Underwater inspection apparatus and method

A holographic recording and reproducing system for recording holographic images of an object positioned in a first medium and replaying said images in a second medium incorporates means for enhancing the relative sensitivity of the system to radiation capable of producing said holographic images.

This invention relates to holography and, in particular, to methods of and 
apparatus for the holographic inspection of underwater objects such as 
pipelines. 
The recovery of oil and gas has presented a significant challenge to the 
offshore industry as regards routine inspection and maintenance of subsea 
installations. As drilling now proceeds to even greater depths the 
problems encountered increase and more emphasis is now being placed on 
remote, rather than diver held, techniques of inspection. Visual 
inspection is extensively utilised with the major part of this being 
carried out using conventional photography, stereo photography and 
closed-circuit television. These methods all, however, suffer several 
drawbacks. Conventional photography produces two-dimensional images of 
moderate resolution but loses parallax information and, particularly in 
close-up, suffers from perspective distortion and limited depth of field. 
Stereo-photography improves this situation by producing a 
three-dimensional image from two fixed viewpoints: parallax information is 
still lost. Furthermore, if precise dimensional measurements are required, 
sophisticated photogrammetric techniques are necessary with limited 
resolution as yet occupancy. Television systems while providing real-time 
operation are essentially low resolution techniques. 
Holography, by comparison, suffers from none of these limitations and gives 
the observer an infinity of viewpoints from which the view the scene. It 
is the ability of holography to reproduce, remote from the original scene, 
a full size three-dimensional image possessing high resolution and low in 
optical aberrations which make it a potentially powerful method of visual 
inspection. Applications which can be envisaged include general archiving, 
measurement of corrosion pitting and cracking, examination and measurement 
of damage sites, structural profiling and examination of marine growth. In 
all such applications the required end product is usually a high 
resolution hologram of a particular scene of interest. From this hologram, 
inspection can be performed directly on an image reconstructed in real 
space. 
Holographic visual inspection or "hologrammetry" as it has now come to be 
known, is becoming increasingly important as a means of making high 
resolution dimensional measurements of engineering components and 
structures. This is particularly true when the inspection site is located 
in a hazardous environment or is an area where access is difficult, such 
as encountered in the nuclear power generating industry or the offshore 
oil and gas industries. 
The basis of holography as a means of high resolution visual inspection is 
the holographic recording of the scene of interest with the subsequent 
replay of the processed hologram in the real image mode of reconstruction. 
Reconstruction of the real image produces an image which is reversed 
left-to-right and back-to-front when viewed from the space in front of the 
hologram. Such a representation of the image is known as "pseudoscopic". 
In general, the holographic interference field is captured on photographic 
film. Other media such as thermoplastic film, photochromic materials, 
non-linear optical crystals and dichromated gelatin may, alternatively, be 
used. Holographic film differs from the film used in ordinary photography 
only in that the grains of silver halide are of the order of a few 
nanometers across as compared to micrometers. Such film is very 
insensitive to light but has the capacity to record the fine detail 
inherent in an interference field. Typical sensitivity is around a few 
millijoules per square meter. The exposed film is chemically processed in 
a similar, but somewhat more elaborate, way to ordinary film to render the 
holographic interference permanent. 
For purposes of visual inspection, however, creation of the virtual image 
is not the most suitable form of holographic reconstruction. It so happens 
that if we turn the plate around and illuminate it from behind with a wave 
which is the exact conjugate of the reference beam, a conjugate image will 
be located in real space in front of the plate. The image so created is 
optically identical to the original save that it appears to be reversed 
left to right and back to front. It is this real, pseudoscopic image which 
forms the basis of a method of visual inspection. 
The utilisation of holography in visual inspection, relies on the creation 
and optimisation of the real image of a scene or object. The real, or more 
correctly the conjugate image, is formed and reconstructed. A parallel 
reference beam is often used in recording the hologram, since 
reconstruction then is a simple case of turning the film around. If a 
diverging reference beam was used in recording, then a converging beam of 
identical curvature would be needed in reconstruction. 
Because the conjugate image is located in real space in front of the 
observer, visual inspection can be carried out directly on this image 
using all the conventional tools of the trade, namely, measuring 
microscopes, photography and TV. Optical sections can be taken through the 
resulting reconstruction by merely placing a piece of film across the 
image and recording directly, without the need for any lenses. This 
concept is sometimes hard to accept without seeing it. An image is 
actually formed in space in front of the observer on which all optical 
tests can be performed as if it were the original subject. 
The usefulness of holography for accurate engineering measurement, whether 
this be in water or in air, is dependent on its ability to reproduce an 
exact image of the object which is low in optical aberrations and 
possesses sufficient resolution to allow detailed measurements to be made. 
In practice, loss of resolution and aberrations can occur at both 
recording and replay stages. There are several recognised factors which 
can give rise to optical aberrations and, ultimately, degrade image 
fidelity. Such factors include, 
(a) distortion of the fringe pattern recorded in the emulsion as a result 
of chemical processing, 
(b) the optical quality of the emulsion substrate 
(c) variations between reference and reconstruction wavefronts 
(d) variations between reference and reconstruction wavelength, and 
(e) the quality of the reconstruction beam. 
The above factors affect the ability to produce optimum conditions for 
wavelength reconstruction but can, with reasonable precautions, be 
controlled to a degree sufficient to produce high resolution images. 
In accordance with the present invention there is provided a holographic 
recording and reproducing system for recording holographic images of an 
object positioned in a first medium and replaying said images in a second 
medium characterised in that it includes gating means substantially to 
reduce the effect on a photosensitive medium of radiation other than 
radiation capable of producing said holographic images.

Referring now to the drawings, FIG. 1 shows an optical arrangement for the 
recording of holograms. An argon ion laser 1 with a shutter 2 serves as 
radiation source. A variable beam splitter 3, separates the radiation into 
recording and reference beams 4,5 respectively. The reference beam passes 
by way of mirror M1,M2,M3 through a microscope objective and pin-hole 
spatial filter 6 to produce a collimated beam 7 which illuminates a 
holographic plate 8. The recording beam 5 passes by way of mirror M4,M5 
and microscope objective and pin-hole filter 9 to illuminate a moveable 
target 10 in a water tank 11. A real image 12 of the hologram is viewed in 
air by means of a travelling microscope 13. The whole arrangement is 
mounted on a pneumatically supported table 14 to reduce vibrations and 
extraneous thermal effects. 
The primary monochromatic aberrations to be found in any imaging system are 
those of spherical aberration (S), chromatic aberration (C), astigmatic 
aberration (A), field curvature (F), and distortion (D). The relative 
coefficients of aberration for each of the above can be given in terms of 
the cartesian co-ordinates relating to object, reference and 
reconstruction wave positions with respect to the hologram plane. 
It can be shown that the aberration coefficients, S, C. A, F and D can be 
minimised if two conditions are met, namely, that 
(a) the reference and reconstruction wavefronts are both located at 
infinity, that is, they should be collimated, and, 
(b) the wavelength of the reference wave must equal that of the 
reconstruction wave. 
Correspondingly these conditions yield the fact that when reconstructed as 
above the lateral, longitudinal and angular magnifications of the real 
image will all be equal to unity. 
The above arguments apply to the paraxial region. For the non-paraxial case 
more rigorous formulations apply. However, we can realistically assume 
that, if we meet the above conditions, then all aberrations will be 
reduced to a minimum. 
In real-image reconstruction, image resolution is limited, in theory, only 
by the quality of the reproduced hologram. The resolution of a holographic 
image is usually defined as the ability to distinguish two points on a 
hologram separated by distance r, given as, 
EQU r=1.22.lambda.z/D (1) 
where .lambda. is the reconstruction wavelength, z is the separation 
between hologram and reconstructed image and D is the effective aperture 
of the hologram. This equation is the standard relationship of optics 
which defines the resolving power of a lens. A lens and a hologram of 
equivalent diameters will produce the same theoretical resolution. Because 
of its reduced susceptibility to optical aberrations, the hologram will 
produce the more highly resolved image. 
The above equation is usually expressed as a resolving power (R) in line 
pairs per millimeter (1 p/mm) by reciprocating and dividing by 10.sup.3. 
Hence, we have 
EQU R=10.sup.-3 /(1.22z.lambda./D)[1 p/mm] (2) 
In holography the presence of speckle effects introduced by the coherence 
of the light and the finite aperture of the viewing system influences the 
resolving power. The speckle size sets the lower limit to the resolution. 
In practice, R is reduced by a factor of 2 to 3 take account of this. 
The resolution obtained in this way assumes that we reconstruct with an 
exact conjugate of the reference beam, that the reconstruction wavelength 
matches that of the recording beam and that the emulsion is 
infinitesimally thin. Conversely, the resolution of a virtual image 
hologram is limited by the effective aperture of the viewing system. 
Emulsion uniformity and substrate quality have a significant effect on the 
quality of the reconstructed hologram. Standard holographic plates 
supplied by the major manufacturers often exhibit a departure from 
flatness of up to a few hundred fringes. To improve the holographic image 
from such plates the reconstructed area can be apertured to cover that 
part of the plate exhibiting best flatness. 
The quality of the recorded hologram depends to a large extent on the 
choice of photographic emulsion and processing techniques. The choice of 
emulsion falls between: 
(a) Agfa 8E56HD and Ilford SP672 
Both these films are sensitised for use with lasers operating in the 
blue-green region of the spectrum and feature fine grain with consequently 
low scatter and high diffraction efficiency. 
(b) Agfa 8E75HD and Ilford SP673 
These emulsions are similar to those mentioned above but are sensitised for 
work in the red region of the spectrum. 
(c) Agfa 10E56HD and Agfa 10E75HD 
These emulsions possess significantly larger grain sizes than any of the 
other emulsions and consequently need less exposure to light. The large 
grain size, however, reduces the resolving power of the film and they can 
therefore not be recommended for high resolution holography. 
Chemical processing of the recorded interference pattern is a crucial step 
in the holographic procedure. Some of the factors which have to be 
considered include image brightness, image resolution, reconstruction 
wavelength, emulsion shrinkage and noise level. 
Several processing procedures have been found to be suitable for processing 
of transmission holograms. These processes include: 
(a) develop only 
(b) develop and fix 
(c) develop and bleach (rehalogenating) 
(d) develop and bleach (reversal) 
(e) develop, fix and bleach. 
It has been established that for bright holograms on silver halide film 
bleaching is desirable in to maximise the efficiency of the hologram. Some 
forms of bleaching, though, can result in non-uniform shrinkage of the 
emulsion, which will give rise to astigmatism in the reconstructed image. 
A rehalogenating bleach process, in which the developed grains of silver 
are reconverted to silver bromide, has been shown to be the most suitable 
process for holography giving rise to a bright image with low scatter. 
Because no silver is removed but is merely redistributed through the 
emulsion it is believed that this type of processing results in the 
dimensions of the emulsion remaining constant before and after exposure. 
A preferred method for processing holograms includes the following steps: 
Pre-wash: 2 min in de-ionised water at 20.degree. C. 
Develop: 2 min in Pyrogallol at 20.degree. C. 
Wash: 2 min in de-ionised water at 20.degree. C. 
Bleach: 2 min in Ferric EDTA at 20.degree. C. 
Wash 1: 10 min in de-ionised water 
Wash 2: 2 min in 50/50 de-ionised water and methanol 
Wash 3: 2 min in 100% methanol 
Tetenal Neofin Blue is prone to emulsion shrinkage and for more exacting 
work it can be replaced by a pyrogallol based developer such as 
Agfa-Gevaert GP62, the formulation of which is given in Table 1. 
TABLE 1 
______________________________________ 
Part A Part B 
______________________________________ 
700 ml water 700 ml water 
15 g metol 60 g Na.sub.2 CO.sub.3 
7 g pyrogallol de-ionised water to 1000 ml 
20 g Na.sub.2 SO.sub.3 
4 g KBr 
2 g Na-EDTA 
de-ionised water to 1000 ml 
______________________________________ 
Working solution made up as 1 part A+2 water+1 part B. The bleach 
formulation used was as in Table 2: 
TABLE 2 
______________________________________ 
700 ml water 
50 g potassium bromide [KBr] 
1.5 g boric acid 
water to 1000 ml 
2 g para-benzoquinone [PBQ] added just before use.. . 
______________________________________ 
. 
An alternative bleach to the PBQ based one described above may be used 
because of its less toxic properties. The formula of this bleach is given 
in Table 3: 
TABLE 3 
______________________________________ 
700 ml de-ionised water 
30 g ferric sulphate [Fe.sub.2 (SO.sub.4).sub.3 ] 
30 g EDTA di-sodium salt 
30 g potassium bromide [KBr] 
10 ml conc sulphuric acid [H.sub.2 SO.sub.4 ] 
de-ionised water to 1000 ml 
______________________________________ 
An alternative means of recording a hologram is to use thermoplastic film. 
Such film is commonly used in holographic interferometry. Its many 
attractive features include rapid electronic processing and reusability. 
The requirement of recording underwater with subsequent laboratory 
reconstruction in air introduces additional factors to those mentioned 
above upon which image fidelity may depend. Such factors include 
(a) thermal gradients in the water, 
(b) turbulence in the water, 
(c) polarisation effects 
(e) scattering 
(f) absorption, and 
(g) mismatch between the refractive index of the medium in which the 
hologram is recorded and that in which it is replayed. 
Inevitably these processes result in an image which will have a resolving 
power below that of the equivalent situation in air. The image will also 
possess optical aberrations brought about by recording in one refractive 
index medium and replaying in another. 
A range of lasers were used in the experiments, as follows, 
(a) Argon-ion 
Manufactured by "Lexel" as Type 90-4. Contains oven stabilised etalon and 
delivers up to 1.5 W, continuous power at 514 nm, single frequency mode. 
(b) Frequency doubled Nd-YAG 
Manufactured by "Quantel". Delivers 250 mJ in a pulse of 15 ns duration at 
532 nm, single-longitudinal mode (SLM). A second output of 50 mJ is also 
available and serves as a reference beam. 
(c) Ruby 
Manufactured by "JK Lasers". Delivers up to 1 J in a 30 ns pulse at 694 nm, 
SLM. 
Agfa 8E56 plates were used with argon and YAG lasers and Agfa 8E75 plates 
and film were used with the ruby laser. Chemical processing of the 
recorded holograms was carried out according to the procedures outlined 
above. 
Whenever real image reconstruction was envisaged, the reference and 
reconstruction beams were collimated using an SORL Fourier lens. This lens 
had a focal length of 300 mm, an f/5 aperture and a wavefront accuracy 
specified as better than .lambda./8 over its central 38 mm at 514 nm. 
Collimation of the beams were achieved to better than 2 mrad. Reference 
beam angles of 30.degree. to the normal with the holographic film were 
typical. This ensured that the spatial frequency of the system was well 
within the cut-off frequency of the film. 
A series of holograms were recorded aimed at establishing the influence of 
thermal gradients and turbulence in the water on the fidelity of the 
reconstructed image. Since thermal gradients and turbulence can both be 
linked to local changes in the refractive index of the water it is to be 
expected that they will have local influences on the path length of light 
travelling through the water. Such an effect, if severe enough, could 
appear in the hologram as localised interference fringes and obscure the 
inspection area. Although the "look around" properties give some measure 
of compensation for this. 
To simulate "worst-case" conditions, an immersion heater operating at 
around 90.degree. C. was placed in the observation tank in front of the 
targets. A series of holograms at exposure times of 20 s down to 150 ms 
(the shortest obtainable with the above set-up and amplitude processing) 
showed that as expected the number of fringes recorded across the field of 
view decreased with exposure duration. 
A photograph of one such hologram, taken from virtual image reconstruction, 
is shown in FIG. 2 corresponding to an exposure duration of 150 ms. Spot 
temperatures in degrees Celsius at various points in the water are also 
shown (numerals 18-21). In any field situation, though, the use of a 
pulsed laser with pulse durations of 10 to 50 ns would be essential 
because of the mechanical stability conditions required by holography. It 
would appear unlikely that, in these short time scales, thermal variations 
of the magnitude likely to be encountered offshore would result in the 
appearance of secondary fringes in the hologram. These conclusions were 
verified by the short analysis which follows and by a series of similar 
holograms taken with both pulsed ruby and frequency-doubled YAG lasers. 
The mechanism by which interference fringes are formed is well known. A 
change .lambda./2 in path length travelled by a given ray between two 
consecutive exposures will give rise to a dark fringe in the hologram. In 
this particular case the path length change is brought about by local 
temperature variations causing a change in local density and hence in 
local refractive index of the water. 
Assuming first order fringes, the refractive index change can be linked to 
path differences by 
EQU x.DELTA.n=.lambda./2 (3) 
where x is the length of medium over which the path length changes, 
.DELTA.n is the refractive index change and .lambda. is the wavelength of 
the laser. 
Equation 3 can be linked to the temperature by assuming that, firstly, the 
spatial variation in refractive index is a step function and, secondly, 
that refractive index is a linear function of temperature over a suitably 
narrow range. Hence, we have 
EQU n=aT+b (4) 
where a and b are constants. 
For small variations in n and T, we can say that 
EQU .DELTA.n/.DELTA.T=a (5) 
and hence, 
EQU ax.DELTA.T=.lambda./2 (6) 
Equation (6) accordingly relates the appearance of fringes to temperature 
changes. 
It has been shown that a temperature change of 20 degrees in pure water at 
a nominal value of 20.degree. C. will give rise to an approximately linear 
change in n of 0.0008, yielding a value of 40.times.10.sup.-6 .degree. 
C..sup.-1 for the constant a. This linear approximation is reasonable 
since a correlation coefficient of 0.96 was obtained for published values 
of n and T over this range. Hence, for a wavelength of 514 nm and assuming 
that the refractive index changes over a 2 cm path, we obtain 
EQU .DELTA.T=0.3.degree. C. 
Thus one dark fringe will appear for a temperature change of 0.3.degree. C. 
over a 2 cm path of water. In contrast to the above the fringes which 
appear in a single exposure hologram correspond to the change in 
refractive index integrated over the exposure duration and may be regarded 
as an ensemble of double exposure holograms. Consequently the number of 
fringes over the field of view must decrease as the exposure duration 
decreases as previously concluded. 
An order of magnitude calculation based on the previous analysis shows that 
unrealistically large temperature variations would need to occur before 
such fringes would appear in the hologram. 
A specially constructed target consisted of a water filled cell 20 as shown 
in FIG. 3 containing a small heating element 21 and three thermocouples 
22,23,24 was used to verify the above analysis. Using the optical 
arrangement shown in FIG. 4 comprising a lens 30, a diffusing screen 31, a 
water cell 32, a collimated reference beam 33 and a holographic plate 34, 
a double exposure hologram was recorded with the heater off and on. A 
photograph taken from a real image reconstruction of the hologram is shown 
in FIG. 5. By examining the resulting hologram under real image 
reconstruction the finite depth of the fringes could be seen. The measured 
temperatures indicate an actual thermal variation of 0.25.degree. 
C./fringe which is in reasonable agreement with the previous analysis. 
Like thermal gradients, the quality of underwater holograms may also be 
affected by the presence of turbulent flow in the water. In such 
situations, turbulence may arise from, for example, surface waves or 
currents around structural members or, perhaps more severely, from the 
operation of underwater inspection vehicles. Since turbulence may be 
interpreted as localised fluctuations in the velocity and pressure of the 
water, such variations would be expected to cause minute changes in 
optical path length resulting in the appearance of fringes in the 
hologram. 
One simulation of the effects of small scale turbulence, utilising a paddle 
revolving at approximately 1 Hz, is shown in FIG. 6. A few localised 
fringes are visible in the hologram. We can interpret the occurrence of 
these fringes as arising from the small amounts of local heating which 
must inevitably be produced by the velocity and pressure fluctuations in 
the water. 
A similar analysis to that conducted for thermal gradients again indicates 
that in the situation likely to be encountered, offshore turbulence is 
unlikely to be a problem if short pulse lasers are used in hologram 
recording. Confirmation of this conclusion was obtained by taking 
holograms similar to those outlined above using both pulsed ruby and 
frequency-doubled YAG lasers. 
All our preliminary holograms showed that bright clear images could be 
recorded of objects submerged in clear tap water. Sea water, however, with 
its salinity, suspended matter and micro-organisms presents a different 
situation. It is to be expected that under these circumstances scattering 
and absorption of light will have an influence on image brightness and 
apparent contrast of the subject. The extent of these phenomena in a given 
medium dictates the limit of visibility. 
Scattering of light in an attenuating medium occurs when light interacts 
either with particles suspended in the medium, or, with inhomogeneities in 
the medium and consequently is deviated from its original path. In 
sea-water, transparent micro-organisms and suspended particles are much 
larger than the wavelength of light. Thus, unlike atmospheric scattering, 
scattering in sea-water is relatively constant over the visible spectrum. 
Absorption, on the other hand, is a thermodynamic process which results in 
the loss of irradiance of a beam of light as it traverses the medium and, 
as such, is strongly wavelength dependent. Other mechanisms such as 
fluorescence and photosynthetic absorption are small enough to be 
negligible. 
When a collimated beam of light, with an initial irradiance E.sub.o. passes 
through an attenuating medium, its irradiance E.sub.x at a distance x from 
the source is given by 
EQU E.sub.x =E.sub.o exp(-.alpha.x) (7) 
where .alpha. is the total attenuation coefficient. In water this 
coefficient is primarily the sum of attenuation due to scattering and that 
due to absorption. Typically, .alpha. is of the order of 0.04 m.sup.-1, 
0.20 m.sup.-1 and 0.34 m.sup.-1 for distilled waters coastal water and bay 
water respectively, all measured at 510 nm. 
A more convenient way of expressing the attenuation of light is in terms of 
the "attenuation length" (.alpha..sup.-1) of the medium. The attenuation 
length is defined simply as the reciprocal of the attenuation coefficient 
and is expressed in "meters". Expressing the above figures in terms of 
attenuation length we have about 25 m for distilled water, 5 m for coastal 
water and about 3 m for bay water. It is generally agreed that the 
visibility limit of a dark object in water, near the surface in daylight 
is around four attenuation lengths. 
The variation in attenuation length with wavelength for distilled water is 
shown in FIG. 7. The peak transmission occurs at around 480 nm. In 
sea-water, with its dissolved yellow substances caused by breakdown of 
animal and plant matter, we would expect this "transmission window" to 
shift towards the green region of the spectrum. At this peak about 60% of 
attenuation is due to scattering by particulate matter, whereas the rest 
is due to absorption. 
The above behaviour indicates that for long-range underwater work, lasers 
such as argon-ion or frequency-doubled Nd-YAG, which have wavelengths in 
the blue-green region of the visible spectrum, will be the most suitable 
choice. 
An effect of scattering common to all underwater optical imaging systems is 
the reduction in contrast and apparent brightness of the image. Two 
mechanisms are responsible for this: firstly, light from the irradiating 
beam can be backscattered towards the film creating a "luminous fog" 
through which the target is viewed; secondly, light reflected from the 
brighter parts of the target is forward scattered into the line of sight 
of darker parts of the target. Both mechanisms cause the darker parts of 
the object to appear brighter than they really are relative to the bright 
areas resulting in a decrease in contrast of the image. This loss of 
contrast ultimately degrades resolution. 
In holography, image contrast is further dependent on fringe visibility in 
the recorded hologram, which is, in turn, dependent upon the relative 
planes of polarisation of the interfering beams. When recording a 
hologram, only object light which is polarised in the same plane of 
vibration as the reference light can actually interfere to produce the 
required hologram. Conventionally the object and reference beams are both 
polarised in the vertical plane of the electric vector. However, light 
scattered back towards the film plane from small particulates in the water 
may suffer some depolarisation resulting in some light of the wrong 
polarisation reaching the film. Although this light cannot contribute to 
the recording of the hologram it does raise the overall background level 
of the film thereby reducing the signal-to-noise ratio. 
Preliminary investigation of the holograms taken earlier did indeed 
indicate a loss of brightness for holograms taken in sea-water over those 
taken in air. Measurements of the polarisation of the object beam 
irradiating the film showed it to contain a component of horizontally 
polarised light at around half the irradiance of the original vertical 
polarisation. 
The magnitude of the depolarisation was studied using the arrangement shown 
in FIG. 8 consisting of a collimated laser beam 40 passing through a water 
tank 41, together with a moveable detector and prism 42. The irradiance 
and state of polarisation of light scattered from a collimated laser beam 
as it traversed a tank of sea-water was measured at various angles around 
the beam direction. The state of polarisation is expressed as the ratio of 
the irradiance of the vertical component, E.sub.V, to the sum of both 
horizontal and vertical components, E.sub.V +E.sub.H, as 
EQU p=E.sub.V /(E.sub.V +E.sub.H) (8) 
A value of p close to unity indicates a beam which is strongly vertically 
polarised, whereas, p tending towards zero indicates a strong degree of 
horizontal polarisation. 
A graph of p and total irradiance, E.sub.H +E.sub.V, against scattering 
angle is shown in FIGS. 9(a) and 9(b) resp. The polarisation state of the 
straight through beam (0.degree.) remained unchanged from its initial 
value of p very nearly equal to one indicating that the beam was not 
suffering any significant depolarisation. At angles greater than 
10.degree. the proportion of horizontally polarised light increased 
peaking at a scattering angle of about 90.degree., corresponding to a 
p-value of 0.62. Light backscattered at angles greater than 90.degree. 
contains as much as 30% of the horizontally polarised component, 
indicating that such light reaching the film would contribute to the 
overall background level. The overall irradiance of the light decreases as 
the scattering angle increases. The high intensity of scattered light 
which is visible at these small forward angles is believed to be due to 
transparent plankton having a refractive index close to that of water and 
small point-to-point variations in refractive index caused by thermal 
effects and salinity gradients as discussed earlier. Wavefronts passing 
through sea-water will, therefore, be distorted and the resolving power of 
any viewing system will be expected to deteriorate over its comparable 
performance in air. Small angle scatter is not expected to have much 
effect on the polarisation of light. Scattering from larger particles, 
however, will have some depolarising effect and this would account for the 
lower values of P obtained at larger angles. 
Another likely cause of depolarisation is that due to the light reflected 
from the target itself. In a series of experiments aimed at establishing 
the magnitude of this effect, parallel laser light was reflected from 
rough surfaces 46 such as corroded aluminium and ground glass and the 
light scattered at various angles monitored using the arrangement shown in 
FIG. 10 comprising a collimated laser beam 44 and polarisation detector 
45. A graph of p and total irradiance, E.sub.H +E.sub.V, against 
scattering angle is shown in FIG. 11. As expected, significant 
depolarisation, up to 20%, of the light is experienced at all scattering 
angles. The extent of depolarisation, however, decreases as the scattering 
angle increases, with p eventually reaching a maximum when the scattering 
angle equals that of the angle of incidence, that is at 45.degree. to the 
normal. The total irradiance is also a maximum at this angle. Hence, at 
that angle the beam suffers the least amount of depolarisation. 
The above effects can be minimised if a vertically orientated linear 
polariser is placed in front of the film plane so that only light 
polarised in the required plane reaches the film. Hence the background 
irradiance will be reduced. Furthermore when exposure values are estimated 
a similar filter placed across the exposure meter will ensure that only 
the correctly polarised light will be measured. 
Advantageously, the concepts of circular polarisation can be exploited to 
improve image contrast. Light which is circularly polarised in a 
particular direction changes its "handedness" each time it is reflected. 
For example, light which is originally "left-hand" polarised will become 
"right-hand" polarised after one reflection and will return to "left-hand" 
polarised after two reflections. Generally, light backscattered from 
particulates in sea water will be reflected once, whereas, light reflected 
from rough objects will experience more than one reflection. If left-hand 
(say) circularly polarised light is used in object illumination, scattered 
light will be predominantly right-hand polarised and light reflected from 
the object scene will possess an approximately even mix of left- and 
right-hand components. Placing a right-hand polariser at the film plane 
will ensure that only light reflected from the object will reach the film. 
Thus the contrast of the image will be improved by removing unwanted 
scattered light. This process, though, is obviously wasteful of energy 
since about half the light reflected from the object is thrown away. The 
process, though, will only be effective with rough objects, specular 
reflectors will experience only one reflection and hence will suffer a 
reduction in contrast. A further quarter-wave plate is needed behind the 
circular polariser to linearise the light. 
Additional techniques for the reduction of back-scatter which have been 
found advantageous in underwater photography include "volume reduction" 
and "range gating". In the former, the scattering volume which is common 
to both source and receiver fields is reduced by increasing the separation 
between source and receiver. For a holographic camera this would be 
accomplished by increasing the distance between the emitted object beam 
and the film plane. In practical terms this solution might put 
unreasonable constraints on system geometry. Range gating is one solution 
to the problem of backscatter. The receiver is electronically gated in 
conjunction with the use of a pulsed laser as the illuminating source. The 
photosensitive medium is exposed to light only at the instant the pulse 
reflected from the target reaches it and then switched off. In this way, 
any light backscattered towards the film by particulates in the water will 
not be seen by the receiver. An alternative concept which can be exploited 
in holography is known as "coherence gating". In this technique, the 
reference beam and object beam paths are matched to within the coherence 
length of the laser. The coherence length is adjusted to correspond to the 
distance between holographic film and subject. Hence only light reflected 
from the object will meet the conditions for interference, light 
backscattered from the water will not meet the conditions and will, 
therefore, not be recorded on the hologram. 
To compare and contrast the resolving power of holograms recorded of 
underwater objects with the equivalent holograms taken in air the optical 
arrangement as previously shown in FIG. 1 was used. Holograms were 
recorded, both, with and without the observation tank in place. The target 
for all resolution measurements was a standard resolution chart possessing 
a series of vertical and horizontal bars in the range 1 to 2281 p/mm. The 
resolution of the reconstructed real image was measured using a travelling 
microscope fitted with a 10x Ramsden eyepiece with an overall system 
magnification of 20x. All holograms were recorded on Agfa holographic 
plates type 8E56HD and processed using a pyrogallol based developer (Agfa 
formulation GP 62) and bleached in a para-benzoquinone (PBQ) based bleach 
(Agfa formulation GP 432). 
Holograms were recorded with the unexposed plate in the plate holder and 
the resolution chart in position A of the optical arrangement. After 
processing according to the method outlined above the hologram was rotated 
through 180.degree. and illuminated through the back of the plate. The 
corresponding real image is reconstructed in position A'. Collimation of 
reference and reconstruction beams was accomplished using a lens with a 
focal length of 300 mm and aperture of f/5. The wavefront accuracy is 
.lambda./8 over the central 38 mm of its aperture at 514 nm. In order to 
illuminate as large an area of the holographic plate as possible the 
entire lens aperture was used to expose a roughly elliptical area of 65 
mm.times.75 mm of the film. The lens was collimated to an estimated 
divergence of no more than 2 mrad. A reference beam to optic axis angle of 
30.degree. kept the spatial frequency of the system below that the cut-off 
frequency of the film. 
The laser used in the experiments was an argon-ion (Lexel type 90-4) 
delivering up to 1.5 W single frequency at 514 nm. The entire set-up was 
mounted on a vibration isolated table. 
A number of holograms were recorded in order to monitor the optical 
resolution achievable when recording the holograms underwater. Initially, 
two holograms were taken under the conditions outlined above but with no 
observation tank in position. In other words the holograms were recorded 
entirely in air in order to establish a reference point. These holograms 
were taken at target-to-film distances of 550 and 1000 mm respectively. A 
second pair of holograms were the recorded at the same target-to-film 
distances but with a perspex observation tank in place. In this case the 
perspex wall was nominally 10 mm thick and the distance between the front 
wall of the tank and hologram plane was 240 mm. A third pair of holograms 
were recorded as above but with turbid water in the observation tank. In 
this situation the holograms were recorded at in-water paths of 300 and 
750 mm respectively. In all cases the film, perspex interface and target 
were parallel to each other and on the same optic axis. 
The experimentally obtained resolving powers are shown in Table 4. As a 
reference point a resolution of 57 lp/mm was measured directly on the 
original resolution chart using the measuring microscope described earlier 
when illuminated by reflected laser light at 514 nm. 
TABLE 4 
______________________________________ 
Resolving Power Measured from Underwater Holograms 
Location Total film Path dist of 
Measured 
of target to target dist 
target in water 
Resolution 
______________________________________ 
In air 550 mm 22 lp/mm 
In air/in tank.sup.1 
550 mm 20 lp/mm 
In water/in tank.sup.1 
550 mm 300 mm 18 lp/mm 
In air 1000 mm 9 lp/mm 
In air/in tank.sup.1 
1000 mm 8 lp/mm 
In water/in tank.sup.1 
1000 mm 750 mm 7 lp/mm 
______________________________________ 
Note 1: This distance includes a film plane to tank separation of 240 mm 
and a nominal tank wall thickness of 10 mm. 
The figures obtained for resolving power of underwater holograms show a 
decrease over the reference holograms recorded in air as, firstly, a 
perspex interface is placed in the optical path and then, secondly, a 
water interface is added to the path. The ultimate reduction in resolving 
power is from 22 to 18 lp/mm. These figures should be contrasted with 
those obtained by underwater photogrammetry which indicate a resolving 
power of around 0.5 lp/mm for similar viewing conditions in sea water. 
In the field, measurement of resolving power is not a very meaningful 
figure considering that the object may be rough, poorly reflecting and low 
contrast. Being able to measure a particular set of bars on a resolution 
target does not really help in determining whether or not a particular 
surface feature can be visualised using holography. To illustrate this a 
hologram was taken of an engineering test piece: a polished titanium block 
with a stress-induced crack in it. The hologram was taken under the 
conditions outlined above with a total object-to-film distance of 550 mm 
and a distance in water of 300 mm. A photograph taken from the 
reconstructed real image is shown in FIG. 12. The crack, which was 
measured to be 40 .mu.m across the root, is clearly visible. A 
reconstruction from a second hologram, taken under identical conditions to 
those above, of a corroded weld specimen is shown in FIG. 13. 
A further range of holograms were taken with the objects submerged in 
"live" sea water. The total attenuation length, .alpha..sup.-1, of the 
water was measured, using a simple arrangement of collimated laser beam 
traversing the water filled tank, as 0.56 m at 514 nm. This attenuation 
corresponds to a light loss of some 80% over a 1 m beam path. The path was 
through a 400 mm length of sea water. In this unoptimised system, a real 
image resolution of 5 lp/mm was determined for the central on-axis parts 
of the object. 
When a hologram is recorded underwater and replayed in air, the 
reconstructed image will suffer from optical aberrations due to the 
difference in refractive index between the two media. For points on-axis 
the image will replay closer to the film plane in the simple ratio of the 
refractive indices. This latter fact will be modified by the presence of 
the perspex interface. 
Measurements of the image shift were made using the optical arrangement 
shown earlier, but this time provision was made to move the target 
position laterally with respect to the optic axis. For each hologram a 
ground glass screen was used to view the real image and the reconstructed 
image position determined. Table 5 shows the measured image shifts for a 
number of target locations. 
TABLE 5 
______________________________________ 
Image Shift Measured from Underwater Holograms 
Object Position 
Image Position Image Shift 
On-axis 
Off-axis On-axis Off-axis 
On-axis 
Off-axis 
______________________________________ 
417 mm 0 mm 309 mm 0 mm 108 mm 0 mm 
417 230 288 227 129 13 
417 435 149 388 268 47.sup.1 
194 432 223 3.sup.1 
______________________________________ 
Note 1: In this case two image positions were obtained, the first one 
corresponding to the vertical bars on the resolution target and the secon 
one corresponding to the horizontal bars of the target. 
The data shows that for on axis points the image shift is in accordance 
with the shift predicted from a simple refractive index ratio. As the 
target, however, is moved laterally with respect to the optic axis the 
measured image shift increases beyond that expected by simple theory. The 
reasons for this being that now the light rays travelling from the 
hologram to the image position are traversing paths which are 
substantially different from those encountered in recording. In 
particular, the refraction of light encountered at the air/perspex/water 
interface during recording does not occur in replay. The replayed rays do 
not converge to the same point from which they emanated in recording. As a 
consequence of this, two distinct image points are formed on 
reconstruction: one for the horizontal resolution bars and one for the 
vertical resolution bars. In other words the image is astigmatic. As the 
object is moved further from the optic axis the difference between the two 
image positions increases. 
Analysis of underwater holograms has shown that when a real image is 
reconstructed in a medium of lower refractive index (air) from a hologram 
originally recorded in a medium of higher refractive index (water) the 
resulting image will suffer from optical aberrations. The origins of this 
may be understood by reference to FIG. 14 which shows the path of two rays 
emanating from an object point, P, located on the optical axis with 
respect to the hologram centre, O. The refraction suffered by these two 
rays at the media boundaries serves to produce a virtual image of P 
located at point, P'. Hence on reconstruction of the real image, the image 
of point P will be located at point P' a distance OP' in front of the 
hologram. The actual position of P' cannot be precisely located by simple 
tracing of the paths of two rays as shown, since, any other two rays which 
subtend a different angle will locate P' at a different position along the 
optic axis. This situation is analogous to spherical aberration produced 
by a lens. In practice, the position of P, will be determined by 
identifying the circle of least confusion located at the waist of the ray 
bundle formed by tracing the path of all rays through the three media. 
Using a paraxial approximation, the expected image shift for on-axis point 
objects has been calculated and confirmed to a limited extent by 
experiment as discussed above. 
For points off-axis, the situation is complicated by the appearance of 
astigmatism in the image as discussed in the following section. 
Experiments involving the reconstruction of real images from holograms of 
off-axis point objects in water have revealed a considerable degree of 
astigmatism. The origins of this astigmatism would appear to lie in the 
fact that the hologram is being recorded in one refractive index medium 
and being replayed in another. 
FIG. 15 shows the basic geometry adopted in recording the hologram. The 
origin O of the co-ordinate system is taken as the centre of the hologram 
H. The object point P is located off-axis in the xoz plane at an arbitrary 
distance z.sub.w in water. Shown in the figure is the refraction of the 
meridional rays, PA and PB, the sagittal rays, PC and PD, and the 
principal ray, PO, at the water/glass and glass/air interfaces. For an 
object point off-axis in only one plane, the two sagittal rays will be 
symmetrically orientated with respect to the principal ray, thereby 
subtending equal angles with the refracting surfaces and, hence, with the 
hologram plane. Whereas, the optical path lengths of the sagittal rays are 
equal in each of the three media this will not be the case for the 
tangential rays, PA and PB. In this case, the angles subtended by the rays 
at the hologram plane OAA' and OBB' are unequal. This situation is 
identical to that pertaining to the origins of astigmatism in a lens 
system. 
In the context of holography, the aberrations discussed will be present in 
the reconstructed real image. The source of these aberrations is, however, 
not connected with the holographic recording process. In order to observe 
the astigmatic effects outlined above it is only necessary to view, from a 
position in air, and object immersed in water. The holographic process 
faithfully records the astigmatic image which can be seen by any observer. 
In the absence of significant monochromatic aberrations a point image would 
be observed at an equivalent position in the negative x and z position. 
Upon illumination of the hologram with the reference beam the original 
wavefronts emanating from the point object are reconstructed, maintaining 
their original orientation with respect to the holographic plate. Thus the 
meridional rays AA' and BB' and sagittal rays CC' and DD' proceed outwards 
from the hologram plane in the (-x,-z) direction and, failing to meet any 
refracting surfaces, form a point image at their intersection. However, 
the previous analysis of the recording stage shows that it is highly 
unlikely that the meridional and sagittal rays will come together at a 
common focal point. The reconstructed image may be expected to exhibit 
some degree of astigmatism. As might be expected from geometrical 
considerations this astigmatism has been found to disappear for axial 
object points leaving, in this case, only spherical aberration to 
consider. 
The variation in path lengths from the object point via the refracting 
surfaces to the hologram plane is analogous to the zone concepts of lens 
imaging. The astigmatic nature of off-axis point images is, therefore, not 
unexpected. 
In considering the aberrations associated with viewing a point object 
located in water it is important to gain an impression of how the degree 
of aberration varies with experimental parameters. This analysis is 
outlined in the following section with particular reference to the 
determination of the location and difference between the astigmatic 
images. 
FIG. 16 shows the refraction of a ray at water/glass and glass/air 
boundaries. A ray originating from a point object P located in the water 
suffers refraction at both water/glass and glass/air interfaces and passes 
through a point R located in air. The angle subtended by the refracted ray 
to the normal as it leaves the point P in water is denoted by 
.theta..sub.w, .theta..sub.g is the angle subtended by the ray to the 
normal at the water/glass interface and .theta..sub.a, is the angle 
subtended by the ray to the normal at the glass/air boundary. The 
co-ordinate distances along the optic axis in the respective media are 
denoted by z.sub.a, z.sub.g and z.sub.w for air, glass and water. Tracing 
the ray back through the media as if refraction were absent it appears to 
meet the optic axis at a point S. 
An equation defining this ray may be given as a function of the angle 
.theta..sub.a, as 
EQU .theta..sub.a =sin.sup.-1 [(n.sub.w /n.sub.a) sin .theta..sub.w ](9) 
More usefully the ray path may be defined in terms of the location of the 
point R above the optic axis. Hence, 
EQU y.sub.a =z.sub.a tan .theta..sub.a +z.sub.g .mu..sub.ag sin .theta..sub.a 
[1-(.mu..sub.ag sin .theta..sup.2)].sup.-1/2 +zw.mu..sub.aw sin 
.theta..sub.a [1-(.mu..sub.aw sin .theta..sub.a).sup.2 ].sup.-1/2(10) 
where .mu..sub.ag =n.sub.a /n.sub.g and .mu..sub.aw =n.sub.a /n.sub.w 
Equation 10 may be solved by iteration and a value determined for 
.theta..sub.a relating to a given point R. 
The ray emanating from P is just one of a family of such rays, which depend 
on the observer's viewpoint, each of which intersects the optic axis at 
some point S. The position of S will get progressively closer to P as the 
divergence of the ray PS decreases. Consider a pair of rays equidistant 
from, but infinitesimally close to R in the meridional (yz) plane of FIG. 
14. These two rays will intersect at a point T at some distance yc above 
the optic axis. Adjacent pairs of rays will map out the loci of all 
virtual image positions and describe a caustic curve as shown in FIG. 17. 
Hence it can be seen that two image positions are obtained for each ray. 
one on the optic axis (the sagittal image) and one on the caustic curve 
(the meridional or tangential image). The equation defining the caustic 
curve, thus contains all the information required to locate the astigmatic 
images associated with viewing the point P from any given location in air. 
It should be realised that if the position of best focus is being looked 
for in reconstruction of the hologram, the observer will select the point 
where the ray bundle converges to its minimum diameter. This is the circle 
of least confusion and it will occur somewhere between the two astigmatic 
image positions. 
In three dimensions, the loci of virtual image positions form a surface 
which is obtained by rotating the curve of FIG. 17 about the z-axis. The 
shape of this virtual caustic surface is a result of the increasing 
divergence of the refracted rays with increasing distance from the z-axis. 
By virtue of this rotational symmetry the projected rays can be seen to 
form a series of cones whose apex lie at increasing distances along the 
z-axis with decreasing radial distance of the projected rays from the 
z-axis measured in the plane of the media interface. The locus of 
intersection of any two adjacent cones (a circle) represents a 
cross-section of the caustic surface in a plane perpendicular to the 
z-axis. The intersections of an infinite series of such cones can be 
considered to generate the entire caustic surface. 
Since the ray RS is actually a tangent to the caustic surface as shown in 
FIG. 16, it is possible to express the co-ordinates of any point (yc' zc) 
of the point T on the caustic surface as a function of .theta.a, as 
follows, 
EQU z.sub.c =Z.sub.g .mu..sub.ag cos.sup.3 .theta..sub.a [1-(.mu..sub.ag sin 
.theta..sub.a).sup.2 ].sup.-3/2 +z.sub.w .mu..sub.aw cos.sup.3 
.theta..sub.a [1-(.mu..sub.aw sin .theta..sub.a).sup.2 ].sup.-3/2(11) 
EQU y.sub.c=z.sub.g .mu..sub.ag (1-.mu..sub.ag.sup.2) sin.sup.3 .theta..sub.a 
[1-(.mu..sub.ag sin .theta..sub.a).sup.2 ].sup.-3/2 +z.sub.w .mu..sub.aw 
(1-.mu..sub.aw.sup.2) sin.sup.3 .theta..sub.a [1-(.mu..sub.aw sin 
.theta..sub.a).sup.2 ].sup.-3/2 (12) 
from a given observation point R in air, Equations 10, 11 and 12 permit a 
determination of the astigmatic image points T and S and hence the 
difference in location between the two images. The extent of the sagittal 
and meridional line images may also be determined. 
In any practical arrangement where a point object P in water is viewed from 
an observation point in air the viewing aperture, in our case the 
hologram, will have finite dimensions. The astigmatic images consist of 
two separate line segments possessing finite length and width. These 
dimensions are determined by those rays of the ray bundle associated with 
the extended aperture in the meridional and sagittal focal planes. If the 
previous analysis is applied to a point at the centre of the aperture the 
location and separation of the line images may be determined since the 
image points T and S locate the centres of both line images. Since often 
an evaluation of the astigmatic difference is of primary concern, the 
point image analysis detailed above proves sufficient as illustrated in 
FIG. 18. This information can be used in assessing the extent of the 
aberrations associated with imaging a finite object through glass and 
water. 
Table 6 shows some data calculated for a hologram of 70 mm dimension. 
TABLE 6 
______________________________________ 
Analysis of Sagittal and Meridional Images 
for Underwater Holograms 
(All dimensions are in millimeters) 
______________________________________ 
Off-axis distance (x.sub.R) 
500 100 
Object point co-ordinates (x, y, z) 
0, 0, 737 0, 0, 737 
Meridional image point (x, y, z) 
57.1, 0, 521.2 
0.6, 0, 624.1 
Length of meridional line 
0, 0, 588.4 
0, 0, 627.8 
Sagittal image point 
8.5 0.4 
Length of sagittal line 
10.6 2.5 
Astigmatic difference 
88.1 3.8 
______________________________________ 
Where it is required to evaluate the dimensions of the line images or 
simply to visualise the convergence of rays from the aperture to the 
meridional and sagittal focal planes then Equation 1 provides the basis 
for a "spot diagram" analysis. The spot diagram serves as an illustration 
of the cross-section of the ray bundle by an array of points at various 
distances from the aperture. This technique serves as a useful method of 
visualising the image forming process. 
On the basis of the foregoing analysis, we have devised cameras for long- 
and short-range under-water holography. By way of example, a long-range 
camera comprises an enclosure 50, having a port 51 for services and a 
window W for observation of an object (not shown). Windows 52,53 are 
provided for the recording beam 54a,b. The radiation source comprises an 
amplified laser 55 with beam splitters and associated half-wave plates 
56,57 to derive the reference and recording beams. Prisms P1-P6 are 
provided to fold the various beams and constrain them within the available 
space. A collimating lens 58 and mirror 59 direct the reference beam 
through a filter assembly 60 on to a film 61 in a carrier 62. Circular 
polarisers 63,64 are provided in the path of the recording beams. 
In the short-range camera shown in FIG. 19 (b), only one recording beam 
outlet is provided. 
To produce successful holograms underwater requires careful selection of a 
laser with the required holographic performance and configuration for 
subsea use. Our experience indicates that two generic classes of laser are 
most appropriate for use in the specific holographic camera envisaged 
here, namely, the ruby laser or the frequency-doubled Nd-YAG laser. Of 
these two classes the ruby laser has its output in the red region of the 
optical spectrum (.lambda.=694 nm) and is most commonly used in industrial 
holography. The frequency-doubled Nd-YAG laser, though rarely used in 
holography up until now, has an advantage over ruby for underwater use 
because its output wavelength is in the green region of the spectrum 
(.lambda.=532 nm) and closely matches the peak transmission window of sea 
water. In normal circumstances for underwater holography of large volume 
subjects from long stand-off distances the Nd-YAG would be the ideal 
choice because of its wavelength advantage. For holography of a small 
subject at a short stand off distance, however, the wavelength advantage 
is not so significant and successful holograms could be made with a ruby 
laser. For that reason, holographic cameras could be envisaged using 
either Nd-YAG or ruby. 
A significant advantage of current Nd-YAG laser over the ruby is in terms 
of its pulse repetition rate. Whereas a maximum repetition rate of 5 
pulses a minute of holographic quality can be attained for ruby, YAG 
systems can be operated at around 1 Hz. Ruby lasers could perhaps be 
designed to operate at faster repetition rates but at a cost and size 
penalty. 
The mechanism by which energy is pumped into the laser medium also has some 
bearing on the size and performance of the laser. Traditionally, solid 
state lasers are pumped by a capacitively discharged flashlamp lying 
parallel to the crystal. Depending on the performance required by the 
laser this generally implies the use of a physically large power supply. 
Recent progress in solid state laser technology has led to the development 
of the diode-pumped Nd-YAG laser. Such lasers appear to offer a more 
efficient and compact means of coupling energy into the laser medium. 
The output energy required for successful underwater holography depends to 
a great extent on both the condition of the water and that of the object. 
We have found that for short-range holography an energy of 50 mJ would be 
sufficient, whereas, for long-range work an energy in excess of 250 mJ is 
needed. The 50 mJ of energy can be obtained using a laser with a single 
crystal rod (the oscillator). For higher energy, a second crystal is 
needed to amplify the oscillator energy. 
With these points in mind a specific laser has been identified as being 
particularly attractive from the point of view of performance and size. 
The laser under consideration is a pulsed system using, at present, ruby 
as its active medium. In most aspects of its performance, for example, 
linewidth, pulse duration and output energy, it is at least as good as the 
competitive lasers from other manufacturers. In some aspects, as indicated 
below, it is vastly superior to any other system currently available: 
(a) Of prime consideration is the fact that this particular laser has been 
designed as a holographic system from the outset rather than as an 
"improved" industrial laser. Hence its performance is optimised in the 
crucial areas of holographic performance and, consequently, could be 
easily configured to holographic camera specification. 
(b) A second important area where this particular laser scores over the 
competition is in its overall size. The laser head, at around 750 mm 33 75 
mm across depending on ultimate configuration, is approximately 1% of the 
volume of the equivalent commercial system. Its power supply at about 600 
mm.times.600 mm.times.100 mm, is barely 2% of the volume of the commercial 
equivalent. Obviously these size advantages are extremely relevant to any 
system which needs to be configured for subsea use. 
(c) A third significant consideration for subsea use, which this laser 
addresses, is that the laser should maintain its holographic performance 
over a wide range of ambient temperatures. Hence, the laser should be able 
to produce high quality holograms with a variation in cooling water 
temperature over several degrees. This particular system claims to be able 
to produce good holograms over a temperature variation of the coolant of 
.+-.7.5.degree. C. This performance should be compared with that of a 
typical system which will only guarantee good quality holograms over a 
.+-.0.5.degree. C. spread. 
The above laser can be configured in either oscillator-only mode or 
amplified mode and can be adapted for ruby or frequency-doubled YAG. 
An alternative method of camera construction, to mounting the entire laser 
and optical components in the camera head, is to mount the laser head on a 
ship or platform and carry the light to the object via an optical fibre. 
The most suitable arrangement is one in which both object and reference 
beams are carried by an optical fibre. The reference fibre carries light 
directly to the film holder, whereas the object fibre carries light to the 
scene of interest. In this arrangement only the filmholder and fibres need 
be taken underwater. To maintain the coherence of the laser light single 
mode fibre should be utilised. Because such fibre has a core diameter of 
the order of only 5 .mu.m, coupling of light into the fibre is difficult 
and when taken together with the high attenuation experienced at visible 
wavelengths, light loss is high. However, successful holograms have been 
taken using 2 m lengths of fibre. The source used was an argon-ion laser. 
To ensure that the planes of polarisation of the exit beams were parallel, 
polarisation rotators have to be included in the optical path. Preferably, 
a pulsed laser is used as the radiation source. Care must be taken because 
the high radiance of such lasers can cause melting of the input end of the 
fibre. 
The choice of film size has a considerable bearing on image resolution: the 
larger the diameter of the film the better is the resolution. For 70 mm 
film the theoretical image resolution is around 8 .mu.m for ruby laser 
light at a target distance of 1 m. For 35 mm film the theoretical 
resolution is about 16 .mu.m. It should also be noted that the viewing 
angle will be less for the smaller film size. 
The quality of the recorded hologram depends to a large extent on the 
choice of film type and processing techniques. Among the factors to be 
considered are exposure sensitivity, contrast, resolution and 
susceptibility to emulsion shrinkage. 
An alternative means of recording a hologram is to use thermoplastic film. 
Such film is commonly used in holographic interferometry. Its many 
attractive features include rapid electronic processing and reusability. 
Ideally the film holder should be able to accommodate holographic film in 
lengths corresponding to around 250 exposures. This latter number would 
allow for most applications of a "survey" nature. The film holder should 
be motor driven at up to 1 fps. The size of film chosen obviously has a 
bearing on size and performance of film holder and eventually holocamera 
size. 
For high resolution holography it is essential that the film be held as 
flat as possible between thin flat glass plates and should not be 
stretched or put under any strain during exposure. This requirement 
dictates that care should be taken in choosing the film transport 
mechanism. 
Generally, high resolution reconstruction of images from holographic 
recordings requires that any optical aberrations in the system be reduced 
to a minimum. For systems employing monochromatic light the aberrations of 
concern are spherical, coma, astigmatism, distortion and field curvature. 
Assuming recording and replay in air. the above aberrations can be reduced 
to zero, for point objects, only if a parallel reference beam is used. at 
both recording and replay stages, and also, if the recording and replay 
wavelengths are identical. 
For the more realistic case of an extended object the situation is similar 
to that mentioned above: although it is now not possible to reduce the 
aberrations to zero, they can be minimised if the aforementioned 
conditions are met. 
The collimating lens is chosen to be of a sufficient diameter to entirely 
illuminate the holographic film. Since it is also not desirable to utilise 
the maximum diameter of the lens because of the introduction of edge 
effects the chosen diameter of lens has a firm bearing on the ultimate 
camera dimensions. To adequately illuminate 70 mm film, for example, a 
lens diameter of 150 mm is desirable. Consequently this determines a focal 
length of around 300 mm. For 35 mm film the respective sizes are 50 mm and 
100 mm respectively. 
A possible method of minimising camera volume would be to carry the 
reference beam through a length of optical fibre. This can be accomplished 
successfully at the lower irradiance levels needed for the reference beam. 
For optimum holographic recording in terms of brightness and contrast it is 
generally necessary to ensure that the paths travelled by the object 
illumination beam and that of the film illumination beam (the reference 
beam) are identical. This condition can, of course, only be fulfilled for 
one specific object plane. In practice, though, provided that the entire 
scene of interest and the reference beam path are matched to within the 
coherence length of the system good, bright holograms will be obtained. 
For the lasers under consideration here coherence lengths of the order of 
a meter or more are typical. 
For optimal matching, path length compensation may be incorporated into the 
camera. Preferably, this would be preset, prior to deploying the camera, 
for a particular target range. On the other hand, it may be thought 
desirable, for reasons of stability, to have the reference beam path fixed 
at the most appropriate length for a majority of situations. 
In recording a hologram only light beams polarised in the same plane of 
vibration can actually interfere to produce the required hologram. 
Conventionally the object and reference beams are both polarised in the 
vertical plane of the electric vector. However, light scattered back 
towards the film plane from small particulates in the water may suffer 
some depolarisation resulting in some light of the wrong polarisation 
reaching the film. Although this light would not contribute to the 
recording of the hologram it could raise the overall fog level of the film 
and is best removed. A similar effect occurs with light reflected from 
specular objects in the observation scene itself. These effects can be 
minimised if a vertically orientated linear polariser is placed in front 
of the film plane so that only light polarised in the required plane 
reaches the film. 
For some objects the concepts of circular polarisation can be exploited. 
Light which is circularly polarised in a particular direction changes its 
"handedness" each time it is reflected. For example, light which is 
originally "left-hand" polarised will become "right-hand" polarised after 
one reflection and will return to "left-hand" polarised after two 
reflections. Generally, light scattered from particulates in sea water 
will be reflected once, whereas, light reflected from rough objects will 
experience more than one reflection. If left-hand (say) circularly 
polarised light is used in object illumination, scattered light will be 
predominantly right-hand polarised and light reflected from the object 
scene will possess an approximately even mix of left- and right-hand 
components. Placing a right-hand polariser at the film plane will ensure 
that only light reflected from the object will reach the film. Thus the 
contrast of the image will be improved by removing unwanted scattered 
light. This process, though, is obviously wasteful of energy since about 
half the light reflected from the object is thrown away. 
The process will only work with rough objects, specular reflectors will 
experience only one reflection and hence will suffer a reduction in 
contrast. A quarter-wave plate is needed behind the circular polariser to 
linearise the light. 
To reduce wavefront distortion to a minimum all ancillary components such 
as mirrors, prisms and beam splitters should have their critical surfaces 
flat to within .lambda./20. Additionally, all transmissive surfaces should 
be anti-reflection coated to minimise light loss. 
The beam splitter as its name suggests divides the intensity of the beam 
into two parts: one part forming the reference beam and the other forming 
the object beam. The split is not equal. Most light is needed for object 
illumination, since much of it is lost in scattering and large angle 
reflection, while only a small portion need form the reference beam. The 
portion forming the reference beam is easier to determine since it is this 
beam which governs overall exposure of the film. 
The beam splitter directs about 2% of the incident beam into the reference 
path. A half-wave plate at the output rotates the plane of polarisation of 
the reference beam through 90.degree. so that it is in the same plane as 
the object beam. A similar half-wave plate at the input to the beam 
splitter controls the relative intensity of both beams such that their 
ratio can be varied. 
Planoconvex lenses are chosen for all applications where a focused laser 
beam may cause air breakdown. The curved portion of the lens is placed on 
the opposite side from the incident beam to ensure that light reflected 
back down the system cannot be refocused in air. 
In conventional photography, to prevent unwanted light reaching the film 
plane a shutter would normally be placed in front of the film and opened 
at the required time. This could also be done for holography. However, 
because of the general insensitivity of holographic film to light and the 
monochromaticity of laser light it is only necessary to position a narrow 
band wavelength selective filter over the film such that only light from 
the laser can reach the film and expose it. It should be possible to 
fabricate the interference as a composite unit together with circular and 
linear polarisers. 
An underwater housing is necessary to protect the camera from ingress of 
water and external pressure effects. The housing should be designed to 
withstand a pressure of 30 bar. An optical window should be incorporated 
to enable emittance and return of light. It is envisaged that the complete 
system be mounted on a remotely operated vehicle (ROV). Power will be 
drawn from the ROV system. 
The performance and parameters of the holographic replay system have a 
crucial bearing on the fidelity of the reconstructed image. Ideally, the 
optical system used in replay should be matched to that of the camera in 
both geometry and wavelength. Thus the reconstruction system must be 
designed in conjunction with the camera to ensure that any compromises 
made in one do not adversely affect the performance of the other. 
Some of the elements required in the reconstruction system share common 
performance specifications with the similar component in the recording 
system such as those relating to ancillary mirrors, collimating lens and 
film holder. 
The specific features of replay worthy of particular mention are those 
relating to the quality of the reconstuction beam and the minimisation of 
optical aberrations. 
We have found that the use of a collimated reference beam at both recording 
and replay stages was desirable for the attainment of high fidelity 
images. It is necessary that a high degree of collimation is maintained 
for both beams with 1 mrad being an acceptable upper limit. The diameter 
of the collimating lens should at least match that of the recording 
collimator. 
We have also found that for high image fidelity recording and replay 
wavelengths should preferably be identical. The pulsed lasers used in 
recording the hologram are unsuitable for reconstructing an image upon 
which high resolution measurement has to be performed. Hence, it is 
necessary to pump a tuneable dye laser with a suitable continuous laser 
such as an argon-ion. Suitable dyes are available to allow reconstruction 
at both ruby and frequency-doubled wavelengths. 
The situation here is of course complicated by the fact that the holograms 
are recorded in water and subsequently replayed in air. Aberrations will 
be introduced into the system, the most severe of these being astigmatism 
and field curvature. Several possible routes present themselves as likely 
solutions to this problem. The possible solutions are to, 
(a) correct for aberrations at the recording stage by incorporating 
correcting elements into the camera configuration, 
(b) record the hologram in the normal manner and correct for aberrations at 
the replay stage by incorporating correcting elements into the 
reconstruction configuration, 
(c) record and replay without correction and correct by computer 
manipulation of output data, or, 
(d) some combination of all three. 
Of the above options it would seem that the most appropriate route is to 
record the hologram in the normal manner and correct for aberrations at 
the replay stage. The reasons for this being that the camera configuration 
is kept as simple as possible, thereby keeping it smaller and more 
reliable, and more complex methods of correction can be employed more 
readily in a laboratory based replay system. 
A specific optic can be designed through which the hologram can be replayed 
such that the aberrations are corrected. 
An initial approach is to replay the hologram through a simple parallel 
plate such that aberrations introduced by recording the hologram through 
an optical window are removed. 
If the hologram can be replayed at a shorter wavelength than that at which 
it was recorded such that the wavelength ratio is the inverse of the 
refractive index ratio between the two media, then it may be possible to 
remove astigmatism caused by recording in water. 
The incorporation of a holographic optical element (HOE) into the replay 
stage is one means of achieving correction. The holographic optical 
element is then substituted for an equivalent element made out of glass. 
An alternative embodiment relies on replaying the hologram back through a 
replica of the distorting medium so that aberrations are cancelled 
resulting in a distortion-free image. 
A possible reconstruction facility is shown diagrammatically in FIG. 20. 
This comprises an argon-ion laser 70 pumping a dye laser 71. The laser 
beam 72 is directed by way of prisms 73,74 and collimating optics 75 to 
illuminate the rear of a hologram 76. A pseudoscopic real image 77 is 
created. Aberrations are corrected by means of an aberration correcting 
element 78.