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Introduction to Optical Liquids
Any liquid can be used for its optical properties, but in practice the need for stability, low toxicity, system compatibility, transparency, and low cost in addition to specific refractive index and dispersion values would make finding a suitable liquid for an application an enormous and complicated task, if Cargille Laboratories had not, over a period of 40 years, gone through the selection process looking for liquid formulas that have the kinds of properties that are most desirable. Cargille Optical Liquids are divided into the following principle groups:
[ Cargille Refractive Index Liquids ]: 1/4 fl. oz. bottles with refractive indices from 1.300 to 2.31 at specific intervals (mostly intervals of 0.005 and 0.002 refractive index units). Specific refractive index and dispersion (including high dispersion) values are maintained by exacting quality control. These liquids are sold individually or in sets (called Series) covering certain refractive index ranges:
[ Master Calibration Liquids ]: very stable, non-toxic liquids with refractive index measured to an accuracy of ±0.00005 at various temperatures and wavelengths.
[ Cargille Immersion Liquids ]: The same kinds of liquids that make up Cargille Refractive Index Liquids Series AA, A, and B, but are generally sold at a lower cost because they are sold in larger quantities, are blended to a wider tolerance, and although their typical dispersion is known, dispersion is not quality controlled as it is for Cargille Refractive Index Liquids.
[ Cargille Immersion Liquid Specials ]: liquids with special properties of transmittance, compatibility, fluorescence, etc. that make them useful alternatives to Immersion Liquids in certain applications.
[ Fused Silica Matching Liquids ]: matches the refractive index at 632.8 nanometers and closely matches the dispersion of fused silica.
[ Cargille Laser Liquids ]: (originally developed for laser applications in the 1960's but now used in many other fields): very clear, stable, compatible, and low in toxicity... These liquids are generally more expensive than the other Cargille Liquids, but are often chosen because of their outstanding qualities.
[ Microscope Immersion Oils ]: Oils used for oil immersion microscopy (various types and viscosities).
[ Brix Liquids ]: Used to calibrate refractometers that read out in Brix (% sucrose).
[ Mounting Media (Meltmount) ]: series of mounting media which are specially formulated optical-quality thermoplastics for use in microscope slide mounting and other optical coupling applications.
[ Optical Gels ]: are typically used for either optical coupling for optical fibers and components or as a mode stripping gel.
Formula Codes: Cargille Optical Liquids are usually blended to a specified refractive index. Often the liquids will be named by a formula code and refractive index. Each code is available in a range of refractive indices. For example Immersion Liquid Code 5040 can be made to have any desired refractive index from 1.459 to 1.570.
Typical Characteristics Sheets: Cargille Laboratories can supply a computer generated printout of the typical optical and physical properties for nearly every Optical Liquid. These useful sheets contain a cauchy dispersion equation, the refractive index and percent transmittance at various wavelengths, the temperature coefficient of refractive index, stability information, compatibility (with common plastics, rubbers, and metals), pour point, boiling point, flash point, density, viscosity, surface tension, coefficient of thermal expansion, solubility with various solvents, and more. These are not specifications but typical values. Usually only the refractive index at one wavelength and temperature is maintained to a specific tolerance.
Optical Properties of Liquids
The optical properties of liquids are calculated as if they were homogeneous solids like optical glasses using the same mathematical formulas.
Refractive Index: A refractive index value has little meaning unless the wavelength and temperature are specified. In this text, when wavelength and temperature are not specified, the refractive index value is at 589.3 nanometers and 25°C. Cargille Optical Liquids completely cover the range in refractive index from 1.293 to 2.31.
Most liquids in general have refractive indices in the range of between 1.45 and 1.55 with the number of liquids with refractive indices outside this range becoming increasingly more scarce as you move further away from the range. Between 1.45 and 1.55 there are liquids that have properties suitable for most applications. Outside this range suitable liquids are rare especially for refractive indices below 1.40 and above 1.64.
The refractive index of liquids is less stable than for solids because of two factors: temperature and evaporation. The temperature coefficient (the change in refractive index for a given change in temperature) for liquids is always negative, almost always much larger than for solids, and almost always around -0.0004 refractive index units per degree centigrade. The accuracy of the refractive index value for a liquid is dependent on the accuracy and control of its temperature. The relatively large temperature coefficient of liquids can be used advantageously to "tune" the refractive index of a liquid to a desired value by changing its temperature. Temperature gradients that cause refractive index gradients, often seen as waviness or swirls in the liquid, can be avoided by keeping the environment at the same temperature as the liquid or by continuous mixing.
Evaporation of a chemically pure liquid will not change its refractive index because evaporation leaves its composition unchanged. However liquids that are mixtures of substances with different refractive indices and different volatilities do change in refractive index through evaporation. As the more volatile ingredients evaporate, the refractive index changes because the proportion of the components in the mixture changes. To obtain a stable refractive index one can select a pure substance, but finding one with the exact refractive index and properties desired is not often practical. An alternative and more successful approach is to select liquid mixtures that contain components of very low and balanced volatility. By varying the proportions of its components such a mixture can be used to make a liquid with any refractive index over a particular range, and a variety of other properties, such as dispersion, viscosity, density, etc. Most Cargille Optical Liquids are of this type.
Water-based liquids are usually not preferred to oil based liquids because of the relatively rapid exchange of water with the open air which causes the refractive index to rapidly change. With sugar or sodium chloride aqueous solutions this exchange is just evaporation that causes the refractive index to rise rapidly, until a saturated solution is reached. Finally when all the water has evaporated just the dried sugar or sodium chloride remains. However, with Cargille water based Immersion Liquids Code OHZB and Code OHGL, each code has a refractive index below which the liquid will increase in refractive index due to evaporation of water into the air, and above which it will decrease in refractive index due to absorption of water from the air. Storage in tight containers will prevent this from occurring.
Dispersion: The refractive index of any substance will be found to vary with wavelength. This variation is called refractive dispersion, or simply dispersion. The dispersion of optical liquids can be described mathematically by a Cauchy, Sellmeier, or another dispersion equation generated from refractive index values at several wavelengths. Useful dispersion curves can be drawn using refractive index values at two wavelengths using specially designed graph paper called Hartmann Dispersion Nets. Dispersion for a liquid can also be expressed in terms of Abbe value v = (nD-1)/(nF-nC) or more simply as nF-nC, where D, F, and C denote the wavelengths of 589.3, 486.1, and 656.3 nanometers. For optical liquids, it will be found that as wavelength goes up, refractive index goes down. Higher refractive index liquids tend to have higher dispersion. For any given refractive index, liquids have generally much higher dispersion than solids which can make them useful additions to a glass optical lens system. Liquids with unusually high dispersion are Cargille Refractive Index Liquids Series E, and Immersion Liquid Code EC31. Liquids with particularly low dispersion, although still higher than optical glass are of Cargille Immersion Liquids Code 5040. Dying a liquid can effect its dispersion in or near regions of strong absorption.
Transmittance and Color: Most optical liquids are used in the visible spectrum between 400 and 700 nanometers, but near-ultraviolet (UV) and especially near-infrared (IR) applications also use optical liquids. Although colorless liquids are usually preferred, many otherwise desirable optical Liquids will absorb slightly in the blue wavelength region giving the liquid a slight yellow color. This is generally unavoidable in optical liquids with refractive indices above 1.55. Typically an optical liquid will transmit greatest from 500 to 800 nanometers; at blue wavelengths below 5000 angstroms and extending into the UV it will typically begin to absorb and reach a cutoff in the near-UV. Transmittance in the near-IR is typically characterized by a series of transmittance peaks and valleys from 800 to 1600 nanometers (0.8 to 1.6 microns) followed by mostly very low transmittance out to 10.6 microns. A remarkable exception to this pattern are Cargille Laser Liquids Code 433 and Code 3421 which do not reach a UV cutoff until below 240 nanometers. They are also highly transparent, without peaks and valleys in the IR, out to 2.5 microns. Percent transmittance curves are available for most Cargille Optical Liquids.
When working with a particular wavelength, especially if it is a high power laser, it is important to select a liquid with good transmittance at that wavelength. Absorption can cause the liquid to heat and form a lower refractive index tunnel along the light path which distorts the beam of light. If enough energy is absorbed, it can cause vaporization or thermal breakdown of the liquid or adjacent components. Path length is also important. For example a liquid which transmits 90% through 1 cm path will approach 100% when used in a very thin layer and will drop to 7% for a 25 cm path.
Absorption of light, especially UV light such as in sun light, will cause some liquids to yellow and some to eventually become quite dark. Light sensitive liquids can remain unaffected if stored in a dark place in dark amber glass bottles and used under low power or with wavelengths that will not affect it. There are liquids that are unaffected by sun light. These include Refractive Index Liquids Series AAA and AA, Immersion Liquids Codes S1050, 4550, and 50350, and all Cargille Laser Liquids except Code 5763. Immersion Liquid Code 1160 may show minimal discoloration but only at refractive indices close to 1.538.
Some optical liquids will darken if heated to an elevated temperature. Ones that show best resistance to heat caused discoloration are all Cargille Laser Liquids and Immersion Liquids Codes S1050, 4550, 50350, 06350, and 1160.
Fluorescence: Fluorescence caused by the liquid absorbing at shorter, usually UV wavelengths, can be a problem in some applications. Particularly low in fluorescence when illuminated by 3560 angstrom black light are Laser Liquids Codes 433 and 3421, and Immersion Liquids Codes S1050, 4550, 50350, 06350, OHGL, OHZB, and 50BN.
Light Scatter: Light scatter is common in liquids, especially of high molecular weight. Nearly all Cargille Optical Liquids when penetrated by a beam of a HeNe laser will show some scatter illuminating the path of the beam, but the actual amount of light lost to scatter is negligible in nearly all Cargille Optical Liquids. Cargille Optical Liquids with relatively low scatter are Immersion Liquids Codes OHGL, OHZB, 4550, 06350,50350, and 1160.
Physical Properties of Liquids
Although optical liquids are chosen because of desired optical properties, the selection process will almost always require consideration of their physical properties.
Viscosity: The viscosity of optical liquids is usually expressed in centistokes (cSt) units. Viscosity of some common liquids are:
Viscosity decreases with increase in temperature at a rate that is usually non-linear and different for each liquid.
With all other properties the same, an optical liquid with relatively high viscosity will have the following advantages over a relatively low viscosity liquid: the high viscosity liquid will hold two components together better, will better bridge a gap, will be less likely to run, and will be less prone to spills. On the other hand, the relatively low viscosity liquid will have the following advantages over a high viscosity liquid: the low viscosity liquid will take less time to reach a uniform temperature, will be easier to pump or mix, will be less likely to entrain air bubbles, and will likely be easier to clean off of surfaces.
People interested in an optical grade liquid with various viscosities might consider Cargille Microscope Immersion Oils. All Cargille Microscope Immersion Oils Types A, B, 300, NVH, and OVH have the same refractive index (1.515 at 23°C) but different viscosities, and they can all be mixed with each other.
Surface Tension (Cohesion) and Adhesion: The balance of the surface tension and adhesion properties in a liquid will affect its performance. For example, low surface tension and high adhesion may cause a liquid to migrate from where it is placed to parts of the system where it is not wanted. In practice, these properties for optical liquids are usually of minor importance.
Density: Density may be important in a system such as one requiring the suspension of particles in the optical liquid or one requiring the formation of layers of Optical Liquids. Density and refractive index are directly related to each other in so far that both are dependent on the concentration of molecules of a liquid in a given volume. When a liquid is heated and expands, both the refractive index and density decrease. Most liquids have densities between 0.8 and 1.3 g/cc. Liquids with densities below 0.8 g/cc tend to have low refractive indices (1.33 to 1.46) but higher densities do not necessarily mean higher refractive indices. For example: Fluorocarbons and chlorofluorocarbons such as Laser Liquids codes 433 and 3421 have high densities (around 1.9 g/cc) and low refractive indices of 1.293 to 1.400. There is little correlation between refractive index and density for liquids as a whole, but for Cargille Optical Liquids those with higher refractive indices tend to have higher densities (if you exclude the fluorocarbons and chlorofluorocarbons). Cargille makes aqueous and organic oil based liquids blended to any desired density from 0.80 to 4.05 g/cc. These are called Heavy Liquids.
Coefficient of Thermal Expansion: The coefficient of thermal expansion in cc/cc/°C units is equal to the temperature coefficient of density g/cc/°C when density equals 1 g/cc. The coefficient of thermal expansion for liquids is usually much larger than for solids. Expansion must be considered if the liquid will be in a sealed portion of the system. An expansion cavity might be considered.
Boiling Point: Generally, optical liquids are chosen that have initial boiling points well above the intended working temperature.
Flash Point: Generally, optical liquids are chosen that have flash points well above the intended working temperature.
Freezing Point and Pour Point: An Optical Liquid will usually be required to have a freezing point that is below the working temperature.