The HSL/HSV color spaces are not considered "perceptually uniform" primarily because they fail to guarantee that numerical differences in the color space are proportional to differences in color perceived by humans. A more detailed analysis follows:
HSL/HSV numerical differences do not match perceptual differences
In HSL/HSV, hue (H) is expressed as an angle, while saturation (S) and lightness/value (L/V) are expressed as percentages. However, two colors can appear similar even if their numerical values differ significantly. For example:
Similar hues but extreme values: H = 0° (red), S = 100%, L/V = 100% vs. H = 359° (close to red), S = 100%, L/V = 100% – the numerical difference is significant, yet the visual perception is almost the same.
Extreme lightness/values but appear dim: H = 60° (yellow), S = 100%, L = 1% vs. H = 240° (blue), S = 25%, L = 1% – the numerical difference is significant, yet both appear close to black.
Comparison of Perceptually Uniform Color Spaces (e.g., CIELAB)
CIELAB quantifies color differences using mathematical formulas (e.g., ΔE = √[(L₂-L₁)² + (a₂-a₁)² + (b₂-b₁)²]), ensuring that numerical differences align with perceived differences. HSL/HSV, however, lacks this mechanism, resulting in numerical changes that fail to accurately reflect human perception of color differences.
The Design Intentions and Limitations of HSL/HSV
HSL/HSV simplifies color description using cylindrical coordinates (e.g., H represents position on the color wheel, S represents saturation, and L/V represents lightness/darkness), but this simplification sacrifices perceptual uniformity. They are better suited for intuitive color adjustments (e.g., in art design) but are unsuitable for scenarios requiring precise perceptual quantification (e.g., calculating color differences).