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# Three channel hemiSpherical Imaging radiometer (TSI)

Mentor: J.H. Schween

## Principle

We use a commercially available fish-eye surveillance camera (AXIS M3027-PVE) to collect images of the sky and determine cloud amount. Images with a resolution of 800×600 pixels (BMP format) are taken every 20sec. Every unsaturated pixel above the horizon is investigated for its blue-to-red-ratio and categorized as sky (blue), thin-cloud (light-blue) or cloud (white or gray). Pixels are counted and stored as relative cloud amount as well as palette based png images. From the number of saturated pixels around the sun a flag for the visibility of the sun is derived.

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). White balance is set to a constant value to ensure that sensitivity of the channels relative to each other is constant. Every 15minutes is varied exposure (collecting time of the sensor) and gain (amplifier within the sensor) which might be used to derive an absolute calibration.

The camera replaced a YES total sky imager which died after some years of operation.

## Data

Current sky image:

Click on figure for further plots.

The image shows the sky image, in the lower right corner the derived cloud mask and the upper right corner a histogram of the three channels within the image.

## History

Period Place
1.10.2015 - today Research Center Jülich, Germany

## Specifications

Parameter Specification Remark
Wavelengths / nm ~560, ~540, ~420 freq. of peak sensitivities of human eye (⇒R,G,B)
Repetition Rate / s 20 Adjustable
resolution / deg 0.15 Resolutiuon in deg/pixel at zenith (see here)
Field-of-View / deg 90 cloud classification downto horizon
image size / pixel 800×600 (=0.46MPx) whole image (adjustable)
image size / pixel 316 (=0.30MPx) radius to horizon
Sensitivity / (mV Lux$^{-1}$s$^{-1}$)) 1300
dyn.range / dB 69
max S/N / dB 37
Size / cm³ ~20x20x15
Average Power consumption / W < 4.5
Weight / kg 0.5 (?)