The distance these lights are visible has been given in the geographical range. The theoretical visibility of a light in clear weather depends upon two factors, the height of the light above water, and its intensity. The height controls what is known as the geographic range, while the intensity controls what is known as the luminous range. As a rule, for the principal lights the luminous range is greater than the geographic, and the distance from which such lights are visible is limited by the earth’s curvature only. Under some atmospheric conditions the glare or loom of these lights, and occasionally the light itself, may be visible far beyond the computed geographic range. On the other hand, and unfortunately more frequently, these distances may be lessened by fog, rain, snow, haze, or smoke.
Some of the terms in this booklet may be new to readers. A short glossary of terms follows:
Candlepower—The luminous intensity of a light expressed in candles.
Lantern—The glassed-in enclosure on the top of an attended lighthouse which surrounds and protects the lens. Sometimes the entire piece of illuminating apparatus is referred to as the lantern.
Prism—A device for refracting light.
Radiobeacon—Electronic apparatus which transmits a radio signal for use in locating a mariner’s position.
Reflector—An optic which by reflection changes the direction of a beam of light.
Classification of lenses—Lenses are classified as to size by “order”, the first order being the largest and the sixth order the smallest. The actual size of a lens is expressed by its inside diameter. The following is a list of the standard lenses:
| Size | Inside diameter | MM | Height |
| Approx. inches | Approx. | ||
| 1st | 727/16″ | 1840 | 7′10″ |
| 2nd | 55⅛″ | 1400 | 6′1″ |
| 3rd | 39⅜″ | 1000 | 4′8″ |
| 3½ | 29½″ | 750 | 3′8″ |
| 4th | 1911/16″ | 500 | 2′4″ |
| 5th | 14¾″ | 375 | 1′8″ |
| 6th | 11¾″ | 300 | 1′5″ |
The numbers in parentheses in the text refer to source of information as indicated in the bibliography on [page 88].