Energy barrier

Energy Barriers (sometimes referred to as Energy Fields) are a type of visible energy field that the Covenant use in airlocks and for boarding parties to board enemy ships.

Energy barriers are useful because they allow solid matter through, but not gasses such as oxygen. However, before matter passes through, the pressure on both sides must be equalized to minimize the risk of explosive decompression.

It is interesting to note that Signal Units work through energy barriers, while COM units do not.

Energy barriers are also present on Covenant Boarding Craft, on the end of the craft's penetrator tip.