Forensic Camera Identification: Effects of Off-Nominal Exposures

TL;DR

This study investigates how off-nominal exposures, such as under- or over-exposed images, affect the error rates of PRNU-based camera identification, emphasizing the need for reliable forensic evidence standards.

Contribution

It provides a new dataset and analysis of the impact of exposure variations on PRNU error rates, highlighting challenges in forensic camera identification accuracy.

Findings

Under-exposed images have a false-positive rate of about 0.54%.

Over-exposed images reduce true-positive rate to 82.90%.

Nominal images achieve 100% true-positive and 99.92% true-negative rates.

Abstract

Photo response non-uniformity (PRNU) is a technology that can match a digital photograph to the camera that took it. Due to its use in forensic investigations and use by forensic experts in court, it is important that error rates for this technology are reliable for a wide range of evidence image types. In particular, images with off-nominal exposures are not uncommon. This paper presents a preliminary investigation of the impact that images with different exposure types - too dark or too light - have on error rates for PRNU source camera identification. We construct a new dataset comprised of 8400 carefully collected images ranging from under-exposed (too dark) to nominally exposed to over-exposed (too bright). We first establish baseline error rates using only nominally exposed images, resulting in a true-positive rate of 100% and a true-negative rate of 99.92%. When off-nominal images are tested, we find striking results: the true-negative rate for under-exposed images is 99.46% (a false-positive rate of roughly one in two hundred, typically unacceptable in a forensic context), and for over-exposed images the true-positive rate falls to 82.90%. Our results highlight the importance of continued study of error rates for the PRNU source camera identification to assure adherence to the high standards set for admissibility of forensic evidence in court.