Oblique Lighting Explained
Shoe print seen with oblique lighting
Have you ever noticed how the CSIs on television seem to examine everything with a flashlight instead of turning on the lights? It drives my wife nuts! Eventually we do turn on the lights but in some scenes it actually helps to look around with a flashlight first. That’s because some items of evidence are easier to see with oblique lighting (also known as side or indirect lighting). Overhead lights don’t create the harsh shadows needed to see certain evidence. Simply stated, the CSI will take a bright flashlight and hold it at a very low angle parallel to the surface they are examining (like a floor). The process works best when all other lights are turned off. You can try this yourself at home. You’ll be amazed at how much dust, pet hair, and other small items you’ll see using this simple technique.
Below are photos of a wooden stool that was found at a crime scene. Looking at the stool in room light no one saw the two sets of shoe impressions from the suspects. They had used the stool to gain access to an otherwise inaccessible area in the crime scene. Without oblique lighting this valuable evidence would probably never have been found. If you want to use this in a book scene you could have your CSI, detective, or private eye find evidence after others have released the crime scene. In any event, don’t be shy about having your characters using flashlights in a crime scene, even during daylight hours.
Stool seen in normal lighting
Shoe prints on stool seen with oblique lighting
Posted on June 18, 2011, in The Crime Laboratory, The Crime Scene and tagged Crime Scene, csi, fiction, forensic photography, forensics, murder, mystery, oblique lighting, side lighting, thriller, tom adair. Bookmark the permalink. 3 Comments.
You could be just the person who I am looking for. Ii estimate that I am 44% of the way through a first draft of my first full lengthh crime novel and Ii occassionally run into questions that I’d love to have someone knowledgable to ask.
Currently, I am working on a scene in which 3 detectives in a medium city with no big crime lab are investigating a series of murders. They are at a victims front porch, which they are quite sure the killer has walked across (something has been afixed to the door). So they want to check foor and lift any foot prints. Would prints left by a person who had walked up the sidewalk be more recoverable … or those of a person who had crossed the lawn?
Also … I read that dust type prints are susceptible to static print recovery methods. How would one recover prints left as a result of moisture? Would one use Ninhydrin, or can THOSE prints even be lifted?
Eagerly await your reply sir.
You came to the right place G.H. I doubt very much that anyone could recover shoe impressions from a lawn (grass) unless they stepped in an exposed area of dirt. The blades of grass aren’t a “smooth” enough surface to capture and retain the detail. A sidewalk is also a very unlikely surface (concrete) to recover a two-dimensional impression unless the impression is in some patent liquid like blood or oil. Ninhydrin reacts with amino acids so it could be used to develop a blood print but other reagents like leuco-crystal violet (LCV) might be better suited. Here is a link to an article I wrote for patrol officers that will explain the difference between “wet” & “dry” origin impressions. You might have better luck if your detectives were to recover a dust impression from the door (kicking) or from an entry rug from an enclosed or covered porch. In such a case they would use the electro-static dust print lifter or what we call a “gel” lifter (from the front door). I hope that helps. http://www.evidencemagazine.com/index.php?option=com_content&task=view&id=385
I was ineffective in explaning my scenario. The shoe prints will be on the wooden floor of a front porch. The question was … were they formed from moisture on the soles (from the lawn) or formed from dust and dirt (picked up from the sidewalk). Hopefully, this explains my dillema better. I watched an article on electrostatic lifting (for dust prints) but I wondered how they would lift prints formed from moisture that had subsequently dried. (Perhaps that’d make better copy)
Also, how possible would it be to actually see the prints at 11:00ish in the morning on an outside porch? Would an ALS have any impact to speak of?