The cosine error of a class of in situ hyperspectral irradiance sensors largely applied for ocean colour investigations has been characterized for both in-air and in-water measurements. Results for in-air measurements indicate a slight wavelength dependence of the cosine error with differences up to 2% in the 412 nm to 865 nm spectral interval at 65° zenith angle (i.e. the angle of incident irradiance with respect to the normal axis of the irradiance collector). However, the dependence of the cosine error on the zenith angle is generally quite marked and significantly varies from radiometer to radiometer with values ranging from -5% up to +7% at 65°. Additionally, apart from the expected increase in the cosine error with the angle of incidence, in-air measurements generally indicate an irregular deviation from the ideal cosine response near the normal angle of incidence. A more pronounced increase in the cosine error is generally observed when radiometers are operated in water with respect to in air.
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