Propagation-based phase contrast uses free-space propagation to get edge enhancement, Talbot and polychromatic far-field interferometry uses a set of diffraction gratings to measure the derivative of the phase, refraction-enhanced imaging uses an analyzer crystal also for differential measurement, and x-ray interferometry uses a crystal interferometer to measure the phase directly. The advantage of these methods compared to normal absorption-contrast X-ray imaging is higher contrast that makes it possible to see smaller details. One disadvantage is that these methods require more sophisticated equipment, such as synchrotron or microfocus X-ray sources, x-ray optics, and high resolution X-ray detectors. This sophisticated equipment provides the sensitivity required to differentiate between small variations in the refractive index of X-rays passing through different media. The refractive index is normally smaller than 1 with a difference from 1 between and .