A passer-by is someone who walks past the front of a store, concession, or display. Counting passers-by tells you how much demand the location delivers before the store itself does anything. It is the top of the physical conversion funnel: passers-by become visitors, and visitors become customers.
Passer-by counts answer a question footfall on its own cannot: when visitor numbers fall, did fewer people walk past, or did the same number walk past and fewer come in? The first is a location problem, owned by the landlord or the centre; the second is a storefront problem, owned by the retailer. Footfall ÷ passers-by is capture rate, the metric that separates the two.
Measuring passers-by is harder than measuring footfall because it means counting externally on the street or concourse, not inside the store at the door. An interior door camera rarely has a usable view of the pavement outside; the angle is wrong and the window display obscures the view. In practice passer-by counting needs either a dedicated camera with an exterior view or a purpose-built sensor mounted on the shopfront. A line drawn across the view, parallel to the frontage, counts everyone who crosses it in either direction.
What counts as a passer-by also needs defining. A person on the far side of a wide mall concourse is not realistically available to a store; someone on the near side is. The counting line is placed where a person could plausibly be drawn in, usually within a few metres of the window, and the same rule applies across every store in an estate so the numbers compare.
Glimpse counts passers-by in two ways, either using a external-facing CCTV camera where one exists, or a Glimpse Sensor: a small motion sensor mounted on the shopfront that counts passers-by events without capturing images. The same sensors can also be used inside the store to count people walking past displays or retail media.