If these aerial objects have the ability to remain invisible to the human eye, at what frequency do they start to become visible? If it's purely the case of hiding away from our eyesight, which is of course extremely feasible, the chances are they're only manipulating the colours of the rainbow. A possible alien craft only has to have an outer shell that doesn't reflect light frequencies between around 400nm-700nm of the electromagnetic spectrum, which is violet through to red . Instead the light bends or flows over the craft very much like wind flowing over a car in a wind tunnel testing for better aerodynamics. Instead, the human only sees what's behind the object, not the object itself. So a one colour background, such as a blue sky, an object using this cloaking method becomes totally invisible.
The video below shows what looks like an invisible UFO being hit by a ball at a football match. You can see the ball take an unnatural curve through the air, seeming to bounce off something that isn't there. At a closer inspection, you can see a distorted area of nothingness which appears to move when the ball drastically changes course.
If this footage was filmed using a camcorder that could see into the infrared ranges, this clip could look very different indeed.