Interface design challenge: How do you present the same information to everyone, yet make sure parts of it are hidden to a select subgroup of viewers?

The ANAR Foundation operates a helpline for children facing aggression and maltreatment. Minors at risk can call the number 116 111, which ANAR wanted to advertise and spread the word about. But here’s the problem: the aggressors are often the same adults that accompany the child. They will prevent the child from taking note of the number, or looking at any ad with the helpline number on it. Worse, they may brutalize the child even worse to scare him or her off asking for help.

How can an ad, or a poster, be made so that an adult cannot see it, yet a child can? More generally, how can you design an interface that transmits a message to a certain group yet makes it “invisible” to all others, without resorting to face recognition, logins, and other explicit checks?

For starters, it’s good to identify where the two groups differ from each other in some physical way that impacts sensory capabilities. For example, children can hear tones up to 20kHz, while adults cannot. Color-blind people can see colors and patterns that are different form what normal-vision ones see. Older women hear, on average, better than older men.

Language is another obvious way to differentiate between groups. What is not so obvious, for example, is that children have their own “game languages” (pig latin is the first that comes to mind). Such languages are not so straightforward for adults to interpret, while being a fun and trusty way of addressing children.

A third way to send a message only to one group is to look for physical differences that prevent one of the groups from accessing the message. Alternatively, look for differences that can be used to physically direct the message one way or another. Highway traffic radio is one way of directing messages: drivers get that information (their radio receivers physically direct the message to the driver), but other listeners don’t (their radio receivers filter the traffic signal as noise). To come back to the initial challenge: adults are taller than children, so the angle at which they see a poster is different from the angle at which a child sees the same poster.

Already in these examples are two ways how ANAR’s message could be spread. You could make an audio advertisement at very high frequencies, audible only to children. Transmitted over the radio, it would be impossible to hear by adults. Or you could exploit the different viewing angle and project one message to the higher angle (adults), while projecting a different message to the lower angle (children). The “different projections at different angles” is the same technique that is used in “magic 3D postcards”, or in “magic rulers”, where the picture changes as you rotate the postcard or ruler. The proper name for the technique is lenticular printing.

This technique is what ANAR decided to use for their poster. The high-angle view, for adults, shows a general message. The low-angle view, for children, shows the phone number. This way, an adult would never even perceive that the child is seeing something else. The helpline number is, effectively, hidden in plain sight.