Various optical apparatus, in particular embodiments, may provide a source of parallel light (7, 75). The parallel light (7, 75) may be generally achieved by directing an incident beam at the apex of a prism (1, 22, 24, 26, 28). The prism may have varying configurations. One configuration has a forward conical face (24). Another configuration has a pyramidal forward end (22). Other configurations are also disclosed. Various optical methods and methods for flow cytometry are also disclosed.