An analyte (e.g., glucose) is tested in a sample (e.g., blood or blood-free interstitial fluid) having both the analyte and other constituents (e.g., protein). The analyte has an absorption peak over a narrow bandwidth at a characteristic wavelength. The other constituents have an absorption over a broad bandwidth. The broad bandwidth includes and is broader than the narrow bandwidth. Radiant energy (e.g., IR radiation) is directed at the sample. The energy has a source bandwidth including the broad bandwidth. The sample absorbs a portion of the energy. A remainder of the energy is available for analysis. The remainder is analyzed by filtering the remainder into an analyte portion and a reference portion. The analyte portion contains substantially only the narrow bandwidth. The reference portion contains substantially only the broad bandwidth. The analyte portion and the reference portion are measured and compared to calculate an amount of the analyte in the sample.