https://link.springer.com/chapter/10.1007/978-1-4613-8896-8_... "The inaccessibility of the retinal blood ves- sels precludes direct, noninvasive measure- ments of retinal blood flow. Indirect optical methods taking advantage of the visibility of the retinal vasculature must be used. A variety of methods have been applied.[...] Methods based on the perception of white blood cells (WBCs) moving in retinal macu- lar capillaries by means of the blue field entoptic phenomenon. Macular capillary blood flow is determined either by counting the number of WBCs passing in a given time through a single capillary 13 or by evaluating the average number and speed of WBCs in the field of observation using the blue field simulation technique. [...]"