The high translucency of embryonic tissue provides a large S/N in absorption measurements, and thus, absorption, rather than fluorescent, VSDs are commonly employed in embryonic preparations. In absorption measurements, signals arise from a baseline of high intensity transmitted light. Under this condition, a photodiode array is preferable to a charge coupled device or complementary metal oxide semiconductor image sensors. The optimal and most commonly used VSD in embryonic tissue is the merocyanine-rhodanine dye, NK2761.10 Although oxonol dyes, such as RH155 (NK3041) and RH482 (NK3630), give strong signals in other preparations, they are inferior to NK2761 for early embryonic preparations. NK2761 was designed by Dr. Kamino as an analogue of Dye XXIII.11 Thus, advances in optical studies on embryonic preparations have benefited from the development of merocyanine dyes in early studies by Dr. Cohen and his colleagues.