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What’s UP with ‘REFLECTORLESS’ (prismless) Distance Measurements? Bottom line:
> Position Instrument
CLOSE to Evidence
In the February, Volume 19, Issue 2 of GIM International, an interesting article caught our attention in their Technology in Focus section. As more and more forensic investigative agencies initially purchase a “reflectorless” electronic Total Station, or trade-in their older Instrument, the persisting question is “how does it work” or “how does it capture the distance without a prism”. It just can’t all be “FM” (!) can it? ((call for what FM means!!))
The process is pretty simple really. You first make sure the Instrument is set for ‘reflectorless’ use (mode) and that the “target height” has been adjusted, if needed, then point the instrument towards the relevant evidence point, say a body location or tire mark, or shell casing. Then you aim the crosshair precisely at the object and press the data collector button to collect the ‘shot’ and ~ as quick as a passing motorist(!), the Instrument has measured the slope distance … WOW, and without a prism or someone standing over it (further contaminating the scene?) or walking through your scene!! S-w-e-e-t!! But, how does it work? As most of us have experienced, the EDM portion of the Instrument permits distances to be measured well beyond what most crash or crime scene investigations really need. Instruments can measure to single prisms, in some cases, thousands of feet away in most all directions. A few prism-less (reflectorless) total stations also can measure to certain objects from virtually right up against its scope to several hundred, and in some instances, a thousand feet or more away from the RP location.
This makes a perfectly tailored Instrument for a variety of scene assignments, from documenting facts about a hard to access location of a reluctant witness, hazardous area as near moving vehicular traffic, or securing blood droplet evidence off a nearby wall. Forensic Mapping Specialists should be knowledgeable about how such measurements are made using this method. Certain underlying characteristics and cautions should be explored.
The article of reference was titled, in part, “…Laser Distance…” measuring. Most of the law enforcement members reading this will recall from their Laser Radar days that L.A.S.E.R. means “Light Amplification by Stimulated Emission of Radiation”. It was in the early 1970’s that many of us recall our departments first being introduced to “laser” and “radar” and other speed measuring/monitoring devices. Total stations, many first used by our military, were generally introduced commercially in the late ’50-60’s although this is always a disputed time period, depending on the manufacturer. Reflectorless total stations, it has been reported, were first introduced commercially in the United States, by Leica Corporation, in the latter portion of the 1990’s. They began to be used by collision & crime scene reconstructionist in 1999*. Since then, nearly all of the commercially available total station manufacturers have their own version of the ‘reflectorless’ instrument on the market. Most of us are accustom to using a prism or retro-prism, mounted on a telescoping prism pole. We would then position the prism pole at a given and documented height, directly over or next to the “object of interest” (shell casing, body, blood, etc.). The distance measured in this fashion from the RP (reference point) location and its associated horizontal angle, indicating relationships between other evidence points, and the vertical angle registering any changes to the scenes’ geometry (or shape), all necessary facts to pin-point where certain evidence was discovered accurately. The article, written by Henk Key, contributing editor, GIM Inter., and Dr. Mathias Lemmens, editor, GIM Inter., indicates that basically, “…two laser principles are common in …” the sort of measurements we and other professions require: phase shift and pulse, both known to Einstein and made, according to the article, operational in the ‘60’s. They continue “…phase shift method is considered to be the most accurate one; it allows a very narrow beam, but its measuring range is limited. The pulse method has a much wider range but has the disadvantage of poor performance on short ranges.” The more recent EDM systems (Sokkia, Leica) reportedly use a “hybrid method” which isn’t as limited to its built-in clock and the width of the spot size. They explain that the prism-less EDM’s in use today require high energy pulses at a level generally in the broad range from “1 to 20” watts. Where, most total station EDM’s that depend on the use of prisms (classified as ‘electro-optical’) only emit their signals at a level “of a few milliwatt.” Comparatively, it’s like trying to get a good reflection off something using just a tiny AAA-battery pen light as opposed to a 12-cell, heavy duty flashlight! “This high energy level..” they continue to explain, “.. enables the detection of reflections on bare surfaces without the need of using prisms.” As illustrated below, another consideration is the size of the emitted dot’s “footprint” on the reflective surface itself.
“Beam divergence as a function of the
distance from the EDM Instrument.
Distance
Spot size
Beam divergence, or the size of the transmitted beam, depends on the “line of sight” distance between the Instrument’s setup position and the ‘target’ itself. See above chart. In the informative article, the authors point to two potential sources of errors associated with the process of documenting any evidence “reflectorless”. An “error” indicated here might only mean not being able to get a return signal, such that the evidence point itself might not be recorded. First, it could be the generally unfavorable makeup of the object/surface itself, and second that the object/surface isn’t ideally situated perpendicular to the Instruments “line of sight”. Another “phenomena” that could occur when documenting interior crime scenes and crush measurements, is the S M E A R I N G-effect (see below) of the visible red-aiming-dot. What does that in itself reveal about the laser’s footprint on the surface?
The article indicates that that portion of the surface (plane) closest to the Instrument will “reflects the beam first, whilst the part that is farthest away will reflect last.” IMPORTANT OBSERVATION- One important topic the article didn’t cover, due to its intended reader outside forensic scene reconstruction, is the absolute and vitally important procedure of verifying the red-aiming-dot’s alignment with the Instruments Crosshair!
The above commercially available ‘red-dot aiming target’ should be placed at nearly the same height as the Instruments’ scope, and approximately 15-20 feet away (where permitted). Then by sighting through the Instruments scope, the operator first lines up the instruments’ crosshair to the small cross seen at the center of the ‘target’ and then second, activates the red-aiming-dot itself. If the visible red-aiming-dot is seen to line up with the small cross seen at the center of the ‘target’, the operator can then be assured that where the ‘red-dot’ is visually seen to project onto the selected evidence point or surface, that is where the Instruments cross-hair is pointed and the resulting coordinate will be satisfactory. J Once the recommended Red-Aiming-Dot Protocol has been completed, then most reflectorless instruments so equipped, can be quickly aimed at the relevant piece of evidence or surface negating the prior need for the Operator to physically focus on each evidence item! The procedure has proven to save time at the scene and permits more flexibility in the overall scene electronic documentation – setup process. An additional exciting & positive feature inherent to the red-aiming-dot from a reflectorless total station, is that the Instruments height ~ whether at its extreme extended tripod height or absolute “spread out the tripod legs and get it down” height ~ won’t matter as the Operator .. working with the red-aiming-dot for aiming at evidence targets .. will not need the ability to focus on the actual object itself, but instead, just aim the red-aiming-dot at the evidence point! Ø Instrument Caution! It has been demonstrated that some Trimble instruments have their “red-aiming-dot” feature affixed to the top of the scope, as shown below (model 5600 DR200+). When the Trimble, as seen here, projects its’ red-aiming-dot, it is NOT in line with the Instruments’ crosshair! All other instruments witnessed (Sokkia/Leica), have the red-aiming-dot literally built in the scope of the Instrument and thus not the parallax effect introduced as with this reflectorless total station.
In conclusion, here is the bottom line when using “reflectorless” total stations.
The authors basically support this by indicating “…the effect is negligible as long as the plane has homogeneous reflectance characteristics.”
As shown below, small white circular stickers can be used to aid in the collection of these important vehicle features.
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