This article reports the development and validation of a test instrument to assess secondary school students' declarative quantum optics knowledge. With that, we respond to modern developments from physics education research: Numerous researchers propose quantum optics-based introductory courses in quantum physics, focusing on experiments with heralded photons. Our test instrument's development is based on test development standards from the literature, and we follow a contemporary conception of validity. We present results from three studies to test various assumptions that, taken together, justify a valid test score interpretation, and we provide a psychometric characterization of the instrument. The instrument is shown to enable a reliable ([alpha] = 0.78) and valid survey of declarative knowledge of quantum optics focusing on experiments with heralded photons with three empirically separable subscales.