Refractometer subassembly method and apparatus

An apparatus and method for creating an aligned subassembly of optically critical components of a temperature compensated, hand-held refractometer prior to installing the subassembly in a refractometer housing is disclosed. The preferred embodiment is a plastic subassembly frame with a critical angle prism reference surface, a temperature responsive member fulcrum, a compensating prism mount, an objective lens mount, and a reticle mount.

BACKGROUND OF THE INVENTION 
The present invention relates to refractometers, more particularly 
temperature compensated, hand-held refractometers, and to methods of 
aligning optically critical components thereof. 
Temperature compensated, hand-held refractometers are well known in 
industry for measuring the refractive index of a test substance. Since the 
refractive index of a liquid substance is related to the concentration of 
elements within the substance, the units of a refractometer scale may be 
calibrated to indicate quantities which are of practical importance. For 
example, the sugar concentration in a soft drink or the lubricant 
concentration in machine tool cutting fluid may be readily measured using 
a suitably calibrated refractometer. 
A common arrangement of optical elements in a temperature compensated 
hand-held refractometer is described in U.S. Pat. No. 3,329,060. The 
optical elements include a critical angle prism, a compensating prism or 
wedge, an objective lens, a reticle, and an eyepiece; a temperature 
responsive bimetallic member moves the compensating wedge to adjust for 
ambient temperature variations. It is known that temperature compensation 
may also be achieved by using a temperature responsive member to move the 
objective lens, as taught in U.S. Pat. Nos. 3,279,309 and 3,625,620, or 
the reticle, as taught in U.S. Pat. Nos. 2,619,003 and 3,447,875. 
Alternatives which do not rely upon mechanically moving parts to effect 
temperature compensation include refractometers having a liquid filled 
compensating prism, as disclosed in U.S. Pat. No. 3,267,795. Regardless of 
the method of temperature compensation, alignment of the optical elements, 
excluding the eyepiece, is necessary for accurately reporting refractive 
values. 
Prior art refractometers require that alignment be carried out after the 
optical elements have been placed inside the refractometer housing. In 
commercial models sold for over twenty years, as well as in currently 
available models known to applicant, the critical angle prism is affixed 
in the housing and the other elements are placed in the housing as an 
unaligned subassembly; alignment is subsequently effected using externally 
accessible adjustment screws. FIG. 1 shows a prior art refractometer 
subassembly with a metal strip 10, reticle mount 16 for holding a reticle 
18, lens mount 26 for holding an objective lens 28, and compensating wedge 
mount 30 for holding a compensating wedge 32. Compensating wedge mount 30 
is fastened to one end of a temperature responsive member 34, while the 
other end of member 34 is fastened to strip 10. 
Various advantages of the present invention distinguish it from the prior 
art. Because the subassembly frame is a one piece unit, it may be formed 
from plastic in a single molding operation. By providing a reference 
surface on the frame for locating and supporting a critical angle prism, 
complete and even permanent alignment of the optically critical elements 
can be achieved before they are installed within the refractometer 
housing. Thus, if the housing is hermetically sealed to protect the 
instrument from possible immersion in a test fluid, the invention 
eliminates the need to access external adjustment screws which may only be 
reached by breaking the seal. 
Accordingly, it is an object of the present invention to allow for complete 
and durable pre-alignment of the optical elements of a temperature 
compensated hand-held refractometer prior to installing them as a 
subassembly inside the refractometer housing. 
This and other objects of the invention will be apparent to those skilled 
in this art from the following detailed description of the invention and 
the accompanying figures. 
SUMMARY OF THE INVENTION AND DRAWINGS 
The present invention comprises a temperature compensated, hand-held 
refractometer with a subassembly frame for supporting a critical angle 
prism, compensating wedge, objective lens, and reticle such that the 
wedge, lens and reticle may be selectively positioned and aligned with the 
critical angle prism prior to installing the entire aligned subassembly 
within a refractometer housing. 
The invention also encompasses a method whereby the optically critical 
components of a refractometer system are aligned prior to installing the 
components as a subassembly inside a refractometer housing. The method 
includes locating a critical angle prism on a subassembly frame; securing 
a compensating wedge mount to one end of a temperature responsive member, 
connecting the opposite end of the member to the frame, and temporarily 
supporting a compensating wedge in the mount. The procedure also requires 
supporting an objective lens and a reticle in the subassembly frame and 
cementing the various optical elements in position. The elements can be 
aligned by selectively adjusting the compensating wedge and reticle 
relative to the critical angle prism prior to, or subsequent to, being 
fixed in place.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to FIGS. 2 and 3, one embodiment of the invention is shown. 
Subassembly frame 108 includes a one-piece elongated body 110 having 
opposing sides 111a and 111b, reference surface 112, reticle mount 116 and 
lens mount 126. Compensating wedge mount 130 is attached to one end of 
elongated temperature responsive member 134, with the other end of member 
134 being fastened to body 110 by fastener 135. Reference surface 112 
defines the location at which a critical angle prism is fixed to body 110, 
preferably by cementing it to surface 112. Reticle mount 116 and lens 
mount 126 are designed to slidably support a reticle and an objective 
lens, respectively, by friction. Wedge mount 130 is provided to 
frictionally support a compensating wedge. This embodiment permits the 
optical elements to be moved into alignment and then fixed in place to 
create a permanently aligned subassembly. 
FIGS. 4 and 5 illustrate a second embodiment of the invention. Subassembly 
frame 208 includes a one-piece elongated body 210 having opposing sides 
211a and 211b, and reference surface 212 for locating and supporting 
critical angle prism 214. Body 210 also has a reticle mount 216 for 
holding reticle 218. Reticle mount 216 is connected to body 210 by elastic 
reticle mount support member 220 which extends from bridge 221. Reticle 
adjustment screw 222 extends upwardly through retainer plate 224 and 
communicates with elastic member 220 at a point proximate to reticle mount 
216. Lens mount 226 is formed as a part of body 210 to hold objective lens 
228. Compensating wedge mount 230 holds compensating wedge 232. Wedge 
mount 230 is fastened to one end of elongated temperature responsive 
member 234 by rivet 236. Temperature responsive member 234 serves as an 
elongated lever which pivots about fulcrum 238 such that its opposite end, 
which has a slot 240, is engaged with biasing spring 242. Biasing spring 
242 is preferably formed as an integral part of body 210 and extends from 
bridge 221. Biasing spring 242 has a protrusion 244 which is received 
within slot 240. Wedge adjustment and retaining screw 246 extends upwardly 
through body 210 into threaded engagement with temperature responsive 
member 234. By locating screw 246 between fulcrum 238 and biasing spring 
242, temperature responsive member 234 is forced into engagement with 
fulcrum 238. 
The second embodiment, like the first, permits alignment of all optically 
critical elements of the refractometer prior to their installation as a 
subassembly within a refractometer housing. To make an aligned 
subassembly, critical angle prism 214 is placed on reference surface 212. 
Compensating wedge mount 230 is riveted to temperature responsive member 
234; member 234 is then set across fulcrum 238 and connected to body 210 
using wedge adjustment and retaining screw 246. Compensating wedge 232 is 
placed in wedge mount 230, objective lens 228 is inserted in lens mount 
226, and reticle 218 is inserted in reticle mount 216. Critical angle 
prism 214 is then fixed to body 210, as are objective lens 228 and reticle 
218, preferably by cement. Compensating wedge 232 is fixed to wedge mount 
230. Pursuant to known conventions in the art of refractometry, final 
alignment is conducted by alternately turning reticle adjustment screw 222 
and wedge adjustment and retaining screw 246 to calibrate the instrument 
for at least two different refractive values. 
Frame 208 is preferably molded as a unit from suitably rigid plastic. Glass 
reinforced polycarbonate will provide locational integrity and durability 
to preserve alignment of the optical elements fixed to it. Black plastic 
is preferred because it does not reflect incident light rays. 
A completed subassembly eliminates the need for further optical alignment 
or adjustment of elements and is ready to be installed, preferably by 
cementing frame 208 to an inner surface of housing 248. An eyepiece 250 is 
connected to the end of housing 248 opposite critical angle prism 214, 
however the eyepiece 250 is used merely to magnify scale markings on 
reticle 218 and is not an optically critical component which needs to be 
aligned with the other optical elements. After focusing eyepiece 250, the 
entire unit may be hermetically sealed by conventional procedures. 
While the present invention has been described with reference to two 
preferred embodiments which show a refractometer wherein the compensating 
wedge is moved by a temperature responsive member, one of ordinary skill 
in the art of temperature compensated, hand-held refractometers will 
understand that the claimed invention is not confined to such an 
arrangement. The spirit and scope of the invention encompass alternative 
refractometers, discussed above, wherein the objective lens or reticle is 
moved by a temperature responsive member, and those wherein all of the 
optical elements remain stationary.