Spectroscopy, the study of the interaction between electromagnetic radiation and matter, is a vital technique in many disciplines. This technique is limited to lab settings, and, as such, sensing is isolated and infrequent. Thus, it can only provide a brief snapshot of the monitored parameter. Wearable technology brings sensing and tracking technologies out into everyday life, creating longitudinal datasets that provide more insight into the monitored parameter. In this paper, we describe Lumos, an open-source device for wearable spectroscopy research. Lumos can facilitate on-body spectroscopy research in health monitoring, athletics, rehabilitation, and more. We developed an algorithm to determine the spectral response of a medium with a mean absolute error of 13nm. From this, researchers can determine the optimal spectrum and create customized sensors for their target application. We show the utility of Lumos in a pilot study, sensing of prediabetes, where we determine the relevant spectrum for glucose and create and evaluate a targeted tracking device.