Photoelectric caliper for noncontact measurement of vascular dynamic strain in vitro

A photoelectric device is described for in vitro semicontinuous contact-free measurement of vascular dynamic strain. The vessel's optical image is projected onto a pair of linear photodiode arrays by use of a projection lens and a set of mirrors. Each array's video signal indicates a focused image of the vessel edge as a steep change of the light intensity. Edge location is defined as the midpoint of the intensity drop across the edge. Knowing the interdiode distance (16 micron), the location of the edge can be determined by counting the number of diodes up to the midpoint. Given both edge locations and the distance between arrays, diameter can be electronically computed. The output voltage is calibrated with a metric grid in place of the vessel and is linearly related to diameter. Resolution varies with magnification and may be on the order of 1 micron, depending on the strain amplitude and the initial unstressed vessel caliber. Frequency response is determined by the array scanning rate and is uniform well beyond the range of physiological considerations.