This disclosure provides a method of determining the viability of at least one cell via quantification of nicotinamide adenine dinucleotide (phosphate) (NAD(P)H), flavin adenine dinucleotide (FAD), and/or collagen. The method includes the steps of contacting the at least one cell with light having a wavelength of from 700 to 900 nm, using two photon excitation, or from 335 to 400 nm, using one photon excitation, to induce an optical response from the NAD(P)H, FAD, and/or collagen and measuring the optical response. The method also includes the step of quantifying one or more of an amount, spatial localization, and/or time-dependent response of the NAD(P)H, FAD, and/or collagen utilizing the optical response.