Optical techniques are ideal for mapping the properties of the solution during a crystal growth experiment. Since a probe need not be introduced in the field of study, they are non-intrusive and practically inertia free. Over the past decade, laser measurement techniques have become popular, though primarily as a flow visualization tool. Recent work however has emphasized the possibility of quantitative measurements as well. A majority of optical techniques are field techniques in the sense that an entire cross-section of the physical region can be mapped. They require the medium to be transparent, and are thus suitable for the measurements in liquids. Optical methods that utilize the dependence of refractive index of light on density (and indirectly on concentration and temperature) can be configured in many different ways. Three available routes are:

• Interferometry, where the image formation is related to changes in the refractive index with respect to a reference environment,
• Schlieren, where light deflection in a variable refractive-index field is captured, and
• Shadowgraph, where the reduction in light intensity on beam divergence is employed.

In the context of crystal growth from an aqueous solution, a unique relationship can be established between the refractive index and the local density of the medium under study. Under normal process conditions, the solution is practically incompressible and the density does not depend on pressure. At any time instant, the solution is also at a spatially uniform temperature. Hence, density changes correlate with those in concentration alone, and the three methods become applicable for concentration field measurement in the fluid medium. The three techniques referred above yield the path integral of the density field in the direction of line-of-sight. The distribution of density in three dimensions can then be extracted from the recorded images using principles of tomography.