With recent advances in multimedia technology, the involvement of digital images/videos in crimes has been increasing significantly. Identification of individuals in these images/videos can be challenging. For example, in cases of child sexual abuse, child pornography, and masked gunmen, the faces of criminals or victims are often hidden or covered and only some body parts (e.g., back, thigh, and arm) can be observed from the digital evidence. Although tattoos and scars can be used for identification in some cases, they are neither universal nor unique. We propose a group of skin marks named Relatively Permanent Pigmented or Vascular Skin Marks (RPPVSM) as a biometric trait for forensic identification. To support the scientific underpinnings of using RPPVSM patterns as a novel biometric trait, the individuality was studied. RPPVSM on the backs of 269 male subjects were examined. We found that RPPVSM in middle to low density patterns tend to form an independent and uniform distribution, while RPPVSM in high density patterns tend to form clusters. We present in this paper an individuality model for the independently and uniformly distributed RPPVSM patterns. When compared to the empirical results, this model fits the empirical distribution very well. Finally, the predicted error rates for verification and identification are reported.

Recent technological advances have allowed for a proliferation of digital images that may be involved in crimes. Using these images as evidence in legal cases like child pornography and masked gunmen can be challenging because usually the faces of the suspects are not visible. To perform personal identification in these images, we propose a biometric trait composed of a group of skin marks including, but not limited to, nevi, lentigines, cherry hemangiomas, and seborrheic keratoses. Due to their biological characteristics, we have grouped these as "Relatively Permanent Pigmented or Vascular Skin Marks," abbreviated as RPPVSM. As statistical study of RPPVSM is essential before investigating their discriminative power, we present in this paper the fundamental statistics of RPPVSM. Back torso images were collected from 144 Caucasian, Asian, and Latino males, and a researcher trained in dermatology manually identified their RPPVSMs. The statistical results show that Caucasians tend to have more RPPVSMs than Asians and Latinos, and over 80 percent of middle to low density RPPVSM patterns are independently and uniformly distributed.