Method for constructing a retroreflective sheet and retroreflective sheet

Abstract

This invention provides a method for constructing a retroreflective sheet, which allows for the creation of a retroreflective sheet by appropriately attaching a low-reflection sheet. [Solution] A low-reflection sheet 22 having a predetermined vertical height H is attached to a retroreflective sheet 21 of a predetermined size. The distance measuring camera 100 photographs the retroreflective sheet 21 to generate an image in which the retroreflective sheet 21 is located in a part of the frame F. The distance measuring camera 100 photographs the retroreflective sheet 21 when it is at a distance from the retroreflective sheet 21 of a first distance L1, which is measured based on the pixels of the surrounding area 23 excluding the area in the frame F where the retroreflective sheet 21 is located. At this time, the vertical height H of the low-reflection sheet 22 is set so that the vertical range of the image of the low-reflection sheet 22 corresponding to the vertical height H in the frame F includes at least one pixel.

Description

Technical Field

[0004] , , , , ,

[0005]

[0001] The present invention relates to a method for constructing a retroreflective sheet and a retroreflective sheet.

Background Art

[0002] A distance measuring device is used that irradiates an object with light, receives the light reflected by the object, and measures the distance from the position where the light was irradiated to the object (see Patent Document 1 or 2).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] When a retroreflective sheet that reflects the light incident from a light source toward the light source using the retroreflective technology is attached to an object to be distance-measured, the incident light is reflected with high intensity, so the sensor value of the sensor that detects the reflected light becomes saturated, and distance measurement cannot be performed or distance measurement is performed erroneously. Therefore, a low-reflection sheet having a lower reflectance than the retroreflective sheet may be partially attached to the retroreflective sheet. There is a demand for the emergence of a method for constructing a retroreflective sheet and a retroreflective sheet to which a low-reflection sheet is appropriately attached.

Means for Solving the Problems

[0005] The present invention provides a method for configuring a retroreflective sheet, which involves attaching a low-reflection sheet having a predetermined vertical height and a predetermined horizontal width to a retroreflective sheet of a predetermined size, generating an image in which the retroreflective sheet is located in a portion of the frame by having a distance measuring camera photograph the retroreflective sheet, and setting the vertical height and horizontal width of the low-reflection sheet so that when the distance measuring camera photographs the retroreflective sheet at a distance from the retroreflective sheet that is a first distance measured based on pixels in the surrounding region of the frame excluding the region in which the retroreflective sheet is located, the vertical range of the image of the low-reflection sheet corresponding to the vertical height and the horizontal range of the image of the low-reflection sheet corresponding to the horizontal width include at least one pixel in the frame.

[0006] The present invention provides a retroreflective sheet to which a low-reflection sheet having a predetermined size, a predetermined vertical height and a predetermined horizontal width is attached, wherein a distance measuring camera photographs the retroreflective sheet to generate an image in which the retroreflective sheet is located in a portion of the frame, and when the distance measuring camera photographs the retroreflective sheet at a distance from the retroreflective sheet by a first distance measured based on pixels in the surrounding region of the frame excluding the region in which the retroreflective sheet is located, the vertical height and horizontal width of the low-reflection sheet are set such that the vertical range of the image of the low-reflection sheet corresponding to the vertical height and the horizontal range of the image of the low-reflection sheet corresponding to the horizontal width include at least one pixel in the frame. [Effects of the Invention]

[0007] According to the method for constructing a retroreflective sheet and the retroreflective sheet of the present invention, a retroreflective sheet can be made by appropriately attaching a low-reflectivity sheet. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a block diagram showing an example configuration of a rangefinder camera. [Figure 2] Figure 2 is a characteristic diagram showing the reflection level of reflected light when near-infrared light is irradiated onto a white chart, and the reflection level of reflected light when near-infrared light is irradiated onto a retroreflective sheet. [Figure 3A] Figure 3A is a plan view showing a retroreflective sheet with a horizontally elongated low-reflection sheet attached. [Figure 3B] Figure 3B is a plan view showing a retroreflective sheet with a low-reflection sheet attached, which has a low vertical height and a narrow horizontal width. [Figure 4] Figure 4 shows the relationship between the distance from the retroreflective sheet to the rangefinder camera and the size of the retroreflective sheet image within the frame. [Figure 5] Figure 5 shows a situation where a rangefinder camera is approaching and taking a picture of a retroreflective sheet to which multiple low-reflection sheets have been attached. [Figure 6] Figure 6 is a block diagram showing an example of a computer configuration for calculating the horizontal width, vertical height, and spacing of low-reflection sheets. [Figure 7] Figure 7 is a flowchart showing the process by which the computer shown in Figure 6 calculates the vertical height and spacing. [Figure 8] Figure 8 shows an example configuration in which a variable ND filter is attached to the surface of a retroreflective sheet. [Figure 9] Figure 9 shows an example of a variable ND filter constructed using two polarizing filters. [Figure 10] Figure 10 shows an example of a variable ND filter constructed using a liquid crystal filter. [Figure 11] Figure 11 is a block diagram showing an example of the configuration of the drive mechanism included in a variable ND filter. [Figure 12] Figure 12 is a flowchart showing the operation of the drive unit shown in Figure 11. [Modes for carrying out the invention]

[0009] The following describes the configuration method and retroreflective sheet according to one embodiment with reference to the attached drawings. First, an example of the configuration of the distance measuring camera 100 will be described using Figure 1. The distance measuring camera is sometimes called a TOF (Time Of Flight) camera. The distance measuring camera 100 includes a lens 11, a light-emitting unit 12, a TOF sensor 13, and a distance conversion unit 14. The distance measuring camera 100 photographs an object 20 to which a retroreflective sheet 21 is attached as the object to be measured. A distance measurement value utilization device 30 that utilizes the distance measurement value generated by the distance measuring camera 100 is connected to the distance measuring camera 100.

[0010] The retroreflective sheet 21 may have a honeycomb structure as an example and may have beads or prisms to increase the amount of reflected incident light. The retroreflective sheet 21 may be used by being attached to an object 20 such as a road sign, a sign or warning sign used inside or outside a building, etc. The distance measurement value utilization device 30 may be an object recognition device, for example. The distance measurement value utilization device 30 may be installed inside the distance measuring camera 100.

[0011] The light-emitting unit 12 is, for example, a vertical cavity surface-emitting laser (VCSEL), which irradiates the object 20 with near-infrared light of, for example, a wavelength of 940 nm within a predetermined irradiation range. The near-infrared light reflected by the object 20 is focused by the lens 11 and incident on the TOF sensor 13.

[0012] The TOF sensor 13 is, for example, an indirect type sensor. In the indirect distance measurement method, the light-emitting unit 12 irradiates the object 20 with pulsed near-infrared light. The reflected light from the object 20 is focused by the lens 11 and forms an image in the TOF sensor 13. The TOF sensor 13 detects the reflected light from the object 20 and obtains an electrical signal level by photoelectric conversion. The TOF sensor 13 changes the phase of the shutter timing pulse multiple times in relation to the timing of the pulsed near-infrared light generated from the light-emitting unit 12. The TOF sensor 13 indirectly calculates the distance to the object 20 based on the ratio of the sensor values ​​obtained at each phase of the shutter timing pulse.

[0013] The TOF sensor 13 generates a sensor image composed of sensor values corresponding to the distances to the object 20 corresponding to each pixel of each frame. Each frame consists of a plurality of pixels arranged in the horizontal and vertical directions. The sensor value is the level of an electrical signal obtained by the TOF sensor 13 detecting light and performing photoelectric conversion.

[0014] As the TOF sensor 13, a direct-type sensor may be used. The TOF sensor 13 may generate sensor values corresponding to each pixel of each frame that directly or indirectly indicate the time from when the light emitting unit 12 irradiates near-infrared light until the reflected light is received.

[0015] The distance conversion unit 14 converts the sensor value corresponding to each pixel of each frame into a distance, and generates a distance measurement image (depth image) composed of distance measurement values corresponding to each pixel of each frame. The distance measurement image is a moving image or a still image. In the case of a moving image, the distance conversion unit 14 generates, for example, 30 distance measurement images per second. Since the method of converting the sensor value into the distance measurement value is well-known, detailed description is omitted.

[0016] FIG. 2 shows the reflection level of the reflected light when the white chart is irradiated with near-infrared light and the reflection level of the reflected light when the retroreflective sheet 21 is irradiated with near-infrared light. In FIG. 2, in order to grasp the reflection level, the sensor value obtained by the TOF sensor 13 receiving the reflected light is shown as a code value as the reflection level. The reflection level when the white chart is irradiated with near-infrared light attenuates as the distance from the distance measurement camera 100 to the white chart increases. The reflection level when the retroreflective sheet 21 is irradiated with near-infrared light is significantly higher compared to the reflection level from the white chart, and varies greatly according to the distance. Moreover, when the distance is more than a predetermined distance, the reflection level becomes too high and saturates, becoming a constant value of the saturation level.

[0017] When the distance measurement camera 100 photographs the retroreflective sheet 21, since the reflection level saturates when the distance is more than a predetermined distance, at distances more than the predetermined distance, the distance measurement value cannot be appropriately generated, and distance measurement may not be possible or may be erroneously performed.

[0018] Therefore, as shown in Figure 3A, by partially attaching a low-reflection sheet 22, which has a lower reflectivity than the retroreflective sheet 21, to the retroreflective sheet 21, it becomes possible to generate distance values ​​even when the distance measuring camera 100 photographs the retroreflective sheet 21. In Figure 3A, the low-reflection sheet 22 is horizontally elongated, extending from the left edge to the right edge of the retroreflective sheet 21. The vertical height of the low-reflection sheet 22 is H, and its horizontal width is W. In this case, it is necessary to properly attach the low-reflection sheet 22 to the retroreflective sheet 21. The following describes how to construct a retroreflective sheet with the low-reflection sheet 22 properly attached to the retroreflective sheet 21.

[0019] In Figure 4, the distance measuring camera 100 is photographing the retroreflective sheet 21. As the distance measuring camera 100 approaches the retroreflective sheet 21, the images of the retroreflective sheet 21 and the low-reflection sheet 22 within the sensor image frame F are enlarged, and as it moves away from the retroreflective sheet 21, the images of the retroreflective sheet 21 and the low-reflection sheet 22 within frame F are reduced. Here, it is assumed that the aspect ratio of the retroreflective sheet 21 matches the aspect ratio of frame F.

[0020] When the distance measuring camera 100 is positioned at a distance L0 from the retroreflective sheet 21 and photographing the retroreflective sheet 21, the area of ​​frame F and the area of ​​the retroreflective sheet 21 are assumed to be the same. The distance from the retroreflective sheet 21 to the distance measuring camera 100 is, strictly speaking, the distance from the retroreflective sheet 21 to the TOF sensor 13. If the distance from the retroreflective sheet 21 to the distance measuring camera 100 is shorter than distance L0, the retroreflective sheet 21 and the low-reflection sheet 22 are displayed in an enlarged state across the entire frame F, as shown in (a). The area ratio Ra of the images of the retroreflective sheet 21 and the low-reflection sheet 22 to the area of ​​frame F is 100% from a distance shorter than distance L0 from the distance measuring camera 100 to distance L0.

[0021] When the distance from the distance measuring camera 100 to the retroreflective sheet 21 becomes longer than distance L0, the area ratio Ra becomes less than 100%, and as shown in (b), the area of ​​the retroreflective sheet 21 and the low-reflection sheet 22 becomes smaller than the area of ​​frame F, creating a peripheral region 23. When the distance from the distance measuring camera 100 to the retroreflective sheet 21 is greater than or equal to the first distance L1, the distance is assumed to be measured correctly by the sensor value of the pixels located in the peripheral region 23 of the sensor image. At this time, the area ratio Ra of the images of the retroreflective sheet 21 and the low-reflection sheet 22 to the area of ​​frame F is, for example, 50%.

[0022] If, within frame F at a first distance L1, the vertical range of the image of the low-reflection sheet 22 includes at least one pixel, then the distance measuring camera 100 can correctly measure the distance using the sensor values ​​of the pixels of the low-reflection sheet 22 in the sensor image at distances less than L. Conversely, if the vertical height H of the low-reflection sheet 22 is reduced to such an extent that, within frame F at a first distance L1, the vertical range of the image of the low-reflection sheet 22 does not include at least one pixel, then a situation will occur where the distance measuring camera 100 cannot measure the distance when it is located less than L.

[0023] If the low-reflection sheet 22 is vertically oriented, then within frame F at the first distance L1, if the horizontal range of the image of the low-reflection sheet 22 includes at least one pixel, then at distances less than L, the distance measuring camera 100 can correctly measure the distance using the sensor values ​​of the pixels of the low-reflection sheet 22 in the sensor image.

[0024] A retroreflective sheet 21, constructed according to the method for constructing a retroreflective sheet according to this embodiment, has a low-reflection sheet 22 attached to it that has a predetermined size, a predetermined vertical height H and a predetermined horizontal width W. The distance measuring camera 100 photographs the retroreflective sheet 21, generating a sensor image in which the retroreflective sheet 21 is located in a portion of the frame. The distance measuring camera 100 photographs the retroreflective sheet 21 when it is at a distance from the retroreflective sheet 21 of a first distance L1, which is measured based on the pixels of the surrounding region 23 excluding the region in the frame where the retroreflective sheet 21 is located.

[0025] In this case, the vertical height H and horizontal width W of the low-reflection sheet 22 are set such that the vertical range of the image of the low-reflection sheet 22 corresponding to the vertical height H and the horizontal range of the image of the low-reflection sheet 22 corresponding to the horizontal width W within the frame include at least one pixel.

[0026] By configuring the retroreflective sheet 21 with the low-reflection sheet 22 attached in this way, it becomes possible to generate a distance measurement value whether the distance measuring camera 100 photographs the retroreflective sheet 21 from a distance greater than the first distance L1, or whether it approaches the retroreflective sheet 21 at a distance shorter than the first distance L1 and photographs the retroreflective sheet 21.

[0027] Minimizing the size of the low-reflection sheet 22 will not reduce the effect of attaching the retroreflective sheet 21 to the object 20. Therefore, as shown in Figure 3B, the low-reflection sheet 22 may have a vertical height H and a horizontal width W such that when the distance measuring camera 100 is at a first distance L1 from the retroreflective sheet 21, the vertical and horizontal range of the image of the low-reflection sheet 22 is 1 pixel.

[0028] As shown in Figure 4, if the field of view of one pixel of the TOF sensor 13 is θ1, the vertical height H can be calculated using equation (1). The vertical height H only needs to be greater than or equal to the height calculated using equation (1). The horizontal width W can be calculated in the same way as equation (1), and the horizontal width W only needs to be greater than or equal to the width calculated in the same way. H = 2L 1·tan(θ 1 / 2) …(1)

[0029] Further consideration will be given to a more preferable configuration of the retroreflective sheet. When the distance measuring camera 100 approaches the retroreflective sheet 21, the low-reflection sheet 22 may move out of the field of view of the distance measuring camera 100, making it impossible to generate distance measurement values. To resolve this, the retroreflective sheet 21 may be further configured as follows.

[0030] The retroreflective sheet 21 includes regions with a retroreflective structure and regions without it. Therefore, when the distance measuring camera 100 approaches the retroreflective sheet 21 very closely, the distance is correctly measured by the sensor value of the pixel that captures the region without the retroreflective structure. As can be seen from Figure 2, distance measurement is possible when the distance from the distance measuring camera 100 to the retroreflective sheet 21 is within 2m. As shown in Figure 5, the distance at which the distance measuring camera 100 approaches the retroreflective sheet 21 very closely and distance measurement becomes possible using the sensor value of the pixel that captures the region without the retroreflective structure is defined as the second distance L2.

[0031] A more preferable retroreflective sheet 21, constructed according to the method for constructing a retroreflective sheet according to this embodiment, has multiple low-reflection sheets 22 attached to it. As shown in Figure 5, when the distance measuring camera 100 approaches the retroreflective sheet 21 at a distance of L2 or less, at least one of the multiple low-reflection sheets 22 is attached to the retroreflective sheet 21 so that it is within the field of view of the distance measuring camera 100. The field of view here is the field of view of all pixels of the TOF sensor 13. Figure 5 shows a state in which horizontally elongated low-reflection sheets 22 are spaced vertically apart by a gap S.

[0032] As shown in Figure 5, if the field of view of all pixels in the vertical direction of the TOF sensor 13 is θ2, the vertical spacing S can be calculated using equation (2). If a vertically elongated low-reflection sheet 22 is attached to the retroreflective sheet 21 with a horizontal spacing S, the field of view θ2 in equation (2) can be set to the field of view of all pixels in the horizontal direction of the TOF sensor 13, and the horizontal spacing S can be calculated similarly. S = 2L²·tan(θ² / 2) …(2)

[0033] When multiple low-reflection sheets 22 are attached to the retroreflective sheet 21, the distance between two adjacent low-reflection sheets 22 is kept within interval S. This allows the distance measuring camera 100 to correctly measure the distance based on the sensor values ​​of the pixels of the low-reflection sheet 22 in the sensor image, even when the distance measuring camera 100 approaches to a second distance L2 or less. The distance measuring camera 100 can measure the distance regardless of the distance from the distance measuring camera 100 to the retroreflective sheet 21.

[0034] The horizontal width W, vertical height H, and interval S described above can be calculated by a computer. Figure 6 shows a computer equipped with an input device 51, a storage unit 52, and a calculation unit 53. The input device 51 can be configured as a keyboard or the like, the storage unit 52 can be configured as a memory for temporary storage, and the calculation unit 53 can be configured as a central processing unit. The user inputs the previously determined first distance L1, second distance L2, field of view θ1 and θ2 using the input device 51 and stores them in the storage unit 52. The horizontal width / vertical height calculation unit 531 of the calculation unit 53 calculates and outputs the horizontal width W and vertical height H. The interval calculation unit 532 of the calculation unit 53 calculates and outputs the interval S.

[0035] Taking the case where multiple low-reflection sheets 22 are attached vertically to a retroreflective sheet 21 as an example, the computer (calculation unit 53) executes the process shown in Figure 7. When the process starts, the computer obtains the first distance L1 in step S1 and the field of view θ1 in step S2. The computer calculates the vertical height H in step S3. Subsequently, the computer obtains the second distance L2 in step S4 and the field of view θ2 in step S5. The computer calculates the interval S in step S6 and terminates the process.

[0036] Figure 8 shows a configuration in which a variable ND filter 24 is attached to the surface of a retroreflective sheet 21. As shown in Figure 9, the variable ND filter 24 comprises, for example, a circular first polarizing filter 241 and a second polarizing filter 242. The variable ND filter 24 includes a drive device 25 that rotates one of the first polarizing filter 241 and the second polarizing filter 242. Here, the drive device 25 rotates the second polarizing filter 242. As shown in Figure 10, the variable ND filter 24 may be composed of a liquid crystal filter 243 whose transmittance can be changed by a voltage applied by a voltage source 244.

[0037] Figure 9 shows the first polarizing filter 241 and the second polarizing filter 242 offset from each other, but in reality, the first polarizing filter 241 and the second polarizing filter 242 are adjacent to each other, facing each other. In Figure 9, the polarization directions of the first polarizing filter 241 and the second polarizing filter 242 are orthogonal, and the variable ND filter 24 has reduced transmittance. In the normal state, the polarization directions of the first polarizing filter 241 and the second polarizing filter 242 are aligned by rotating the second polarizing filter 242 by 90 degrees, and the variable ND filter 24 has increased transmittance. In this case, light incident on the retroreflective sheet 21 is reflected with high intensity.

[0038] Figure 11 shows an example of the configuration of the drive unit 25. The drive unit 25 includes an infrared light detection unit 251, a control unit 252, a memory 253, and a drive unit 254. The flowchart shown in Figure 12 illustrates the operation of the drive unit 25. When the drive unit 25 starts operating, the infrared light detection unit 251 detects infrared light in step S11. In step S12, the control unit 252 determines whether the detected infrared light is near-infrared light emitted by the distance measuring camera 100 (light-emitting unit 12). As described above, the light-emitting unit 12 irradiates the object 20 with pulsed near-infrared light, so the control unit 252 can determine whether or not the light is near-infrared light emitted by the light-emitting unit 12 based on the pulse frequency.

[0039] If the detected infrared light is not near-infrared light emitted by the distance measuring camera 100 (NO), the infrared light detection unit 251 and the control unit 252 repeat steps S11 and S12. If the detected infrared light is near-infrared light emitted by the distance measuring camera 100 (YES), the control unit 252 determines in step S13 whether the intensity of the detected infrared light is above a predetermined intensity. If the detected infrared light is not above a predetermined intensity (NO), the infrared light detection unit 251 and the control unit 252 repeat steps S11 to S13.

[0040] If the infrared light detected in step S13 is above a predetermined intensity (YES), the control unit 252 calculates the transmittance of the variable ND filter 24 and the rotation angle of the second polarizing filter 242 (or the applied voltage to the liquid crystal filter 243) to achieve that transmittance in step S14. Here, transmittance refers to the transmittance of the variable ND filter 24 reduced to below a predetermined transmittance. A table showing the correspondence between transmittance and rotation angle (or applied voltage) is stored in the memory 253, and the control unit 252 may read the rotation angle (or applied voltage) from the memory 253.

[0041] In step S15, the control unit 252 controls the drive unit 254 to rotate the second polarizing filter 242 or to apply voltage to the liquid crystal filter 243. This drives the variable ND filter 24. In step S16, the control unit 252 determines whether or not to terminate control of the variable ND filter 24. If control of the variable ND filter 24 is not terminated (NO), steps S11 to S16 are repeated. If control of the variable ND filter 24 is terminated (YES), the drive unit 25 terminates its operation.

[0042] Thus, the transmittance of the variable ND filter 24 is reduced by the drive unit 25 only when the distance measuring camera 100 photographs an object 20 to which the retroreflective sheet 21 is attached. Therefore, under normal conditions, the retroreflective sheet 21 performs its function, and when the distance measuring camera 100 photographs an object 20 to which the retroreflective sheet 21 is attached, the high-intensity reflection of light by the retroreflective sheet 21 is suppressed, enabling the distance measuring camera 100 to measure distances correctly.

[0043] The low-reflection sheet 22 shown in Figures 3A, 3B, and 5 may be configured as a variable ND filter 24 that increases the transmittance when light (near-infrared light) is not being emitted from the distance measuring camera 100, and decreases the transmittance when light is being emitted from the distance measuring camera 100.

[0044] The present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention. [Explanation of symbols]

[0045] 11 lenses 12 Light-emitting part 13 TOF sensors 14 Distance conversion unit 20 objects 21 Retroreflective sheet 22 Low-reflection sheet 24 Variable ND Filter 25 Drive unit 30 Distance measurement value utilization device 100 Rangefinder Camera

Claims

1. A low-reflectivity sheet having a predetermined vertical height and a predetermined horizontal width is attached to a retroreflective sheet of a predetermined size. The distance measuring camera captures the retroreflective sheet, generating an image in which the retroreflective sheet is located in a portion of the frame. When the distance measuring camera captures the retroreflective sheet at a distance from the retroreflective sheet equal to a first distance measured based on pixels in the surrounding region of the frame excluding the area in which the retroreflective sheet is located, the vertical height and horizontal width of the low-reflection sheet are set such that the vertical range of the low-reflection sheet image corresponding to the vertical height and the horizontal range of the low-reflection sheet image corresponding to the horizontal width include at least one pixel within the frame. Method for constructing retroreflective sheets.

2. Multiple low-reflectivity sheets are attached to the retroreflective sheet. The distance measuring camera photographs the retroreflective sheet while it is close to the retroreflective sheet, so that the distance from the distance measuring camera to the retroreflective sheet is measured based on pixels that photograph areas of the retroreflective sheet that do not have retroreflective structures. The distance measuring camera is positioned to be at least a second distance from the retroreflective sheet, and at least one of the multiple low-reflection sheets is attached to the retroreflective sheet so that it is within the field of view of the distance measuring camera. A method for constructing a retroreflective sheet according to claim 1.

3. The method for constructing a retroreflective sheet according to claim 1 or 2, wherein the low-reflection sheet is configured with a variable ND filter that increases the transmittance when light is not being irradiated from the distance measuring camera and decreases the transmittance when light is being irradiated from the distance measuring camera.

4. A retroreflective sheet having a predetermined size, predetermined vertical height and predetermined horizontal width, to which a low-reflection sheet is attached. When a distance measuring camera photographs the retroreflective sheet, it generates an image in which the retroreflective sheet is located in a portion of the frame. When the distance measuring camera photographs the retroreflective sheet at a distance from the retroreflective sheet that is equal to a first distance measured based on pixels in the surrounding region of the frame excluding the region in which the retroreflective sheet is located, the vertical height and horizontal width of the low-reflection sheet are set such that the vertical range of the image of the low-reflection sheet corresponding to the vertical height and the horizontal range of the image of the low-reflection sheet corresponding to the horizontal width include at least one pixel within the frame. Retroreflective sheet.

Images