Aerial Lights

AERIAL LIGHTS Light signals are needed by air craft in night flying, just as they are required by the mariner. Air craft travel in three dimensions and also at higher velocities than a ship. Light signals for the guidance of an airman must therefore be at short intervals and must be visible from the horizon up to the zenith. One form of optical apparatus for an aerial lighthouse comprises a lower section similar to the por tion below the focal plane of a marine revolving light ; above this is a section of a fixed-light optic with its axis horizontal in stead of vertical as in a marine fixed-light. The lower revolving lens emits a high-power beam of light of small angle with its axis just above the horizontal while the upper segment throws a nar row band of light of lower power from the horizontal to the verti cal. Although an airman would pick up the main beam emitted by such an apparatus at, say, 3o m., he would be above it (if fly ing at a height of about 6,000 ft.), when at a distance of about 20 m. and not near enough to pick up the weaker beam. To compensate for this the lower lens and the illuminant are made of such dimensions and power that the main beam is stronger than is actually required in the direction of its axis and a portion of the rays from this beam is bent upwards by means of re fracting prisms placed in front of the panel, thus reinforcing the light in the direction where additional power is required (fig. 16). The effect of this adjustment is shown by the range curve. The optical apparatus is rotated on a mercury float or on ball bearings as in a marine lighthouse.

In some aerial apparatus, as at Cranbrook (Kent), reflecting mirrors are used as well as refractors in front of the lower part of the lens, and refracting prisms are employed instead of the fixed-light lens in the upper part of the apparatus. .

Both catoptric and dioptric apparatus have been employed in France in the construction of aerial lighthouses, and in some of them powerful electric-arc lights have been used. The aerial light

at Dijon (1925) is of great power and is made up of four sets of double dioptric lenses (8 in all) of 500 mm. focal distance, having an arc lamp at the focus of each lens, projecting parallel beams of light in groups of four. The lighthouse at Mont Valerien, near Paris (1926) is provided with catoptric projectors of gilded metal, a portion of the series of mirrors being arranged to direct the light rays in directions between the margins of the main beams and the zenith. The electric arc is also employed in this case. At Le Bour get a fourth-order dioptric apparatus is installed with a gas-filled electric lamp of 8,000 c.p. in focus. Lanterns containing the apparatus of aerial light have glazed roofs as well as the side glazing usual in marine lighthouses.

Neon tubes emitting a red light have been used at the Croydon aerodrome since 1924 for the guidance of airmen in fog because of the large percentage of red rays which have a higher power of penetration in fog. The Croydon beacon is of 13,500 c.p. and con sists of clusters of Neon tubes, 16 ft. long, without lenses or re flectors of any description. A flashing characteristic is given to the signal by interrupting the current. Many of these lamps are in use in the United States, Germany and France. It is doubtful whether the Neon tube under fog conditions is superior to other methods of aerial lighting by means of which beams—both white and red— of much higher total intensity can be projected.

In Great Britain and France the guiding principle of airways lighting is (in 1929) to provide powerful lights at comparatively long distances apart, which can be picked up by airmen at long range flying either low down or at comparatively high altitudes. In the United States and in Germany the practice is to lay a series of comparatively low-power lamps at short intervals along the air route, the airmen being expected to follow the line marked out.