  Improved miniaturization capabilities for complex fisheye camera systems have
recently led to the introduction of many compact 360-degree cameras on the
consumer technology market. Designed primarily for recreative photography,
several manufacturers have decided to allow users access to raw imagery for
further editing flexibility, thereby offering data at sensor level that can be
directly exploited for absolute-light quantification. In this study, we
demonstrate methodologies to carefully calibrate a consumer-grade 360-degree
camera for radiometry use. The methods include geometric calibration,
assessment of the illumination fall-off across the image plane,
spectral-response determination, absolute spectral-radiance calibration,
linearity, and dark-frame analysis. Accuracy of the calibration was evaluated
by comparing sky radiance measurements with a co-localized Compact Optical
Profiling System (C-OPS, Biospherical Instruments Inc.), which gave mean
unbiased percentage differences of less than 21 %. Using the photon-transfer
technique, we calculated that this camera consisting of two fisheyes with a
182$^\circ$ field of view in air (152$^\circ$ in water) has a limit of
detection of at least $8.0 \times 10^{-7}$ $\mathrm{W \cdot sr^{-1} \cdot
m^{-2} \cdot nm^{-1}}$ in its three spectral channels. This technology, with
properly stored calibration data, may benefit researchers from multiple
scientific areas interested in radiometric geometric light-field study. While
some of these radiometric calibration methods are complex or costly, this work
opens up possibilities for easy-to-use, inexpensive, and accessible radiance
cameras.
