Convergence
Written by H.W. “Rus” Ruslander, CSCSA, CLPE, CBPE, D-ABMDI   

How to use inexpensive laser torpedo levels to determine points of convergence in bloodstain patterns

THERE ARE MANY METHODS that can be used to determine the point of convergence in a bloodstain pattern. And as a bloodstain pattern examiner, I have used most of them. The methods include the conventional stringing method, as well as a number of other techniques such as mason’s chalk lines, artist’s tape, drawing a line through the long axis of stains with a straight edge, and creating an overlay of the pattern area using a sheet of clear plastic with lines drawn with a marker.

While all of these methods work equally well, I have found that using inexpensive laser torpedo levels is an easy, quick, and extremely accurate way to do the job.

These small lasers come with a tripod and, most importantly, a special magnetic cap that converts the laser dot to a visible, continuous line. They also have an on-off switch instead of a momentary switch. These laser torpedo levels can be purchased in a variety of hardware stores, department stores, or on the Internet. Since they are already threaded for 1/4-in. x 20 threads, they can be mounted onto conventional tri-pods instead of using the small tripods that come with the original product.

The lasers I use are equipped with a bubble level on the top so you can make sure it is aligned to a perfectly horizontal plane. This provides true orientation to the plane of the surface that holds the bloodstains.

Figure 1; Figure 2

The author set up a demonstration to show how managing the ambient lighting can make a big difference in photographing the visible laser beams. Figure 1 shows beams on a door with afternoon sunshine illuminating the door. Figure 2 shows the same door but the natural lighting from outside has been eliminated. Simply eliminating the outdoor light coming in through the door has made a big difference in the results.

By utilizing a number of these lasers, one can adjust the lines they project and then align the laser beams through the long axis of each bloodstain to establish a point of convergence.

Photographing the results of the laser setup is fairly easy. All you have to do is darken the room slightly in order to brighten the projected laser light. The resulting photos will document the point of convergence with remarkable clarity.

Figure 3; Figure 4

Figure 5; Figure 6

Once you have determined the point of convergence for the bloodstain pattern, you can find the point of origin by using trigonometric math calculations. Or you can follow the author’s suggestions on Page 36. It involves using a laser mounted on a protractor (Figure 3). You just position the zero point of the protractor on each major bloodstain (Figures 4, 5, 6) and let the laser beams fall on the yardstick. They will intersect at the point of origin on the leading edge of each major bloodstain.

In order to demonstrate some of the procedures that are involved with using torpedo laser levels, I set up several levels in a hypothetical example. In Figure 1 on Page 34, you see the image with laser beams that have been projected onto a white metal door. This picture was captured inside a residence at 3 p.m. on a bright, sunny day with ambient daylight illuminating the surface. A Nikon D70 digital SLR camera was used. Notice that while the laser beams are not particularly vibrant and bright, they are still visible.

Figure 2 is a photo that shows the same door. But the door has been closed, eliminating the natural light from the outside. About four feet away, a window allowed outside light to come streaming through. Across the room was another window providing natural light that illuminated the room from behind. No flash was used in either of these photos. Simply eliminating the light through the door, however, was enough to make the laser beams much more visible in the image.

Using a flash is not recommended when photographing a display such as this. When working in reduced light, simply mount the camera on a tripod, set the camera to a longer exposure and/or a wider aperture—and use a shutter-release cable or time-delayed trigger. This will allow for more image saturation with a CCD digital camera or a conventional film camera.

The projected image is very easy to see and use for the establishment of a point of convergence. I have not found any drawback to using this method in rooms with average lighting. However, attempting this technique outside in the direct sunlight would present a problem. This problem could be overcome by using tarps or some other types of material to provide shade for the area being examined.

Deciding on the number of stains you use in determining a point of convergence will dictate the number of laser levels you need. By using the tripods included with the laser levels or conventional photographic tripods—both with legs retracted or fully extended—it is conceivable that as many as 10 or 20 lasers could be used without too much trouble.

Once the point of convergence for the bloodstain pattern has been established, it is very easy to then determine the point of origin. All you need to do is mount a yardstick on a tripod as shown in Figure 4. Then you can use either the normal trigonometric calculations of width divided by length to find the inverse sine, or you can use the tangent method.

The tangent method can be used to extrapolate the point of origin, which can then be shown graphically on a computer using any one of a number of drafting or crime-scene diagramming programs available on the market.

By using the lasers and then placing a yardstick mounted on a tripod at the point of convergence, you can use the tangent method to determine how far the source of the bleeding was from the surface. It is then relatively easy to place a marker of some sort on the yardstick at that point to graphically illustrate the general area where the bleeding originated. This will eliminate the need to use actual strings, while still providing a graphic image for the viewers of the diagram or photograph.

Figure 7; Figure 8

The author suggests using a styrofoam head (as seen in Figure 7) when you are determining the point of origin in bloodstain pattern analysis. According to the author, the styrofoam head creates a readily recognizable image for the jury when it is a central part of photographs taken at the scene. With the styrofoam head in place on the yardstick and the laser beams positioned at the point of convergence, the photographs are self-explanatory.

Another way to determine point of origin is to use a laser mounted on a protractor (Figure 3). Place the zero point of the protractor at the edge and along the long axis of each stain and project it onto the yardstick as in Figure 4, Figure 5, and Figure 6. This will designate the point of origin on the yardstick. Bear in mind that the actual point or area of origin would be determined by the area (or areas) of impact. That area would be considered the area of origin. This could require the tripod to be lowered or raised so the yardstick is on the same plane as the area of actual injury sustained by the victim. (See Figure 7)

In Figure 7 of my example, I have used a tripod, a yardstick, and a styrofoam head that I obtained from a wig store. With these items, I have created for the jury a readily recognizable image (Figure 8) of where the event occurred. The use of the styrofoam head adds a human touch to the scene.

A word of caution when you are using lasers:

There has been some research showing that in some instances a prolonged exposure to the light that is emitted from lasers could have a negative effect on evidence containing DNA. Personally, I don’t think the lasers I have mentioned emit a strong enough beam of light to have that type of negative effect. However, as with any investigative process, adequate written, photographic, and sample collection methods should be performed prior to any process or procedure you might use to document the point of origin or the point of convergence.

Some final comments:

Using this method is much quicker than any of the other methods I have used in the past. It took me less than five minutes to set up the exercise that was described in this article. This entire amount of time even included the time it took to load batteries in the lasers.

This article refers specifically to the laser torpedo levels, but there are other models of lasers on the market that could be used equally well on this procedure. One laser level (Figure 9) can be adjusted both vertically—to any angle between 0 and 90 degrees—as well as horizontally. They are also inexpensive. The laser torpedo level like the one I used is shown in Figure 10.

Figure 9; Figure 10

There are literally dozens of laser levels available on the market, ranging in price from only $10 or so up to hundreds of dollars. One laser level mentioned by the author (Figure 9) can be adjusted both vertically and horizontally. The one used in the author’s demonstration photographs is a laser torpedo level (Figure 10) that comes complete with a tripod. Both of these laser levels are relatively inexpensive

 

About the Author

Harold W. “Rus” Ruslander is the chief investigator for the Palm Beach County (Florida) Medical Examiner’s Office. He is recognized by the International Association for Identification (IAI) as a Senior Crime Scene Analyst; a Certified Bloodstain Pattern Examiner; and a Certified Latent Print Examiner. He is also a Registered Medicolegal Death Investigator with the American Board of Medicolegal Death Investigators. Ruslander is actively involved in teaching with other law-enforcement organizations and has written more than 20 forensic articles. He can be reached by e-mail at this address: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

 


ORIGINALLY PUBLISHED:
"Convergence," written by H.W. “Rus” Ruslander
March-April 2008 (Volume 6, Number 2)
Evidence Technology Magazine
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