Patent GB-1209036-A (GB)
Title: Improvements in or relating to refractometers
Inventor: GEAKE JOHN EDWARD
IPC: G01N21/43
Filed: 1967-06-19
Abstract: 1209036 Critical-angle refractometer NATIONAL RESEARCH DEVELOPEMENT CORP 14 June 1968 [19 June 1967] 28275/67 Index G1U 18 [Also in Division G4] A refractometer comprises a block of transparent material 3 with a concavity in the surface in the shape of the curved portion of a cylinder, which measures the refractive index of a liquid by bringing the curved portion 10 into contact with the liquid and observing the said curved portion with observing means by means of light coming only from a direction perpendicular to the axis of the cylinder. Light from a source 1 is reflected by total internal reflection at a 45 degree face 2 down a transmitting section of the block 3 in the form of a rod of polymethylmethacrylate which is divided into two sections 5 and 6 by an opaque screen in the form of a longitudinal saw cut 7 filled with an opaque plastics material. The end portion of the block has a bevel surface 8 painted white and an aluminized bevel surface 9 both at 45 degrees to the axis of the block and an inner cylindrical surface having its axis parallel to the longitudinal axis of the block. The bevel 8 has its lower edge 11 coinciding with the projection of the screen 7 to reflect the light diffusely from the transmitting section 5 of the rod 3 on to the cylindrical surface of the rod in such a manner that it is reflected by total internal reflection from the cylindrical surface 10. The light is then specularly reflected by the bevel 9 and leaves the receiving section 6 of the rod 3 via a magnifying lens 12 and a small exit pupil 13. The image of the bevel 8 seen through the lens 12 is cut off at a position related to the refractive index of the liquid under test and the value of this refractive index can be read off a scale which may be engraved on the cylindrical surface 10 or preferably is in a flat plane at an apparent distance from the eye-piece such that there is no parallax between the scale and the image of the bevel. The scale may be a graticule (121) Fig. 12 (not shown) at the top of the block viewed by reflection of a metallized strip (122) on the base or it may be in a pit (121) Figs. 13, 14 (not shown) and may be viewed directly Fig. 14 or by reflection Fig. 13, or a graticule may be positioned in the eyepiece of a compound microscope. In a modification Fig. 7 (not shown) the shaped end portion 4 of the rod 3 as shown in Fig. 1 is replaced by separate block of material (71) the material being chosen so as to have a value of its refractive index greater than that of the liquid to be measured. Bevels 8 and 9 are formed on the block (71) and act to transmit and receive light into and from the block of material (71) in the same manner as before. In yet another embodiment Fig. 8 (not shown) a separate block of material (81), similarly to the block of material (71) is attached to an opaque supporting rod (82). In this case light is transmitted into and received from the block (81) by means of polymethyl methacrylate sheets (83 and 84) respectively which are attached to the opaque supporting rod (82). The surfaces that are not used optically are painted matt black to reduce reflection. This arrangement may be used to measure the state of charge of a battery. In an alternative embodiment Fig. 9 a cell or a part of a conduit has a transparent wall 92 with one opalescent face 93 illuminated by an extended light source. Light is reflected through the edge 95 and cut off as before, a scale engraved on the face 93 giving a measure of the refractive index of a liquid in the conduit. Walls of material of different dimensions or quality may be made to study the variations in refractive index with depth to reduce the parallax error. The scale can be observed using a fiducial line to line up the observers's eye or through a coherent fibre optics viewing system. An electrical output signal may be obtained by observing the image with a photocell or a stack of sheet light guides each one of which has a photo-cell associated with it. A second method is to use a split cathode photo-cell which is arranged to lock on a mask carried on a moving pointer to the boundary between the light and dark areas. A third method gives a bright line at the demarcation edge which is observed through a digitizing mask. A mask 181 Fig. 18 with two holes 182, 183 is placed against the face 93 and a combination of total internal reflection and refraction through the liquid gives the pattern shown in (c) and the two light areas are superimposed to provide a narrow bright line the range of values that can be measured may be increased by stacking a number of arcs (191) of increasing cylindrical area, Fig. 19 (not shown) and by providing more than one direction for viewing. If the refractometer is to be used with a very turbid liquid the surface 10 may be illuminated through the liquid.