Theguardian_ Roy Lichtenstein never shied away from the fact that his paintings were based on copies of comic strips. But some critics of one of the founders of American pop art have long argued that “copying” was too polite a term. Now, a new film alleges that hundreds of his pieces of work can be traced to other artists.

“It’s called stealing,” said comic strip artist Hy Eisman, who has just turned 96 and only recently discovered that Lichtenstein had reproduced one of his images in the 1960s. “I worked like a dog on this stupid page and this guy has $20m to show for it. If it wasn’t so tragic, it would be [funny].”

He vents his anger in a new documentary, titled WHAAM! BLAM! Roy Lichtenstein and the Art of Appropriation, which shows that the original comic artists had lived austere and even destitute lives. While Lichtenstein’s paintings sold for tens of millions of dollars, their originals had earned them just a few dollars.

Eisman, who has worked for almost 75 years on a huge range of publications, most recently the Popeye comic strip, is among more than 30 comic artists “appropriated” by Lichtenstein and who believe that they were cheated of recognition.

Recalling that his Private Secretary comic of 1963 was cribbed for Lichtenstein’s Girl in Window of the same year, he told the Observer: “I got paid very little for the page, something like $4. He was able to turn it into a painting and make millions. When I saw that he did that to other people, I thought it was a lousy thing to do. But until now I never thought I was involved.”

James L Hussey, the documentary’s director, said: “Eisman was unaware that Lichtenstein had appropriated his work until I contacted him. He was very shocked.”

Lichtenstein, who died in 1997, is widely regarded as one of the foremost artists of the 20th century. Along with Andy Warhol, he created the pop art movement by appropriating imagery from modern American culture. His work now changes hands for millions of dollars, with the most expensive, a painting called Masterpiece, reportedly selling for $165m.

The documentary features comic book expert David Barsalou, who has traced about 300 Lichtenstein works to other artists: “I thought, maybe there’s only four, five or six paintings that he did close to the originals. But it was hard to describe my feelings as I started discovering more and more over the years that his images were like just direct copies.”

They include Lichtenstein’s Blam of 1962, in which a pilot ejects from an exploding plane, just like Russ Heath’s All-American Men of War #89 of the same year.

Heath was appreciated for the high quality of his work, but he struggled financially. He died in 2018, having ended his days relying on charity for his food.

Michael Daley, a cartoonist, said: “Comic book artists are rightly aggrieved. Aside from outright plagiarisms, technically and artistically, they are generally far better draughtsmen than Lichtenstein, who couldn’t even correctly copy the properly drawn ellipse of a jet engine’s air intake. They were also superior in their artfully dynamic and expressive compositions, every one being a master of the graphic potency of a cropped image.”

But Bradford R Collins, author of the book Pop Art and professor of art history at the University of South Carolina, defended Lichtenstein: “It’s not plagiarism. It’s appropriation. With plagiarism, you’re stealing somebody’s work and using it for the same purpose they did. If Lichtenstein made comic books out of it, that would be stealing. But appropriation means you’re taking something and reusing it for a very different purpose, taking something out of a comic book and making it into a painting.

“I can understand why Eisman would feel angry. I would feel the same way. But artistically, it’s not plagiarism.”

Before his death, Heath had expressed frustration: “I couldn’t do anything because all the characters that I did draw for comic books were, at that time, owned by the comic book company. So, if they want to sue, they could sue and have a legitimate reason to sue. But they wouldn’t make enough to bother having a suit.”

In the documentary, a lawyer argues that Lichtenstein’s work would be considered transformative because in most instances he added new elements rather than making an exact copy.

Lichtenstein himself said: “My work has been accused of looking like the things that I copy, and it certainly does look like the things I copy… But it is that quality, whatever art is, that transforms the work of art to be something different.”

Hussey said: “When Lichtenstein did this in the 1960s, he was selling these paintings for thousands of dollars. That was an irritant to a lot of these comic artists. But the bitterness grew.

“As the years went by and as they moved into retirement, not only were they struggling financially, but they were also watching these paintings rise dramatically in value.”