We used visible spectroscopy of fat to discriminate lambs that were pasture-fed (n = 76), concentrate-fed (n = 79) or concentrate-finished after pasture-feeding (n = 69). The reflectance spectrum of perirenal and subcutaneous caudal fat was measured at slaughter and 24 h post mortem. In Method 1 (W450–510), the optical data were used at wavelengths in the range of 450–510 nm to calculate an index quantifying light absorption by carotenoids. In Method 2 (W400–700), the full set of data at wavelengths in the range of 400–700 nm was used to differentiate carcasses using PLS-DA as a classification method. W400–700 proved more reliable than W450–510 (P < 0.0001). The proportion of correctly classified lambs using W400–700 was 95.6% and 95.9% for measurements made on perirenal fat at slaughter and 24 h post mortem. The intensity of light absorption by carotenoids decreased exponentially with live weight gain during the finishing period