Air-side walk-through light boxes. What are they?

To disembark in many European terminals, passengers must pass through a corridor with a bottleneck. On either side of the hallway there are tall, chubby boxes. The white or gray boxes are powered and sometimes illuminated.

Usually they are unattended, with red lettering reading "one person at a time" and ignored for the most part by passengers. Not to be confused with:

• light box ads
• body scanners
• metal detectors
• luggage screeners
• one way gates or airlocks

But referring specifically to these oversized panels which passengers must pass between. They are usually near passport control or near customs, prior to the arrivals lobby exits.

What are these?

(This image is the nearest can find to an example -- sometimes they are adjoined by overhead bar, sometimes not. Notice the absence of the boxes at the "goods to declare" passage)