FIELD OF VIEW CONSIDERATIONS. Hyperbaric chamber viewports are
restrictive to both area and viewing angle. This subsection presents a
simplified approach to treatment of viewing angles for the designer. The
examples given are simple graphical techniques for handling field-of-view
problems. The techniques may be extended to three-dimensional form with
additional variables at the designer's option.
Refractive Index. The refraction of light in thick windows viewed
at close range can produce noticeable distortion due to refraction. An
example of the effect is shown in Figure 3-8. A ray passing through a 2-inch
thick acrylic window at a 30 deg. angle is deviated about 3/8 inch. The
effect is governed by Snell's Law, as given in the figure. The deflection
will be parallel provided the refractive indexes of the fluids on both sides
of the window are equal. Since the conditions governing refraction vary with
the gas in the chamber, and since the effect is generally not large enough to
seriously affect rough estimates of viewport size requirements, the
simplified field of view diagrams given here assume undeviated line-of-sight.
Effective Limiting Aperture. Every viewport design will contain an
aperture which restricts the field of view more than any other part of the
viewport structure when sighted from a point on the central perpendicular to
the window. As illustrated in Figure 3-9, the location of the limiting
aperture may be on the near side, far side, or within the window structure.
It may also change location depending on the viewing distance chosen, and
whether viewing is from inside or outside the chamber.
Monocular and Binocular Fields. Most observing through viewports
is done by personnel at close range, using both eyes. This results in a
compound field of view in the horizontal plane. The compound field consists
of a central binocular portion bounded by two monocular segments. The
observer has full stereoscopic vision only in the binocular portion. The
monocular segments cannot be viewed as comfortably, and depth perception is
possible only by indirect clues. The observer can, of course, shift the
binocular field by moving his head, but this involves a loss of stereoscopic
(1) Horizontal Plane. Figure 3-10 illustrates the determination
of projections of the monocular and binocular fields in the horizontal plane
for a given window aperture and viewing distance. Such diagrams can be drawn
to scale using a typical eye separation of about 2-1/2 inches.
DESIGN OF ACRYLIC WINDOWS. The design of safe acrylic windows for
hyperbaric chambers has progressed to the point where tested procedures are
available (see Reference 2, Stachiw, Critical Pressure of Conical Acrylic
Windows, 1967). The recommended procedures are based on experimental data
giving catastrophic failure pressures under rapidly applied hydrostatic
loading as a function of thickness to diameter ratio. As a result of further
experiments with sustained loading at elevated temperatures, and through the
determined to allow adjustment of short-term failure data to more realistic
conditions. The recommended methods are very conservative in that individual
proof tests are specified for multipliers less than 10 or 12. Acrylic
windows must be designed per Reference 1 (ANSI/ASME PVHO 1, Appendix A,
Design of Viewports) and are briefly described in the following paragraphs.