Illuminating means for refractometer

Details Illuminating means for refractometer Illuminating means for refractometer

1999-10-14

2001-02-27

Evans, F. L (Department: 2877)

Optics: measuring and testing

Refraction testing

Prism engaging specimen

C356S136000

Reexamination Certificate

active

06195160

ABSTRACT:

TECHNICAL FIELD
This device relates generally to critical angle type hand-held refractometers used for testing the physical properties of unknown liquids and more specifically to refractometers with illuminating means.
BACKGROUND OF THE INVENTION
A refractometer is a device for measuring the refractive index of a liquid substance. Refractometers are commonly used in industry where it is advantageous to know the specific relationship between two chemical species in a binary mixture.
When light passes from one medium (n
1
) into another (n
2
) with a refractive index different than the first, the relationship between the angle of incidence and the angle of refraction can be represented by Snell's Law:
Refractive index(
n
1
)·Sin(Angle of Incidence)=Refractive index(
n
2
)·Sin(Angle of refraction)
When the angle of incidence exceeds a certain angle, all light ceases to be refracted and is instead total internally reflected at the boundary between the two mediums. This angle of incidence is the “critical angle” and is a well documented scientific principle.
Critical angle is defined as:
&phgr;
C
=sin
−1
(
n
2
/
n
1
)
Where n
1
is the refractive index of the first, light incident, medium and n
2
is the refractive index of the second medium.
Known critical angle type hand-held refractometers typically incorporate a series of optical elements arranged to measure the refractive index of a solution under test. These optical elements may consist of a primary critical-angle prism, a transparent sample cover, an objective lens, a reticle, and a focusing means. All the elements are combined together and properly positioned and aligned in a rigid body or housing. Numerous other configurations of the internal components are possible including the addition of temperature compensating means, and various arrangements of optical components.
In operation, the liquid measurand is sandwiched between the measuring surface of a prism and an externally mounted transparent cover. Light incident upon the measuring surface of the prism passes through the exit surface, at an angle relative to the measuring surface. An objective lens focuses this light on an image plane or reticle. The reticle is usually a small piece of glass or other transparent material with an analog scale image etched vertically across its face. Tiny horizontal divisions divide the vertical scale and are representative of some specific unit of measure related to the refractive index of the solution.
Light focused on the image plane creates a shadowline or demarcation, at the critical angle, which separates the field of the reticle into light and dark regions. Very small changes in the refractive index of the liquid under test will create corresponding small vertical movement in the shadowline. It is not necessary to describe in any greater detail the path of the light energy through the optical elements or the formation of the image on the image plane, since it is elucidated in numerous patents and textbooks and is well known to those skilled in the art of refractometry.
A few refractometers with illuminating means have previously been disclosed in U.S. Pat. Nos. 2,601,128 and 4,650,323 and most notably is U.S. Pat. No. 4,890,916. These prior art refractometers have included illuminating means attached to a refractometer body or housing in a hinged sample cover superior to the measuring surface of a prism.
One disadvantage of the prior art refractometers is that the illuminating means is positioned in a hinged sample-cover suspended above the measuring surface. When the power source and electronics are housed in the sample cover, the cover becomes bulky and these elements can in fact impede ambient light passing through the cover, as well as create secondary shadows and/or reflections.
Another disadvantage to the prior art is that the sample cover is prone to damage since it is mounted externally on the refractometer body. Most importantly, the prior art provides for general illumination perpendicular to the plane of the measuring surface instead of at a preferred grazing angle of incidence.
A further disadvantage of prior art is that pressure induced by pressing on the sample cover to actuate the light source can squeeze the liquid sample from between the sample cover and the measuring surface or cause enough mechanical pressure on the prism to actually move the prism with respect to the other optical components causing an erroneous reading. Another disadvantage of this invention is that the amount of space available for mounting a light source and battery limits the size and life of the battery.
Various advantages of the present invention distinguish it from prior art. Because the illumination of the present invention is supplied at either a grazing incidence to the measuring surface or from below the measuring surface, light is incident relative to the prism in the direction necessary to best enhance the measurement of refractive index. Further, since the illumination means is provided in the body or housing of the refractometer it is sturdier and not prone to breakage or damage as in an externally mounted illuminating means.
DISCLOSURE OF THE INVENTION
The present invention concerns an illuminating means for a hand-held critical angle type refractometer having a body housing, a sample cover assembly, a prism assembly and measuring assembly for measuring the composition and density of a substance. An illuminating means for the refractometer in accordance with a preferred embodiment the present invention includes a light source, a power source for operating said light source, and an operating means for selectively activating the light source. The illuminating means is housed in the refractometer body to introduce light at a grazing incidence to a measuring surface of the prism. While a grazing incidence is preferred, other angles of incidence are contemplated by the present invention. The sample cover may be transparent so as to allow ambient light to light the measuring surface when possible.
The illuminating means of the preferred embodiment may be selectively powered, as needed, by a switching means being brought into electrical contact with a power supply, both of which are integral to the instrument. For the purpose of this embodiment the illuminating means consists of an LED providing visible light directly in front of the primary prism, however it is still within the scope of this invention to use other types of illuminating devices.
In another form of the invention, the actual illumination device is located remotely from the prism area and light energy is conducted to the prism area by means of an optical element, light pipe, wave guide, or optical fiber.
In still another form of the invention, the sample cover is comprised of a transparent oxide glass doped with a rare earth ion or alternatively molded from a resin containing a phosphorescent pigment or material. Optical energy from the illuminating means, as well as ambient light, will excite the sample cover causing it to luminesce or phosphoresce, providing visible illumination for the reticle scale lasting for several hours after exposure to light.
In still another embodiment of the present invention, the illuminating means is housed within the body of the instrument and positioned underneath the forward edge of the primary prism providing illumination for the scale.
These and other objects, features and advantages will become clearer from the following description of the preferred embodiment read in connection with the accompanying drawings.


REFERENCES:
patent: 2601128 (1952-06-01), Rosenthal et al.
patent: 4243321 (1981-01-01), Okuda et al.
patent: 4650323 (1987-03-01), Nakagawa
patent: 4890916 (1990-01-01), Rainer
patent: 5859696 (1999-01-01), Nicholas et al.
patent: 5969808 (1999-10-01), Cotton et al.

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