Abstract. All-sky cameras are frequently used to detect cloud cover; however, this work explores the use of
these instruments for the more complex purpose of extracting relative sky radiances. An all-sky
camera (SONA202-NF model) with three colour filters narrower than usual for this kind of cameras
is configured to capture raw images at seven exposure times. A detailed camera characterization of
the black level, readout noise, hot pixels and linear response is carried out. A methodology is
proposed to obtain a linear high dynamic range (HDR) image and its uncertainty, which represents
the relative sky radiance (in arbitrary units) maps at three effective wavelengths. The relative
sky radiances are extracted from these maps and normalized by dividing every radiance of one channel
by the sum of all radiances at this channel. Then, the normalized radiances are compared with the
sky radiance measured at different sky points by a sun and sky photometer belonging to the Aerosol
Robotic Network (AERONET). The camera radiances correlate with photometer ones except for
scattering angles below 10∘, which is probably due to some light reflections on the
fisheye lens and camera dome. Camera and photometer wavelengths are not coincident; hence, camera
radiances are also compared with sky radiances simulated by a radiative transfer model at the same
camera effective wavelengths. This comparison reveals an uncertainty on the normalized camera
radiances of about 3.3 %, 4.3 % and 5.3 % for 467, 536 and 605 nm, respectively,
if specific quality criteria are applied.
Correcting the Defects of the Default Tone Mapping Operators Implemented in the JPEGXT HDR Image Compression Standard
