Optical immersion probe incorporating a spherical lens

Details Optical immersion probe incorporating a spherical lens Optical immersion probe incorporating a spherical lens

2002-01-23

2004-12-14

Stafira, Michael P. (Department: 2877)

Optics: measuring and testing

For light transmission or absorption

Of fluent material

Reexamination Certificate

active

06831745

ABSTRACT:

BACKGROUND OF THE INVENTION
On-line and in-line analytical measurements are routinely performed for environmental and industrial process monitoring and control. Many of the specific measurements made in these fields are performed using spectroscopic-probes, which are inserted into the sample to be analyzed. These types of probes are generally referred to as ‘immersion probes.’ These probes are found in many shapes, sizes and optical configurations specific to a certain process or sample being analyzed. The need for the multitude of different optical probe designs stems from the varying samples they are designed to measure. These samples range from pure liquids, pastes, slurries, powders, solids and gases at varying temperatures, pressures and pH's.
Many immersion probe designs are intricately engineered with numerous moving parts and optical components. The addition of moving parts to allow an operator to align/focus a probe leads to imprecision during repeated analyses. The measurement errors may be due to misalignment, focus/alignment drifting over time or numerous operators having different optimization criteria. The addition of multiple optical interfaces can also lead to measurement imprecision when using immersion probes. Flat faced optical probes have a tendency to collect material on the optic in contact with the sample, thereby degrading performance over time. Many probes use a focusing (movable) optic in the barrel of the probe that is focused through a flat window that is in contact with the sample. Most immersion probes operate with a focusing lens that illuminates a portion of the sample that is some fixed distance from the physical tip of the probe (either window or lens). This common probe design leads to imprecision due to changing focal length and tip fouling and increases light scattering due to particles in a sample, changes in optical density and other physical variations in sample properties.
There is a need in the art for a single robust, straightforward, versatile and precise optical probe for use with various spectroscopic techniques to analyze all types of samples. The fact that the focal volume of the probe is a constant at the surface of the optical element in contact with the sample ensures accurate optical focus with whatever type of sample is present.
This invention provides a number of attributes not available in known optical immersion probes: 1) precise focus on any surface or material; 2) no need for sample alignment; 3) ease of sampling—simply place probe into or onto sample; 4) ability to be used in flowing/static sampling systems; 5) analysis not affected by directional flows or variable contact points; 6) analysis not affected by differential light scattering or particle distribution of solid particles; and 7) fully sealed probe element that is highly durable in harsh process/analytical environments. Thus, this invention circumvents the need for a multitude of imprecise complicated optical probes to measure samples ranging from gases to liquids to solids.
SUMMARY OF THE INVENTION
A novel optical probe is provided utilizing a spherical lens as both the optical and sample interface for applications including laboratory and process analysis applications. The spherical lens optical immersion probe (also called a ball probe) is an efficient sampling interface for the analysis of many types of samples including solids, powders, slurries, suspensions, particles, vapors, liquids and the like. The samples may be homogeneous, heterogeneous, or comprised of multiple phases. The probe design is compact, durable and straightforward with no moving or easily fouled components. The spherical lens probe has been demonstrated to greatly improve the precision of spectroscopic measurements (e.g. ultraviolet/visible (UV-Vis), near-infrared (NIR), mid-infrared (FTIR), fluorescence, and Raman) of a variety of samples over other known optical immersion probes. Importantly, this invention has broad applications to any optical analytical technology that necessitates an optical immersion probe.
The precision of the optical immersion probe of this invention is due to its novel design whereby a spherical lens is used as both the light focusing element and the optical interface with the sample. As such, the probe may also serve as a light collecting lens or device for optical signal collection. For example, in Raman spectroscopy, scattered light is collected by the spherical lens and directed to the instrumentation for analysis. This optical design provides a constant and precisely positioned focal volume, located directly on the proximal face of the spherical optic, for the excitation source of the various optical analyses, which leads to greatly increased measurement precision.
Further precision enhancement is gained by choosing a spherical lens having a focal point close to the surface of the spherical lens. Typically, the focal point is from about 50 &mgr;m to about 200 &mgr;m from the apex of the spherical lens. This ensures that any sample in contact with the spherical lens is properly focused to perform an optimal optical measurement. This design element eliminates the measurement imprecision due to path length variations inherent in other optical immersion probe designs.
FIG. 1
shows the theoretical optical path of a collimated optical beam through the spherical lens. For example, a focal length of about 200 &mgr;m from the apex of the spherical lens surface has been determined when a collimated 3 mm diameter 785 nm laser beam illuminates the surface of the spherical lens opposite the surface in contact with the sample. There is also no need for optical focusing of this probe onto/into the sample because the sample is optimally focused when it is in contact with the spherical lens. This makes the spherical lens optical immersion probe of this invention a focus free immersion probe with the only sampling condition being that the spherical lens itself must be in contact with the sample.
In its simplest embodiment, the immersion probe of this invention comprises a spherical lens attached to one end of a cylinder (the probe tip) in such a way that the end of the cylinder immersed in the sample (herein the ‘proximal end’) is substantially sealed and leak-proof. The seal can be provided by welding or braising the lens to the probe tip, or by using epoxy or other adhesives to fix the lens to the cylinder. Preferably the seal is provided by braising a sapphire lens to a metal or alloy cylinder. In other preferred embodiments, the lens is secured at the proximal end of the probe tip by using a combination of gaskets or o-rings and additional threaded tubes to provide force to the gaskets sufficient for a leak-proof seal.
As used herein, the term ‘gasket’ is used to refer to a pressure/tight seal made of any deformable material such as polymers, rubber, plastic, metals such as copper and gold, etc. Gaskets can be any shape, including the specific round shape of an o-ring.
Throughout the specification the term “leak proof seal” or substantially leak proof is used to describe a seal sufficient to close the interfaces in the optical immersion probe so as to prevent material from entering (or leaving) the interior of the optical immersion probe. The seal must be sufficient to prevent corruption of the analytical results. The quality of the seal is a measure of how much pressure the seal can withstand without leaking and is dictated in part by choice of sealing material (epoxy, weld, o-ring composition, etc. Those skilled in the art are readily able to recognize how to choose and apply materials that will provide sufficient seals for a given application. For example, the immersion probe described in this disclosure was constructed using 316 stainless steel tubing, Chemraz® 505 o-rings (Green Tweed, Inc.), and a synthetic sapphire spherical lens. The use of Chemraz® 505 o-rings has been shown to provide an optical immersion probe that is leak-proof to greater than 600 psi Helium. Alternative embodiments using sapphire braising resulted in an immersion probe that is leak

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