Acrylic vs Glass – It’s Really No Comparison When it Comes to Optical Qualities!
Posted by admin in Articles on July 31, 2018
Wondering how acrylic and glass compare when it comes to their optical qualities? According to the handbook of acrylics, there really is no comparison at all.
Here’s what that treasured resource has to say about the subject:
Acrylic is far superior to glass in regards to optical quality.
Unlike glass, acrylic is colorless at any thickness. This is especially important when viewing the colorful world of the coral reef and the species that inhabit those waters through panels whose thickness, in many cases, exceeds 12 inches.
The index of refraction for acrylic (1.49) is much closer to water (1.37) than glass (1.58); therefore, there is less distortion to the image of the exhibit or to the animals as they swim past the viewer.
Because of the clarity and the refractive index, when the public views an exhibit through an acrylic panel, it is not possible to see the thickness of the panel and instead there is only the appearance of a wall of water.
Acrylic for aquaria use is generally colorless and clear with a minimum of haze. (> 92% total transmission with a 2-inch [51-mm] cubed sample per ASTM D-1003). Colored acrylic is available upon request. Opaque black acrylic is often used for a “high-tech” seamless transition when fabricating all-acrylic tanks.
As stated before, the refractive index of clear acrylic is 1.49 and water is 1.37. Air is 1.00. Using Snell’s law, it is possible to calculate the exact view angle through the acrylic.
When the viewer is looking straight through the acrylic, there is no angle shift (distortion) seen. As one looks through the acrylic at an angle, then the difference in refraction causes some distortion.
The worst case refraction angle through the acrylic is when the viewer is approaching the surface and looking across the panel. At this severe view the angle of refraction is approximately 45
Acrylic doesn’t just excel over glass in the aquaria setting, it also offers a superior viewing experience in submersibles.
From the early 1960s there was a growing consensus in the underwater community that a transparent material could be found that would solve the panoramic visibility enigma and allow submersibles to be built from such a material.
At several Navy laboratories around the country glass seemed to be the material of choice. Many experiments and experimental hulls were proposed and, in some cases, model-scale testing occurred. Unfortunately, it was found that the attributes of an acceptable material from which to construct an underwater vessel did not match up with commercially available glass compositions.
There are six attributes an acceptable material must have, ranked in order of importance. They are:
- Reproducibility of material’s response to identical loading conditions.
- Predictability of material’s response to different loading conditions.
- Availability of test criteria for establishing and maintaining quality assurance and control procedures during fabrication.
- Accessibility to proven design criteria for the particular material.
- Availability of fabrication technology capable of producing large-scale spherical shells and shell sectors free of residual tensile stresses with the same physical properties as small-scale models.
- Low fabrication costs.
Glass simply did not have these attributes in the 1960s and doesn’t have them all even today.
With only limited success with test programs focusing on glass, Dr. William McLean of NOTS, China Lake; Edwin Link; Will Forman of NOTS, China Lake; J.G. Moldenhauer of Pacific Missile Range, Pt. Mugu, CA; and Jerry Stachiw, to mention a few, decided to turn their attention to another material: polymethyl methacylate—or acrylic plastic.
The first reaction to acrylic plastic is that it does not appear to be a feasible material for constructing pressure hulls. Its low compressive strength, low fracture toughness, and sensitivity to stress risers in tension make it a poor replacement for metal. Fortunately, however, its strength-to-weight ratio makes it equivalent to low carbon steel; its plasticity permits it to tolerate large stress concentrations in compression; and its ability to adhere well to specially formulated adhesives makes it feasible to join acrylic structural shapes by bonding.
In addition, acrylic has very desirable optical qualities and is inexpensive in comparison to glass.
The fabrication of the early spherical shells was limited to free forming of small hemispheres from thin acrylic sheets by compressed air. But even with all the limitations, the ocean engineering community saw the advantages acrylic offered to panoramic visibility.
Actual diving systems with the all-acrylic pressure hulls which demonstrated this potential were HIKINO, KUMUKAHI, and NUCOTE.
HIKINO was conceived in 1962 by the late Dr. William McLean and engineered by D.K. Moore of NOTS, China Lake. The two-person vehicle had the shape of a catamaran with the acrylic sphere suspended between the two hulls. The acrylic sphere itself was assembled from two free-formed hemispheres mated at the equator to a metallic joint ring.
The vehicle successfully demonstrated the design concept of panoramic visibility but because it was free formed from thin commercially available sheets, the design depth was only 20 feet. As a result of this severe limitation, it was subsequently used only as a concept demonstrator in a shallow swimming pool.
KUMUKAHI was conceived in 1967 by T.A. Pryor, engineered by Will Forman, fabricated by Fortin Plastics, and delivered to Oceanic Institute in Hawaii in September 1969. The submersible was configured as a self-propelled diving bell with the batteries and a variable displacement tank contained in a pod suspended directly under the sphere.
KUMUKAHI experienced extensive manned operational evaluation at snorkel diver depth and test data seemed to indicate that the pressure hull would be safe to 300 feet for manned dives. Unfortunately, the vehicle was damaged in a freak collision with the support ship and its depth was derated to 90 feet.
However, it was never certified by the Navy or American Bureau of Shipping for commercial use. Lack of further financial support terminated this project.
NUCOTE was conceived in 1969 by E. Rosenberg and engineered by Mike Cook and Ron Reich at the Naval Undersea Center, San Diego. The two-man capsule was assembled from two free-formed acrylic hemispheres suspended by cable from an overhead winch on the NUC offshore tower. Air was supplied to the sphere by a flexible tube from the tower. The vertical movement of the undersea elevator was controlled from the interior of the sphere by its occupants. It was placed in service in 1973 to a depth of 56 feet but after two years of service was taken out and placed in storage.
As successful as the individual acrylic plastic hulls were in their application, they were not copied and applied by other designers to later diving systems.
The major reasons were (1) shallow design depths and small sizes, (2) absence of published design and test data applicable to larger acrylic spheres and greater depths, and (3) lack of approval for acrylic plastic pressure hulls for manned vehicles by the U.S. Navy, American Bureau of Shipping, Lloyd’s Register of Shipping, or Det Norske Veritas.
Pioneering work in the development of design and fabrication criteria for acrylic plastic submersible hulls was initiated by Stachiw in 1961 and continued until 1990. While at the Ordnance Research laboratory, Pennsylvania State University, Stachiw built several torpedo-shaped acrylic plastic instrumentation capsules that performed exceedingly well in a sea environment. The opportunity to apply this experience to large submersible hulls presented itself after he moved to the Naval Civil Engineering Laboratory, Port Hueneme, CA.
While investigating the performance of acrylic plastic in windows for submersibles, he was approached in 1964 by J.G. Moldenhauer of the Naval Missile Center, Pt. Mugu, to assist the center in the design and construction of a tethered undersea observatory.
Sensing the opportunity to match the transparency of acrylic plastic to a system requiring panoramic visibility, he submitted to NMC a revolutionary hull design in which the use of acrylic plastic was maximized and the use of steel was minimized. The design called for an acrylic sphere with a 1000-foot (305-meter) design depth and a 4200-foot (1280-meter) implosion depth.
At this time (1964), there were no data available on the mechanical properties of the thick acrylic castings needed for man-sized pressure hulls, the strength of bonded joints between thick castings, the plastic creep of acrylic under sustained loading, and the corrosive effects of seawater on acrylic. In addition, there was no engineering design or fabrication technique available for making a thick-walled pressure hull of acrylic and data were needed to make performance predictions.