Tunnel brightness measurement method based on point cloud intensity

Abstract

The invention discloses a tunnel brightness measurement method based on point cloud intensity, which belongs to the technical field of tunnel illumination, and comprises the following steps: establishing an object reflection model, determining received laser power Pr based on a Lambert model, and correcting the distance d between the object surface and a laser reflector based on the Lambert model, and then introducing a BRDF model to correct the included angle theta between the incident light and the surface of the object so as to eliminate the influence of the two factors of the incident angle and the distance on the laser intensity value to obtain the object reflectivity rho1 and fr under the condition of the point cloud intensity I, and finally calculating the road surface brightness after reflection of the side wall of the tunnel. According to the invention, brightness measurement is carried out from the perspective of a three-dimensional laser scanning technology, a new brightness measurement method is provided, a tunnel brightness detection model considering multi-dimensional space parameters is established, the change of tunnel light environment characteristics is comprehensively analyzed, and a multi-dimensional space brightness detection model is provided in an active light detection form.

Description

Technical Field

[0001] The present invention belongs to the technical field of tunnel lighting, and particularly relates to a method for measuring tunnel brightness based on point cloud intensity. Background Art

[0002] About 90% of the information during a driver's driving process is obtained through vision. However, the visibility inside a tunnel is low. Good road conditions can prevent traffic accidents caused by low visibility when a driver is driving. When a driver enters a tunnel, the vision changes from bright to dark, and vice versa when exiting the tunnel. This is the "black hole effect" and "white hole effect". If the road surface brightness inside the tunnel is uneven, there will be alternating dark bands and bright bands on the road surface, which is the "zebra crossing effect". The importance of lighting for highway tunnels is self-evident. Among the various evaluation indicators of tunnel lighting, brightness is an important parameter. Detecting the illuminance and brightness uniformity of the road surface and side walls of the tunnel plays an important role in ensuring the safe driving of vehicles and reducing the system energy consumption of tunnel lighting.

[0003] In existing research, various detection methods have been proposed to obtain brightness, which can be classified into four categories: point luminance meter measurement, imaging luminance meter measurement, video / photo-based measurement, and other methods. According to the brightness calculation and measurement methods described in the European Road Lighting Calculation Standard (EN13201-3), a point luminance meter is usually used for single-point measurement. The spot luminance meter uses the principle of lens imaging and measures the light spot signal as an electrical signal through an optical detector for further brightness calculation. Although this instrument provides the advantages of high precision, portability, and easy availability for brightness measurement, it lacks the analysis of brightness change situations and requires point-by-point operation, which takes a long time for large-scale evaluation. Therefore, the imaging luminance meter provides an alternative solution for the brightness analysis of the entire scene. Although this measurement method can quickly obtain the brightness within a large field of view, it requires complex brightness calibration and has a limited brightness measurement range. To expand the brightness measurement range, high dynamic range (HDR) imaging technology has been applied to brightness measurement. The so-called high dynamic range imaging is to calculate the brightness by mathematically deriving the camera's response function by considering multiple photos with different exposures of the same scene. Mikko developed a measurement method for intermediate brightness estimation using the S / P of digital camera pixels, and HCai developed a method for obtaining scene brightness and three-dimensional coordinates using dynamic range imaging technology. However, all these methods require cumbersome brightness calibration, and errors will also be generated by the camera's lens and aperture.

[0004] In addition, Constantinos A. Bouroussis used an unmanned aerial vehicle system to evaluate outdoor lighting devices. Shuguang Li proposed a method for brightness detection using tunnel monitoring videos. Matti T. Vaaja further proposed a brightness measurement method based on three-dimensional laser scanning by using a three-dimensional laser scanner to provide three-dimensional coordinate information and the rgb of camera photos for brightness calculation, but did not fully utilize the characteristics of obtaining surface features of three-dimensional laser scanning technology. Essentially, it is still an image-based brightness measurement method.

[0005] Due to the different limitations of devices in processing brightness, it is necessary to develop a new method in which the defects of the devices (including operations and results) should be fully considered in the efficiency of brightness measurement to achieve more accurate and faster measurement.

[0006] Therefore, it is very necessary to develop a set of fast, efficient and acceptable-precision brightness detection system. By actively scanning with lasers to obtain point cloud data and then automatically detecting the tunnel brightness based on the point cloud intensity, it can avoid the influence of vehicles and achieve rapid detection of long tunnels. The required cost is low and continuous measurement can be carried out. Summary of the Invention

[0007] In view of this, the purpose of the present invention is to provide a tunnel brightness measurement method based on point cloud intensity. On the basis of calculating brightness using the reflection coefficient, the relationship between the point cloud intensity and the reflectivity is constructed to improve the description of the reflection characteristics of objects. The point cloud intensity is used to optimize the brightness calculation model, and the automatic detection and analysis of the tunnel pavement brightness are realized based on point cloud technology. It has the advantages of high precision, simple measurement operation, short time consumption and applicability to various environments, can make up for the deficiencies of existing brightness measurement methods, and realize the efficient evaluation of tunnel operation and maintenance.

[0008] To achieve the above purpose, the present invention provides the following technical solutions:

[0009] The present invention provides a tunnel brightness measurement method based on point cloud intensity, including the following steps:

[0010] S1: Establish an object reflection model. Based on the Lambert model, assume that the object surface is a Lambert body. Then, when observing the surface from any direction in space, the brightness remains unchanged and the light absorption rate is 0. Then, the received laser power P is determined by the following formula r :

[0011]

[0012] In the formula, P r is the received laser power, P t is the emitted laser power, ρ λis the reflectivity of the object surface for the defined wavelength λ, D r is the receiver aperture, R is the distance from the scanner center to the scanned target point, η sys is the transmission coefficient of the scanning system, η atm is the atmospheric transmission coefficient, cisθ is the cosine value of the incident angle;

[0013] S2: According to the relationship between the laser intensity value and the reflectivity of the target object: I(ρ, θ, d) = f1(ρ)·f2(θ)·f3(d), and based on the Lambert model, correct the distance d between the object surface and the laser reflector, and then introduce the BRDF model to correct the angle θ between the incident light and the object surface, so as to eliminate the influence of the two factors of the incident angle and the distance on the laser intensity value to obtain the object reflectivity ρ1,f under the point cloud intensity I r :

[0014]

[0015] In the formula, ρ1,f r is the object reflectivity obtained by BRDF calculation corresponding to the point cloud intensity of I, f ρ，BRDF is the function between the reflectivity measured by BRDF corresponding to ρ material and the incident angle, K is the coefficient of the point cloud intensity and the BRDF measurement value of the object under the angle parameter, α is the exit angle, θ is the laser incident angle, D is the laser scanning distance, I (θ，D) is the point cloud intensity after distance correction;

[0016] S3: Calculate the road surface brightness after reflection from the tunnel side wall:

[0017]

[0018] In the formula, L is the road surface calculation point brightness; E p is the illuminance value generated by the lamp at the road surface calculation point; E1 is the illuminance of the road surface calculation point under the side wall reflection; ρ I is the corrected road surface point cloud intensity, where ρ I = ρ1,f r .

[0019] Furthermore, in step S2, correcting the distance d between the object surface and the laser reflector based on the Lambert model includes the following steps:

[0020] A1: Correct the angle effect of the point cloud intensity, select the point cloud data within the preset measurement range and apply the following formula to correct the incident angle θ, where the incident angle θ is the angle between the plane normal vector and the laser incident angle, and the plane normal vector is defined as the normal vector perpendicular to the preset plane fitted by the adjacent point clouds generated by the laser on the object surface:

[0021]

[0022] Among them,

[0023]

[0024] In the formula: I(θ) is the point cloud intensity after angle correction, f(θ) is the functional formula of the point cloud intensity at different angles, f′(θs) is the functional formula of the point cloud intensity at the set angle θs, and I d is the original point cloud intensity at a fixed distance;

[0025] A2: Determine the point cloud intensity correction formula under the distance diffuse reflection model according to the following formula:

[0026]

[0027] Among them,

[0028]

[0029] In the formula, I(θ, d) is the point cloud intensity after distance correction, f(θ) is the functional formula of the point cloud intensity at different angles, f′(θs) is the functional formula of the point cloud intensity at the set angle θs, f(d) is the functional formula of the point cloud intensity at different distances, f′(ds) is the functional formula of the point cloud intensity at the set angle ds, I is the original point cloud intensity, β is the function fitting parameter, and i is the polynomial degree.

[0030] Further, in step S2, introducing the correction of the angle θ between the incident light and the object surface under the BRDF model includes the following steps:

[0031] B1: Establish the relationship between the distance and the original laser intensity:

[0032]

[0033] B2: By measuring the variation laws of the point cloud intensities of the road surface material, the highly diffuse sidewall material, and the road surface material at different distances, and performing distance correction on different materials, the correction formula is as follows:

[0034]

[0035] B3: Analyze through the semi-empirical bidirectional reflection distribution mathematical model FOI, and the specific expression is:

[0036]

[0037] B4: Convert f r cosθ obtained from the FOI model into the relational expression of the reflectance with respect to the incident angle, and the specific conversion formula is as follows:

[0038]

[0039] B5: Determine the point cloud intensity I after angle and distance correction according to the following formula (θ，D) :

[0040]

[0041] where d is the distance between the object surface and the laser reflector, β is the function fitting parameter, i is the polynomial degree, I(d) is the point cloud intensity after distance correction, f(d) is the functional formula of the point cloud intensity at different distances, f′(d0) is the functional formula of the point cloud intensity at the set distance, I(θ) is the point cloud intensity after angle correction, I is the original point cloud intensity, A is the diffuse reflection amplitude; m is the diffuse reflection coefficient; B is the specular reflection amplitude; s is the specular reflection coefficient, f r is the BRDF measurement value, E is the illuminance; ρ is the reflectivity, and θ is the angle between the incident light and the object surface.

[0042] Furthermore, in step S2, the function between the reflectivity of the BRDF measurement corresponding to the ρ material and the incident angle is determined by the following formula:

[0043]

[0044] where b i and c i are both coefficients for formula derivation.

[0045] Furthermore, in step S3, calculating the road surface brightness after reflection from the tunnel side wall includes the following steps:

[0046] C1: Discretize the side wall point cloud. According to the point cloud coordinate information, use the finite element idea to discretize the tunnel side wall into multiple rectangular units with the same size that can describe the influence of the light reflected by the lamp on the road surface to enhance the illumination;

[0047] C2: Conduct a spatial analysis on the discretized rectangular units. Combine the spatial positions of the lamp and the rectangular units to calculate the light intensity and illuminance values, including the calculation of the distance and angle between the lamp and the rectangular units. The specific formulas are as follows:

[0048]

[0049]

[0050]

[0051] C3: Based on the calculated direct brightness value of the lamp in the rectangular unit on the side wall and combined with the light reflection performance of the tunnel inner wall material, convert the brightness of the rectangular unit on the side wall into the light intensity value in the direction of the road surface calculation point;

[0052] C4: Calculate the illuminance value unit E at the road surface calculation point according to the spatial position relationship of the rectangular unit li :

[0053]

[0054] In the formula, E li is the illuminance value generated by the side wall unit at the road surface calculation point; I is the light intensity value of the side wall unit in the direction of the road surface calculation point; τ is the angle between the line connecting the center of the side wall unit and the road surface calculation point and the road surface normal direction; D1 is the distance between the side wall unit and the road surface calculation point; among them,

[0055] C5: Combine the corrected road surface point cloud intensity to obtain the road surface brightness affected by side wall reflection:

[0056]

[0057] In the formula, E b is the total illuminance value (lx) generated by all lamps on the discrete unit; E bi is the illuminance (lx) generated by one lamp on the rectangular unit; I(φ,γ) is the light intensity value (cd) obtained according to the angle calculation on the lamp rectangular unit, and it is taken according to the lamp light intensity table; θ is the angle between the connecting line between the lamp and the center of the rectangular unit and the normal direction; D0 is the distance between the lamp and the center point of the rectangular unit, x0, y0, z0 are the distances between the lamp and the center point of the rectangular unit in the X, Y, and Z directions, L is the road surface calculation point brightness; E p is the illuminance value generated by the lamp at the road surface calculation point; E1 is the road surface calculation point illuminance under side wall reflection; ρ I is the corrected road surface point cloud intensity, among which, ρ I = ρ 1，fr .

[0058] Furthermore, the light intensity value in the direction of the road surface calculation point is calculated by the following formula:

[0059]

[0060] I f = L b ab

[0061] In the formula, L b is the total brightness of the discrete unit; r is the angle between the reflected light, the calculation point and the horizontal direction of the side wall unit; L b is the total brightness of the discrete unit, If is the point cloud intensity after angle and distance correction; a and b are the length and width of the discrete unit.

[0062] The beneficial effects of the present invention are as follows:

[0063] (1) Measuring brightness from the perspective of 3D laser scanning technology, providing a new brightness measurement method, establishing a tunnel brightness detection model considering multi-dimensional space parameters, comprehensively analyzing the changes in tunnel light environment characteristics, and proposing a multi-dimensional space brightness detection model in the form of active light detection.

[0064] (2) Developing a brightness calculation method based on point cloud intensity, including the correction of point cloud intensity. Compared with traditional methods, this method has better effects in terms of measurement accuracy and data acquisition speed.

[0065] Other advantages, objectives and features of the present invention will be described in the subsequent specification, and to some extent, will be obvious to those skilled in the art, or those skilled in the art can obtain teachings from the practice of the present invention. The objectives and other advantages of the present invention can be achieved and obtained through the following specification. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] In order to make the objectives, technical solutions and beneficial effects of the present invention clearer, the present invention provides the following drawings for illustration:

[0067] Figure 1 Schematic diagram of the rectangular unit in the present invention where the lamp reflects light onto the road surface to enhance the lighting effect;

[0068] Figure 2 Schematic diagram of the discretization of the point cloud coordinate information in the present invention;

[0069] Figure 3 Schematic diagram of the spatial position of the lamp and the rectangular unit in the present invention;

[0070] Figure 4 Schematic diagram of the spatial relationship between the size of the sidewall rectangular unit and the calculation points on the road surface in the present invention;

[0071] Figure 5 Schematic diagram of the brightness geometric model in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0072] As Figures 1 to 5 shown, the present invention provides a tunnel brightness measurement method based on point cloud intensity, including the following steps:

[0073] S1: Establish an object reflection model,

[0074] Among them, the point cloud intensity information can represent the absorption and reflection degree of the measured object to the electromagnetic wave in the laser band, representing the reflectivity characteristics of the object surface. It is the digitally represented laser echo energy, which is proportional to the number of photons incident on the detector.

[0075] The lidar equation can represent the echo intensity. Since the intensity value obtained by the scanner is proportional to the total received laser power, and factors such as distance, incident angle, and atmospheric attenuation effect will cause deviations in the target reflection intensity. Since laser and radar electromagnetic waves follow the same principle, using the radar equation to describe the relevant parameters of the received signal power, the received laser power P r can be expressed as:

[0076]

[0077] Among them, P r is the received laser power, with the unit of W, P t is the transmitted laser power, with the unit of W, D r is the receiver aperture, with the unit of m, R is the distance from the scanner center to the scanned target point, with the unit of m, η sys is the scanning system transmission coefficient, η atm is the atmospheric transmission coefficient, σ cross is the target cross-section, with the unit of m 2 , β t is the angle between the incident light and the surface normal;

[0078] Assuming that the target is larger than the laser beam and the surface is a perfect diffuse reflector, then

[0079] σ cross = πρ λ R 2 β t 2 cosα

[0080] In the formula, ρ λ is the reflectivity of the object surface for the defined wavelength λ, and α is the exit angle, that is, the angle between the exit beam and the surface normal;

[0081] However, the actual object reflectance is not an ideal diffuse reflector. According to the research of relevant scholars, the above formula should be expressed as:

[0082] σ cross = πρ λ R 2 β t 2 f(α)

[0083] In the formula, f(α) is the functional relationship between the actual object reflectivity and the angle in the illumination model. Then the received laser power P r can be expressed as:

[0084]

[0085] Through the radar equation, the relevant influencing factors can be divided into three categories: the instrument itself (power, aperture, laser wavelength), the scanning state (atmospheric factors, distance, angle), and the object reflectivity.

[0086] S2: Establishment of the relationship between point cloud intensity and reflectivity under the Lambert model

[0087] For the phenomenon that the light reflected from the surface of a diffuse reflection object is uniformly scattered in all directions, assuming that the object surface is a Lambert body, the brightness (radiant intensity) remains unchanged when observing the surface from any direction in space and the light absorption rate is 0. Then the radar equation can be simplified as:

[0088]

[0089] In the same scanning task, the atmospheric attenuation can be ignored, the characteristics of the scanning instrument can be regarded as stable and unchanged, there is a linear relationship between the received power of the laser and the point cloud intensity, and the laser intensity data is mainly affected by three factors: target reflectivity, incident angle, and distance.

[0090] In order to convert the laser intensity value into the reflectivity of the target object, it is necessary to correct it to eliminate the influence of other factors. Based on the Lambert hypothesis, for a laser scanner in a single scanning task, it can be considered that the reflectivity of a specific point is a fixed property of the target object surface and is a fixed value that does not need to be corrected. The cosine value of the incident angle is theoretically proportional to the intensity, and the square of the distance is inversely proportional to the intensity. Assuming that the influences of target reflectivity, incident angle, and distance are independent of each other, then the intensity after correcting the incident angle and intensity is only related to the reflectivity. Then the laser intensity value I can be expressed as the product of a function of the target reflectivity ρ, incident angle θ, and distance d:

[0091] I(ρ, θ, d) = f1(ρ)·f2(θ)·f3(d)

[0092] Among them, the incident angle effect is theoretically that the cosine value of the incident angle is proportional to the laser point cloud intensity. To obtain the actual functional relationship, the relationship between the incident angle and the original laser intensity is established through an angle experiment. The functional formula for the influence of the incident angle can be written as:

[0093]

[0094] The incident angle θ is the angle between the plane normal vector and the incident angle of the laser. The plane normal vector can be defined as the normal vector perpendicular to the optimal plane fitted by the adjacent point clouds generated by the laser on the object surface. According to the variation law of the point cloud intensity at different angles of several commonly used tunnel materials collected during laboratory measurements, a relational expression of intensity with respect to the angle was fitted and corrected. The correction formula is as follows:

[0095]

[0096] In the formula: I(θ) is the point cloud intensity after angle correction, f(θ) is the functional expression of the point cloud intensity at different angles, f′(θ) is the functional expression of the point cloud intensity at the set angle θ, and I d is the original point cloud intensity at a fixed distance.

[0097] For the correction of the distance effect, an actual tunnel was selected for the experiment. As can be seen from the actual situation, the incident angle also changes with the change of the distance. Therefore, to study the influence of the distance effect, the angle effect of the point cloud intensity was first corrected. On this basis, the point cloud data within a large measurement range was selected to apply the angle correction formula to eliminate the angle effect, and a function fitting was performed. The function formula of the distance influence can be written as:

[0098]

[0099] In the formula, d is the distance between the object surface and the laser reflector, β is the function fitting parameter, and i is the polynomial degree;

[0100] Then the point cloud intensity correction formula under the final obtained diffuse reflection model is:

[0101]

[0102] In the formula, I(θ, d) is the point cloud intensity after distance correction, f(θ) is the functional expression of the point cloud intensity at different angles, f′(θs) is the functional expression of the point cloud intensity at the set angle θs, f(d) is the functional expression of the point cloud intensity at different distances, f′(ds) is the functional expression of the point cloud intensity at the set distance ds, and I is the original point cloud intensity.

[0103] Since the correction based on the Lambert diffuse reflection model is based on the assumption that the radiation intensity on the object surface is isotropic, it will result in the invariance of the corrected point cloud intensity at different observation angles, and only solves the problem of "same object, different spectra" well. However, the description of the reflection characteristics of the object surface does not conform to the actual situation. Therefore, further correction improvement is needed.

[0104] S3: Establishing the relationship between the point cloud intensity and the reflectivity under the BRDF model:

[0105] On the basis of the Lambert model correction, the BRDF model is introduced to improve the intensity correction. The BRDF model mainly describes the reflection characteristics of the object surface from the perspective of angles. Therefore, before using the BRDF model for correction, it is necessary to eliminate the influence of distance on the point cloud intensity, so that the point cloud intensity is only related to the angle and reflectivity. The distance effect is theoretically that the square of the distance is inversely proportional to the laser point cloud intensity, but this is not the case in reality. To obtain the actual functional relationship, the relationship between the distance and the original laser intensity is established through a distance experiment:

[0106]

[0107] In the formula, d is the distance between the laser emitter and the object surface. By measuring the variation law of the point cloud intensity of the road surface material, the high diffuse reflection sidewall material and the road surface material at different distances and fitting the relational formula of the intensity with respect to the distance, the fitting function is applied to correct the distance for different materials. The correction formula is as follows:

[0108]

[0109] In the formula, I(d) is the point cloud intensity after distance correction, f(d) is the functional formula of the point cloud intensity at different distances, f′(d0) is the functional formula of the point cloud intensity at the set distance. The average value is selected as the set distance in the research, and I is the original point cloud intensity.

[0110] On the basis of the distance correction, the BRDF model is introduced to improve the influence of the angle effect.

[0111] Since the 3D laser scanner is a single-station active detection and scanning, the FOI model is selected for analysis. This model is a semi-empirical bidirectional reflection distribution mathematical model. It is generally considered that the direction of the light received by the laser detector is the same as the incident direction, that is, θ i =θ r , where, θ i , represent the incident angle and azimuth angle of the incident light; θ r , represent the scattering angle and azimuth angle of the scattered light; The specific expression is:

[0112]

[0113] In the formula, A is the diffuse reflection amplitude; m is the diffuse reflection coefficient; B is the specular reflection amplitude; s is the specular reflection coefficient. Therefore, based on the above assumptions, the theoretical model of the FOI model is simplified and transformed for f r cosθ into the relational formula of the reflectivity with respect to the incident angle. The specific conversion formula is as follows:

[0114]

[0115]

[0116] By solving the two equations simultaneously, we can obtain:

[0117]

[0118] In the formula, f r is the BRDF measurement value; L is the luminance, E is the illuminance; ρ is the reflectivity, and θ is the angle between the incident light and the object surface.

[0119] In this paper, with the angle as the unified parameter, the bidirectional reflectance distribution function (BRDF) is introduced to improve the light propagation theory on the object surface, so that the point cloud intensity can describe the reflection characteristics of the object surface. Therefore, the reflectivity based on the correction of the point cloud intensity and combined with the BRDF theory can be expressed as:

[0120]

[0121] Among them,

[0122]

[0123]

[0124] In the formula, ρ1, f r is the object reflectivity obtained by BRDF calculation corresponding to the point cloud intensity of I; I (θ，D) is the point cloud intensity after angle and distance correction, θ is the laser incident angle, D is the laser scanning distance, f ρ，BRDF is the function between the reflectivity measured by BRDF corresponding to the ρ material and the incident angle, which can be expressed by a Gaussian polynomial through function fitting. K is the coefficient of the point cloud intensity and the object BRDF measurement value under the angle parameter, and the meanings of d and D are the same.

[0125] S4: First, discretize the sidewall point cloud. According to the point cloud coordinate information, use the finite element idea to discretize the tunnel sidewall into a series of rectangular units that can describe the influence of the reflected light of the lamp on the road surface to enhance the illumination. The sizes of all rectangular units are exactly the same. After discretization, the point cloud is as Figure 1 , and then perform a spatial analysis on the discretized rectangular units. Combine the spatial positions of the lamp and the rectangular units to calculate the light intensity and illuminance values. The spatial position relationship between the two is as Figure 2 . According to the light propagation principle of the lamp, the illuminance value of a certain rectangular unit can be calculated, mainly including the calculation of the distance and angle between the lamp and the rectangular unit. The specific formula is as follows:

[0126]

[0127]

[0128]

[0129] In the formula, E b is the total illuminance value (lx) generated by all lamps on the discrete unit; E bi is the illuminance (lx) generated by one lamp on the rectangular unit; I(φ,γ) is the luminous intensity value (cd) obtained according to the angle calculation on the lamp rectangular unit, and is taken according to the lamp luminous intensity table; θ is the angle between the connecting line of the lamp and the center of the rectangular unit and the normal direction; D0 is the distance between the lamp and the center point of the rectangular unit; x0, y0, z0 are the distances between the lamp and the center point of the rectangular unit in the X, Y, and Z directions.

[0130] Based on the calculated direct brightness value of the lamp on the sidewall rectangular unit and combining the light reflection performance of the tunnel inner wall material, the brightness of the sidewall rectangular unit is converted into the luminous intensity value I in the direction of the road surface calculation point. Specifically, the size of the sidewall rectangular unit and the spatial relationship between the calculation points and the road surface calculation point need to be considered, such as Figure 3 , at this time, first convert the calculated sidewall brightness according to the size of the rectangular unit into a similar light source. At this time, the obtained is the luminous intensity value in the direction perpendicular to the rectangular unit. Next, calculate and obtain the reflected brightness calculation considering the sidewall rectangular unit as the light source, that is, calculate and obtain the luminous intensity value in the direction of the road surface calculation point along the rectangular unit. Calculate and deduce according to the spatial relationship between the rectangular unit and the tunnel road surface calculation point. The specific calculation formula is as follows:

[0131]

[0132] I f = I b ab

[0133] After calculating the luminous intensity value of the rectangular unit in the direction of the road surface calculation point, calculate the illuminance value at the road surface calculation point according to the spatial position relationship of the rectangular unit shown in the figure below. Finally, combine the corrected road surface point cloud intensity to obtain the road surface brightness affected by the sidewall reflection. The calculation formula is as follows:

[0134]

[0135]

[0136] In the formula, E li is the illuminance value generated by the sidewall unit at the road surface calculation point; I is the luminous intensity value of the sidewall unit in the direction of the road surface calculation point; τ is the angle between the connecting line of the center of the sidewall unit and the road surface calculation point and the road surface normal direction; D1 is the distance between the sidewall unit and the road surface calculation point.

[0137] Finally, the illuminance value of the calculation point on the road surface is obtained by accumulating the illuminance generated by the direct light of the lamp on the road surface and the illuminance generated by the sidewall reflection on the road surface. Combining the correction result of the point cloud intensity of the road surface point, the road surface brightness L considering the influence of the sidewall is calculated, and the calculation formula is as follows:

[0138]

[0139] In the formula, L is the brightness of the calculation point on the road surface; E p is the illuminance value generated by the lamp at the calculation point on the road surface; E1 is the illuminance of the calculation point on the road surface under the sidewall reflection; ρ I is the corrected point cloud intensity of the road surface.

[0140] This solution measures the brightness from the perspective of 3D laser scanning technology, provides a new brightness measurement method, establishes a tunnel brightness detection model considering multi-dimensional space parameters, comprehensively analyzes the changes in the characteristics of the tunnel light environment, and proposes a multi-dimensional space brightness detection model in the form of active light detection. A brightness calculation method based on point cloud intensity is developed, including the correction of point cloud intensity. Compared with the traditional method, this method has better effects in terms of measurement accuracy and data acquisition speed.

[0141] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made in form and details without departing from the scope defined by the claims of the present invention.

Claims

1. A tunnel brightness measurement method based on point cloud intensity, characterized in that, Including the following steps: S1: Establish an object reflection model. Based on the Lambert model, assuming that the object surface is a Lambertian body, the brightness remains unchanged when observing the surface from any direction in space and the light absorption rate is 0. Then, the received laser power P is determined by the following formula r : Wherein, P r is the received laser power, P t is the transmitted laser power, ρ λ is the reflectivity of the object surface for the defined wavelength λ, D r is the receiver aperture, R is the distance from the scanner center to the scanned target point, η sys is the transmission coefficient of the scanning system, η atm is the atmospheric transmission coefficient, and cosθ is the cosine value of the incident angle; S2: According to the relational expression between the laser intensity value and the reflectivity of the target: I(ρ, θ, d) = f1(ρ)·f2(θ)·f3(d), and based on the Lambert model, correct the distance d between the object surface and the laser reflector, and then introduce the BRDF model to correct the angle θ between the incident light and the object surface, so as to eliminate the influence of the two factors of the incident angle and the distance on the laser intensity value and obtain the object reflectivity ρ1 under the point cloud intensity I, f r : where ρ1, f r is the object reflectance obtained by BRDF calculation corresponding to the case where the point cloud intensity is I, f ρ，BRDF is the function between the reflectance measured by BRDF corresponding to ρ material and the incident angle, K is the coefficient of the point cloud intensity and the object BRDF measurement value under the angular parameter, α is the exit angle, θ is the laser incident angle, D is the laser scanning distance, I (θ，D) is the point cloud intensity after distance correction; S3: Calculate the road surface brightness after reflection from the tunnel sidewall: where L is the luminance at the pavement calculation point; E p is the illuminance value generated by the luminaire at the pavement calculation point; E1 is the illuminance at the pavement calculation point under the sidewall reflection; ρ I is the intensity of the corrected pavement point cloud, where 2. The tunnel brightness measurement method based on point cloud intensity according to claim 1, characterized in that: In step S2, the correction of the distance d between the object surface and the laser reflector based on the Lambert model includes the following steps: A1: Correct the angular effect of the point cloud intensity. Select the point cloud data within the preset measurement range and apply the following formula to correct the incident angle θ, where the incident angle θ is the angle between the plane normal vector and the laser incident angle, and the plane normal vector is defined as the normal vector perpendicular to the preset plane fitted by the adjacent point clouds generated by the laser on the object surface: Where, Where: I(θ) is the point cloud intensity after angle correction, f(θ) is the functional formula of the point cloud intensity at different angles, f′(θs) is the functional formula of the point cloud intensity at the set angle θs, and I d is the original point cloud intensity at a fixed distance; A2: Determine the point cloud intensity correction formula under the distance diffuse reflection model according to the following formula: Where, In the formula, I(θ, d) is the point cloud intensity after distance correction, f(θ) is the functional formula of the point cloud intensity at different angles, f′(θs) is the functional formula of the point cloud intensity at the set angle θs, f(d) is the functional formula of the point cloud intensity at different distances, f′(ds) is the functional formula of the point cloud intensity at the set angle ds, I is the original point cloud intensity, β is the function fitting parameter, and i is the polynomial degree.

3. The tunnel luminance measurement method based on point cloud intensity according to claim 2, wherein: In step S2, the correction of the angle θ between the incident light and the object surface introduced under the BRDF model includes the following steps: B1: Establish the relationship between the distance and the original laser intensity: B2: By measuring the variation law of the point cloud intensity of the road surface material, the high-diffuse reflection sidewall material, and the road surface material at different distances, and performing distance correction on different materials, the correction formula is as follows: B3: Analyze through the semi-empirical bidirectional reflection distribution mathematical model FOI, and the specific expression is: B4: Convert the f r cosθ obtained from the FOI model into a relationship between reflectance and incident angle. The specific conversion formula is as follows: B5: Determine the point cloud intensity I after angle and distance correction according to the following formula (θ，D) : Where d is the distance between the object surface and the laser reflector, β is the function fitting parameter, i is the polynomial degree, I(d) is the point cloud intensity after distance correction, f(d) is the functional formula of the point cloud intensity at different distances, f′(d0) is the functional formula of the point cloud intensity at the set distance, I(θ) is the point cloud intensity after angle correction, I is the original point cloud intensity, A is the diffuse reflection amplitude; m is the diffuse reflection coefficient; B is the specular reflection amplitude; s is the specular reflection coefficient, and f r is the BRDF measurement value, E is the illuminance; ρ is the reflectivity, and θ is the angle between the incident light and the object surface.

4. The tunnel brightness measurement method based on point cloud intensity according to claim 3, characterized in that: In step S2, the function between the reflectivity of the BRDF measurement corresponding to the ρ material and the incident angle is determined by the following formula: where b i and c i are both coefficients in the formula derivation.

5. The tunnel brightness measurement method based on point cloud intensity according to claim 4, characterized in that: In step S3, calculating the road surface brightness after reflection from the tunnel sidewall includes the following steps: C1: Discretize the sidewall point cloud. According to the point cloud coordinate information, use the finite element idea to discretize the tunnel sidewall into multiple rectangular units with the same size that can describe the influence of the reflected light of the lamp on the road surface to enhance the illumination; C2: Perform spatial analysis on the discretized rectangular units. Combine the spatial positions of the lamp and the rectangular units to calculate the light intensity and illuminance values, including the calculation of the distance and angle between the lamp and the rectangular units. The specific formula is as follows: C3: Based on the calculated direct brightness value of the lamp in the sidewall rectangular unit, combine the light reflection performance of the tunnel inner wall material to convert the brightness of the sidewall rectangular unit into the light intensity value in the direction of the road surface calculation point; C4: Calculate the illuminance value unit E at the pavement calculation point according to the spatial position relationship of the rectangular units li : where E li is the illuminance value generated by the sidewall unit at the pavement calculation point; I is the luminous intensity value of the sidewall unit in the direction of the pavement calculation point; τ is the angle between the line connecting the center of the sidewall unit and the pavement calculation point and the pavement normal direction; D1 is the distance between the sidewall unit and the pavement calculation point; among them, C5: Combine the corrected road surface point cloud intensity to obtain the road surface brightness affected by the sidewall reflection: Where, E b is the total illuminance value (lx) generated by all lamps on the discrete unit; E bi is the illuminance (lx) generated by a single lamp on the rectangular unit; I(φ,γ) is the luminous intensity value (cd) obtained according to the angle on the rectangular unit of the lamp, which is taken from the lamp luminous intensity table; θ is the angle between the connecting line of the lamp and the center of the rectangular unit and the normal direction; D0 is the distance between the lamp and the center point of the rectangular unit, x0, y0, z0 are the distances between the lamp and the center point of the rectangular unit in the X, Y, and Z directions, and L is the luminance of the road surface calculation point; E p is the illuminance value generated by the lamp at the road surface calculation point; E1 is the illuminance of the road surface calculation point under the sidewall reflection; ρ I is the corrected road surface point cloud intensity, where, 6. The tunnel brightness measurement method based on point cloud intensity according to claim 5, characterized in that: The light intensity value in the direction of the road surface calculation point is calculated by the following formula: I f = L b ab Where L b is the total luminance of the discrete unit; r is the angle between the reflected light, the calculation point and the horizontal direction of the sidewall unit; L b is the total luminance of the discrete unit, and I f is the point cloud intensity after angle and distance correction; a and b are the length and width of the discrete unit.

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