A method for determining a characteristic of tissue in a biological sample comprising: directing light at the biological sample at a first depth and receiving that light reflected from the biological; directing the light at a reflecting device and receiving that light reflected from the reflecting device. The method also includes: interfering the light reflected from the biological sample and the light reflected from the reflecting device; detecting light resulting from the interfering; and determining a first phase associated with the light resulting from the interfering based on the first depth. The method further includes: varying an effective light path length to define a second depth; determining a second phase associated with the light resulting from the interfering based on the second depth; and determining the characteristic of the biological sample from the first phase and the second phase.