In physics, the phase problem is the problem of loss of information concerning the phase that can occur when making a physical measurement. The name comes from the field of X-ray crystallography, where the phase problem has to be solved for the determination of a structure from diffraction data. The phase problem is also met in the fields of imaging and signal processing. Various approaches of phase retrieval have been developed over the years.
Overview
Light detectors, such as photographic plates or CCDs, measure only the intensity of the light that hits them. This measurement is incomplete (even when neglecting other degrees of freedom such as polarization and angle of incidence) because a light wave has not only an amplitude (related to the intensity), but also a phase (related to the direction), and polarization which are systematically lost in a measurement. and electron crystallography.
Not all of the methods of phase retrieval work with every wavelength (X-ray, neutron, and electron) used in crystallography.
Direct (ab initio) methods
If the crystal diffracts to high resolution (<1.2 Å), the initial phases can be estimated using direct methods. and electron crystallography.
Density modification (phase improvement)
Solvent flattening
Histogram matching
Non-crystallographic symmetry averaging
Partial structure
Phase extension
See also
- Coherent diffraction imaging
- Ptychography
- Phase retrieval
External links
- An example of phase bias
- An appropriate use of 'molecular replacement'
- Learning crystallography