Spattered blood intentionally hidden under layers of paint can be detected with a standard digital camera that’s been tweaked to record infrared light. The approach could become an important tool for cold-case investigators sizing up an old crime scene.
“We hope it gives law enforcement the ability to go on hunches,” says Glenn Porter, an expert in forensic photography at the University of Western Sydney in Australia. Blood is potentially powerful evidence, as it may harbor DNA that could allow a killer or victim to be identified.
Porter, formerly a forensic photographer with the Australian Federal Police, had heard of cases where investigators suspected that a crime had taken place in a now-remodeled house. So he and his colleagues decided to see if infrared photography might reveal blood hidden under paint. The researchers took a digital camera and swapped the light filters so the camera’s sensor would record only infrared light. With slightly longer wavelengths than visible light, infrared is better at penetrating layers of paint.
The researchers diluted horse blood to one-tenth its original strength and put 200-microliter drops onto plasterboard that had been painted over with primer. After allowing the blood to dry for 48 hours, they painted over the bloodstained plasterboard, testing several colors of acrylic paint and three different types of white paint.
Under two layers of black paint, the blood was completely invisible to a standard digital camera. But in photos taken with the infrared camera, the blood could be seen even under six layers of black, Porter and his colleagues report online July 30 in the Journal of Forensic Sciences. The camera also revealed blood under layers of purple, orange, blue, yellow and green. The approach revealed blood under three layers of white oil-based paint and white spray paint. Red paint, perhaps an obvious choice for a would-be killer, didn’t conceal the blood from the naked eye — even using six layers.
The technique could allow investigators to test a hunch about a crime scene with little effort and disturbance, says Porter. “You can have your suspicions and then get a hit behind the paint,” says Porter. “Then you decide if you want to start scraping paint off or take out a wall.”
The camera couldn’t see through more than three layers of white acrylic paint, probably because of the effectiveness of particles used in the pigment — often lead or titanium — at scattering light, says Dan Kushel, a specialist in art conservation and imaging at Buffalo State in New York.
The art world has used infrared imaging to reveal drawings hidden under paintings since the 1930s, a technique that often works because the underdrawings were done using black pigments made of carbon, such as charcoal. These “carbon-blacks” are very good at absorbing infrared light. The black paint in the current study must have contained a noncarbon-based dye or it would have absorbed the infrared light in the same way the blood did, creating no contrast, says Kushel. The size and concentration of pigments in the paint also play a role. “It’s a very complex mix of variables that determines whether you are going to see what’s below,” he says.
Criminals hoping to make a blood stain invisible to an infrared camera would be well-advised to go with Mars brown, a family of iron oxide-based pigments, says Kushel. Because the iron in blood’s hemoglobin is probably what makes it visible to the infrared camera, paint with a lot of iron oxide might absorb light similarly and eliminate the contrast that makes blood stand out.