Abstract: A vein locator comprises an energy source for directing incident energy toward organism tissue. At least a portion of the incident energy is reflected from structures within the organism tissue as reflected energy. The vein locator comprises an energy detector connected to detect energy reflected from the organism tissue. The incident energy is moved across a scan region a first time and the energy detector produces a corresponding first signal representative of at least one characteristic of the first reflected energy. A controller receives the first signal, identifies an extremum in the first signal and determines a threshold level based on the identified extremum. The incident energy is then moved across the scan region a second time and the energy detector produces a corresponding second signal representative of at least one characteristic of the second reflected energy.