Spectral Model eXplainer: a chemically-grounded explainability framework for spectral-based machine learning models

2026-05-04

AI Summary

This work addresses the limitations of existing explainable AI methods in interpreting spectral machine learning models, which often overlook chemically meaningful contiguous spectral regions. To bridge this gap, the authors propose SMX, a novel framework that incorporates chemical prior knowledge into model interpretation by leveraging expert-defined spectral intervals to deliver global, post-hoc, and model-agnostic explanations. SMX integrates principal component analysis (PCA), logical predicates, and perturbation analysis, and employs a directed weighted graph to aggregate region-wise importance scores. Furthermore, it introduces a threshold-based spectral reconstruction technique to intuitively visualize explanations directly in the original spectral domain. Experimental results across eight real-world datasets—comprising six X-ray fluorescence (XRF) and two gamma-ray spectra—as well as one synthetic benchmark demonstrate that SMX significantly outperforms current approaches in both explanatory fidelity and interpretability.

Abstract

Spectral-based machine learning models have been increasingly deployed in chemometrics and spectroscopy, where predictive accuracy is as important as explainability. Current employed eXplainable Artificial Intelligence (XAI) methods are largely adapted from tabular or generic multivariate domains, assigning relevance to isolated spectral variables rather than to the chemically meaningful spectral zones. Widely adopted tools such as SHapley Additive exPlanations (SHAP), Permutation Feature Importance (PFI), and Variable Importance in Projection scores (VIP) were not designed for the physical continuity and high collinearity of spectral data, and their variable-level outputs require post-hoc aggregation to recover zone-level information. This study introduces the Spectral Model eXplainer (SMX), a post-hoc, global, model-agnostic XAI framework that explains spectral classifiers through expert-informed spectral zones. SMX summarizes each zone via PCA, defines quantile-based logical predicates, estimates predicate relevance with perturbation in stochastic subsamples, and aggregates bag-wise rankings in a directed weighted graph summarized by Local Reaching Centrality. A key component is threshold spectrum reconstruction, which back-projects predicate boundaries to the original spectral domain in natural measurement units, enabling direct visual comparison with measured spectra. The method was evaluated on eight real spectral datasets (six based on X-ray Fluorescence--XRF and two based on Gamma-ray Spectrometry) and one synthetic benchmark with known gr

Problem

• spectral data
• explainable AI
• chemically meaningful zones
• variable importance
• spectroscopy

Innovation

• Spectral Model eXplainer
• chemically-grounded explainability
• spectral zones
• threshold spectrum reconstruction
• model-agnostic XAI