Abstract:  Objective Due to the development of inkjet printing technology, in most cases, discernible traces are hardly found on printed documents nowadays. To solve this problem from the perspective of forensic document examination, this study used two-dimensional fast Fourier transform (2D FFT) to convert the ink dot images of inkjet-printed documents into spectral diagrams, and then to analyze the halftone ink dot spectral characteristics of these images as a basis for identifying the types of inkjet printed documents. Methods Through analysis of the patterns of change in halftone ink dot spectral characteristics of documents printed from the same color inkjet printer under continuous printing and various influencing conditions, this study evaluated the stability of spectral characteristics, identified influencing factors, and demonstrated their forensic value. Results The halftone ink dot spectral characteristics from one same color inkjet printer exhibit relative stability across different printing times. Changing the image resolution, ink cartridges, the method of opening word processing software, PDF conversion modes, as well as the brand, material, and weight of printing paper did not significantly affect the halftone ink dot spectral characteristics. However, the halftone ink dot spectral characteristics of different printed contents showed significant differences. Therefore, halftone ink dot spectral characteristics possess high practical value in the examination of inkjet printed documents. Conclusion The application of halftone ink dot spectral characteristics in the examination of inkjet printed documents is feasible and provides a new approach to feature-based examination for the authentication of color inkjet printed documents to a certain extent.

Key words: two-dimensional fast Fourier transform (2D FFT), inkjet printed document, document examination, halftone ink dot

摘要： 目的 针对当前喷墨打印技术迭代致使文件检验工作出现“无痕可鉴”的情况，利用二维快速傅里叶变换将喷墨打印文件墨点图像转换为频谱图，分析半色调墨点频谱特征作为喷墨打印文件种类识别依据的可行性。方法 通过对同台彩色喷墨打印机连续打印以及不同影响因素条件下的打印文件半色调墨点频谱特征变化规律进行实验分析，评估频谱特征的稳定性以及影响因素，并揭示其鉴定价值。结果 同台彩色喷墨打印机不同打印时间的半色调墨点频谱特征具有相对稳定性，改变图像分辨率、墨盒、文字处理软件打开方式及PDF转换模式、打印纸张的品牌、材质、克重均未对半色调墨点频谱特征产生明显影响，但不同打印内容的半色调墨点频谱特征存在明显差异。因此，半色调墨点频谱特征在喷墨文件检验中具有较高的应用价值。结论 半色调墨点频谱特征运用于喷墨打印文件检验工作具有可行性，在一定程度上为彩色喷墨打印文件鉴定提供了新的特征检验思路。

关键词: 二维快速傅里叶变换, 喷墨打印文件, 文件检验, 半色调墨点