In the vastly growing field of remote sensing, automated field spectrometer systems are of high value as ground reference, e.g., when measuring reflectance or solar-induced fluorescence. To calculate reflectance of a target surface, these instruments are typically equipped with cosine receptors to measure irradiance corrected for the cosine function of the incident lights angle. However, available cosine receptors suffer from poor cosine characteristics or low light throughput. We created an innovative cosine diffusor concept, which follows simple design rules and allows iterative manufacturing of a diffusor with accurate cosine response. Using a specifically constructed goniometer, we characterized common cosine diffusors and compared their performance against the innovative type. The new cosine receptor was able to produce a near-perfect cosine response even at lower light angles and higher light throughput. The novel diffusor concept uses robust materials, which promise long-term stability in outdoor use. The presented research improves the collection of precise, hyperspectral irradiance measurements toward recording of extended time-series.