Fig. 2. The reflective signals at 650 nm and 532 nm are shown while the stylet reached ligamentum flavum and epidural space in an in vivo  study. The y-axis is the magnitude of light reflected from the tissues (the peaks represent 650 nm wavelength whereas the troughs indicate the 532 nm wavelength). The x-axis indicates the time course of the modulation for 650 and 532 nm during the insertion of the needle into the tissue. (Dotted line ) The reflected light when the needle is located in the ligamentum flavum and (solid line ) the light reflected from the epidural space. The two lines illustrate the reflectance that are obtained at different times (i.e. , when the needle is in two different locations) but are superimposed to provide a visual comparison.