Research on the Recovery Technology of Damaged Characters Based on the Magneto-optical Method

This work intends to address the issue of surface damage to significant characters on ferromagnetic materials for the assessment of physical evidence. For the identification of altered metallic characteristics, a nondestructive magneto-optical imaging recovery approach based on the magneto-optical effect is presented. The results of the experiments demonstrate that the technique may successfully repair the damaged metallic features. It can be used as a nondestructive test for the examination and restoration of altered and damaged metal handwriting in criminal technology, which offers a fresh approach and method to deal with this issue.


Introduction
This work intends to address the issue of surface damage to significant characters on ferromagnetic materials for the assessment of physical evidence.For the identification of altered metallic characteristics, a nondestructive magneto-optical imaging recovery approach [1][2] based on the magneto-optical effect is presented.The results of the experiments demonstrate that the technique may successfully repair the damaged metallic features.It can be used as a nondestructive test for the examination and restoration of altered and damaged metal handwriting in criminal technology, which offers a fresh approach and method to deal with this issue.In commonly used methods, the chemical corrosion method recovery impact is influenced by the acid concentration, which is difficult to control and may harm the object being tested.Additionally, the original number can be reproduced by using the electrolytic corrosion method, but the test object's surface will be damaged, and the operation is laborious and slow.Resolution criteria for optical magnetic particle inspection [3] and magnetic induction methods of recovery are not reached due to poor performance and low resolution [4] .
Therefore, this study suggests a new nondestructive inspection method based on the Faraday magneto-optical effect: the magneto-optical imaging detection method for the recovery identification of damaged characters [5] .This method aims to increase the resolution, effectiveness, and nondestructiveness of the recovery technique.With the use of magneto-optical imaging technology, it enables nondestructive inspection and recovery of significant surface ferromagnetic material characteristics including scratches and embossing.The advantages of the magneto-optical inspection approach over the traditional method include its ease of use, effectiveness, and nondestructive inspection.Through experimental confirmation, we discovered that the magneto-optical inspection approach has the qualities of high resolution and high efficiency, as well as the ability to maintain the integrity of the object being scanned, avoiding the damage that may arise from conventional methods.This offers a fresh perspective and approach to the issue of ferromagnetic materials' surfaces being damaged and altered.

Analysis of magnetic circuit and magnetic leakage mechanism
The distribution of magnetic fields in magnetic materials is referred to as the magnetic path.The layout of the magnetic circuit plays a significant role in the construction of the magnetic domain structure and the production of the leakage signal in magneto-optical imaging techniques.The magnetic properties of the magnetic material and the impact of the external magnetic field must be taken into account during the design of the magnetic circuit.A leakage signal is produced in the magnetic circuit as a result of the magnetic material's magnetic domain structure changing in response to the external magnetic field.The relationship between magnetic flux and magnetic field strength in a magnetic circuit satisfies the equation: where Φ denotes the magnetic flux in Weber (Wb); H denotes the magnetic field strength in amperes per meter (A/m); L denotes the magnetic circuit length in meters (m).
The leaking of magnetic signals in magneto-optical imaging technology is essential for information reading.A magnetic circuit forms inside a magnetic substance when an external magnetic field is applied to it.The magnetic circuit will be locally deformed at these discontinuities, producing magnetic poles and ultimately causing magnetic field leakage if the workpiece is no longer coherent inside, for example, if there are defects such as fractures and holes inside the workpiece [6] .The degree of variation in the magnetic domain structure is correlated with the strength of the leakage signal.In addition to the magnetic circuit, the magnetic characteristics of the magnetic material and the external magnetic field also have an impact on the development of magnetic poles.To accurately identify the defective characteristics inside the workpiece, the influence of magnetic path, magnetic material, and external magnetic field must be taken into consideration in magneto-optical imaging technology [7] .

Faraday spin effect
The Faraday rotational effect serves as the foundation for the magneto-optical imaging technique, which detects magnetic materials by measuring how the magneto-optical medium reacts to a magnetic field.As shown in Figure 1, the line polarized light will rotate when it is incident on the rotational medium, when a magnetic sensor is placed on the surface of the inspection material and a magnetic field parallel to the direction of polarized light propagation is added to the magneto-optical medium.This phenomenon is known as the Faraday magneto-optical effect or magneto-rotational effect.Adding a magnetic field B to the propagation direction of the line polarized light propagating in the medium, the angle of deflection can be expressed as: where L is the length of light propagation in the medium; B is the magnetic induction intensity;ψ is a coefficient, known as the Verdet constant, which characterizes the magneto-optical properties depending on temperature, the working wavelength of light, and the material properties of the magneto-optical medium.The characteristics of the medium and the magnetic field's direction determine a direction.

Magnetic induction strength
Linearly polarized light Deflection angle

Figure 1. Faraday spin effect
The study demonstrates that the information about the magnetic field can be described in terms of information about light intensity, and the CMOS/CCD camera receives the line-polarized light that contains the information about the test object's defects to create a magneto-optical image.After that, the computer examines the magneto-optical image processing to find any imperfections in the test object and can pinpoint their precise location, size, shape, and depth.

Experimental materials and equipment
We prepare metal strips, damaged character strips, metal lettering non-destructive inspection systems, and Nikon 7, 100 camera.
The damaged character steel bar is stamping numbers on the surface of the metal steel bar and has been polished, cut, and welded, and the individual characters are no longer recognizable by the naked eye.The specific realization effect is shown in Figure 2. The steel bar above is based on the traits of the steel bar below to simulate the conventional tampering of the car frame number, scraping putty and spray paint treatment, and re-stamping the numbers 0, 1, 2, 3, 4, 5, and 6 on the surface.

Magneto-optical imaging recovery experiments of damaged characters
The induced magnetic anisotropy of a steel plate is caused by plastic deformation, slip, and lattice twinning when an external force acting on the steel plate exceeds its yield limit.The magnetic induction of the steel plate at the deformed point differs greatly from that of other portions due to the magnetic anisotropy produced by the slip, which is the most noticeable.Even if the character's surface is destroyed during the embossing and scribing process, the lattice slip by plastic deformation still survives, demonstrating slip-induced magnetic anisotropy.The magneto-optical imaging recovery recognition of damaged characters on the surface of steel plates can be achieved when combined with the Faraday magneto-optical effect.
The magnetic strip from the metal handwriting nondestructive inspection instrument is first covered on the ready steel sheet by using the aforementioned principle, and then it is transferred to the inspection material to see if there is any magnetic leakage.The magnetic strip is then inserted into the device to read the data information.This method involves placing the sensor on the surface of the target item, using a permanent magnet to create a magnetic field, scattering light and linear polarized light to illuminate the sensor, and placing a fixed polarizer in front of the scatterer.To enable the sensor to detect the reflected light, the distance between the light source and the camera is changed.We utilize the camera to capture the image that the sensor has mapped and a rotating filter to improve it.
Finally, the brightness change of the image is used to determine if the magnetic field emission area has faded and/or been destroyed.The magneto-optical imaging recovery identification of the damaged characters on the surface of the steel plate may be realized through the analysis and comparison of the images.It should be mentioned that maintaining the consistency of the experimental setting throughout the process will help to prevent outside influences from an impact on the outcomes of the experiment.To get better imaging results, the experimental conditions must be changed for various sorts of damaged characters.

Results
The letters and numbers and their forms on the sample's surface before being artificially tampered with can be seen by reading the information on the magnetic strip of the sample by using a magneto-optical detection method.The revealed results are displayed in Figure 3.It should be noted that the iron sample is austenitic steel, which is scarcely ferromagnetic and cannot be drawn to a magnet.Martensitic steels, on the other hand, have strong ferromagnetism and can be drawn to magnets.The process of choosing and testing a material must be done in accordance with the intended qualities and attributes.The magneto-optical detecting technology may achieve high-precision imaging recovery identification of damaged characters on the surface of steel plates without using any physical touch [8] .The method has a wide variety of potential applications and can be used not only in the sphere of criminal investigation but also in the fields of insurance and quality inspection [9] .

Example of a car tampering with VIN code
The space between the digits 9 and 5 in the VIN code, as seen in Figure 4, is bigger than that for the other numbers, but it is impossible to identify the middle numbers.After being scanned by magneto-optical equipment, Figure 5 depicts the actual condition between the digits 9 and 5, demonstrating that there is still a digit 1 present between the digits 9 and 5, and demonstrating the veracity of the tampering.

Discussion
The surface of objects to be detected changes traces on the sensor producing a clear magneto-optical effect and the formation of a clear image or pattern, which helps to carry out the subsequent analysis and comparison work.Related experimental results surface from cars, guns, cast iron, and stainless steel on the fading and destruction of the word can be detected by the magneto-optical inspection method.
The magneto-optical inspection method is a practical non-destructive inspection technique with a wide range of potential applications.The magneto-optical inspection approach, meanwhile, still has some drawbacks in practical applications, including strict environmental requirements, specific demands on the physical and chemical properties of materials, and inapplicability to non-magnetic substances.Therefore, consideration should be given to sample preparation, magnetic field management, sensor selection and calibration, data processing, etc. while utilizing the magneto-optical inspection method.For instance, by rotating the sensor or adding extra magnets, the contrast of change traces can be improved, and its ideal circumstances need to be further explored.
The scope of applications for the magneto-optical inspection method will expand with ongoing technological advancements, and it will be crucial in domains such as criminal detection, cultural relics protection, and metal product quality control.However, further study and development of the technology are required to find the best excitation method for various defects to improve its detection accuracy and applicability.The focus and difficulty of the research is to think about how to construct three-dimensional information on the two-dimensional magneto-optical image.It is also necessary to consider its combination with other technologies, such as artificial intelligence [10] , to achieve better detection accuracy and applicability.

Figure 2 .
Figure 2. The ruined metal lettering steel strip

Figure 3 .
Figure 3.The information of the check material after reading the magnetic stripe

Figure 4 .
Figure 4. Photos of a car part of the VIN code by naked eye observation

Figure 5 .
Figure 5. Magneto-optical device reading results5.2.Example of an imported vehicle tampering with specific digits of the VIN codeFigure6illustrates how a vehicle's VIN code portion to the left of the ninth digit 8 font differs from other identical figures and has a light scratch, although it is impossible to identify with absolute certainty.Figure7demonstrates that the original form of the car VIN code should have been the number 3 starting from the left of the ninth digit 8, which is a clear intentional destruction.

Figure 6 .Figure 7 .
Figure 6.An imported small car part of the VIN code