The central technological conceit of the movie is the lifeclock, a rosette crystal that is implanted in each citizen’s left palm at birth. This clock changes color in stages over the course of the individual’s lifetime.

Though the information in the movie is somewhat contradictory as to the actual stages, the DVD has an easter egg that explains the stages as follows.

White		Birth to 8 years
Yellow		9 to 15 years
Green		16 to 23 years
Red		24 years to 10 days before Lastday (30 years)
Blinking Red		from 10 days before Lastday to Lastday
Black		End of Lastday (Carousel/death)

Lifeclocks derive their signal and possibly power from a local-area broadcast in the city. When Logan and Jessica leave the city their lifeclocks turn clear.

The signal of the lifeclock is so central to life that most citizens dress exclusively in colors that match their lifeclock color. Only certain professions, such as Sandmen and the New You doctor, are seen to wear clothing that lacks clear reference to a lifeclock color, even though the individuals in these professions have lifeclocks and are still subject to carousel at Lastday. We can presume, though are not shown explicitly, that certain rights and responsibilities are conferred on citizens in different stages, such as legal age of sexual consent and access to intoxicants, so the clothing acts as a social signal of status.

As an interface the lifeclock is largely passive, and can be discussed for its usability in two main ways.

Color

The first is the color. Are the stages easily discernable by people? The main problem would be between the red and green stages since the forms of red-green color blindness affects around 4% of the population. To accommodate for this, reds are made more discernable with a brighter glow than the green. As a wavelength, red carries the farthest, and blinking is of course a highly visible and attention-getting signal, which makes it difficult for an individual to socially hide that his or her time for carousel has come.

Black is a questionable signal since this indicates actual violation of the law but does not draw any attention to itself. Casual observation of a relaxed hand with a black lifeclock might even be mistaken for a colored lifeclock in shadow, but as the citizenry has complete faith in the system and a number of countermeasures in place to ensure that everyone either attends carousel or is terminated, perhaps this is not a concern.

But if we’re just going on human signal processing, the red should be reserved for LastWeek, and a blinking red for after LastDay. That leaves a color gap between 24 and 30. I’d make this phase blue, since it looks so clearly different from red. The new colors would be as follows.

White		Birth to 8 years
Yellow		9 to 15 years
Green		16 to 23 years
Blue		24 years to 10 days before Lastday (30 years)
Red		from 10 days before Lastday to Lastday
Blinking Red		End of Lastday (Carousel/death)

Location on the body

The second question is the location of the lifeclock. Where should it be placed? It is a social signal, and as such needs to be visible. The parts of the body that are most often seen uncovered in the film are the hand, the neck, and the head. The neck and head are problematic since these are not visible to the citizen himself, useful for reinforcing compliance with the system. This leaves the hand.

Given the hand, the palm seems an odd choice since in a relaxed position or when the hand is in use, the palm is often hidden from view of other people. The colored clothing seen in the film show that a citizen’s life stage is not really considered a private matter, so a location on the back of the hand would have made more sense. To keep it in view of its owner, a location on the fleshy pad between the thumb and the forefinger would have made a better, if less cinematic, choice.