Logarithmic 3-Band Color Encoding: Robust Method for Display and Comparison of Compositional Maps in Electron Probe X-ray Microanalysis

Abstract

Electron-excited X-ray maps recorded with the scanning electron microscope (SEM)/electron probe X-ray microanalyzer (EPMA) are a major method of presenting compositional information. Digitally recorded maps are processed in a variety of ways to improve the visibility of features. Scaling of the recorded signal to match the 8-bit gray-scale intensity range of a typical computer display system is almost always necessary. Inherent limitations of gray-scale displays have led to other intensity-encoding methods for X-ray maps, including clipping, histogram normalization, and pseudocolor scales. While feature visibility is improved by applying these scales, comparisons among image sets are difficult. Quantitative comparisons must be based on standardized intensities corrected for background to produce intensity ratio (k-value) maps. We have developed a new logarithmic, multiband color-encoding method to view these k-value maps more effectively. Three color bands are defined, starting with a dark primary color and grading to a bright pastel: blue = trace (0.001 to 0.01); green = minor (0.01 to 0.1); and red = major (0.1 to 1.0). Within each band, the color is assigned according to a logarithmic scale that depends on intensity ratio or compositional measurements. Logarithmic multiband color encoding permits direct comparisons of maps, such as maps of different elements in the same field of view or maps of the same element in different areas, because the color scale is identical for all maps.