Abstract. The continuous monitoring of industrial and environmental processes is becoming an increasingly important aspect with both economic and societal impact. So far, spectral-optical sensors with their outstanding properties in terms of sensitivity and reliability have not been considered as a potential solution because of the cost-intensive and bulky readout hardware. Here we present a card-size, inexpensive, and robust readout platform based on a wavelength-sensitive photodiode. In test and characterization experiments we achieved a wavelength shift resolution of better than 0.1 nm and a detection limit of 0.001 AU for ratiometric measurements. We furthermore discuss the capability and current limitations of our readout unit in context with interrogation experiments we performed with a photonic crystal-based fluid sensor. In sum we expect the presented readout platform to foster the exploitation of spectral-optical sensor technology for gas monitoring, chemical analytics, biosensing and many others fields.