Application of fluorescence lifetime imaging (FLIM) in latent finger mark detection L.K. Seah * , P. Wang, V.M. Murukeshan, Z.X. Chao School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received 14 April 2005; received in revised form 1 August 2005; accepted 24 August 2005 Available online 22 September 2005 Abstract Fluorescence lifetime imaging (FLIM) in frequency domain enables the mapping of the spatial distribution of fluorescence lifetimes of a specimen. It has been extensively applied in biology. In this paper, a theoretical analysis for the fluorescence lifetime determination of latent finger mark samples is described, which is followed by the feasibility study of using FLIM in frequency domain for latent finger marks detection. Preliminary experiments are carried out with latent finger marks treated with a fluorescent powder on two different substrates. The resulting fluorescence lifetime image of finger mark revealed a good contrast, and was able to detect the latent finger marks clearly. # 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Fluorescence lifetime imaging; Frequency domain; Latent finger mark; Homodyne 1. Introduction Finger marks are one of the most valuable types of physical evidence and hence its detection plays a significant role in criminal investigation and forensic science. There are three types of finger marks that occur at a crime scene: visible, plastic and latent. Latent finger mark is the most difficult to be detected. The existing detection techniques for latent finger marks have their own limitations. Most con- ventional detection methods (physical and chemical) cannot detect older marks [1,2]. Laser induced fluorescence detec- tion of latent finger mark was initially explored in 1976 and involves the detection of fluorescence intensity, color and lifetime [9]. In general, conventional methods for detection of latent finger marks take the first two properties into consideration. However, the existing techniques based on fluorescence filtering fail when there is strong fluorescence emission from the background, and are ineffective when the emission wavelengths from the finger mark and that of background fall in the close wavelength range. In this context, FLIM method would be beneficial. Fluorescence lifetime is the average decay time of the fluorescence emitted by a molecule after excitation with a short laser pulse. Fluorescence lifetime imaging (FLIM) has received considerable attention and has been widely used in biophysics and medical diagnosis [3–6]. FLIM could obtain the unique and quantitative information available from dynamic fluorescence measurements, and possibly distinguish several fluorescence species with dissimilar lifetimes even though they may have overlap- ping spectra. This paper deals with determination of average fluores- cence lifetimes of latent finger mark sample on a pixel-by- pixel basis using homodyne detection method. A number of images of the sample are collected at different phase delays relative to the excitation light. A subsequent fit of fluores- cence intensity on a pixel-by-pixel basis yields the fluores- cence lifetime distribution image of the latent finger mark, which is independent of the fluorescence intensity. www.elsevier.com/locate/forsciint Forensic Science International 160 (2006) 109–114 * Corresponding author. Tel.: +65 6790 4824; fax: +65 6795 4632. E-mail address: mlkseah@ntu.edu.sg (L.K. Seah). 0379-0738/$ – see front matter # 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.forsciint.2005.08.018