In this study we describe the second-order optical properties of retinal and related molecules. Hyper-Rayleigh scattering was used to measure the first hyperpolarizability of several retinal derivatives and to determine the solvent dependence of the first hyperpolarizability of retinal. This technique is also shown to be sensitive to fluorescence induced by multiphoton absorption, which precluded the measurement of trans retinol. By comparing the experimental results with the theoretical predictions obtained with the semi-empirical INDO/SCI/SOS method we show the validity of the two-state model for beta. The two-state model is extended to explain the large solvent dependence of the first hyperpolarizability of retinal, that is also related to the degree of bond-length alternation.