Abstract

Blood is the key evidence for forensic investigation because it carries critical information to help reconstruct the crime scene, confirm or exclude a suspect, and analyze the timing of a crime. Conventional bloodstain detection uses chemical methods. Those methods require cautious sample preparations. They are destructive to samples in principle. Some of them are carcinogenic to investigators. They require experienced investigators and constrained conditions. Spectral imaging methods are an emerging technique for bloodstain detection in forensic science. It provides a non-destructive, non-contact, non-toxic, and real-time methodology for presumptive bloodstain searching, either in the field or in the laboratory.

This thesis prototyped two crime scene bloodstain detection imaging systems. The first generation crime scene imaging system is a LCTF based visible hyperspectral imaging system. Detection results of a simulated indoor crime scene show that bloodstains can be highlighted. However, this system has some drawbacks. First of all, it only records spectral information at 400 nm to 700 nm spectral range. Bloodstains on some substrates may not be detected. Second, it has a low SNR. This is mainly due to the low transmittance of the LCTF. Third, its FOV is only $pm$7 degrees from normal. Fourth, it cannot work continuously for a long time. The lighting module is attached next to the camera, which emits excessive heat and warms the camera quickly. Fifth, it is not calibrated into physical units such as radiance or reflectance. Therefore, a second generation crime scene imaging system was developed.

The second generation crime scene imaging system is a VNIR multispectral imaging system. It uses interference filters to construct the spectral bands. Blood reflectance spectral features were extracted from the comparison of blood and visually similar non-blood substances on various common found substrates. Three spectral features were used to construct the VNIR spectral bands of the multispectral imaging system. A linear regression pixel-wise model was used to enhance the spatial uniformity of the CMOS sensor. The lens falloff was corrected. The transmittance spectra of interference filters with various incident angles were calibrated. The system was calibrated to reflectance with the required 10\% accuracy from first principle based modeling.

Two verification tests were carried using the MSI system. The first test is a systematic test where 9 substrates were laid radially symmetric to study the spectral shift effect introduced by the interference filters. Comparing with the reflectance error, the spectral shift is found to be not as influential for bloodstain detection using RBD, RX, and TAD methods. The result from the first principle based modeling agrees with the test. The second test is a semi-realistic indoor crime scene test where different sizes of blood spatters were directly applied to off-white carpet, dark carpet, door, and painted wall under various daytime environmental conditions. The image processing workflow includes noise reduction, bloodstain detection, and data fusion. Bloodstains were detected in a large FoV in the semi-realistic crime scene on most substrates, except for a small spatter on dark color thick carpet. SNR is the major factor impacting the detectability, the 2nd major factor is the extra visible lighting which lacks of near infrared information.

The contribution of this work are three aspects: the first is the developing and prototyping an interference filter based multispectral imaging system used at a large FoV for forensic bloodstain detection. The second is the discussion of the spectral shift impact compared with the reflectance calibration error suggests that spectral shift is not as influential as reflectance calibration error. Third, the discussion of the impact of environmental conditions towards bloodstain detection on this MSI system.

Abstract (Chinese)

血是一项关键法庭证据,因为它携带的关键信息可用于重建犯罪现场,确定或排除嫌疑人,或者分析犯罪发生的时间。传统的血迹检测运用化学方法。这种方法在准备样品试剂时需要很小心,它在原则上会破坏样本。其中的一些化学方法还对检查员的身体有害。它需要有经验的证据检查员和受限制的条件。运用光谱成像方法检测血迹在司法科学领域是一项新技术。它非破坏、不接触、无毒、和实时的特点可用于实验室或现场的初步血迹寻找。

这篇毕业论文介绍了两个犯罪场景血迹检测成像系统的样机。第一代系统是一个基于液晶可调谐滤波(LCTF)的可见光高光谱成像系统。对室内仿真犯罪场景的检测结果表明血迹是可以被凸显出来的。然而,这个系统存在几个缺点。首先,它只记录了400纳米到700纳米的光谱信息。某些基质上的血迹可能不能被检测到。第二,它的信噪比很低,主要原因是LCTF的透过率低。第三,它的视场角只有正负7度。第四,它无法长时间持续工作。光源模块与感光器件接近,它辐射出的热量能很快把相机加热至高温。第五,它没有被校准成诸如辐射度或反射率的物理量。因此才有了第二代光谱成像系统。

第二代犯罪场景成像系统是一个近红外多光谱成像系统。它运用干涉滤光片来构成光谱波段。通过比较几种常见基质的血迹和看上去相似的非血迹物质的光谱可提取出血的反射光谱特征。这个近红外多光谱成像系统采用了三个光谱特征波段。本研究采用一个线性回归逐像素模型来提高CMOS传感器的空间均匀性,在一定程度上矫正了镜头晕影,并且标定了干涉滤波片的透过率谱随着不同入射角的变化。基于第一原理的模型给出了这个系统对于反射率标定精度的要求是10%,该系统也被标定成反射率单位。

利用这个多光谱系统进行了两次验证性实验。第一次实验是个系统性检测。实验时,为研究干涉滤光片的光谱位移现象,9种基质呈对称辐射状分布于场景中。与反射率标定误差相比,光谱位移在利用RBD, RX, 和TAD的方法来检测血迹的实验中并没有影响太大。基于第一原理的模型与实验结果相吻合。第二项检测是一项半真实性室内犯罪场景实验,在不同的白天光照条件下,在近乎白色的地毯、暗色地毯、门上、和漆墙上都施用了不同大小的血迹。图像处理过程包括去噪,血迹检测,和数据融合。结果,除了在暗色厚地毯上的一个小血迹没有检测出来,这个半真实性实验在相对很大的视场和多数基质上都检测到了血迹。分析认为信噪比是影响检测效果的主要原因,其次是缺少近红外信息的光照条件。

这项研究的贡献主要有三方面: 第一是制成了一部基于干涉滤光片的多光谱近红外血迹检测原理样机,它可用于大视场角的司法血迹检测。第二是关于光谱位移的影响的讨论,与反射率校准误差相比,它的影响没有想象中的大。第三,本次研究讨论了实际血迹检测时环境条件的影响。

Publication Date

7-17-2019

Document Type

Dissertation

Student Type

Graduate

Degree Name

Imaging Science (Ph.D.)

Department, Program, or Center

Chester F. Carlson Center for Imaging Science (COS)

Advisor

David Messinger

Advisor/Committee Member

John McClusky

Advisor/Committee Member

Roger Dube

Campus

RIT – Main Campus

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