3D Laser Scanning Techniques for the Crime Scene Investigator: Part 2 - Executing the Scan
Written by Brenda Butler   

SO, YOU’VE DONE THE WORK of planning your scans and positioning the scanner. You are almost ready to get started with your first scan, right? Not quite yet. The purpose of scanning a crime or crash scene is to complete a measurable diagram in a 3D world. Stop and think of the reason we create a measurable diagram of any kind: It is to preserve the crime, crash, or fire scene at that moment in time, as it was on that day. But we don’t need measurements for that. The reason we collect measurements, regardless of the diagramming method, is to enable presentation in court, to know where everything was located, and to be able to accurately put everything back in its place if asked to do so by the judge.

This article is the second in a series intended to answer many of the questions that are continually asked of the author as a forensic solutions specialist for FARO Technologies. The author has more than 20 years of experience as a police officer and 14 years as a crime scene investigator and evidence technician. This article series will provide helpful techniques and best practices compiled from the author’s experience and the experience of other crime scene investigators across the United States. Even though the technique examples may refer to the FARO laser scanner, the scanning techniques can be utilized by anyone who has a 3D laser scanner and may be applied to any scene, regardless of the manufacturer. Remember, it is always best to refer to your manufacturer’s guidelines and your agency’s protocol when determining the best scanning methods for your specific scanner.

Find Part One: Planning and Positioning of the Scanner in the Fall 2019 issue of Evidence Technology Magazine.

Placement of Validation Scales

Obviously, you intend to use your diagram in court. If it was a 2D diagram, you would be required to include the words Not to Scale on your diagram, or you would take the time and effort to scale it and include the scale in your diagram. The same court rules apply to a 3D diagram. You need to have a validation scale in your diagram, because we are not going to spend the time scanning a scene and then say it is not to scale! It is to scale! In a 3D world, we don’t wait until we get back to the software to add the scale. We need to add the scale to the 3D virtual world, which means we need to do it while on scene. There are several tools that can be used as a validation scale, such as a scale bar, a yardstick, or a pocket rod.

Some options for a validation scale.

When considering what to use as your validation scale, just remember that it needs to be large enough to find in a 3D diagram while you are on the stand, under pressure, with members of the jury watching you. No, a 2-in. sticky scale is not enough, and it is difficult to find in a big scene! Give yourself at least 3 ft. if not 6 ft. It is easier to find—trust me. It is best to use a NIST/NITSA-certified scale, but if you do not have that available to you, document the scale you are using and be prepared to articulate what the NIST/NITSA certification is and explain that your validation device does not bear that certification. You can also take a known measurement and use that as your validation scale, if you document that measurement. I do not encourage this method, as this introduces human error into the equation. I can get five people to measure the same doorway and, if it is carried out to two decimal places, they will all have slightly varying measurements. Make sure you are avoiding anything cloth, as material stretches and is difficult to keep taut. Also avoid any sticky-tape scale that is pliable enough to stretch, and anything that is not removable from its contracting case, as it could cause bending or turns. Lastly, make sure your validation scale is not reflective. A shiny metal yard stick in bright sunlight might not be visible in your scan data if there is a high reflection issue. This will defeat your purpose.

When considering where to place your validation scale, remember you need to be able to find it in court. It is intended for your verification reference and should not draw the attention of the jury. For example, you do not want to place it directly in the middle of all your evidence and have your jury staring at the validation scale and not looking at the evidence when you are showing it to them. The validation scale should be placed off to the side and out of the way in a vertical or horizontal position.

There are many schools of thought on where to place your validation scale. I recommend always placing it in your scene off to one side of where most of your evidence is located, and make sure the complete scale appears in at least one scan. If you are scanning methodically with the appropriate overlap, you will notice that pieces of the scale are captured by multiple scans. The main thing is, do not move it. Remember, crime-scene processing rules apply as they pertain to evidence that you intend on processing for DNA. Do not create a situation where you are defending yourself against potential cross contamination in your scene.

If you have a large scene, you may need to consider placing two or more scales in your scene. For example, if you are documenting both floors of a large colonial home, you may want to separate the floors into individual projects. This would require a minimum of one scale on each level. In another example, if you are scanning the interior and exterior of a building, you will require at least one scale both inside and outside. The point is, you can use multiple scales when you need to, but avoid falling into the move the scanner, move the scale mentality. If you move the validation scale every time you move the scanner, you can end up with a scene that is drowning in scales and it can prevent the jury from seeing the actual crime scene itself. Moving the scale for each scan can also cause registration issues in the software if it is excessively moved around in a small area.

Some suggest placing a validation scale in the first and last scans. This is to ensure the scanner was not damaged while it was utilized on the scene. I have never personally prescribed to this technique, but I understand the methodology that if it is accurate at the beginning and the end, then the middle is considered accurate as well. The scans are placed together from a registration process. If your last scan is significantly off in data collection, it will not work with the remaining scans. If you have a larger scene, then use two or more scales and, if you desire, place them at the beginning and end—although they could be placed at any point throughout your scanning. Refer to your policies and procedures and adhere to the method your court venue has chosen for the purposes of validation.

Threshold Scanning

Now that you are scanning, remember that you want your final project to be a “true and accurate representation” of the scene on that day… but not necessarily “as you found it,” because you have processed it and identified evidence. If you are scanning a structure that had the doors closed and you also want to scan the outside of the structure to document it (for purposes such as the potential entry and escape routes of the suspected offender), then you need to make sure you have sufficient overlap of your scans so that the software can link together the interior and exterior scans. However, achieving that may be a challenge with the doors closed. You could open the door and scan it, but that would not necessarily be a true and accurate representation. This could be explained through your testimony, but I do not prefer that method as it could lead a jury member to see an open door and not hear the explanation. This could lead to a misunderstanding, especially if it is a domestic-related case and there was no intruder or forced entry. Instead, it is better to use a method of threshold scanning.

Threshold scanning allows the crime scene investigator to scan the interior of the structure in the desired higher-resolution and higher-quality settings of the scanner with the door closed. Open the door and scan in the threshold of the door. You don’t want your finished project to include an open door and a closed door, so a threshold scan should be comprised of extreme low-resolution (1/10) and low-quality settings, and it should be scanned in a grayscale setting, so no photographs are taken to apply color to the data points. Remember, the items of importance for this scan are the two opposing areas, so the 0° / 360° point should be placed on the actual threshold of the door. Once the threshold is scanned, the scanner can be moved to the new area and the settings can be changed to the desired higher-resolution and higher-quality settings, and color can once again be applied if desired. Close the door and continue scanning your scene.

It is important to familiarize yourself with your scanner settings, so you do not waste valuable time trying to change the settings of the scanner for a threshold scan. Likewise, you should practice and train yourself so that you remember to return the scanner settings to the desired higher-resolution and higher-quality to prevent problems with your scans in your finished product. If you are not comfortable changing your scanner settings or feel you may forget to return the scanner settings to the desired scan resolution and quality, it is best to leave the settings as they were when scanning the threshold and explain in your testimony why there are two doors—one open and one closed.

The threshold method can be applied from room to room, or room to hallway, or even from the outside of a vehicle to the inside of a vehicle. Scanning the outside of a vehicle and expecting the scanner to provide enough overlap through the windows is a mistake you will only make once. You need to scan the threshold of a car door before scanning the inside to accurately complete the diagram and reduce your error rate from manual registration. This can also be applied in doorways with no door or even an open door if you do not feel you have enough overlap between rooms.

Bodies, Vehicles, and Hidden Objects

Deceased bodies and vehicles pose a different scanning situation. Although in some cases it may not be possible, you should always scan a body before it is removed from the scene. This can be a very valuable piece of documentation, as bloodstain patterns on the body can be observed in the scene as the body was positioned, and you can also take important measurements.

Once the body is removed, the area under the body is searched for evidence and, if any is found, typically additional evidence markers are placed. The area under the body should be scanned after the body is removed to document the surface underneath. In the event the judge requests the body be hidden from the view of the jurors, it can be hidden inside a clipping box in the 3D documentation software. If you have scanned the ground underneath, you are able to present a pleasant view, whereas if you did not scan under the body, a hole of missing data will be in the area of the hidden body; this can become a great distraction to the jury. The same applies to motor vehicles.

Sometimes it is important to prove the negative—that is, that no additional items of evidentiary value were located under the body or under the vehicle. Skid marks and other valuable data can be captured if you scan once the vehicle is removed from the location.

The rule of scanning over and under applies to more than just bodies and vehicles. It applies to anywhere there is additional, obscured evidence. An example is a bedroom where evidence is located on the bed and under the bed. This can be difficult to document in any fashion. Scan the room with the bed in place, remove the bed completely from the room (maybe leave the frame and just remove the mattresses) and get it completely out of the sight of the scanner. Rescan the room where the bed was. This enables you to have the bed in the room and the evidence under the bed. The bed can be placed in a clipping box and hidden, turning it on and off as you desire.

Resolution and Quality Settings

When scanning indoors and the spaces are smaller, a lower resolution can be used since there will be significant overlap between your scans. This enables you to decrease your scan time per individual scan and still obtain the same high-quality product.

When scanning outdoors or in larger indoor areas, the resolution should be increased to enable more data points. You will need the distance between the data points to be less since you will not have as much overlap as in the smaller indoor scenes. Remember, the resolution settings should be determined by how far apart your scan positions are and how much overlap you are intending on obtaining for each scan. It is not based on how large your scene is. But, typically, the larger the scene the less overlap there is as we spread the scan positions further apart.

Resolution is how far apart the laser data points will be considering the distance from the scanner to your desired scan area. The higher the resolution, the closer together the data points will be. The lower the resolution, the further the data points will be apart.

You also need to adjust your quality settings. These are determined by your environment. If you have good lighting and good weather and time is of concern to you, a lower-quality setting is acceptable. If you are indoors or outside with overcast conditions, a 3x quality setting is typically enough for basic scanning. If you are outside in sunny conditions when range is needed, if you are experiencing inclement weather, or if you have reflective surfaces, you should increase your quality setting. When in doubt, use a higher setting than you think you need.

FARO Scanner Resolution Guidelines

1/1 or 1/2 Objects and small areas
1/4 or 1/5 Outdoors and large indoor spaces
1/8 or 1/10 Indoors and small outdoor spaces

FARO Scanner Quality Guidelines

2x Optimal conditions (good lighting, good weather) and when time is a concern
3x Indoors or outdoors with overcast conditions
4x Outdoors in sunny conditions when range is needed, or in inclement weather

Various factors at each scene will affect your decision-making process when choosing quality settings.

Metering, Color, and Quality

Metering needs to be considered when color is applied to your scans. The metering of your scanner follows the same rules as digital photography. As crime scene investigators, we typically receive some type of photography training and learn about metering the camera in order to document scenes and evidentiary items in situations where the lighting is challenging. We learn to paint with light, angle or deflect the flash and, of course, we learn how to bracket our shutter speed.

The metering of the scanner camera follows the same principals. We have three basic options for metering: Even Weighted, Horizon Weighted, and Zenith Weighted. Learning which setting to use is easy if you follow a simple guide. Even Weighted is just that—even. If you have an evenly lighted scene and you do not see any exposure challenges, then this a great setting to use as it meters a little from everywhere. If you have very bright lights on the ceiling of your structure, or if you are outside and it is a very bright, sunny day and your evidence is closer to the ground than the treetops, then Horizon Weighted metering is best, as it meters horizontally. If you have bright light spots at eye level or below, then Zenith Weighted metering is best, as it meters from height.

When you are not sure if you can resolve your metering issues due to having complicated lighting situations, remember you can always not apply color to your scans and scan in grayscale—even in darkness. If you choose to scan in grayscale, you should be aware that it scans in infrared, which means the rules of infrared apply. This is an important consideration because blood behaves differently under infrared than other bodily fluids or objects. If you are scanning in grayscale and you need to document blood in your scene, you need to understand the infrared rules and consider the surface where the blood is located in order to determine if it will react to the infrared scan and appear in your scan data. When in doubt, scan it in grayscale and check the scan on your scanner screen at the completion of the scan to ensure that it was captured.

FARO Scanner Metering Guidelines

Even Weighted Ideal if lighting is relatively uniform. This setting uses light coming from the entire scene and takes less time.
Horizon Weighted Best for scenes with bright overhead lighting. The camera meters light from the horizon level.
Zenith Weighted Best for scenes where bright lighting comes from eye level or below, the camera meters light from height.

Additional options to resolve lighting issues include HDR settings (high dynamic range). Using this setting provides a greater dynamic range of luminosity or quality. This is basically bracketing, just like you would bracket with your camera when your lighting situation is challenging. Bracketing is a method of taking several pictures at different camera settings automatically and in a methodical manner, without the photographer stopping and making manual adjustments for each picture. Bracketing two or three stops down and two or three stops up to adjust your camera shutter speed means you are moving two or three steps brighter and then two or three steps darker, so you capture the subject at the perfect light.

The FARO scanner has an indoor HDR mode, an outdoor HDR mode, and an object HDR mode. It accomplishes the same thing to resolve very challenging lighting situations. This is only applicable if you are scanning in a color-applied setting. This is a great option to assist with lighting variances such as under a shadowed overpass looking out to the bright sunny highway, or the inside of a garage looking out the open garage door onto a bright, sunny driveway. Object HDR is a great setting when you require a very high resolution of a nearby object or surface. We will discuss high-detail scanning later in this article. And you should note here that it is great to use HDR when you need it, but it should not be used when it is not necessary, as it can increase your scan time.

FARO Scanner HDR Guidelines

Indoor HDR Indoors, distance between scanner and object is under 20 m with HDR quality
Outdoor HDR Outdoors, distance between scanner and object of interest is over 20 m with HDR quality
Object HDR High-resolution scan

Additionally, there is a nighttime mode on some scanners. The nighttime mode is used to reduce noise in the photographs at night. There is an increase in capture time but not as much as using HDR. As an example, a scene took place in a plaza parking lot at night. The storefronts were all glass, causing reflection from the headlights and overhead lights of the emergency vehicles. There were randomly placed light posts providing blotchy patches of light, and the store signs were all lighted in neon colors. The nighttime mode assists in cleaning up the difficult scene if color is desired. One could also solve this issue by scanning in grayscale, as it would decrease the scanning time significantly if time was a concern.

If scanning in the dark or in a poorly lit area indoors or outdoors, scan in grayscale by turning the color application off. Do not bring in lights to scan, as this can create areas of over-exposure or under-exposure. If lights are brought in for processing the scene, it is sometimes best to turn them off or remove them for scanning. If emergency vehicles can safely do so, turn off their headlights and overhead emergency lights that face the area you are scanning, as this can cause purple, red, and blue streaking and overexposed areas.

If scanning a large outdoor scene, consider using grayscale on areas of less importance and using color at the main scene area or area of most importance, such as the location of the vehicles or evidence. This saves significant time and provides needed details and documentation. If scanning a large indoor scene, such as a colonial home where evidence was only located on the second floor, consider scanning the outside and other levels in grayscale and using color only on the second floor where the areas of most importance are located. This also saves significant time and provides needed details and documentation.

Watch for Part 3: High-Detail Scanning in the February 2020 issue of Evidence Technology Magazine.

About the Author

Brenda Butler is a private consultant and a former commissioned police officer. She is a state-recognized expert in multiple Ohio counties in areas of investigation, crime scene investigation and reconstruction, 3D laser scanning, bloodstain pattern analysis, and shooting-incident reconstruction. She has processed countless homicide scenes, officer-involved shooting scenes, and everything in between. Butler has worked as a police officer in Ohio for 20 years and a crime scene investigator for 14 years. She is a graduate of the National Forensic Academy Section XXXIV and has a Master’s Degree in Justice Administration from Tiffin University, as well as a Bachelor’s Degree in Executive Public Safety Management (summa cum lade) from Cleveland State University. She served in the United States Army (military police). Among other certifications, Butler is a certified crime scene analyst with the International Association for Identification and is a previous deputy director and instructor with the Ohio Peace Officer Training Commission. She is an experienced college instructor and has trained hundreds of police officers on a multitude of topics. Butler currently works as a forensic solutions specialist at FARO and has taught 3D laser scanning and crime scene documentation to police agencies across the United States.

This article appeared in the Winter 2019 issue of Evidence Technology Magazine.
Click here to read the full issue.

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