Seibert Q. Duntley

Light in the Sea

Light in the sea may be produced by the sun or stars, by chemical or biological processes, or by man-made sources. Serving as the primary source of energy for the oceans and supporting their ecology, light also enables the native inhabitants of the water world, as well as humans and their devices, to see. In this paper, new data drawn from investigations spanning nearly two decades are used to illustrate an integrated account of the optical nature of ocean water, the distribution of flux diverging from localized underwater light sources, the propagation of highly collimated beams of light, the penetration of daylight into the sea, and the utilization of solar energy for many purposes including heating, photosynthesis, vision, and photography.

The Reduction of Apparent Contrast by the Atmosphere

A veil of atmospheric haze reduces the visibility of all distant objects by decreasing their apparent contrast. In this paper equations are derived which describe the manner in which the apparent contrast of any object depends upon the distance of the observer. The treatment is not limited to horizontal paths of sight, but applies also to the apparent contrast of objects on the ground as seen from the air, and to the apparent contrast of objects aloft as viewed from the ground. The equations are not limited to the case of a homogeneous standard atmosphere; they may be applied to many kinds of non-standard atmospheric conditions. For every path of sight there exists a luminance level which will be transmitted unchanged. The apparent luminance of any receding object approaches this equilibrium level. For many paths of sight the equilibrium luminance is matched by the luminance of some portion of the horizon sky.

Image Transmission by the Troposphere I

Quantitative treatment of the apparent luminance of distant objects and the reduction of apparent contrast along inclined paths of sight through real atmospheres has been accomplished by means of optical data taken from an aircraft in flight. Sample data from a single flight are used to illustrate some of the principles involved. Correlation has been found between the humidity profile of the atmosphere and its optical properties.

The Visibility of Distant Objects

For thousands of years, thousands of mariners have sighted thousands of ships, and have made appropriate entries in their logs. Even so, this mass of miscellaneous information is of little use in predicting the range at which a specified object will be just visible under a new set of circumstances. The purpose of this paper is to identify the principal factors involved in the visibility of an object, to indicate how each factor affects the range of visibility, and to supply charts which, by combining these factors, enable the limiting range to be found under any set of prevailing conditions. (This paragraph has been lifted, almost verbatim, from some material prepared during the war by Professor Arthur C. Hardy, then Chief of the Camouflage Section (16.3) of the NDRC.)

THE REDUCTION OF CONTRAST BY ATMOSPHERIC BOIL

It is shown that the probability of receiving light from an object viewed through a turbulent atmosphere follows a normal Gaussian distribution. Furthermore the root-mean-square angular deflection of the points of any object will be proportional to the square root of the object-to-observer distance. From relations of the type described in the examples it is possible to predict the apparent contrast throughout a given scene, provided the inherent contrast distribution, the optical air state, and the range of the target are known. The optical air state for a given condition of atmosphere can be measured by using a telephotometer and a series of long thin black bars of varying widths.

Underwater laser scanning system

A system is described that produces high quality images through turbid waters by means of time encoded reflected light transmitted by scattering. The system consists of a compact battery operated laser scanning unit that scans the underwater scene with the laser beam in a manner similar to a television raster. Light reflected from any object in the scene varies in accordance with the reflectance of the minute spot being illuminated. This time varying intensity (TVI) signal is transmitted through the water to a remote receiver by both scattered and unscattered light where the received signal may be stored and/or displayed. The underwater laser scanning unit can be moved freely about the field of interest by scuba diver or ROV, unencumbered by entangling umbilicals, and can send real-time images over distances of 15 to 20 attenuation lengths to observers in a shirt-sleeve environment for critical viewing on an image display monitor. This previously undescribed system was developed in the early 1970s for proof of concept tests and used technology that is now 18 or more years old. The physical principles and the experimental hardware are described and examples are given of images providing exquisite detail that were made in an experimental tank together with some images obtained in ocean trials.

Measuring earth-to-space contrast transmittance from ground stations.

Inherent object and background radiance, beam transmittance through the atmosphere, and sky radiance are all measurable from a ground station. It is shown how, in the visible sectrum, the sky radiance and beam transmittance may be used to compute the path radiance for orbital downward paths of sight. Thus, the components for calculating apparent radiance, apparent contrast, and the contrast transmittance from earth to space are measurable from a ground station. our in-flight experiments were conducted to obtain measurements of contrast transmittance for comparison with measurements made simultaneously at ground stations. The in-flight and ground-based measurements are in good agreement.

Signal-light nomogram

A nomogram is presented for predicting the sighting range for white, steady-burning signal lights. The theoretical and experimental bases are explained and instructions are provided for its use for a variety of practical problems concerning the visibility of signal lights. The nomogram is appropriate for slant path as well as horizontal sightings, and the gain of range achieved by utilizing binoculars can be predicted by use of it.

Atmospheric Limitations on Missile Photography

Optical data taken from an aircraft in flight as well as data secured at ground level have enabled the obscuration of high-flying missiles by the atmosphere to be ascertained and the requirements to be met by telescopic finders and cameras to be specified for several typical weather and lighting conditions.