Described herein are devices and methods for making extremely accurate measurements in a medium by continuously measuring the index of refraction of the medium such as water or biological tissue. Also described herein is a device for constantly measuring the index of refraction, and using the index of refraction data to constantly calibrate the optical measurement device. In addition, a primary measurement device (a ladar) that is optimized for data collection in a volume backscattering medium such as water or biological tissue is described, along with data results from the lab.