A time-domain measurement can be equivalently expressed in the frequency domain. Some researchers have developed imaging systems which acquire frequency-domain data directly using a source that is amplitude modulated at a high frequency (a few hundred MHz), and measuring the reduction in amplitude and phase shift of the transmitted signal. While time-resolved measurements have the advantage of acquiring information at all frequencies simultaneously, frequency-domain systems can employ light sources and detectors which are significantly less expensive than those required for time-resolved systems.