Dimension: 32mm (1-1/4") in diameter approximately. A typical use would be a blue filter when using low levels of light in a brightfield application. (Like dry layer blood analysis.) When low levels of light are used, you usually get a yellowing effect because the lamp is turned low. The blue filter brings the light back to a more natural white light. Green filters are often placed in the light path to improve the performance of achromat and planachromat objectives. When using phase contrast the best images are often produced in green light. A typical use would be a yellow filter when the microscopy images are blue. The yellow filter brings the light back to a more natural white light.