CN101532825A

CN101532825A - Method for measuring thickness of sea surface spilled oil film based on differential laser triangulation method

Info

Publication number: CN101532825A
Application number: CN200910068627A
Authority: CN
Inventors: 葛宝臻; 吕且妮; 王向南; 王春谊; 魏耀林; 姚文达
Original assignee: Tianjin University
Current assignee: Tianjin University
Priority date: 2009-04-27
Filing date: 2009-04-27
Publication date: 2009-09-16
Anticipated expiration: 2029-04-27
Also published as: CN101532825B

Abstract

The invention relates to a method for measuring thickness of sea surface spilled oil film based on differential laser triangulation. The method uses dual laser triangulation ranging system, and laser beams emitted by two lasers are respectively irradiated to the upper and the lower surfaces of an oil film to be measured in any vertical direction; the off-plane displacements of the upper and the lower surfaces can be obtained simultaneously according to the size of image drift and configuration parameters of imaging system; the thickness of the measured oil film can be determined by subtracting; and the thickness distribution of oil film in spilled oil area can be obtained by multi-point sampling. The non-contact measuring method has the characteristics of simple theory, convenient measurement, low cost, high precision, small volume and the like, and can be applied to dynamic on-line measurement of oil film thickness at the surface of aqueous solution.

Description

Sea surface oil spill oil film thickness measuring method based on differential laser triangulation method

[ technical field ]:

the invention belongs to the technical field of marine environment three-dimensional monitoring, and particularly relates to a technology for measuring the thickness of a sea surface spilled oil film.

[ background Art ] A method of:

the measurement of the thickness of the oil film on the sea surface is one of indispensable works of marine environment management, and is an important problem in the fields of environmental protection, chemical analysis, oil film dynamics research and the like. The ocean monitoring department and experts have always sought a method for accurately measuring the thickness of the water surface oil film, but until now, a credible method for measuring the thickness of the water surface oil film cannot be found. The oil film thickness provides effective and reasonable guidance for oil spill treatment (such as correct and reasonable use of a dispersing agent, field incineration and the like), provides important scientific basis for understanding the basic theory of oil layer extended dynamics, and has important significance for weather, climate and marine environment treatment. The following methods have been proposed for measuring the thickness of different oil and oil films on the surface of aqueous solutions:

the method can simultaneously realize oil product identification and oil film thickness measurement by analyzing the fluorescence life of different oil products and the spectral intensity under different oil film thicknesses, and is suitable for measuring the thickness of a thin oil film.

The microwave radiometer obtains the oil layer thickness information by comparing the measured brightness temperature, but the method needs to determine some environmental parameters and oil property parameters in advance.

In the multi-beam interference method, the oil film forms two parallel plane layers of oil/gas and oil/water on the sea surface. Light is incident on the oil-gas interface, part of the light is reflected to the air, and part of the light is transmitted to enter the oil layer. The light beam entering the oil layer is reflected/transmitted at the oil-water interface, and the reflected light beam is reflected back to the oil layer through the oil-water interface and then is transmitted back to the air. The light waves in the air are superposed to generate interference, and the thickness of the oil film is calculated according to the interference phase difference.

The method is characterized in that oil quality identification is carried out by analyzing the peak ratio of interference envelopes formed by different oil products on a water surface and an air interface, and the oil film thickness is calculated according to the distance between two groups of interference fringe peaks formed by an oil-gas interface and an oil-water interface. The technology based on the interference method is suitable for measuring the thickness of a transparent oil layer with a flat oil-water interface.

The basic mechanism of the laser acoustics/laser ultrasonic technology is based on laser radar and ultrasonic principle, and the thickness of an oil layer is calculated by utilizing the time of sound pulse transmitted through the oil layer and then returned to an oil-gas interface and the sound wave transmission speed in the oil layer. A laser ultrasonic remote sensing oil film thickness measuring sensor has been developed, and the sensor has high measuring precision and great potential in the field of aerial oil spill remote sensing measurement, but three lasers are needed: a CO₂Pulsed lasers, a Nd: YAG laser and a He-Ne laser, and are large in volume.

[ summary of the invention ]:

the invention aims to provide a method for measuring the thickness of a sea surface spilled oil film based on a differential laser triangulation method, which is used for solving the problem of measuring the thickness of the sea surface spilled oil film, filling the blank of the technical research in the aspect of marine oil monitoring in China, promoting the development of the existing oil monitoring level in China and providing a technical basis for the measurement of the thickness of the sea surface spilled oil film, the oil layer extension dynamics and the marine environment science.

The invention firstly proposes that the thickness measuring technology of the laser triangulation method is applied to the measurement of the thickness of the oil film of the spilled oil, the fluctuation of the seawater and the oil film is considered, and a double-optical-path differential system, namely a symmetrical oblique-incidence double-laser triangulation distance measuring system, is adopted. Laser beams emitted by the laser device are converged on the upper surface and the lower surface of the oil film by the converging lens, and an elliptical laser spot is formed at an incidence point on the upper surface and the lower surface of the oil film. The imaging system images the light spot on the light receiver CCD to form an image point. By using a double-laser triangulation system of the upper and lower surfaces, according to the image motion size h and the structural parameter k of the imaging system, the out-of-plane displacement of the upper and lower surfaces at the point can be obtained simultaneously, and the oil film thickness of the measured point can be determined by subtraction. In the optical path system, the laser beam, the optical axis of the imaging lens and the CCD are ensured to be in the same plane, the included angle between the optical axis of the laser beam and the optical axis of the imaging lens is strictly ensured to be 90 degrees, the included angle between the CCD and the optical axis of the imaging lens is 90 degrees, the object distance is ensured to be unchanged after the position of the measured surface is changed, the measured point in a certain depth of field can be ensured to be focused and imaged on the detector, and the measurement precision is ensured.

The specific measuring process of the measuring method comprises the following steps:

first, light path System building

Building a symmetrical oblique double-laser triangular distance measuring system according to the figure 1, wherein each laser triangular distance measuring system comprises a light source part consisting of a laser and a converging lens, and a receiving imaging system consisting of an imaging lens, a detector CCD (charge coupled device) or a position detector PSD (position sensitive detector) and a data acquisition and processing module; like parts are designated by like reference numerals in the drawings;

second, light spot image acquisition

The laser beam emitted by the first laser device is converged and obliquely incident on the upper surface of the oil film to be detected through the converging lens, an elliptical laser spot is formed at an incident point on the upper surface of the oil film (the position of the lens is adjusted, so that the focus is positioned near the upper surface of the oil film to be detected, and the spot has strong brightness and small radius to form a bright point), and the imaging lens in the receiving imaging system images the spot on the CCD; the laser beam emitted by the second laser is converged by the convergent lens and obliquely enters the seawater through the air, the seawater is reflected and refracted at the air-water interface, the refracted light is obliquely incident on the lower surface of the oil film, an elliptical laser spot is also formed at the incident point on the lower surface of the oil film (the position of the lens is adjusted, the focus is positioned near the lower surface of the oil film to be detected, the spot has very strong brightness and very small radius at the moment, a bright spot is formed), the imaging system images the spot on the CCD, and the upper illuminating point and the lower illuminating point are made to correspond to the upper surface and the lower surface of any point of the object to be detected in. And collecting light spot image points of the two systems on the CCD. The reference plane is set as a virtual reference plane, and the selection can be arbitrary theoretically;

third, image displacement measurement

The collected light spot image is processed by adopting filtering technologies such as morphological filtering and the like, the centroid of two image points on the CCD is obtained by utilizing methods such as a centroid method and Hough transformation, and the image displacement h of the upper and lower light spot image points relative to the reference plane light spot image point can be obtained by utilizing a distance formula between the two points₁And

fourthly, establishing a mathematical model for calculating out-of-plane displacement

According to the corresponding displacement h of the light spot on the CCD obtained in the previous step₁And h₂Establishing a mathematical model for calculating the out-of-plane displacement, and calculating by a data processing module to obtain the out-of-plane displacement d of the reference plane assumed by the distance between the upper surface and the lower surface of the point₁And d₂(displacement square)To the direction shown, the upward arrow is positive and the downward is negative). Wherein d is adopted for the first optical path system₁＝k₁h₁Formula (I) is calculated, wherein k₁Cos α/f (u-f). For the second optical path system

In the formula

u and a are object distances of two imaging systems, alpha is an included angle between the optical axis of the laser beam and the normal line of the measured surface, and theta₁Is the incident angle of the laser beam incident on the air-sea interface, n₀Is the refractive index of air, n₁Is the refractive index of seawater, and f is the focal length of the imaging lens;

fifth, data processing and output

For two out-of-plane displacements d₁And d₂Subtracting the oil film thickness d ═ d of the point₁-d₂。

The measurement principle of the method of the invention is as follows:

fig. 2 is a basic schematic diagram. In the oblique type shown in fig. 2(a), the laser beam is incident at the measurement point a at an angle α and forms an elliptical spot at that point. On the other side of the normal line at A, the optical axis of the imaging lens forms an angle beta with the normal line of the imaging lens, and the camera captures an image of the light spot and images the light spot at a on the CCD. When the incident light spot moves from the point A to the point M along the laser beam, the camera captures the laser light spot irradiated on the measured object surface M by the laser without any change and images the laser light spot on the M position of the CCD. The optical axis of the laser beam and the CCD are vertical to the optical axis of the imaging lens. According to the image shift h and the structural parameter k of the imaging system, the displacement of the measured object surface can be determined.

If the displacement of the light spot on the imaging plane is h₁The displacement of the measured object surface is

Where u is the object distance of the imaging system and f is the focal length of the imaging lens. In the above formula, only h is measured₁Then d is obtained₁。

For FIG. 2(b), the laser beam from the laser is at an angle of incidence θ₁The air obliquely enters the point A on the air-sea water interface, and the reflection and the refraction occur at the air-water interface. The camera takes an image of the reflected light spot and images it at point D on the CCD. The refracted light is obliquely incident on the lower surface of the oil film, an elliptical laser spot is also formed at an incident point B on the lower surface of the oil film, and the camera captures an image of the spot and images the image at a point C on the CCD. If the displacement of the point C and the point D on the imaging surface is

The included angle between the CCD and the optical axis of the imaging lens is 90 degrees, then

In the formula n₀Is the refractive index of air, n₁Is the refractive index of seawater, a is the object distance of the imaging system, and f is the focal length of the imaging lens.

When the incident light spot moves from the point A to the point M, the imaging point correspondingly moves on the CCD. From the analysis of FIG. 2(a), the displacement of the object surface is shown

Wherein

The image motion is the size of the image motion.

The displacement of the lower surface of the oil film relative to point M is then

In the formula (2), only need to measureAnd

then d can be obtained₂

The invention has the advantages and beneficial effects that:

the invention provides a non-contact measuring method, which has the characteristics of simple principle, convenient measurement, low cost, small volume, capability of eliminating errors introduced by an imaging system, liquid level change, oil film surface gloss and color and external environment, capability of effectively overcoming the influence of background light on measurement precision and the like, and can be used for dynamic online measurement of the thickness of an oil film on the surface of an aqueous solution.

The measuring method can quickly, accurately and quantitatively realize dynamic on-line automatic measurement of the thickness of the oil film of the spilled oil on the sea surface, provides a reliable measuring tool for ocean spilled oil detection, promotes the development of the existing spilled oil monitoring level in China, and provides important basis and technical support for spilled oil treatment.

[ description of drawings ]:

FIG. 1 is a schematic diagram of a symmetrical oblique-type dual-laser triangulation distance-measuring optical path system according to the present invention;

FIG. 2 is a schematic diagram of laser triangulation measurement according to the present invention, in which FIG. 2(a) is a first path of oblique single optical path system, and FIG. 2(b) is a second path of oblique single optical path system;

fig. 3 is an experimental light path diagram of a symmetrical oblique-incidence type double-laser triangulation measurement system.

Fig. 4 is an example of a light spot image obtained by a symmetric oblique-type dual-laser triangulation measurement system, fig. 4(1) is an image acquired by a first path system, and fig. 4(2) is an image acquired by a second path system.

Wherein, 1 is a laser, 2 is a convergent lens, 3 is an imaging lens, 4 is a CCD, 5 is a virtual reference plane, 6 is a data acquisition and processing module, 7 is a sample cell, 8 is a spectroscope, and 18, 19, 28 and 29 are all reflectors.

[ embodiments ] of the present invention:

example 1

The specific measurement process of the measurement method of the present invention is described below by taking the experimental optical path shown in fig. 3 as an example;

optical path system construction

A symmetrical oblique-incidence type double-laser triangular distance measuring system shown in figure 3 is built, and a light source part of the system comprises a laser 1, a converging lens 2, a spectroscope 3 and reflecting

mirrors

18, 19, 28 and 29. The laser 1 is a He — Ne laser having a wavelength λ of 632.8nm, and the focusing lens 2 is a focusing lens having a focal length f of 300 mm. The receiving (imaging) system is composed of an imaging lens 3, a detector CCD 4 and a data acquisition and processing module 6. The imaging lens 3 is an imaging lens with the f being 250mm, the detector CCD 4 is an area array CCD, and the data acquisition and processing module 6 comprises an image acquisition card and a computer. The sample cell 7 is a transparent glass cell without a cover, water is filled in the cell, and oil floats on the water surface. In the optical path system, the laser beam, the optical axis of the imaging lens and the CCD are ensured to be in the same plane, the optical axis of the laser beam is strictly ensured to be vertical to the optical axis of the imaging lens, and the CCD is strictly ensured to be vertical to the optical axis of the imaging lens. Adjusting the light path to enable the first path of light beam and the second path of light beam to be intersected in the air, and enabling the upper and lower illuminating points to correspond to the upper and lower surfaces of any point of the object to be measured in the vertical direction;

② light spot image acquisition

Laser beams emitted by the He-Ne laser 1 are reflected by

reflectors

18, 19, 28 and 29 after passing through a spectroscope 3, and are converged at a point S on the upper surface of an oil film to be measured by air obliquely entering through a first path of converging lens 2, so that scattering and refraction are generated at an air-oil interface. And adjusting the position of the converging lens 2 to enable the focal point to be positioned near the surface of the oil film to be detected, so that the light spot has strong brightness and a small radius to form a bright spot. The scattered light at point S forms an elliptical laser spot which is imaged by imaging lens 3 onto CCD 4. The reflected light is converged by the converging lens 2 of the second path, enters water through the glass (the bottom surface of the container) by air obliquely incident, is reflected and refracted at an interface point A of the air and the glass, is reflected and refracted at an interface point C of the glass and the water, reflected light at the point A, C forms an elliptical laser spot respectively, and is imaged on the CCD 4 by the imaging lens 3. The refracted light obliquely enters a point B on the lower surface of the oil film and is scattered at an oil-water interface. And adjusting the position of the converging lens 2 to enable the focus to be positioned near the lower surface of the oil film to be detected, forming an elliptical laser spot at an incident point B on the lower surface of the oil film, and imaging the spot on the CCD by an imaging system. Light spot image points of the two-path system on the CCD are collected, as shown in FIG. 4. The oil used in the experiment is petroleum, the image collected by the first path is shown in figure 4(1), and the image collected by the second path is shown in figure 4 (2). In the figure, a shows an image point corresponding to an air glass interface, b shows an image point corresponding to an oil-water interface, C shows an image point corresponding to a glass-water interface (not shown in fig. 1 and 2, if the refractive index of the selected sample cell material is close to that of water/seawater, the point a coincides with the point C), s shows an image point corresponding to an oil-gas interface, and a reference plane is selected as an M point in an oil layer, is a virtual reference plane, and is obtained by calibrating an experimental system. Meanwhile, in the measuring process, the relative spatial positions of the two distance measuring systems are fixed to be unchanged;

③ measuring image displacement

And carrying out image processing on the acquired light spot image by adopting filtering technologies such as morphological filtering and the like, and obtaining the centroid coordinate of the image point on the CCD by utilizing a centroid method. Fig. 4(1) acquires coordinates of the centroid of the light spot at the point S as (1896.9, 1186.2), and coordinates of the centroid of the light spot at the previously calibrated reference plane point M as (1581.1, 1127.6). The coordinates of the centroid of the spot at point B in fig. 4(2) (941.1, 1302.1), at point a (1669.3, 1352.1), at point C (1511.1, 1321.2), and at point M (205.0, 1159.3) are collected. The image displacement quantities h of the upper and lower light spot image points relative to the reference point can be obtained by utilizing a distance formula between the two points₁2.3768mm and

data processing

According to the corresponding displacement h of the light spot on the CCD obtained in the previous step, the data processing module calculates by using the formula d ═ kh, and the displacement d of the upper surface and the lower surface of the oil film relative to the point M can be obtained₁And d₂In the formula d₁＝(u-f)h₁ cosα/f＝k₁h₁，

Wherein,

the object distance a, u, 508.38mm, a, 44.24 °, θ of the imaging system is obtained by calibration₁44.45 degrees, f 250mm, air refractive index n₀1, refractive index of water n₁1.33, glass refractive index n₃Calculated as 1.5, d in fig. 4₁＝1.7599mm，d₂1.4963mm, and 3.26mm oil film thickness d.

Claims

1. A sea surface oil spill oil film thickness measuring method based on a differential laser triangulation method is characterized by comprising the following steps:

the method comprises the following steps that firstly, symmetrical oblique-type double-laser triangular distance measuring systems are selected, each laser triangular distance measuring system comprises a light source part consisting of a laser and a converging lens, and a receiving imaging system consisting of an imaging lens, a detector CCD or a position detector PSD (position sensitive detector) and a data acquisition and processing module;

secondly, designing an oil film thickness measuring optical path system, enabling a laser beam emitted by a first laser to be converged and obliquely incident on the upper surface of the oil film to be measured through a converging lens, forming an elliptical laser spot at an incident point on the upper surface of the oil film, and imaging the spot on a CCD (charge coupled device) through a receiving imaging system; the laser beam emitted by the second laser is converged by the convergent lens and obliquely enters the seawater through the air, reflection/scattering and refraction occur at an air-water interface, the refracted light obliquely enters the lower surface of the oil film, an elliptical laser spot is also formed at an incident point on the lower surface of the oil film, the imaging system images the spot on the CCD, and the upper incident point and the lower incident point correspond to the upper surface and the lower surface of any point of the oil film to be measured in the vertical direction; the reference plane is set as a virtual reference plane, and the selection of the reference plane can be arbitrary theoretically;

thirdly, determining the image displacement h of the upper and lower two light spot image points relative to the reference plane light spot image point according to the light spot image points of the two systems on the CCD₁And

fourthly, according to the corresponding displacement h of the light spot on the CCD obtained in the previous step₁And h₂The data acquisition processing module calculates to obtain the out-of-plane displacement d of the assumed reference plane of the distance between the upper surface and the lower surface of the point₁And d₂(ii) a Wherein for the first optical path system d₁＝k₁h₁In the formula, k₁The angle between the optical axis of the laser beam and the normal line of the measured surface is (u-f) cos alpha/f, alpha is the included angle, u is the object distance of the imaging system, and f is the focal length of the imaging lens; for the second optical path system

In the formula

a is the object distance of the imaging system, θ₁Is the incident angle of the laser beam incident on the air-sea interface, n₀Is the refractive index of air, n₁Is the refractive index of seawater;

fifthly, two out-of-plane displacements d are obtained according to the steps₁And d₂Subtracting the oil film thickness d ═ d of the point₁-d₂。

2. The measuring method according to claim 1, wherein for the first optical path system, an included angle between an optical axis of the laser beam and an optical axis of the imaging lens is 90 degrees, and the CCD is perpendicular to the optical axis of the imaging lens; for the second optical path system, the optical axis of the laser beam is vertical to the optical axis of the imaging lens, and the angle formed by the CCD and the imaging optical axis is 90 degrees.