TECHNICAL NOTE GENERAL; CRIMINALISTICS John Albanese, 1 Ph.D. and Ronald Montes, 1 H.B.A. Latent Evidence Detection using a Combination of Near Infrared and High Dynamic Range Photography: An Example Using Bloodstains ABSTRACT: In this paper, we use bloodstains to illustrate an approach for identifying latent evidence on dark cloth using near infrared (NIR) photography combined with high dynamic range (HDR) photography techniques. NIR photography alone has been used to capture latent evidence that cannot be seen in normal ambient light. HDR techniques combine multiple bracketed photographs of the same image to increase the dynamic range of the photograph which can provide greater contrast. Using NIR photography alone, we were able to detect a bloodstain up to a 1 ⁄ 16 dilution, an improvement over previous studies. Combining NIR photography with the HDR process resulted in a noticeable increase in visibility up to 1 ⁄ 16 dilution when compared to NIR photographs alone. At 1 ⁄ 32 dilution, we were able to detect bloodstains that were not visible using NIR alone. NIR is a useful tool for imaging latent evidence, and combining NIR with HDR consistently provides better results over NIR alone. KEYWORDS: forensic science, forensic photography, near infrared photography, latent evidence detection, high dynamic range photogra- phy, bracketing photographs The detection of latent evidence on black or dark mottled cloth can present specific challenges to crime scene investigators. In this paper, we investigate the utility of combining near infrared (NIR) photography with high dynamic range (HDR) photography tech- niques for the detection of latent evidence using bloodstains on black cloth. Digital cameras record a scene by capturing light either transmit- ted or reflected into the light capture device, normally a charged couple device (CCD) or silicone equivalent (CMOS). Camera brands such as Nikon and Fuji use CCDs while Canon uses the CMOS variant. The instant preview function and the capture of images onto memory cards have increased the ease of use of pho- tography in many fields including forensics (1,2). Tethering a digi- tal camera to a laptop computer takes the on-site preview and analysis of images to a whole new level. Traditional infrared (IR) photography used IR-sensitive film that needed to be loaded in the dark. Focus and exposure was estimated by the photographer and results could only be observed after develop- ment. Thus, the costly and inaccurate procedures made IR photogra- phy too cumbersome for practical use in many forensic contexts. With the advent of digital cameras, focusing and exposure can be compensated on-site by reviewing each image in real time on the pre- view screen or a tethered laptop computer. In most digital cameras, a ‘‘hot’’ filter or IR ⁄ UV cut filter is installed in front of the CCD or CMOS capture device to filter out any IR ⁄ UV that degrades the qual- ity of normal visible spectrum photographs. When used for IR photography, the hot filter must be removed and replaced by a clear glass cover to allow full spectrum sensitivity (1,3) (http://www.life- pixel.com/forensics/, accessed January 15, 2010). Filters are then fitted in front of the lens to block specific wavelengths of light. The NIR spectrum commonly used in forensic photography is c. in the 700–900 nm range (1). The Wratten rating system denotes the spectrum range which is blocked, and the 89B filter is the most common IR filter available which blocks light below 680 nm wavelength. Using NIR photography can provide greater contrast to visually separate latent evidence such as a bloodstain from the background material (4). Lin et al. (5) demonstrated the utility of NIR photography for detecting latent evidence, including gunshot residue, ink, and bloodstains on dark cloth. They were able to iden- tify dilutions of up to 1 ⁄ 8 for bloodstains. Previous studies have used image fusion to combine images with differing flash exposures (6,7). In contrast, HDR tone mapping con- sists of creating an 8-bit image from the 32-bit image using the HDR process. HDR photography is a proven technique that has been used extensively in landscape and architectural photography. The HDR process involves combining three or more photographs of the same scene but with differing exposure. Usually under- exposed ()1), normal, and overexposed (+1) photographs are taken. Variations in this method include taking up to five photographs or exposures ranging from )2 to +2. The photographs can be easily combined using available software such as Adobe Photoshop CS3 Ó (San Jose, CA) or Photomatix Ó (Montpellier, France). Combining the three images results in a 32-bit image that exhibits an HDR compared to each individual image. Tone mapping must then be performed to reduce the photograph back to an 8-bit image as most displays can only output the detail of an 8-bit image. 1 Department of Sociology, Anthropology, and Criminology, University of Windsor, 401 Sunset Avenue, Windsor, ON N9B 3P4, Canada. Received 4 June 2010; and in revised form 9 Sept. 2010; accepted 3 Oct. 2010. J Forensic Sci, November 2011, Vol. 56, No. 6 doi: 10.1111/j.1556-4029.2011.01850.x Available online at: onlinelibrary.wiley.com Ó 2011 American Academy of Forensic Sciences 1601