I have been doing police work with a body-worn camera for 10 years, 8 of those as a full-time DUI hunter. If your agency is getting BWCs soon, or you have not done many DUI investigations with a BWC yet, this article can help you both on the side of the road and in court. The BWC is a powerful tool, but it is not your eyes or your ears, and that difference can become glaring. Without proper articulation, it can leave a judge and jury doubting your credibility.
The jury will watch the video, read your report, and listen to your testimony. Their attention will lock onto one issue fast: the moments when you, or your report, say something happened that they do not see or hear on camera. That gap makes your credibility the issue, and credibility is the foundation every case is built on.
Body-worn cameras are powerful evidence, but they also have limits. Your report and your testimony must complement the video, not compete with it.
Why the BWC Can Miss What You Document
The microphone is not the human ear.
Wind and traffic mask mild to moderate speech changes. Microphone compression and placement can flatten both volume and clarity. A quiet, slightly slurred phrase can be difficult to discern on playback.
The camera is not the human eye.
Body-worn cameras struggle with subtle visual cues in low light, high contrast, or while you are moving. Wide-angle lenses distort distance and depth. Slight sway can look like normal movement, or no movement at all. Exposure changes, particularly when a bright flashlight is used in a dark environment, can wash out detail entirely.
Frame rate matters more than most officers realize.
Most body-worn cameras capture video at standard rates. Thirty frames per second is common in digital video and is the rate at which most BWC technology records. At that rate, the camera is sampling motion in slices, not continuously. Research on human temporal perception shows that people can perceive flicker artifacts at rates far beyond the commonly cited 50 to 90 Hz range under certain conditions, including when high-frequency edges are present and natural eye movements occur.¹
For the sake of brevity, I will reference Hz throughout this article, though some measurements are technically expressed in frames per second. They are similar concepts, but not identical.
Horizontal Gaze Nystagmus
This test carries a regular risk of not showing what you will testify is present.
Because your eyes perceive at roughly 50 to 90 Hz, your BWC captures at 30 Hz, and your stored BWC footage may be compressed even further, capturing the clues of HGN on a body-worn camera is, in practical terms, nearly impossible. That is precisely why training videos for HGN are captured using high-frame-rate cameras mounted close to the subject’s face at approximately 60 Hz or higher.
So how do you explain this in court?
I have had defense attorneys play my BWC footage and ask me to identify the moment I observed HGN clues. Most of the time, I can see Lack of Smooth Pursuit. Under near-perfect conditions, I can sometimes identify Distinct and Sustained Nystagmus at Maximum Deviation. I have never been able to capture Onset of Nystagmus Prior to 45 Degrees on BWC footage in a way that is clearly visible on playback; particularly when you capture this clue at true onset.
When I testified to seeing Lack of Smooth Pursuit, the defense replayed the video and told the jury they could not see it. This is where analogies become your best tool, and your DA should set it up with this question on redirect:
“Officer, can you explain how you were able to observe HGN that may not be clearly visible on the video?”
Rather than getting into frame rates, Hz, and recording capabilities (the defense will likely object that you are not an electronics expert anyway), use an analogy most jurors can understand.
The Olympic Diver Analogy
Imagine you are watching Olympic diving with a friend. You have never participated in the sport and only watch it once every four years. Your friend, however, dove competitively for years and studied it in college. You both watch a diver leave the board, perform several athletic twists and turns, and enter the water gracefully. You think, “That looked pretty good.”
Your friend, however, says: “Meh. Good height and solid commitment. He performed a back dive with 2½ somersaults and 3½ twists from pike. There was some looseness in his form during the twisting phase, and the pike position was not consistently tight. His entry was short of vertical with noticeable splash.”
You both watched the same video, but had very different observations, because one of you has training and experience and the other does not. The HGN video is no different. Having administered HGN and reviewed BWC footage many times throughout my career, I can confidently identify nystagmus on that video, because my training and experience allow me to recognize how what I observed in person translates to what appears on the footage.
Walk and Turn
This test carries the risk of failing to explain the conditions under which the camera captures observations.
Two important things happen during the Walk and Turn that officers need to be prepared to address.
First, the instruction phase contains two clues that will not be captured on your BWC unless you have a cover officer recording from a different angle, or you deliberately position your camera to capture the subject while you are demonstrating. As a full-time DUI hunter who was not regularly involved in physical altercations, I kept my camera mounted with only the magnets. This meant that during the demonstration, I could detach it and point it directly at the subject.
Second, there is no single position during the walking stage that will capture all possible clues on camera. If you stand directly in front of the subject, you will capture Uses Arms for Balance and Steps Off Line, but you will have difficulty capturing Does Not Touch Heel-to-Toe. If you stand to the side, it is the opposite. Being able to explain to a jury how you observed the hand on the far side of the subject’s body rise while the camera missed it, particularly in low light, is extremely important.
One Leg Stand
This test carries a regular risk of overstating what the video will show.
The clue most likely to cause trouble here is Sways While Balancing. Your report and your on-scene observations may document 2 inches of front-to-back sway, but when the BWC footage is played, that sway may be barely visible or not visible at all. How do you prepare for this?
First, position your camera strategically. Standing at a 45-degree angle to the subject gives you the best chance of capturing both front-to-back and left-to-right sway on video.
Second, explain the one-eye problem. You have two eyes. Your camera has one. Two eyes give you depth perception, and a single lens cannot replicate that. You can explain this to a jury without getting technical: “It is harder to catch a pen thrown at you with one eye closed than it is with both eyes open.”
You can count on a few pens being tossed around in the deliberation room.
Closing
The video is going to be played. That is not a threat. It is an opportunity, provided your report is written to complement what the jury will see and to explain what the camera cannot reliably capture. When your report and the video tell the same story, jurors focus on impairment, supported by both the footage and your credible explanation of what they see…or do not see.
- Davis J, Hsieh YH, Lee HC. Humans perceive flicker artifacts at 500 Hz. Sci Rep. 2015 Feb 3;5:7861. doi: 10.1038/srep07861. PMID: 25644611; PMCID: PMC4314649.
