It is based on the transmission of ultrasonic plane waves into the brain tissue. (sciencedaily.com)
«Previous works on nonlinear optical effects in plasmonic oligomers have been based on using plane waves or focused beams with homogenous, i.e., uniform, states of polarizations. (sciencedaily.com)
If you know that your electrons are fairly free for example, it is sensible but not essential to start with a basis which includes that property as built in (e.g. augmented plane waves). (realclimate.org)