Exothermic oxidation of the chip. The delay in the exothermic reaction causes the chip to glow brighter after it has left the cutting zone than it did while in the cutting zone. This increase in brightness is far more than could be explained by a change in emissivity alone. Steel workpiece, 1200 m/min surface speed. Camcorder movie on the left is 60 frames per second and triggers a strobe to help freeze motion. Visible light movie on the right is 300 frames per second. Field of view is about 30 mm wide. Even though it is a visible light image, heat causes the chip to glow enough to be seen by the camera. Visible light movie in the center is the same as the movie on right, except that it updates the screen only when the image is sufficiently bright. In the beginning, this middle movie only updates when the strobe flashes. Towards the end, the chip glows so brightly that the image updates every frame. The bottom of the frame shows cutting forces and a once per revolution indicator. For more information on strobing, see NIST publication "Simultaneous Visible and Thermal Imaging of Metals During Machining".