The MacAdam ellipse (Standard Deviation of Color Matching, or SDCM) measures how much a light source's color can vary from a target color point before the human eye detects a difference. A 1-step SDCM is imperceptible; 2-3 steps are barely noticeable; 4-5 steps show visible color differences; 6+ steps are obvious mismatches. For commercial and industrial projects where multiple fixtures are installed side by side, specifying SDCM ≤3 (sometimes labeled '3-step binning') ensures all fixtures appear the same color to the naked eye. Budget fixtures may use 5-7 step binning, resulting in noticeable color patchwork on ceilings. Always check the datasheet for the SDCM specification.