Photo-optical technology has exceptionally varied uses in the sea ranging from imaging to communications and remote sensing. The optical region has many inherent advantages which will insure its continued application in ocean-related tasks. Optical systems, however, are limited to relatively short water paths due to the strong absorption and scattering of even the clearest water. Better understanding of light propagation in a scattering/absorbing medium and the development of techniques to extend operating range are among our most important tasks.
This is a review of some of the more effective means for calculating the distribution of radiance in the sea. The first section reviews the fundamentals of scattering and absorption. The second section briefly reviews the concepts of irradiance and radiance. The third and fourth sections describe a method of estimating radiance near the surface, where little of the power has scattered more than once through a large angle. The fifth section describes the exact solution for radiance transport by expansion in spherical harmonics, and approximate means for simplifying this solution. Finally, the sixth gives some mathematical background for the fifth.
Presentation of the metrical fundamentals of underwater lens system that are associated with the recording phase and are applicable for an object space of water and an image space of air. The metrical fundamentals are presented for cardinal points; dome window ; air lens; lens system as combination of dome window and air lens; water refractive index as a function of wavelength, temperature, salinity, and pressure; water depth and pressure relationship ; and water refractive indexes versus lens system parameters, nodal image distances, and back focal distances in the form of tables for the C201 ELCAN lens.
The optical transmission characteristics of deep ocean waters show a broad maximum between 450 and 500 nm. The most efficient incoherent sources are electric discharges of elements with transitions into this spectral region. Lamps with emission efficiencies of 6% and with output powers of tens of watts have been constructed. A review of the spectral emission properties of discharges of mercury, thallium, zinc, and cadmium as underwater sources is presented. Efficiency of emission and spectral line widths are shown to increase rapidly with metal number density. The efficiency also increases with electrode spacing; however, spectral radiant emittance is independent of electrode spacing. Measurements of time-resolved spectra of pulsed zinc discharges show a dominance of emission in the blue-green.
This paper discusses the present status and future possibilities of blue-green laser technology. Laser developments that are potentially most useful for ocean optics are emphasized. Historically, argon ion lasers have been used for virtually all underwater experiments requiring coherent cw sources, while frequency doubled Nd: YAG lasers have provided pulsed blue-green radiation. Neither of these lasers is appropriate for high-average-power field use. The best present candidate for appli-cations where high pulse energy is of primary importance is the flashlamp pumped dye laser. For range-gated applications, where 5-20 nsec pulses are needed, the copper vapor laser is the best immediate choice. A single pulsed laser that is suitable for both applications is not yet available. A cw laser with reasonable efficiency is also unavailable. With recent UV/visible molecular gas laser developments, it is likely that a single blue-green source capable of satisfying most ocean optics needs will be available within three to five years. Promising possibilities include KrF laser and XeF laser pumping of a blue-green dye laser and Raman downshifting of XeF laser output. Several longer term possibilities for in-band blue-green molecular gas lasers have been identified.
The design of underwater television viewing systems is dictated to a large extent by the optical properties of the water and by the mechanical and dynamic constraints imposed on the sensor package by the application. The system design is usually also limited in lens selection to available commercial lenses. When the system designer includes the lens design as an integral part of the viewing system development, significant improvements in system performance can be achieved. This paper describes three lens designs which have been produced for three different underwater remote viewing applications: a remote controlled underwater inspection vehicle for offshore operations; a miniature underwater camera for internal inspection of water cooled nuclear reactors; and an underwater camera for submersibles, search systems, and work vehicle application. A unique feature of these lenses is the ability to orient the line of sight by rotatio of the optics rather than the camera. The added complexity of the optics in each case is more than offset by resulting advantages in the overall viewing system performance. The paper emphasizes the necessity for a system approach. which coordinates the design of the optics, sensor and illumination with other elements of the system to achieve optimum performance.
Remote sensing of ocean color offers the potential of monitoring some of the characteristics of the upper layer of the ocean, on a global basis, by use of sensors on satellites or aircraft. Color sensing has been shown to be of use for parameters such as chlorophyll and sediment concentration and the location and motion of pollutants due to ocean dumping. A major obstacle to remote sensing of ocean color from satellites or high altitude aircraft is backscatter of sunlight by the atmosphere. This backscatter, consisting of reasonably predictable Rayleigh scattering and unpredictable Mie scattering, will constitute the majority of the signal seen by a high altitude sensor. In addition, some of the sunlight scattered from the ocean is scattered out of the sensor field of view by the atmosphere. The result is a low contrast signal with the oceanographic information contained in 20% or less of the total detectable signal. Aircraft flight tests, with concurrent surface truth measurements, have been carried out at altitudes up to 19.8 km with spectrometers and multispectral imagers. Analysis techniques have been developed that allow meaningful, quantitative calculation of ocean water content to be made from the remotely sensed color. The results have led to a satellite sensor, the Coastal Zone Color Scanner, to be flown on Nimbus G in 1978.
By combined photographic and photovoltaic photometry, the transmission of the light from an argon-ion laser beam through aerated tap water was studied in an approximately coaxial active optical system. The transmittance of the aerated water in the tank was varied from 0.0027 to 0.52%. The approximate range of detectability through the water tank and return for a target having a diffuse reflectivity of 11.5% was 6.1 attenuation lengths (tank transmittance = 0.22%). The signal and backscatter intensity were then both about 10-1° watt/cm2, the signal power about 2 x 10-12 watt and the integrated exposure energy from the signal about 0.06 erg/cm2. A range of at least 7 attenuation lengths is predicted, if a crossed polarizer is used at the receiver. The limiting range of detection of a specular reflection of about 0.3% off the interface between the water and plastic exit window was 10 attenuation lengths. The calculated signal was then about 10-12 watt, the light intensity about 10-10 watt/cm2, and integrated film exposure about 0.1 erg/cm2.
The possibility of collapsing a train of pulses, by delayed super-position, into a single pulse using a grating to obtain a very high energy density is described. The same principle can also be used to produce a pulse of desired shape out of a given single or a train of pulses.
A semi-quantitative method for the assessment of optical holographic interference fringes is developed. It is found that the fringe loci caused by a general three-dimensional translation of a flat object can be approximately described by a quadratic equation. The equation represents off-centered concentric circles with the location of the center determined by the displacements and the geometry of the holographic system. The location of the center is shifted opposite to the direction of the in-plane component of the displacement with an amount increasing with the amplitude of that component. The radii of the circular fringes decrease as the out-of-plane component of the displacement increases.
The light pen described in a previous paper has been improved in functions and made more economic in realization. The principle of operation of the light pen is unchanged: a square marker with a black cross is displayed on the bistable screen of an unmodified storage oscilloscope at an intensity level high enough for good operator viewing and low enough to avoid unwanted spot writing.
Real-time data storage and processing using optical techniques have been considered in recent years. Of particular interest are photosensitive electro-optic crystals which permit volume storage in the form of phase holograms, by means of a charge transfer process. A survey of the state of the art of such holographic memories is presented. The physical mechanism responsible for the formation of phase holograms in such crystals is discussed. Attention is focused on various aspects of materials characterization, development and utilization. Experimental reversible holographic read-write memory systems with fast random access and high storage capacity employing this new class of photosensiitive materials have already been demonstrated.
A method is described for eliminating the Doppler effect by the counterpropagating-beam two-photon technique. Emphasis is placed on gas-phase molecular electronic-state spectroscopy. Past accomplishments with this method are summarized and applications of the method are discussed.
A novel spectroscopic tool is presented for measuring the incoherent resonance decay (IRD) of selectively (6 MHz) prepared electronic states. The method utilizes electro-optic switching of a single laser mode that is on or off resonance with respect to the homogeneous molecular packets in the excited ensemble. The technique is applied to a variety of systems: gases at low and high pressures, molecular beams, and solids at low temperatures. The coherent transient of solids observed in the forward direction of the laser gives the phase memory (optical T2 processes) while the IRD measures directly the radiative and radiationless decay (optical T1 processes), and the optical transition moment between the ground state and the prepared electronic state. The theory of coherent and incoherent states is also given, and related to the different aspects of time-resolved spectroscopy of molecular beams and solids.
The integrated radiance of several types of flashbulbs has been measured and compared with the maximum permissible exposure for published laser standards. At close distances several flashbulb types have integrated radiance values that exceed those allowed for lasers.
A substantial body of literature is evolving which reports on several optical devices and systems applicable to the processing of radar signals. In addition, considerable research and procurement funds are being allocated to advancing optical techniques for radar signal processing. Review of the published literature, RFPs and proposals points to the fact that a common set of definitions, specification criteria and test standards has not been established. The BMDATC Radar Directorate has taken an initial step by assembling a baseline document for that purpose.
Spectral characteristics of bulk extrinsic photo-conductive detectors have been measured under conditions that only partially illuminated the photo-sensitive areas. The data presented were obtained at background photon flux levels of 10" to 10" photons/sec-cm'. The geometry of the detectors was such that the applied electric field was perpendicular to the incident radiation. Substantial changes in spectral responsivity have have been measured. These changes are dependent upon which portion of the detector's sensitive area is illuminated.