Yea light pens were originally a way to add something like a touch screen to CRTs way back in the "old days" long before alternative displays were feasible let alone better. As far as touch screens using power, they don't as I've known, or at least they don't have too. the same tech that makes up touchpads is usable to make touch screens (and in fact it is on your smartphone and tablet). That may not have been true back with the resistive touch sensors such as those on the palm pilot and such.