A system and method are disclosed for cleanliness of transmission windows for directed energy devices. A positive pressure gas chamber is fixed to the transmission window of a directed energy device, e.g., a laser or high power microwave. A gas source is connected to the positive pressure chamber in order to provide pressurized gas. The gas flows into the positive pressure chamber, and exits the positive pressure chamber through openings, thus pushing particulates out of the positive pressure chamber. The gas continues to flow during directed energy use, thus preventing undesirable material, e.g., dirt and moisture, from depositing on the transmission window.