Embodiments of the present invention provide a system and method for determining the location of the bottom of a fluid chamber in a surgical cassette to calibrate the surgical system. According to one embodiment, the surgical cassette is shaped to allow light from a light source to pass unobstructed to a linear sensor array in an area that is a known distance from the bottom of a fluid chamber. Light rays intersected by solid cassette material, on the other hand, will insufficiently illuminate the linear sensor array. Because the slot is a known distance from the bottom of the chamber, the location of the bottom of the chamber can be determined by determining the transition between illuminated and insufficiently illuminated portions of the linear sensor array.