How Do I Use PPF Appropriately?

In the IPVMU class, we are studying PPF using our PPF guide. Now that we have gone through the detailed math and issues involved, one attendee had a very good question that I wanted to answer here:

Should you determine your PPF requirements first and then lighting, lenses and real world issues? Or the other way around?

The challenge here is that both sides are intricately connected. Here's how I recommend approaching the problem:

1 - What is the expectation of the end user for the camera? Do they expect to get detailed images of suspects that they can send to the police to find an unknown intruder or is it ok to get a fuzzy outline of people? This tells me where to start. We may need to come back to the end user and reconsider later but start here. If they detailed images, you are easily looking at 50ppf+.

2 - How wide and deep an area does the end user want to cover? Is it a narrow short path or an entire parking lot, etc.? This helps in understanding lens options.

Of course, you may have an issue where the end user wants something totally unrealistic, like a single camera getting facial details across a 300 stall parking lot. Even before you get into lighting issues, you'll need to educate and reset expectations.

3 - At this point, I would start looking at lighting issues: Where does the sun rise? Where does it set? How strong of an impact will it have? At night, what lights are on? How strong are they?

If there are issues with the sun, I might keep ppf requirements about the same but require a true multi-imager WDR camera. If the scene is very dark (no direct street lighting), you almost certainly are going to either need to (1) add IR, (2) add street lighting or (3) set expectations of very poor images at night. Even with more light, the images still won't be as good but they typically will deliver a clear outline of subjects.

Ultimately, I feel it is an iterative process whereas your education of the end user helps them better appreciate the real world tradeoffs and allows them to chose what they want to give up.

The reality is you can't have super wide FoVs, super high detail, and no concerns about lighting - even though understandably every user would love that. Work through these elements, explain them and find the most acceptable tradeoffs given the end user's preferences.

There's a lot more we can discuss here. I just wanted to start with a few thoughts.


When I'm getting down to that level of detail on something I tend to follow a similar process. I first want to find out "what does the person writing the check expect from the job?". A not uncommon quote of mine is that "we can solve any problem, it's just a matter of money". Usually then things will veer towards a sort of "here's what you can get for your budget, here's what it would take to match your expectations", and so forth. If you approach it reasonably with good explanations, most people will understand that you're not trying to rip them off or sell them the wrong thing.

Once we've figured out a practical budget, and a practical set of expectations you can look at the environment and find the right combnation of cameras, positions, lenses and lighting. I think it's hard to have a real solid recipe to follow here because you're going to be dealing with different versions of the same variables with every job. I guess a big part of my approach is always trying to educate the customer enough so that we can have a meaningful dialog and know that we're working with a practical budget towards reasonable expectations. Tackling that early on seems to provide more efficiencies for me.

Good article. This really helped me understand PPF better. Thank you.