Mentor: J.H. Schween
A Total Sky Imager (TSI) is used to identify cloud types and evaluate cloud coverage. Images from a common electronical webcam are evaluated for the 'blueness' of single pixels. Sky pixels are 'blue', thin clouds appear 'light blue' and thick clouds appear 'white'. The details of the classification are proprietary to the manufacturer.
The instrument could also be called a Three channel hemiSpherical Imaging radiometer: channels are in the visible range with average wavelengths of ~650nm (red), ~510nm (green), ~465nm (blue), have a broad spectral sensitivity and do overlap. Peak sensitivity wavelengths are at 560nm (red), 530nm (green) and 430nm (blue) (at least these are the peak sensitivities of our visual system which a camera tries to mimic). But unfortunately the system is not calibrated: the camera adapts its exposure to received light (i.e. sensitivity changes in time) and it also performs a white balance, i.e. the sensitivity of the three channels relative to each ohter is not constant. We even do not know whether the sensitivity is linear, logarithmic or anything else.
YES Inc. TSI-880 at Forschungszentrum Jülich on the roof of the IEK-8 (Institut für Energie- und Klimaforschung: Troposphäre). In the background the 120m met-tower.
Current sky image:
Click on figure for further plots.
The image shows on the left side the sky image and on the right side the cloud mask as provided by the instrument.
|Wavelengths / nm||~560, ~540, ~420||freq. of peak sensitivities of human eye (⇒R,G,B)|
|Repetition Rate / s||20||Adjustable|
|resolution / deg||0.67||Resolutiuon per pixel at zenith (see here)|
|Field-of-View / deg||87||cloud classification only downto 3deg above horizon|
|image size / pixel||291 x 352||whole image|
|image size / pixel||~130||radius to horizon|
|Size / cm³||41 x 36 x 71|
|Average Power consumption / W||50-500||Heater off-on|
|Weight / kg||32|