit 12 obtains a light receiving signal S2 shown in FIG. This light reception signal S2 is the same as the light reception signal shown in FIG.

  Then, the control unit 13 calculates a phase difference φ for each color from the light reception signals S2 of all colors, that is, green light, red light, and blue light, and based on the phase difference φ, the distance measurement target 17 is calculated. The distance D is calculated.

Further, when the reflectance of the distance measurement target 17 is extremely low with respect to a certain color, for example, red, the light reception signal S2 has a level that is almost zero with respect to red as shown in FIG. 6B. Become.
In such a case, the phase difference φ is not measured for red, and the phase difference φ is calculated only for the other colors, that is, two colors of green and blue, and the distance measurement is performed based on the phase difference φ. A distance D to the target 17 is calculated.

In this way, for example, for one or two colors, even if the reflectance on the surface of the distance measurement target 17 is low, if the reflectance on the surface of the distance measurement target 17 is high for the remaining colors, the color The level of the received light signal S2 is sufficiently high.
Therefore, with respect to the color, the phase difference φ can be reliably measured from the light emission signal S1 and the light reception signal S2. Thereby, even if the distance measurement target 17 has wavelength selectivity regarding reflection, it is possible to measure the distance D reliably and accurately.

Next, the sensitivity due to the frequency of the CCD camera used in the light receiving unit 12 will be considered.
For example, in a monochrome CCD camera “ICX279AL” manufactured by Sony, as shown in FIG. 7, there is a peak wavelength of sensitivity in the wavelength region of 550 nm, that is, in the green region, and the sensitivity decreases to 0.4 times or less in the infrared region. End up. The sensitivity in blue and red is about 0.8 times that in green.
Therefore, as described above, when red light, green light, and blue light are used as the measurement light, the sensitivity is approximately doubled for each color in the case of infrared light.

In addition, in the monochrome CCD camera “ICX419ALB” manufactured by Sony, as shown in FIG. 8, there is a peak wavelength of sensitivity in the shorter wavelength side of 500 nm, that is, in the green region, and 0.3 times or less in the infrared region. The sensitivity will decrease. The sensitivity in blue and red is about 0.8 times that in green.
Therefore, as described above, when red light, green light, and blue light are used as measurement light, the sensitivity is about three times as high as infrared light for each color.

  Here, when the sensitivity of the light receiving unit 12 is doubled, the measurement error is generally improved by about 1.4 times, and the distance measurement target 17 is more easily recognized. That is, when the light receiving sensitivity is doubled, the light emission intensity is halved with respect to the irradiation range of the same error and the same distance, so that power consumption is reduced.

In addition, since red, green, and blue light are used as measurement light, the entire mixed color light is almost white light and can be used as illumination light without any sense of incongruity. It does not become light or disturbing light.
Here, as described above, the light receiving sensitivity of the green light is the maximum, but the green monochromatic light is visually unsuitable as the irradiation color. For this reason, other colors are used in combination, and the light emission intensity of each color is appropriately adjusted to obtain white light by color mixing.
In addition, white LEDs are generally not suitable for use as measurement light because the light of a blue LED or an ultraviolet LED is converted into white light by wavelength conversion with a phosphor.

  Therefore, by using red light, green light and blue light, each color is emitted 20 times or more per second as discontinuous intermittent light at different timings, thereby measuring the light from each light emitting part for distance measurement. In addition to the measurement light, white light is obtained by color mixing and can be viewed as continuous light by the human eye, so that the user does not feel uncomfortable or uncomfortable due to blinking.

Next, the difference in reflectance depending on the material of the distance measurement target 17 will be considered.
In FIG. 9, as the distance measurement target 17, incident light is applied to light brown paper, copy paper, black cloth 1 (fleece material), black cloth 2 (curtain material), yellow plastic board, green cloth, work clothes, and hands. The reflected light is incident on the surface of the distance measurement target 17 from an angle of 45 degrees and reflected in the direction of an angle of 45 degrees on the opposite side using red light, blue light, green light and infrared light as The amount of light was measured.

As a result, as shown in FIG. 9, light brown paper, copy paper, yellow plastic board and work clothes have the highest reflectance of red light, and black cloth No. 1 and hand have the highest reflectance of blue light. It was. The green cloth had the highest green reflectance except for infrared light.
According to this, only the black cloth 2 and the green cloth have the highest infrared light reflectance, and the average reflectance of red light, blue light, and green light is compared with the average reflectance of infrared light. Visible light has a reflectivity of about 1.5. Therefore, based on this difference in reflectance, coupled with the difference in sensitivity of the light receiving unit, by using visible light as measurement light, the light emission amount of the light emitting unit 11 can be reduced to about half and the same light emission amount. If so, the measurement error is reduced to about half.

Furthermore, the control unit 13 can obtain an external light signal based on disturbance light based on the light reception signal of the light receiving unit 12 in the rest period t0. Based on the level of the external light signal, the control unit 13 corrects the light reception signal S2 by subtracting the external light signal from the light reception signal S2 at the time of distance measurement, and the distance that is not affected by disturbance light. Can be measured.
When the level of disturbance light is high and exceeds the saturation level of the CCD camera 12a of the light receiving unit 12, detection of disturbance light and correction of the light reception signal S2 cannot be performed. Therefore, when using this optical distance sensor 10, it is desirable to avoid using it in a place where direct sunlight or its reflected light is strong.

FIG. 10 shows a configuration of a second embodiment of the optical distance sensor according to the present invention.
10, since the optical distance sensor 20 has substantially the same configuration as the optical distance sensor 10 shown in FIG. 1, the same components are denoted by the same reference numerals and description thereof is omitted.

The optical distance sensor 20 is different from the optical distance sensor 10 shown in FIG. 1 in that the light emitting units 14, 15, and 16 of each color are arranged in a compliment and are provided with a light diffusion plate 21 in front of them. It has a different configuration.
The light diffusing plate 21 is disposed in a path of light emitted from the light emitting units 14, 15, 16.

  According to the optical distance sensor 20 having such a configuration, the optical distance sensor 10 operates in the same manner as the optical distance sensor 10 shown in FIG. 1, and light emitted from the light emitting units 14, 15, 16 passes through the light diffusion plate 21. As a result, the light is diffused, mixed well with each other, and irradiated as white light without unevenness.

FIG. 11 shows the configuration of the second embodiment of the optical distance sensor according to the present invention.
10, since the optical distance sensor 30 has substantially the same configuration as the optical distance sensor 10 shown in FIG. 1, the same components are denoted by the same reference numerals and description thereof is omitted.

The optical distance sensor 30 is different from the optical distance sensor 10 shown in FIG. 1 in that a light emitting unit 31 is provided instead of the light emitting unit 11.
The light emitting unit 31 includes a plurality of six three-color LEDs 31a, 31b, 31c, 31d, 31e, and 31f arranged in a dispersed manner.
Each of the three-color LEDs 31a to 31f has a red LED chip 31r, a green LED chip 31g, and a blue LED chip 31b, and is arranged in a triangle, for example, in a range of about 1 mm square.

  According to the optical distance sensor 30 having such a configuration, the optical distance sensor 30 operates in the same manner as the optical distance sensor 10 shown in FIG. 1, and light of one color is emitted from each of the three-color LEDs 31a to 31f arranged in a distributed manner. Therefore, even if there is no light diffusion plate, the colors are sufficiently mixed with each other and white light without unevenness is obtained.

  In the above-described embodiment, the light emitting units 14, 15 and 16 of the respective colors of the light emitting unit 11 emit light at different timings as shown in FIG. 4, thereby reducing the power consumption of the light emitting unit 11. When the amount of light as the white illumination light is insufficient, the light emitting units 14, 15, and 16 of the respective colors may emit light so that the light emission timings of the respective colors at least partially overlap each other.

  In this way, according to the present invention, a distance measurement target having high sensitivity and wavelength selectivity with respect to reflection can be accurately and stably measured with a simple configuration. Thus, an optical distance sensor can be provided.

  The optical distance sensor according to the present invention is visually recognized as white light as a whole due to the color mixture of light from each light emitting section, so that it is not only used as a distance sensor, but also, for example, a security sensor with a nightlight, a sensor with a street light, It can be used as a sensor with illumination.

10, 20, 30 Optical distance sensor 11 Light emitting unit 12 Light receiving unit 13 Control unit 13a Display unit 14 Red light emitting unit 14a, 15a, 16a Light emitting circuit 14b, 31r Red LED
15 Green light emitting part 15b, 31b Green LED
16 Blue light emitting part 16b, 31b Blue LED
17 Distance measurement target 21 Light diffusing plate 31 Light emitting part 31a to 31f Three-color LED

Claims (1)

  1. Based on the phase difference between the light emitting unit, the light receiving unit that irradiates light from the light emitting unit and receives the light reflected by the distance measurement target, and the phase difference between the light emission signal from the light emitting unit and the light reception signal of the light receiving unit In a time-of-flight optical distance sensor including a control unit that calculates the distance to the measurement target,
    The light-emitting unit emits red, green, and blue light, which are the three primary colors of light, as discontinuous intermittent light at different timings, and the light of each color is mixed with each other and visually recognized as continuous white light It emits light when possible ,
    The light receiving unit receives the reflected light from the distance measurement target as a continuous light receiving signal without dispersing the light for each color,
    The optical distance sensor, wherein the control unit calculates a phase difference for each color from the received light signal and calculates a distance to the distance measurement target based on the phase difference .
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