Method for displaying latent fingerprints based on nanosecond laser preparation and oxide film transfer

Abstract

The invention relates to a method for displaying latent fingerprints based on nanosecond laser preparation and oxide film transfer. Belongs to the field of latent fingerprint development. The method comprises the following steps that a transparent material is attached to metal, nanosecond laser penetrates through the transparent material and irradiates the surface of the metal through multi-line scanning, the metal is subjected to phase change, generated metal oxide molten particles are attached to the surface of the transparent material, and a compact metal oxide layer is formed. The oxide layer is arranged above the latent fingerprint, and after laser multi-line scanning, the oxide film and the transparent material interface form a high-voltage area. Through heat conduction, the oxide attached to the latent fingerprint is heated to be subjected to phase change, is driven by high pressure to be transferred to the surface of the fingerprint, is in contact with a latent fingerprint sample at a relatively low temperature and then deposits to push the molten mass to the surface of the latent fingerprint, and the molten mass deposits to form a film. But as skin secretions are attached to the ridge line, the deposition capacity of a molten mass is weak, the contrast ratio of the skin secretions and the molten mass is enhanced, and fingerprints appear. The method has the advantages of simplicity in operation, no pollution, low cost and the like. The method has potential application prospects in the criminal investigation field.

Description

Technical Field

[0001] This invention relates to the field of latent fingerprint development technology, and in particular to a method for preparing and transferring oxide films to develop latent fingerprints based on nanosecond lasers, which is expected to be practically applied in criminal investigation. Background Technology

[0002] Fingerprints are the unique patterns formed by the raised and recessed textures of the skin on the fingertips. When a finger touches an object, the pores on the skin continuously secrete sweat, which mixes with the oil secreted by the sebaceous glands to form secretions. These secretions adhere to the surface of the object, forming fingerprint patterns that are difficult to observe directly with the naked eye—latent fingerprints. Human fingerprints possess highly specific and immutable characteristics. Due to these biological properties, latent fingerprints are hailed as the "king of evidence," and their identification technology plays an irreplaceable and crucial role in solving cases. Currently, there are many methods for visualizing latent fingerprints, each with its own limitations. For example, in the 2023 issue of the *Journal of Electroanalytical Chemistry*, Volume 941, 117526 (Electrochemical deposition of copper films to develop the latent sebaceous fingerprints on metal substrates), Yao et al. deposited copper films on metal substrates with latent fingerprints using an electrochemical deposition method. Because the sebum adhering to the fingerprint area isolates the metal surface from the electrolyte, the deposition ability of copper in the fingerprint area is weakened, resulting in a greater contrast between the fingerprint and the creases between the fingers. However, this method is only applicable to conductive materials such as metals, limiting its application range. For example, in 2024, in ACS Applied Optical Materials, Volume 2, pp. 433-444 (Investigation of Latent Fingerprint Detection and Cheiloscopy Development Using Li2SrSiO4:Tb),... 3+ Phosphor for Forensic-Based Applications proposes a configuration for Li2SrSiO4:Tb 3+Fluorescent agents are used to make the fingerprint outline fluoresce and become visible. However, this method has the following limitations: first, the developing solution used contains toxic components, which may harm the health of operators; second, it relies on a chemical reaction process, resulting in a long developing time, affecting the timeliness of criminal investigations; and third, some of these methods require repeated rubbing of the fingerprint surface with tools such as brushes during the operation, which can damage the fingerprint pattern. Therefore, the continuous development of fingerprint development technologies applicable to diverse scenarios has significant research value and practical significance. This study proposes a new method for developing latent fingerprints based on nanosecond laser preparation and transfer of oxide films: a metal oxide film is pre-deposited on a glass substrate using a nanosecond laser, and then the same laser device is used for precise energy control to drive the oxide film to be directionally transferred to the latent fingerprint surface. By utilizing the difference in deposition characteristics of the metal oxide film between fingerprint ridges (areas with attached secretions) and valleys (areas without secretions), high-contrast development of latent fingerprints is achieved. This method is not only simple to operate and highly efficient, but also has low cost, non-toxic and pollution-free environmentally friendly characteristics. It is also applicable to latent fingerprint detection on various substrate materials, making it widely applicable. In summary, this technology provides a novel solution for the rapid and efficient development of latent fingerprints in the field of criminal investigation, and has high practical application value. Summary of the Invention

[0003] The purpose of this invention is to provide a method for developing latent fingerprints based on nanosecond laser-induced oxide film preparation and transfer, achieving clear, efficient, and pollution-free development of latent fingerprints. This invention leverages the characteristic that the deposition behavior of molten metal oxides can be altered in areas where skin secretions are present. An oxide film is prepared using a nanosecond laser, and the spatial distribution of this oxide film on the sample surface is controlled after laser-induced transfer. By adjusting the laser parameters, latent fingerprints on the sample surface are rapidly developed. This method provides a novel technical approach for latent fingerprint detection in the field of criminal investigation.

[0004] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0005] A method for preparing and transferring latent fingerprints using nanosecond lasers, characterized by comprising the following steps:

[0006] (1) Obtaining latent fingerprints: latent fingerprints are formed on the surface of a carrier by applying slight pressure to the sample surface with a finger;

[0007] (2) Preparation of the metal oxide film: A transparent material is tightly bonded to the surface of a metal substrate. Laser parameters are controlled by a computer program, and a multi-line scanning method is used to allow the laser beam to penetrate the transparent material and focus on the underlying metal substrate. Upon absorbing the laser energy, the metal undergoes instantaneous high-temperature melting. Due to the tight bond between the transparent material and the metal, the molten metal material firmly bonds to the glass surface, forming a thin metal oxide layer. The laser irradiation parameters include: an average laser power of 36.4 W, a laser scanning speed of 1000 mm / s, a distance of 0.05 mm between adjacent laser scanning lines, a laser wavelength of 1064 nm, a laser pulse width of 250 ns, a laser repetition frequency of 100 kHz, and a Gaussian distribution of laser energy.

[0008] (3) Laser-induced transfer process: A transparent material with a metal oxide layer is covered on the latent fingerprint sample, so that the fingerprint is in direct contact with the oxide layer. Under the same nanosecond laser equipment and specific parameter settings used in step (2), multi-line scanning processing is performed on the sample surface: The laser beam penetrates the upper transparent material and focuses on the contact interface between the oxide film and the transparent material. After absorbing laser energy, the region undergoes a rapid phase change, forming a high temperature and high pressure environment. Through heat conduction, the contact surface between the oxide film and the latent fingerprint is also heated and phase-changed, and under the drive of high pressure, it is transferred to the fingerprint surface. After contacting the relatively low temperature latent fingerprint sample, it is deposited again. Since the ridge area contains skin secretions, its material deposition ability is lower than that of the valley area, thus forming a significant contrast difference, and finally realizing the appearance of the latent fingerprint. The laser irradiation parameters include: the average laser power is 5.3W, the laser scanning speed is 1000mm / s, the distance between adjacent laser scanning lines is 0.04mm, the laser wavelength is 1064nm, the laser pulse width is 250ns, the laser repetition frequency is 100kHz, and the laser energy is Gaussian distributed.

[0009] Furthermore, the sample mentioned in step (1) is a material such as glass, metal, ceramic, or wood.

[0010] Furthermore, the latent fingerprint described in step (1) is mainly composed of sweat and oil, which inhibit the deposition of molten material on its surface.

[0011] Furthermore, the transparent material mentioned in step (2) is a highly transparent material such as fused silica or glass.

[0012] Furthermore, the metal substrate material mentioned in step (2) is stainless steel, zinc, brass, etc.

[0013] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0014] This invention proposes a method for developing latent fingerprints based on nanosecond laser-induced oxide film fabrication and transfer, providing a novel solution for latent fingerprint visualization in the field of criminal investigation. First, a metal oxide film is deposited on the surface of a transparent material, and then transferred using nanosecond laser-induced transfer. The difference in deposition between ridges and valleys allows for visualization of the latent fingerprint. It has the following advantages:

[0015] 1. It possesses excellent environmentally friendly characteristics, with no toxic or harmful substances generated throughout the entire process, and has no negative impact on the environment.

[0016] 2. The solution is simple and efficient, requires minimal equipment, and has controllable costs. It has significant practical application value and promotion potential in the field of latent fingerprint detection in criminal investigation.

[0017] 3. There is no need to repeatedly rub the fingerprint sample with tools during the operation, which will not damage the fingerprint pattern. Attached Figure Description

[0018] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate the invention and are used to explain it, but do not constitute an undue limitation of the invention.

[0019] Figure 1 This is a schematic diagram illustrating the operation of a method for preparing and transferring latent fingerprints based on nanosecond lasers to fabricate and transfer oxide films, as provided by the present invention.

[0020] Figure 2 The X-ray diffraction (XRD) analysis results of the oxide film deposited on the surface of a glass substrate by laser irradiation according to Embodiment 1 of the present invention;

[0021] Figure 3 These are digital photographs of latent fingerprints on glass, ceramic tile, and wood surfaces before and after laser-induced transfer, as described in Embodiment 1 of the present invention.

[0022] Figure 4 The results are energy color emission spectrometry (EDS) analysis results of the latent fingerprint on the glass surface after processing, which are involved in Embodiment 1 of the present invention. Detailed Implementation

[0023] To further clarify the purpose, technical solution, and advantages of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0024] Using typical glass as the transparent material for depositing oxide films, glass, ceramic tiles, wood, etc. as carriers of the original latent fingerprints, and brass as the donor of the deposited oxide film as an example, this paper illustrates the implementation process and beneficial effects of a method for preparing and transferring oxide films to reveal latent fingerprints based on nanosecond lasers proposed in this invention.

[0025] Example 1:

[0026] (1) Obtaining latent fingerprints: latent fingerprints are formed on the surface of a carrier by applying slight pressure to the sample surface with a finger;

[0027] (2) Laser deposition of oxide film: A clean glass plate is tightly bonded to the brass surface. It is placed on the sample stage, and parameters are set in the computer program, using a laser beam for multi-line scanning. The laser irradiation parameters include: average light power of 36.4W, laser scanning speed of 1000mm / s, distance between adjacent laser scanning lines of 0.05mm, laser wavelength of 1064nm, laser pulse width of 250ns, laser repetition frequency of 100kHz, and laser energy distribution in a Gaussian manner.

[0028] (3) Laser-induced transfer process: A glass with a metal oxide layer is placed on the latent fingerprint sample, allowing the fingerprint to directly contact the oxide layer. Multi-line scanning is performed under specific laser parameters: the average laser power is 5.3W, the laser scanning speed is 1000mm / s, the distance between adjacent laser scanning lines is 0.04mm, the laser wavelength is 1064nm, the laser pulse width is 250ns, the laser repetition frequency is 100kHz, and the laser energy exhibits a Gaussian distribution.

[0029] See Figure 2 The figure shows the X-ray diffraction (XRD) analysis results of the oxide film deposited on the surface of the glass substrate by laser irradiation in this embodiment. The substance formed on the glass surface is Cu2O.

[0030] See Figure 3 The image shown is a digital photograph of latent fingerprints on surfaces such as glass, ceramic tiles, and wood before and after laser-induced transfer, according to this embodiment. It can be observed that after laser processing, the fingerprint outline is clearly discernible, and its display effect meets the practical application requirements for latent fingerprint detection in the field of criminal investigation.

[0031] See Figure 4 As shown, the energy colorimetric (EDS) analysis results of the latent fingerprints on the glass surface after processing show that copper is enriched in the fingerprint groove region, proving that the ridges, which are rich in skin secretions, show less accumulation of metal oxides compared to the grooves, thus amplifying the optical contrast between the ridges and the grooves.

[0032] The experimental results show that the method proposed in this invention can quickly and conveniently reveal latent fingerprints clearly, and has a wide range of applicability.

[0033] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made to the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing and transferring latent fingerprints using an oxide film based on nanosecond laser, characterized in that, Includes the following steps: (1) Method for collecting latent fingerprints: Press your finger lightly on the sample surface to allow skin secretions to adhere to the surface and form a latent fingerprint sample; (2) Preparation of metal oxide film: A transparent material is placed on a metal substrate, with the lower surface of the transparent material adhering to the upper surface of the metal. After the laser irradiation parameters are preset by a computer program, a laser beam is used to perform multi-line scanning on the metal substrate. Due to the high light transmittance of the transparent material, the laser can penetrate the transparent material and focus on the metal substrate. After absorbing the laser energy, the metal substrate rapidly heats up and undergoes a phase transition. The molten metal material, due to its close contact with the lower surface of the transparent material, will adhere to the surface of the transparent material and deposit to form a metal oxide layer. The laser irradiation parameters are: laser wavelength of 1064nm, pulse width of 250ns, repetition frequency of 100kHz, average power of 36.4W, scanning speed of 1000mm / s, distance between adjacent laser scanning lines of 0.05mm, and laser energy with a Gaussian distribution. (3) Laser-induced transfer processing: A transparent material with a metal oxide layer is applied to the latent fingerprint sample so that the fingerprint is in direct contact with the oxide layer. Under the same nanosecond laser equipment and specific parameter settings used in step (2), multi-line scanning processing is performed on the sample surface: The laser beam penetrates the upper transparent material and focuses on the interface between the oxide film and the transparent material. After absorbing laser energy, the region undergoes a rapid phase change, forming a high-temperature and high-pressure environment. Through heat conduction, the interface between the oxide film and the latent fingerprint is also subjected to a thermal phase change and transferred to the fingerprint surface under the pressure. After contacting the relatively low-temperature latent fingerprint sample, it is deposited again. Since the ridge area contains skin secretions, its material deposition ability is lower than that of the valley area, thus forming a significant contrast difference, and finally realizing the appearance of the latent fingerprint. The laser irradiation parameters include: the average laser power is 5.3W, the laser scanning speed is 1000mm / s, the distance between adjacent laser scanning lines is 0.04mm, the laser wavelength is 1064nm, the laser pulse width is 250ns, the laser repetition frequency is 100kHz, and the laser energy is Gaussian distributed.

2. The method for preparing and transferring latent fingerprints based on nanosecond laser based on oxide film according to claim 1, characterized in that: The sample mentioned in step (1) is glass, metal, ceramic or wood.

3. The method for preparing and transferring latent fingerprints based on nanosecond laser based on oxide film according to claim 1, characterized in that: The main components of the skin secretions mentioned in step (3) are sweat and oil.

4. The method for preparing and transferring latent fingerprints based on nanosecond laser based on oxide film, as described in claim 1, is characterized in that: The transparent material mentioned in step (2) is any one of glass or fused silica that has high light transmittance.

5. The method for preparing and transferring latent fingerprints based on nanosecond laser based on oxide film, as described in claim 1, is characterized in that: The metal material mentioned in step (2) is stainless steel, zinc or brass.

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