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The refractive index (RI) (or index of refraction) of a medium is a measure of how much the speed of light is reduced inside the medium. For example, typical garden variety glass has a refractive index around 1.5, which means that in glass light travels around 1/1.5 or 2/3 the speed of light in a vacuum (the RI of air is a bit higher than vacuum, but is often rounded to 1.0). When light crosses a boundary between materials with different refractive indices, the light beam will be partially refracted (bent due to the change in light velocity), and partially reflected at the boundary surface. The best scenario for light transmission is where the photons approach the boundary straight on (0°), referred to as normal incidence. As the angle of incidence increases from normal to the surface (closer to being parallel to the boundary) up to the critical angle of reflectance, light will stop crossing the boundary altogether and instead be totally reflected back internally (this can only occur where light travels from a medium with a higher refractive index to one with a lower refractive index; it will occur when passing from glass to air, but not when passing from air to glass though there are reflective losses in both directions). Although we do not notice the losses due to the dynamic response of the eye, these reflective losses are present in windows and even eyeglasses at normal incidence (both around 8%, 4% for each side, though higher in uncoated high RI lenses).
Optical coupling compounds are used to bridge the gap or boundary for light signals between different media (optical couplant is an interchangeable term). These materials are also known as index matching compounds because they optimize light transmission by matching the refractive index of mating elements. Optical coupling compounds may be oils or other fluids, greases, or gels; they may retain their original characteristics, or they may be of the type that cures into a true glue. Immersion oil used for high magnification microscopy is one example, another is the optical gel used for splicing or terminating fiber optic cables.
Scintillation detectors used in well logging tools require an optical coupling compound between the scintillation crystal and the photomultiplier tube (PMT) for optimal results (there are actually two interfaces in a scintillation detector assembly, glass / air / glass). Optical coupling grease has been the most common traditional solution in the well logging tool industry, but a few manufacturers have used optical cements or curing silicone compounds (epoxies can be problematic at higher temperatures). Non-curing optical coupling materials simplify later service work should a crystal or PMT require replacement, but a mechanical means of insuring assembly integrity is necessary (tape being the most common solution). Separating a crystal and PMT that have been glued together can be a nightmare.
One rule of thumb estimates a 10-20% loss of light transmission for two uncoated optical surfaces in contact without a coupling agent. The Fresnel equations, deduced by Augustin-Jean Fresnel, describe the behavior of light when moving between media of differing refractive indices. The reflection of light that the equations predict is known as Fresnel reflection. Fresnel's simplified formula for reflection at normal incidence is as follows:
R = ((n2-n1) / (n2+n1))2
T = 1-R
where R=reflectivity, n1 and n2 are the refractive indices of the two materials (glass and air in this case), and T=transmissivity. For two interfaces as are present in a crystal / PMT assembly, the two transmissivities are multiplied together. Assuming a refractive index for the PMT end window material and for the crystal window material of around 1.4 to 1.5, and the interstitial air 1.0, our rule of thumb is obviously a fair estimation of Fresnel reflective losses (keeping in mind that not all photons will leave the crystal at the normal incidence angle). If an optical couplant is used between the PMT and crystal with a refractive index of around 1.4 to 1.5, then the improvement in transmission will be significant. If the refractive indices of the two optical surfaces differ, then the optical coupling compound refractive index should be the geometric mean of the two (not really an issue here unless an exotic window material is present like sapphire).
In truth the optical couplant used is not terribly critical, especially in simple gross count gamma ray tools. A physics professor friend was fond of using Vaseline brand petroleum jelly for trial couplings of PMTs and scintillation crystals in his lab, and at least one adventurous experimenter has used breast implant silicone gel. Vaseline is probably not a good choice for well logging tools and no comment is offered on the breast gel, but there are other inexpensive yet effective alternatives. A matched refractive index is probably more important than optical clarity. A perfectly clear compound with the wrong refractive index might be worse than say DC-4 or DC-111 which both look cloudy in thick sections. Of course we want a very thin layer of optical couplant (on the order of a few microns thick) so the clarity of bulk compound can be deceptive. Stated otherwise, almost any clear or translucent silicone fluid, gel, or grease with an RI in the range of 1.4 to 1.5 would probably work reasonably well. In this document non-curing gel and optical coupling grease are taken to be one and the same. The optical couplants discussed below are all silicones of one type or another.
Premium Options
The following premium options are all marked as optical copuplants and/or index matching materials. Dow Corning Q2-3067 optical coupling compound at $243.11 for a four ounce jar is regarded the "gold standard" for optical greases by many (and it is priced as if it were made of gold). GE G-688 optical grease was a competing product for many years, but seems to no longer be available. Visilox V-788 was another competing premium product; it is now manufactured by Blue Star Silicones exclusively for Precision Converting. The astronomical mark-up for these products has attracted other players as well. Rexxon markets their RX-688 at $115.00 for a one ounce tube (they are presently looking for a replacement manufacturer). Saint-Gobain sells their BC-630 for $85.00 for two ounces (back before Saint-Gobain acquired Bicron, BC-630 was rumored to be repackaged GE G-688). Nye Lubricants sells their NyoGel OC-459 for $87.85 for a 10cc syringe. Boro 1000 is offered by Boro Technologies, but the sample they promised us never arrived and they seemed unable to even provide pricing information. There are several other products available including a new entry from Silicone Solutions using the old Visilox "V-788" trademark (now changed to SS-988).
These premium products are often water clear (very pretty), but a few are a bit cloudy in appearance. They usually purport to be solvent free, and usually claim to be highly filtered and free of light absorbing microscopic particles. While these premium products are expensive, it is only fair to mention that a little optical coupling material goes a long way. An ounce or two would constitute a near lifetime supply for most logging technicians doing service work.
Economical Options
The following economical options are not usually marketed as optical couplants and/or index matching materials, but have proven to be effective as such. Ludlum Measurements uses Dow Corning 4 silicone grease for their survey meter scintillation detectors. DC-4 was Dow Corning's very first product developed during WWII to solve the problem of ignition system corona discharge at high altitude, thus making high altitude flight practical. DC-4 can be had for around a dozen bucks for a 5.3 ounce tube. One of the major logging companies uses Dow Corning Sylgard 184 to couple their scintillation detector assemblies. Sylgard 184 is a two part clear curing silicone available at around $50.00 for a .5 kilogram kit. GE RTV-615 is a more expensive two part curing optically clear material. (AnaLog prefers silicone grease for ease of later service as discussed above.)
Parker Super O-Lube is popular with Canadian logging companies (it is pretty good for O-rings, too). It is a high viscosity silicone oil and not a silicone grease (the less expensive Parker O-Lube is a petroleum based grease). Parker Super O-Lube sells for less than $30.00 for a two ounce tube. Dow Corning 200 (100,000 cSt), now Xiameter DMX-200 (100,000 cSt), is also a high viscosity silicone fluid, not a silicone grease. It has demonstrated superior light transmission in trials comparing various optical couplants (in one government lab test, DC-200 was the very best optical couplant tested for scintillation detectors). 100,000 cSt viscosity silicone fluid has the consistency of refrigerated honey; it should be noted that silicone fluids come in various viscosities, but the thick stuff is what is needed for an optical couplant. Both Parker Super O-Lube and DC-200 are water clear products. There have been rumors that Parker Super O-Lube is actually repackaged DC-200 (100,000 cSt); Parker will not confirm nor deny, and Dow Corning claims not to know.
Dow Corning 111 valve lubricant is a heavy bodied silicone grease that a few technicians like as an optical couplant (others complain it is not suitable in their view). Those who like DC-111 are impressed by its tacky quality that helps hold components in place, thus making detector tape-up less painful. Another candidate is Dow Corning High Vacuum Grease. It is a highly purified silicone grease with about the same consistency as DC-111 and is available for less than twenty bucks for a 5.3 ounce tube. While not our first choice, General Cement (GC) 10-810 silicone dielectric grease at around six bucks for a one ounce tube is adequate for trial couplings on the workbench (this material is also referred to as "Type Z5" silicone). Due to space constraints, not every option can be addressed here. One area where there are a number of entries is the fiber optic market; however, these products tend to be priced high (Dow Corning Q2-3067 is also sold as a fiber optic couplant).
Some Comments and Tips
The old advertising slogan "a little dab'll do ya" applies to optical coupling greases and fluids. We like to smear a thin film on both the crystal and PMT faces, then join the two with a slight twisting motion. It is important not to use too much as an excess extruding out of the crystal / PMT interface can be problematic (tape will not adhere to surfaces with a silicone film on them). A few logging tool manufacturers pulled vacuum on coupled scintillation detector assemblies in an effort to remove all air bubbles in the couplant, but that process is overkill and comparative testing reveals it is normally unnecessary. The shelf life of typical optical couplant products is given as one or two years. Some technicians store their optical coupling compounds in the refrigerator in the belief it prolongs shelf life (after all, it works with batteries).
Thanks to the membership of both the Wireline Reflector Mailing List and the RadSafe Reflector Mailing List (especially Robert Atkinson) for their contributions to this page.
Happy Coupling!
FTC Disclosure: Neither AnaLog Services, Inc. nor the author has an economic interest in any of the companies or products discussed above, and no monetary compensation was received. Free samples were received from Dow Corning (but not of Q2-3067). None of the manufacturers / distributors was aware this page would be written.
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10-17-09
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