Mars Soiling Sensor measurement principle and calibration

The Mars™ Soiling Sensor determines transmission loss through its soiling collection window using a camera system which looks up at the sky through the window. The camera is focused to see the outer layer of the window, including any soiling particles present. When soiling particles collect on the window they block light from the sky from reaching the camera. 

Measurement consists of determining the amount of light from the sky lost at each pixel of the images collected by the camera. The Mars sensor automatically performs its analysis each day around sunset time, when conditions are optimal. The sensor must be positioned outdoors with a clear view of the sky to perform a measurement, as shown in the Mars 810230-20 User Guide.

Calibration fiducial marks are printed on the inside of the Mars sensor's window. These marks allow the sensor to automatically determine reference pixel intensity levels for performing the measurement, including intensity levels corresponding to totally absorbing soiling particles (black marks) and intensity levels corresponding to totally reflecting soiling particles (white marks). By referencing each pixel intensity between the black and white marks, the relative transmission of light through each pixel is determined. Note that both black and white marks appear dark in the Mars camera images because both marks cast shadows and block light from the sky.

Calibration of the Mars Soiling Sensor has two components:

1. Determination of alignment mark positions on the camera: This step is performed at Atonometrics prior to shipment. The Mars unit window is cleaned and images are acquired. Automated software determines the positions of the marks and these are recorded in the calibration settings of the device.
2. Determination of reference intensity levels for image analysis: This step is performed each time Mars does an analysis. It automatically extracts the intensity levels of the fiducial marks on the window as seen in the image and uses these as reference values for analyzing the image pixels corresponding to soiling particles, determining the relative transmission loss of each pixel in the image by comparison to the known fiducial marks. Thus Mars is self-calibrating at each analysis. 

At Atonometrics prior to shipment, each unit is tested with test samples to ensure readings are in the correct range.

The Mars principle and self-calibration approach is covered by multiple patents, including those listed on our Patents page.

For more information please see this 2018 EU PVSEC research article.