How thermal image looks when sand or mud deposited on the camera lens?

[Suganthi]

IR radiations wont pass through glass material. For automotive applications, if mounted near bumper, say for example, there is a possibility of sand or mud getting deposited in due course. I am interested to see or understand , how thermal/ IR image looks when some sand or dust or mud or water deposited on camera lens? And also, the conditions both climatic and environment conditions that are likely to degrade the contrast of the image. Any suggestions and insights are highly appreciated. thanks in advance.

[DiodeDipShit]

Infrared sensors detect heat. Therefore can not be obstructed. Best bet is to recess the sensor out of the environment.
Would a water repellant on the IR lens help by beading ?
Automotive proximity sensors will sense magnetic fields or utilize ultrasonic waves that are similar to sonar.
 These are much lest effected buy road dust.
Of coarse blobs of off road mudding may reduce the effectiveness of detection, but who does this in their shiny $65K Jeep?

[Gyro]

From Fraser's various threads on automotive night vision cameras ( Raytheon and others), the coated Germanium lenses look to get fairly rapidly beaten up.

[DaJMasta]

A cheap way to get an idea: use a standard camera and bin the pixels or post process it to be much lower resolution - something in the ballpark of the camera you're intending to use.  Ideally pick something with a similar diameter lens and a similar resolution after processing.

Since the image plane is not in focus at the lens elements, you won't be getting one-to-one obstructions as you wouldn't get with any lens (unlike dust on the sensor itself, which is much worse), and because of the large pixel size, thermal cameras are generally more tolerant of lens imperfections, but accumulated debris on the lens will certainly effect performance.

[CatalinaWOW]

There are several effects.  Details depend on the camera.

1.  Transmission of mud is near zero, so mud globs and thick mud films decrease sensitivity proportional to the fraction of the aperture covered.

2.  Resolution may be reduced.  If the optics are the resolution limiter instead of the FPA and if the optics are diffraction limited there will be a reduction. 

3.  The emissivity of the mud is high, and it is unfocused so it will provide a thermal background.  In cooled sensors this can be a significant noise increase.  Uncooled sensors may have negative impacts on the non- uniformity correction.

In many ways muddy windshields provide similar affects, and thinking about looking through a muddy window with the sun in front of you can give you some good intuition about the situation.