Small scale fluctuations in refractive index can affect visibility and image quality in ocean optics. Such fluctuations are a result of temperature and salinity microstructure. Ocean mixing proceeds by the stirring together of dissimilar water types at finer and finer scales until diffusion creates a water type intermediate to the original components. Optically, the most important scale in the mixing cascade is microstructure because it consists of the highest gradient and smallest scale structures. Two classes of mixing process have been distinguished by shadowgraph images made in conjunction with profiles of temperature, salinity, and velocity shear. One class is diffusive and depends on the vertical distribution of temperature and salinity. The other class is turbulent and depends on velocity shear.

Intermediate nepheloid layers were observed by a beam transmissometer operating at 650 nm during 3-8 November 1976 over the continental shelf and slope off Oregon. Two well defined intermediate nepheloid layers were observed. One was located at about 150 m depth and extended westward from a point 10 NM offshore. The second was at about 375 m depth and went westward from a point 25 NM offshore. They were at least 15 NM wide (east--west dimension), and 50-150 m thick. They closely followed isotherms in the zonal section. The overall structure of the nepheloid layers remained unchanged during the period of the cruise. The intermediate nepheloid layers appear to have been generated by lateral diffusion and advection of turbid water from bottom nepheloid layers over the continental shelf and slope toward the deep water. Possible processes of diffusion and advection of bottom nepheloid layers to generate intermediate nepheloid layers are discussed.

Measurements of the volume scattering function were made from 0.1° to 1.0° from the main beam for two types of phytoplankton (Amphidinium C. and Thalassiosira F.) and one quartz suspension. The functions obtained were quite flat within that angular range as predicted by theory. Using particle size distribution measurements and tables of theoretical scattering values the theoretical volume scattering functions of the samples used were also determined. The theoretical volume scattering functions for particulate indices of refraction of 1.02 and 1.15 were close in value to the obtained functions for the phyto-plankton but the measured scattering function of the quartz particles was much higher than the theoretical prediction. The explanation for this probably lies in the fact that the quartz particles have a significantly higher composite index of refraction than either of the phytoplankton. The results of this experiment are part of a system of experiments that will be made to be able to determine the index of refraction distribution of oceanic particulate matter.

The development of multispectral scanners flown on aircraft and satellites have led to the use of these for measuring ocean color remotely. Potentially, this remote sensing of color will substantially increase the oceanographer's ability to monitor chlorophyll and particulate concentration in the upper layers over large spatial areas. Research conducted over the past several years has developed the techniques and laid the ground work for the quantitative interpretation of the remote sensed signals. It has been shown the meaningful corrections to these signals for atmospheric scattering can be made and that the resultant inherent signal can be correlated with in-situ measurements of chlorophyll and particulate matter concentration. A review of these experiments and techniques is presented.

Spectral beam attenuation measurements (400-650 nm) and spectral volume scattering functions (400-550 nm; 45°, 90° and 135°) were made during cruises in May and September, 1977 in and near Monterey Bay, California. Preliminary analyses indicate that the clearest waters display the greatest spectral dependence even if the effects of pure seawater are subtracted. The shape of the volume scattering function was also markedly different in clear water. Scattering at 135° showed the largest spectral variation and attenuation displayed the least. Nonrandom variation other than that dependent on the turbidity of the water has not yet been found, but extensive statistical analyses are planned.

Apart from the region covered by extremely low frequencies (λ≥3 x 103 km), ocean water is in general very absorptive for electromagnetic radiation, the only exception being the blue-green part of the visible spectrum. Even within this so-called window region, however, losses are high and can range from 0.1 dB/m to 1 dB/m, depending on the water clarity. For purposes of optical underwater communication, ranging, and illumination, powerful blue-green laser sources are therefore required with high efficiency and reliability. This paper discusses some recent progress in the area of blue-green laser technology.

Measurements have been made of the light scattering properties, including all polarization effects, ofoseveralocultured samples of phytoplankton. The range of scattering angles is from 8° to 168°. The measurements indicate the extent to which these phytoplankton may be regarded as spherical particles with respect to their light scattering properties.

The factors contributing to the visibility of submerged targets are discussed and the data requirements for calculating submerged contrast and contrast transmittance are developed. An instrument capable of making the measurements of the optical properties of seawater needed for visibility calculations is described. The instrument incorporates proven features of previous spectral transmissometers and oceanographic illuminometers into a single unit that can be operated through standard STD cables up to 8000 meters in length. It can, as a result, be used on most oceanographic vessels without requiring the addition of a special winch and cable. The instrument system utilizes a unique microprocessor-controlled, digital data acquisition and command system. This system performs the control and data transmission functions for eight underwater data channels through the armored, single wire STD cable. The total instrument system includes an HP9830B desk-top computer for data processing and recording, with an interconnected plotter for the real-time plotting of the measured variables.

Laboratory measurements were made of the sizes, shapes, and settling speeds of microscopic ocean-type particles by recording sequential transmission holograms of clay aggregates and glass shot in a settling chamber. Density values for particles that were approximate spheroids were determined using a modified Stokes settling equation. The successful application to this problem of inexpensive, off-the-shelf components and low power, He-Ne lasers improves the feasibility of remote, in situ measurements of ocean particle settling dynamics.

Pulse transmission mode (PTM) Q-Switching of an Nd:YAG oscillator was investigated for use in airborne laser bathymetry system transmitters. Peak powers of 4.7 to 6.3 MW were generated at 1.06 μm at repetition rates up to 50 Hz with a pump energy of 6.19 joules. Frequency doubling with CD*A produced a peak power of 2.46 MW at 0.532 µm with a conversion efficiency of 52%. Pulse widths of 6 nanoseconds were obtained at 1.06 µm, and 5 nanoseconds at 0.532 µm. The pulse width was limited by the commutation time of the Q-switch driver. Operation at increased PRF is also discussed.

An in-situ Optical Transfer Function measurement system has been developed and was successfully tested in the Sargasso Sea to 1000+ m depths in water 5000+ m deep. Concurrently obtained water samples were analyzed with a 16-channel Coulter counter for particulate content. Mie theory was applied to obtain the Volume Scattering Function σ (or β) for 8 samples at various depths, and the Fourier transform relation first developed by Wells was then used to obtain Modulation Transfer Functions H for comparison with those directly measured. Finally the transfer functions were inverse-transformed to obtain Volume Scattering Functions for comparison with those obtained by Mie calculations. Agreement was excellent in almost all cases, indicating that the equipment worked correctly, that Mie theory is applicable and the Wells' transform relation is valid. The experimental data display particle scattering plateaus which directly yield the beam attenuation coefficient a (or c). Integration of the Volume Scattering Function yields the total scattering coefficient s (or b) whereby the absorption coefficient a is obtained by simple subtraction from α. The range of spatial frequencies employed in the single successful experiment precluded specification of the diffuse attenuation coefficient γ (or k∞ the coefficient of attenuation of the asymptotic distribution at great depths). But the theoretical calculations indicate that H plateaus (again) near zero spatial frequency and that this plateau extends sufficiently beyond ψ = 0 to permit a reasonable determination of γ with obtainable optics. But the range of spatial frequencies employed did indicate the existence of an inverse-square roll-off of H with Li. Despite the lack of ancillary observations, this behavior is attributed to so-called "turbulence" or patches of "coherent" refractive index. The Coulter analyses themselves permit selection from among reported values of the slope of the so-called hyperbolic distribution of particle sizes. And the MIE scattering σ to H calculations suggest a selection from among similarly reported values of an average particle refractive index. Finally, evidence indicates, surprisingly, that although the larger particles have a large effect on σ near θ = 0 (greatly affecting the forward scattering peak) their effect on both the particle scattering (and absorption) coefficient and H is nil.

Numerous applications exist for a beam transmissometer that is low in cost, relatively stable, and consumes very little power. In this paper we present the design and calibration of a beam transmissometer that is constructed with P.V.C. to simplify the design and minimize costs, contains stable temperature compensated electronics, and consumes less than 150 milliwatts of power. Accuracy and stability with proper calibration and careful use will provide data with an error of less than 0.5% transmission.

This paper presents the development of the model and the analysis for downlink optical propagation between atmospheric and underwater terminals. The model incorporates the effects of atmospheric attenuation, boundary layer aerosols, wave slopes at the air-water interface, and the absorbing and multiple-scattering properties of the water. For a laser transmitter in the atmosphere, algebraic expressions are presented for the power, intensity, and ray-angle distributions underwater, together with iso-irradiance contours showing the fall-off of irradiance with distance from the surface spot center.

It is the purpose of this paper to demonstrate how the matrix operator method can be effectively implemented to couple the radiation fields of the atmosphere and ocean. Azimuthally averaged radiances and irradiances are presented as a function of optical depth for a conservative Rayleigh scattering medium of total optical thickness τmax = 1000 with a dielectric interface placed at optical depths of 0.01, 0.1, 1.0, and 10.0, and for various solar incident angles.

Quantitative analytical procedures for relating selected water quality parameters to the characteristics of the backscattered signals, measured by remote sensors, require the solution of the radiative transport equation in turbid media. In this paper, we present an approximate closed form solution of this equation and based on this solution discuss the remote sensing of the sediments. The results are compared with other standard closed form solutions such as quasi-single scattering approximations.

The general problem of a laser beam entering the ocean at near normal incidence (within 45°) is treated from the point of view of beam steering for small beams and beam spreading for large beams. The degrading effect of the surface on the irradiance is compared with the volume scattering effect and the initial beam size. It is shown that compensating for the surface would be of marginal benefit. The mean height of wind driven capillary waves for wind speeds in the range 2.2 m/sec to 7.6 m/sec is determined to be in the range 0.06 mm to 8.8 mm. The spectral content of these waves is mostly below 500 Hz with the maximum intensities occurring below 100 Hz. The higher frequency waves occur more at higher wind speeds.

This paper describes optical coherence loss and very small angle forward scatter mea-surements performed in Atlantic coastal waters. The technique used to measure coherence loss involved the use of an argon-ion laser source and a self-aligning interferometer capable of projecting two confocal beams to ranges of approximately 10 and 20 meters. The interference patterns formed at the common focal point of these beams were used to determine the normalized degree of coherence at spatial frequencies ranging from 10,000 to 100,000 cycles/radian. The measurement of very small angle forward scatter was also accomplished using the argon laser source. However, instead of using two apertures to produce two interfering beams, a single 50 mm aperture was used. The results of these measurements for Florida waters show that coherent optical behavior is present at all spatial frequencies measured. The response (i.e., the value of the normalized degree of coherence) tends to fall at the higher spatial frequencies, but often this behavior is dominated by significant temporal variations. Very small angle forward scattering experiments (1-300 microradians) have provided some insight into these temporal variations. These measurements show in-water beam diameters varying between 3 and 10 times their diffraction limited spot diameters within relatively short time intervals. Also observed in these experiments was a granulation (spatial spiking) of the beam at very small angles. This granulation possesses a scale size approximately equal to that of the diffraction limited spot size of the 50 mm source aperture used in this test. The optical parameters of the water, its T-S microstructure, and other oceanographic parameters were also recorded.

It has been experimentally shown that a two-color LIDAR is capable of detecting submerged objects to depths of 15 feet at angles of incidence greater than 85 degrees. The basic limitation appears to be extraction of the signal from the surface and volume backscatter sea clutter return in the LIDAR receiver. The experiment is briefly described, the results shown and areas of possible improvement discussed.

Flight testing of an airborne, scanning lidar bathymetric system has been conducted to determine vertical accuracy, operational constraints, and the effects of system variables. Test results are described, and an analytic performance model based on optical interactions is presented.

The return signals from a pulsed air/underwater laser radar system can have dynamic ranges that exceed the capabilities of normal photomultiplier tubes, creating spurious signals. By actively controlling the grid voltage of a photomultiplier tube, the usable dynamic range of a receiver can be extended. Careful active control of the grid voltage on an ITT 6 stage 4084 photomultiplier has resulted in gain changes in excess of 50 dB in a time frame of several hundred nanoseconds. This programmed STC (sensitivity time control) has allowed the recording of enhanced signal returns covering larger dynamic ranges than previously possible, without spurious signals resulting from multiplier overload.

This paper describes an underwater optical flying-spot scanning sensor system designed to provide an optical map of the ocean floor. Designed primarily as aid in underwater search and mapping, it provides a real-time optical picture of a 400-foot-wide swath width of the sea floor when towed from a 120-foot height off the bottom. The system inherently incorporates a means of sufficiently reducing the detrimental effects of backscatter to allow fast area search through the use of a wide (120°) field-of-view. Reasons for the choice of the approach taken to reduce backscatter are given along with details of the specific approach and a sample of preliminary data taken in an underwater test facility.