The PRR-800 is designed to have a very good cosine response in water, and the PRR-810 is designed to have a very good cosine response in air — each is optimized for the environment they are intended to be used in. By "very good," we mean ±2% from 0° to 65°; ±10% from 65° to 85°, respectively. Both sensors have a "field of view" of 180 degrees. A good discussion of this cosine response is found here.
If you are using a PRR-800 in air because you do not have the normal PRR-810 (or PUV-2510) surface ("deck") versions, make sure that you don't apply the immersion coefficient. The accuracy of the cosine response will be less than optimum in this case, and the loss of accurate cosine correction will probably not be significant when the Sun is high in the sky. At lower solar angles, you will begin to see the effects of the cosine response error. Typically, the reported values will be somewhat higher than they should be because the Sun is lower and departures from the cosine law become significant. This effect will be more pronounced in the long wavelengths, and minor in the UV.