a device , system , and method for measuring the depth of a material layer such as a blood vessel plaque layer is disclosed . a fiber optic bundle housed in a balloon catheter projects a laser dot toward a conical mirror , which reflects the dot perpendicularly onto the surface of the plaque . the laser dot is reflected back from the plaque layer with a substantially gaussian intensity profile . the conical mirror directs the reflected image back to the fiber optic bundle , which delivers the image to a sensor . the depth of the plaque layer can be determined by comparing the diameter of the image intensity profile to a pre - obtained normalized data set .