The beam (volume) attenuation coefficient, a, and diffuse attenuation coefficient, K, of optical radiation have been measured in selected U.S. East Coast, Bahama, and Puerto Rico Trench waters. The objective was to determine what, if any, empirical relationship exists between these two optical properties over a wide range of turbidity. a was determined from transmittance versus depth profiles made with a beam transmissometer. K was determined from relative irradiance versus depth profiles made with a relative irradiance meter. In order to eliminate data bias introduced by spatial movement, temporal fluctuations, and spectral dependency, a and K were measured simultaneously and at a common wavelength (A = 535 nm). "Effective" (depth averaged) values of a and K, designated a and K respectively, were calculated in order to characterize the variable data versus depth profiles at each station with single values. The empirical expression: K = 0.2 7+ 0.04, applicable for the range 0.11 m-1 < 7< 1.6 m-I, has been determined for data combined from all locations. The degree of concurrence of this relationship with data obtained by others is discussed. Definitions of a and K, and discussion related to their differences are also presented.

The depth at which a Secchi disc disappears provides a measure of visible range which is not applicable to many practical situations. Most other methods of determining in-water visibility measure one or more water parameters, which are used in a theoretical or empirical model to predict visible range. The proposed Contrast Meter measures the apparent contrast of a known target at a fixed distance and relates the loss of contrast to visible range by empirical measurements. This instrument lends itself to remote sensing and could be adapted for measuring contrast loss in other turbid mediums. A prototype instrument has been built and tested.

Experiments have been performed which measure the intensity distribution pattern of an optical beam which originates below the surface of the water. A narrow beam (5°) light source was submerged at varying depths up to 16 attenuation lengths (alpha = 2.4 m^-1) and pointed in a vertical direction. An optical detector was flown through the beam at altitudes to 3000' and pointed downward at the underwater source. Data was obtained which provides plots of the optical beam intensity distribution as a function of angle. Three analytical models are presented for predicting beamwidth based on optical properties of the ocean. Comparison with the experimental results shows deficiencies in each of the models developed.

Particulate matter of the sizes, concentrations, and refractive indices found in situ renders the Optical Transfer Function of water a real quantity. Modulation Transfer Function (MTF) is measured in vitro by spatial filtering of the projected image of a slit in the Fourier Transform plane. Analysis is by Moire' fringes with a smooth and continuous variation of spatial frequency (nominally 0-40,000 cycles/ radian) obtained by counter-rotating Ronchi rulings. The analogue of convolution of impulse responses is tested as the cascadability in a scattering medium of true sinusoidal MTF's. Coulter Counter techniques are used to measure differential particulate count in 15 channels up to 100 gm. Experimental data for various ranges and particle distributions are compared to theoretical predictions based on volume scattering functions (VSF) obtained by Mie scattering calculations and the Fourier transform conversion relating MTF and VSF first obtained by Willard Wells. The equipment is being repackaged for insitu measurements to accompany forthcoming flood-illuminated SEGAIP (SELF GATED IN-WATER PHOTOGRAPHY) trials.

A new instrument has been developed for the study of those optical properties of ocean water that affect the transmission of image-forming light. The instrument performs simultaneous measurements of the volume attenuation coefficient and the volume scattering function at three angles. Any of ten wavelengths covering the spectral range from 400 to 670 nanometers may be used. A depth capability of 500 meters permits the examination of water below the euphotic zone and of the bottom waters on the continental shelf. The considerations leading to the design of the instrument, its capabilities and the unique features it incorporates are discussed. Some examples of the data obtained with the instrument are presented.

The refractive index of marine diatoms is measured by immersing them in a solution of pyridine and water. As the refractive index of the solution approaches that of the diatoms, the diatoms tend to disappear. Using this method, the refractive index of Skeletonema !R. was measured to be 1.457 * 0.005

To simplify the analysis of a typical underwater imaging system--illuminator, target, and receiver adjacent to the illuminator--the combined optics-water response to a point reflectance is treated as a system response function. This is shown to be the product of the separate transmitter and receiver response functions. Each of these may be expressed as a convolution of the optics and water response functions. The latter are the point and beam spread functions. By an expansion of the principles derived, the backscatter arising from points in the region between the illuminator-receiver assembly and the target is characterized analytically. Then the total backscatter from the water is obtained by a simplified three dimensional integral. Values of the total backscatter obtained by this method are compared with values obtained by other means and with measured values. Finally an assessment of the utility of various analytical descriptions of underwater optical imaging is given.

This paper presents the concept of an effective attenuation coefficient (EAC) which considerably simplifies underwater multiple-scattering calculations. Concise expressions for the total flux through an aperture, the beam-spread function, and on-axis irradiance are derived in terms of the EAC. These expressions are compared with both Monte Carlo results and experimental data. Good agreement is obtained for distances and angles of practical interest.

A solution to the radiative transfer equation is presented which describes the apparent radiance of underwater optical targets of simple shapes. The solution is an exponential, the coefficient of which varies with distance and depends on four quantities: the volume attenuation coefficient, the integrated volume scattering function, the target size, and the illuminating beam size. The solution is compared with data taken by the authors and explains a discrepancy with the generally employed constant coefficient exponential dependence. A comparison with Monte Carlo calculations of point spread functions further supports the validity of the solution.

Experiments have been performed which measure the intensity of an optical signal which originates from a "collimated" underwater light source. A narrow beam (5°) light source was submerged at varying depths up to 16 attenuation lengths (alpha = 2.4 m-1) and pointed in a vertical direction. An optical detector was flown through the beam at altitudes to 3000 feet and pointed downward at the underwater source. The signal intensity measured through a vertical path is found to be in good agreement with the following expression: P e -kD 2 Signal Intensity = o watts/meter (at Airborne Receiver) ir(A tan 0s )2where: k = Diffuse attenuation coefficient (meters) s = Scattering coefficient (meters- 1) D = Water path length (meters) A = Air path length (meters) Po = Transmitter power (watts) Os = Half beamwidth (function of s) The data was obtained under conditions of a calm sea such that wave contributions to spreading appear negligible. Methods are discussed of predicting 0(s) from easily measured parameters.

The problem of communicating between a subsurface terminal and one above the ocean surface is investigated. A multiple scattering model developed by Heggestad and Arnush is modified to characterize the propagation in the ocean environment. Good agreement with data is observed over a fairly large range of values. The surface effects are investigated and the effects of wave motion are incorporated. By linearizing Snell 's law a working model for transmission through the air-sea interface is developed which is accurate out to zenith angles of +45°. Calculations are presented which indicate that in 10 meter water a low data rate (100 bps) communication system is feasible to depths of several hundred feet on the downlink and from a depth of 100 feet on the uplink, both employing a stationary satellite. Some component development would be necessary in the area of wide field of view narrowband filters.

The original research program at sea and underwater was done and published by marine biologist Louis Boutan at the Banyuis Laboratory between 1893 and 1900. His fascinating book on the subject reveals that Louis Boutan had found out about the most important problems of underwater photography: The water-air refraction effects, the necessity to protect the photographic chamber in a dry air environment, the necessity to use powerful lighting underwater, the necessity to protect the camera lens from unwanted light noise scattering.

This paper describes the viewing system design requirements for two somewhat dissimilar applications; namely, an underwater remote controlled vehicle and a towed sensor platform. The low light level television camera viewing systems evolved to satisfy the performance objectives are explored and operational experience is presented which demonstrates the appropriateness of the designs.

Two techniques for modifying the spectral irradiance from a pulsed long arc xenon lamp to make it more comparable with the attenuation minimum (460-495 nm bandwidth) of deep ocean water were successfully tested. One technique applied the concept of shaping the current pulse by limiting the maximum amplitude and pulse width and produced a 50% improvement in irradiance in the 460-495 nm bandwidth over the present output of the NRL LIBEC photographic strobe. The second technique resulted in an additional improvement over the pulse shaping technique. In this technique, a metal was added to the interior of the flash lamp envelope. The three metals investigated were selected based upon the physical properties of low melting temperature and their prominent spectral lines in the 460-495 nm bandwidth. The three were zinc, cadmium, and zinc-cadmium. Improvements in irradiance of 24%, 28%, and 29% for zinc, cadmium and zinc-cadmium respectively were observed.

Underwater applications such as optical RADAR, communications, and imaging have motivated researchers to develop improved, higher power pulsed lasers for the blue-green spectrum. This paper reviews the state-of-the-art of current blue-green laser technology with consideration given to the underwater window and system performance parameters.

The first MTF measurements made with the Elcan underwater simulator, as described by Mandler in Applied Optics, Vol. 9, March 1970, are presented. The combination of the underwater simulator and the EROS IV OTF analyser for finite and infinite object distances is described. The measurements were made on four lenses constructed to the identical prescription designed specifically for underwater use. The results are compared to the design predictions.