I’m not entirely sure how popular this guy is these days, but I’d like to throw in my two cents on his work. I’ll preface this by saying that I myself began working on sketches and engravings of circular forms after experiencing anew a recuring hallucination in which I looked at a person’s face, and all I saw was rotating wheels of color in place of eyes, mouths, noses, etc (don’t worry, I have very benign hallucinations that occur regularly 
). When I discovered Delaunay, it was like I understod myself better than I could have without him.
Anyhow, so, this painter Robert Delaunay was a neo-impressionist at first. He wanted to be the next Seurat- a goal I could never find fault with. The more he studied and the more he painted, the brush strokes became increasingly large and no longer served the purpose of mixing in the beholders eye to create Seurat’s “ghost colors and shades.” He became increasingly fascinated, much like the Symbolists Alphonse Mucha, Frantisek Kupka, and the young Vassilii Kandinskii in pure colors. Of course, Gauguin and Maurice Denis had a lot to do with this.
The amazing thing that happened was this: based on Symbolist theory, Delaunay, Kupka, etc., started developing “Orphic” perspective- that is, a “universal identification with all places and things,” or identification with the unconscious all consciousnesses in the world. The late Symbolist, early Modernist Guillaume Apollinaire, one of Delaunay’s closest friends, dubbed the art “Orphic,” based on his Symbolist take on the decapatation of Orpheus- Orpheus becoming the purified Prophet/poet in his decapatation- and appropriated the “Orphic” perspective into his poetry. Orphic painting uses pure color fields, related closely to Maurice Denis and Gauguin’s art theory, to express the “essential truth” of an object. That is, if an object is green, you should paint it as green as it seems to you, regardelss of the darker and lighter patches of shadow and light, as the green is the “truth.”
Delaunay moved on to paint in circular forms, largely influenced by the Wagner cult’s ideas of “music” in art, the brilliant colored circle being the rhythmic truth of many objects in the world. I find this fascinating. I rate Delaunay higher than the later Kupka, although the early work of Kupka and Mucha is some of the best Art Nouveau ever conceived of. Sorry for the Art History rant, but I really had to put all this in one place, as I’ve read from three books on him and another on his friendship with Apollinaire lately. Sometimes, one discovers and artist and can’t figure out how so few ppl know about him/her. Hopefully that was interesting for you Thanks for reading. Check out this painting of his!
