Visibility measurement and wipers efficiency for road safety.

Modern wipers of vehicles windshields now can be considered as systems composed of many complex mechatronic subsystems: the blade, the rubber, electric engines, windshield itself, the rain sensor, the light sensor, the software of rain recognition, the software of automatical triggering, ...

Each of these sub-systems can take several states: example: 10 qualities of rubber, 10 settings for parameter threshold for triggering the wipers ... There are millions of possible systems, and the work of
R & D, integrators engineers is to find an acceptable solution in terms of cost and performance.

However, the most commonly used method to test the performance of these systems is to "wait for rain", and then, to let an expert drive and fill a qualitative rating scale. This method is obviously inefficient : weather is capricious, the experimental conditions are neither known nor reproductive (it's never the same rain, never the same light) cycle is long which limits the number of trials (when there are millions of possible settings), the measurement performance is very poor quality (the tester is not always in the same mental state, it is not always the same tester, ...).

To overcome this problem, some companies eased the work by sprinkling a windshield in the laboratory. But the water generated by sprinlers generally has physical characteristics very different from those of natural rainfall. In the absence of a rain machine capable of generating statistics of rainfalls like those of rain  (drop size, rainfall, impact angle, ...), the adjustment in industrial laboratories is inefficient.

Similarly, if one has such a rain machine, then remains to measure quantitatively the effectiveness of wiper system. However, this task is quite complex. Indeed, the windshield wipers have a mission to push the water out of the driver's sight as rain has the effect of misting our detection capabilities and pattern recognition (essential to car driving). But the discomfort of visibility is not a simple and increasing function of the amount of water deposited on the windshield. The drops of water can be seen as a zero-focal lenses that transmit in all directions. They are then very embarrassing for the vision through the windshield. On the opposite, clusters of water and plans runoff  are semi-transparent films that relatively few distort images in transmission.

Finally, the physical measurement of the amount of water on the windshield (wiped or remaining) is not a good indicator of the effectiveness of the wiping.

The only solution would be to measure the visibility for the driver through the windshield.

There is a complete system of measurement that includes a rain machine (RainNex ™) and a measure of visibility through the windshield (VisiNex ™). This system was developed by the French company NEXYAD.

The rain machine can be used for different applications (disruption of visibility by watering the windshield, watering capacitive systems for opening car doors, ...). It generates rains which are calibrated, repeatable, representative of natural rain, and known (to the smallest features: drop size, rainfall, ...).

The measure of visibility through a windshield uses mathematical models of human vision and applies psychosensory criteria. Broadly, we can say that our vision system needs a certain amount of information to be able to detect, recognize or identify shapes.

This information is distributed on three directions :
- The angular accuracy of images
- The contrast of objects from the background
- The number of possible gray levels  (eg: 256 levels of gray to a depth of 8 bits)

One can see very small objects provided that their contrast is very strong. On the opposite, if an object is very large and that many variations of gray are possible (for gradients), we detect it even with extremely low contrast.

All the compromises are not equivalent and VisiNex™ synthesizes, in view of an image captured through a windscreen (the latter being watered and dried), a quantitative score of visibility. This score is correlated with human impressions (by construction).

At such a measure, we find that the rain reduced the visibility score, and each sweep recovers a better visibility. The level where visibility goes, depends on the quality of rubber, of the windshield, of  wiper speed, ...  By varying these parameters, engineers can optimize their wiper systems (best compromise performance / cost).