One, or both of the screens would have to be dirty enough to affect the optical sensors' ability to detect the change in light as the projectile passes over them. I think the dirty screen can only delay the detection of the projectile. A detection delay at the first screen would cause the calculated speed to be higher than it really is. A delay at the second sensor would cause a slower reading. If you have delays at both screens, then the larger delay would be the controlling factor, but to a lessor degree.
Now we can ask how much error is caused? Assuming 900 feet per second speed, since we don't know if you're shooting pellets or slugs, we can ballpark the error in speed calculation. If the detection is delayed by 1/4 inch of travel, and assuming a 12 inch distance between sensors, that would cause an error of 18.75 FPS in calculated speed. Again, faster or slower is determined by which sensor the detection delay occurred at.
So, to answer your questions. Yes, dirt on the screens can affect the result, if there is enough to cause a detection delay. High or low depends on which delay is greater. You will only get correct results when the delays, whether none or some, are equal or close to equal.
Hope that helps.
James
