A corporal in the US armed forces, Ledger was selected to attempt to harness the thought-controlled power source from Hyperion's spaceship. Previous subjects were not able to be single-minded or focused enough to control it, and so it was hoped that Joe would be able to control the crystal without letting his mind wander and the disastrous effects that would follow.
Ledger was successfully able to use it as intended, destroying targets. Firing energy bursts through his hand, the crystal caused him serious pain, but no injuries. Further use caused the crystal to fuse into his hand, all attempts to remove it failing, and killing the one trying to touch it. The accident caused Joe to fall into a coma for a lengthy period of time, during which he seemed to be having a battle of wills with the crystal. Despite having emerged from the coma, able to control energy emitted from the crystal, it has yet to be seen if he really won that battle.
While in the armed forces, he had gained a reputation for being a "Surgeon" of sorts during covert operations. That coupled with the colorful effects of the energy he controls earned him the name "Dr. Spectrum." The original (and perhaps continued) goal of the project was to have a counter to Hyperion and a means to control him.
This version of Doctor Spectrum was created by Roy Thomas and Sal Buscema in 1969 and first appeared in The Avengers #85. 16 issues earlier, Kinji Obatu appeared as the Doctor Spectrum of the Squadron Sinister. Unlike the other members of Squadron Sinister, this version of Doctor Spectrum isn't the same person.
Powers and Abilities
With the Squadron Supreme originally being a parallel of the Justice League, Dr. Spectrum fills the Green Lantern role.
He controls energy from the crystal power source of Hyperion's spaceship. He is able to use it to encase himself with the energy, protecting himself, and giving himself an air supply for space or underwater travel. This also lets him fly at incredible speeds.
He can fire blasts of energy to devastating effect.
The Doctor Spectrum(s) of Earth-712 and Earth-616 can also create solid objects with their power prisms, which is often used to great effect.
The crystal seems to be much more than just a power source. It responds to the thoughts of the one closest to it, and since merging with Joe Ledger, has responded only to him. However, the crystal itself has a mind of its own, and when it wishes, can apparently take control of Ledger, though he doesn't remember these occasions when it does. " Power Princess" has seemed to know much more about the crystal than she has let on, speaking with it while its in control. What these two are scheming toward is still vague and unknown.
Ledger has had his background fleshed out much more than most members of the Squadron Supreme, especially during the time when he was in a coma, speaking with the crystal.
Though promising, he never really did return to his girlfriend from before he joined the military. He takes his duty very seriously, and is extremely loyal to his superiors in the military/government.
Once Hyperion was forced to go public with his abilities, the government used Spectrum for black ops missions. They could then deny their own involvement, since Hyperion had alibi's doing "the superhero thing." Hyperion, in an attempt to track down others like himself, and believing the government may be hiding others like him, tracked Spectrum down, and the two faced off in a devastating battle, and destroyed a large chunk of the rain forest.
Spectrum befriended " Kingsley," an aquatic mutant female while healing after his battle with Hyperion at the bottom of the ocean. Bonding mentally with her Ledger took her to the surface world and ensured they were in the Squadron Supreme when it was started.
In Other Media
Doctor Spectrum appears alongside the rest of the Squadron Supreme in the Avengers Assemble episode "Dark Avengers," sporting his original appearance. As an in-joke, he is voiced by Phil LaMarr, the actor who voiced Green Lantern (the character Doctor Spectrum is a take-off of) in Justice League and its sequel, Justice League Unlimited. This version of the character draws his power from the Reality Gem.