How Many Cameras Can One Monitor?

All things equal, what number of simultaneous cameras should one operator be expected to monitor?

By monitor, take this to mean checking for general 'civil disobedience' type threats in a populated room / area, i.e. public courtyard, gaming floor, or the lobby and surrounds of a highrise building.


The number of views one operator can be expected to monitor is directly related to the level of activity on screen and the threat exposure of the facility or activity being monitored. There's no general rule of thumb I know of as the level of attention required to identify an abandoned object in a high pedestrian traffic area is much higher than gate traffic at a international port authority maintenance gate. One client may be able to monitor 100 views in a setting normally devoid of activity while monitoring entrants at the Prudential building in Chicago an operator might max out at 4 screens focused on a single door each.

This was posted on line in December 2004 all sums are equal if there is no cost impact from failure to detect, so this simply comes down to good risk management, as all CCTV systems are not proscriptive then the same must be said about viewing habits http://www.securitysa.com/article.aspx?pklarticleid=3313

Throwing my hat into the ring to answer the question no more than 32 live images per operator, with screen shots set at quad across 8 x 17"-20" monitors with aprox 2.5m viewing distance. This will fit nicely into most security system concepts, space is normaly the domanint factor on how many you can offer, as little though goes into the control end. (usualy cost driven)

I recall an ASIS study that found operators showed significant fatigue after only 20 minutes watching 16-18 cameras, and their effectiveness dropped immensely. I generally think if an operator is expected to catch activity without assistance (no alarms, or analytics, etc.), if you ask them to watch more than 4 or MAYBE 8, it's useless. That sounds really extreme, because it is. I think it's been it's a topic that's been ignored for a really long time. If the area is typically low-activity (not occupied like you described), then maybe you can get 9-16 in there.

Link or this study didn't happen! Seriously, people always quote this 20 minute number but I've never seen the actual test. It's not that I find it to be untrue, I am just very curious to see if there was a real test, how it was done, etc.

I'll try and find it. I might actually have it in paper somewhere.

If not, I am sure ASIS can teletype or pony express it to you...

I hope they have pigeons, because ponies don't reach these parts.

Seriously, IPVM should do a real study/test on this. Hire a few guards and regular people, set them up, monitor how they perform.

Here is a relevant study although it does not state the number of cameras monitored rather the amount of time (20 mins):

Maggie, thanks, that's helpful to see a reference, though... "Experiments were run at Sandia National Laboratories 20 years ago"

And the article is undated, who knows... this test might be from 1985, eons ago

I'd love to see the full details!

I have a few friends that work there, I'll see what I can do...

Someone email Mary Lynn Garcia!

Lol.. I'll send her an email.

The reference below supports Ethan's statement about the 20 minutes. I cound't find an online version of the article unfortunately. However, I hereby quote one of the authors' statementes:

"Wood & Clarke state that operators remain more attentive when working in 20 minute blocks with 5 minutes rest in between."

Wood, J., Clarke, T.; Practical guidelines for CCTV ergonomics. In: Meeting Diversity in Ergonomics. Proceedings of the International Ergonomics Association congress; Elsevier, Amsterdam (2006).

Tiago, did Wood & Clarke actually do a study? What is their basis for this claim? Opinion? Common sense? Field study with random people or professional guards?

No doubt, I've heard dozens of people make the 20 minute statement. My question is: What's that grounded in? Is there real data or is this just folk wisdom?

Hi John, As far as I know Wood & Clarke carried out some field experiments with operators monitoring traffic as well as railway stations. Unfortunately I don't have more information than that.

It actually depends on the views , and what triggers are they relying on to get thier attention .

How much information to process , or what they must they must do with it when alerted .

Our facility has 30+ cameras. We are realistic and don't really expect our security staff to watch them 24x7. Instead they are used during actual events, and for evidence playback. We do catch about 75% of the incidents, call us lucky or the criminals stupid.

I'm very interested in video analytics, but the prices are rather high, and the reliabiltiy doesn't seem to be quite there yet. My facility produces lots of steam (direct and cloud forming) which is very difficult for analytics to deal with. Adding a second (thermal) camera is expensive. I'm waiting for the analytics segment of the industry to mature.

Sorry for my comment, but, It all depends on the scenario to be seen.

For instance, I think that the night shifts of public buildings could "monitor" more cameras of the halls, rather than the shifts of the laboral hours. Other example is a park, the activity on sunday during the day or on holidays is not the same that on winter night. etc etc etc. This is beside the 20 minute of focus.

Interesting anecdote. Recently I spoke with an integrator in Qatar who says that their laws/regulations restrict operators from monitoring more than 9 cameras and more than 2 monitors.

Coming back to the original question of this thread. Wood & Clarke concluded that:

- "no more than 9 simultaneously displayed pictures should be observed if pictures show considerable movement and the task primarily involves general surveillance;"

- "no more than 16 simultaneously displayed pictures should be observed if pictures show little movement and the task consists of general surveillance and observation."

Wood, J., Clarke, T.; Practical guidelines for CCTV ergonomics. In: Meeting Diversity in Ergonomics. Proceedings of the International Ergonomics Association congress; Elsevier, Amsterdam (2006).

From Mary Lynn Garcia, author of Design and Evaluation of Physical Protection Systems:

"A complex question and I will try and explain as clearly as I can via written message!

So, I think the question is really how many monitors can an operator effectively observe; in a well-designed video assessment (not surveillance) system, there can be as many cameras as needed. What is important is how much video is presented to the operator to make a decision. When Sandia designs a system, we use 4 monitors, only 1 of which present video of the current highest priority alarm. The other 3 are used to present a live image of the highest priority alarm sector, the next highest priority alarm sector and a free monitor for operator choice. Note that an assessment system integrates sensors and video and uses a human operator to assess the causes of an alarm in real time.

As to a surveillance system (the operator is monitoring a number of cameras and sensors are not used), where an operator is observing live video from all cameras, and the images are usually scanning all cameras, it is harder to say. We do know that humans are bad at monitoring video over an extended period, as you note using the 20 minute number. This is because we have a limited ability to stay alert for a long time, especially when doing something as tedious as looking at a bunch of video images being presented all at the same time. By the way, that number is not the result of Sandia research but by a group of people from the UK (I think); recent research shows that the period of time is actually less, more like 15 minutes! This same group demonstrated that humans could successfully observe up to 9 large monitors BUT monitor size, distance of incident from camera (which is sort of resolution), duration of the incident, and disruptions such as telephone calls or other activity around operator (think a large control room) reduce operator effectiveness. And this is only for the 20 minutes noted. Other research has shown that the complexity of the video scene also contributes to operator ineffectiveness. This means that if there is a lot going on in the scene (maybe an exterior view of a bust street or a parking lot at shift change), and bad night lighting, it will be very hard for the operator to pick out anything of importance.

Think about it this way--suppose you are the video operator, sitting in a typical control room. There will be others in the room, most likely, doing their tasks (answering the phone, dispatching security personnel via radio, supervisors checking on operators, etc). the operator is generally seated looking at a bank of monitors, often above their head and in front of them by some medium at best distance. Then, the large monitors are broken into smaller, 16/32 or whatever, "screens" and video is canning on each screen at some rate, say 5 seconds per view. If there are 50 cameras and 16 screens displayed on a monitor, that means that it will take 3 times through to get all 50 cameras sequentially scanned onto the large monitor with the 16 "screens". if there is something happening in the scene, it will be hard for the operator to distinguish this due to all the factors noted above. if we combine all the factors (distance, resolution, complexity of scene, lighting, duration of event, disruptions and a 20 minute period of highest alertness), you can see it adds up to something not very effective. If we also consider that the operator is often told to "look for something suspicious" with little explanation as to what this means, it makes it very tough for the operator. Since as security people, we are primarily concerned with malevolent human acts of theft or sabotage, surveillance systems give us little "bang for the buck" in terms of preventing successful adversary attacks. There can always be some value in the recorded video for an after-the-fact investigation, but it is too late to protect the asset at that point. This is acceptable as long as the consequence of loss is acceptable.

Sorry this took so long, but the short answer is not many, and it will not be effective for a long time or for many events. If anyone in your class has a copy of the Design book (either edition), all of this is explained in better detail and with pictures in Chapters 8 and 9. Chapter 9 in particular gives details about how monitor layout can affect operators and gives some info on how many things humans can deal with at the same time."

From Mary Lynn Garcia- "Use of Fixed vs. PTZ cameras makes monitoring video harder"

"So, combine the whole sequential scanning thing with the fact that most systems use PTZ cameras, which by their nature may not always be pointed in the same direction. Now you have the 50 cameras reporting to 16 small "screens" on a large monitor, and not even looking at the same scene all the time--what a mess. No wonder the operators feel overwhelmed; who would even want that job? And I am using a fairly small number of cameras (at least in my experience). Of course this depends on how the PTZs are used--in some cases they aren't moved at all. In this case, a fixed camera would cost less and do the same. If the PTZ is used by the operator as it can be (that is, panned, tilted, zoomed or a combination of the 3), it is highly likely that any event will be missed if that event is happening out of camera view. Oh, and we often see the PTZ cameras aimed into the sun or pointing up into the sky, or not working due to lack of maintenance.

Really all of this just points out why we need to focus our attention on system design, not individual components. We start by determining the consequence of loss of assets and identifying threats, then do a VA so we can see how effective the system is in protecting the asset from the threat. then someone makes a decision about how much risk they are comfortable with and we go from there. and there are other ways to manage risk than by implementing a security system (see my Chapter on risk!)."

Thanks, Maggie! I totally forgot that I joked about you emailing her. I think that most of what she looks at are far more complex systems than the everyday installs most integrators see, but I think they're sort of "trickle down" theories, and you can find something useful out of it. That's how I feel about her books. They're enlightening reading, and dated in sections, but there's still a lot to find useful.