Coherence Scanning Calibration Procedures for High Aspect Ratio Tool Steel Sidewalls
Calibrating optical coherence scanners on tool steel sidewalls requires specular tilt mapping, transfer function correction, and noise floor subtractive adjustment.
Surface Height Bias
Meaning
Variation in material reflectivity and optical phase shifts can introduce systematic errors in the height measurements of multi-material surfaces. A surface height bias occurs when the optical profiler registers an incorrect height difference because of varying phase shifts upon reflection from different regions. This bias can distort the characterization of mold inserts containing distinct metallic and carbide phases.
Optical Shift
The reflection of light from a metallic surface is accompanied by a phase change that depends on the complex refractive index and the extinction coefficient of the material. When a tool steel contains embedded primary carbides, the phase shift differs significantly between the carbide phase and the surrounding iron matrix. This spatial variation in phase shift generates a false surface height bias during optical profiling, making a physically flat surface appear artificially textured or stepped on the resulting digital topography.
Polishing Analysis
Assessing the polish quality of injection molds requires distinguishing between true physical relief and optical artifacts. An unrecognized surface height bias can lead engineers to believe a mold cavity is rougher or has more carbide relief than it actually possesses. This incorrect diagnosis may result in unnecessary polishing cycles that over-round the cavities and degrade the mold geometry.
Accuracy Restoration
Metrologists correct this bias by applying phase correction algorithms based on the known optical properties of the constituent phases. Alternatively, a thin coating of gold or chrome can be deposited on the sample to unify the surface reflectivity and phase shift. This coating eliminates the phase shift variation, yielding the true physical profile of the mold.