Optics: measuring and testing – Refraction testing – Prism engaging specimen
Patent
1985-04-02
1987-07-28
Evans, F. L.
Optics: measuring and testing
Refraction testing
Prism engaging specimen
G01N 2141
Patent
active
046828893
DESCRIPTION:
BRIEF SUMMARY
The subject of the present invention is a refractometer for measuring the refractive index of a liquid, comprising a prism, of which at least one of the faces is in contact with the liquid to be measured, a light source to direct a light beam through the prism and passing through the said face in contact with the liquid and through one face in contact with another medium.
Refractometers of this type are well known. They usually have the characteristic feature of measuring the refractive index for a given temperature. Although this feature is acceptable when the temperature of the liquid can be controlled, it is not an accurate measure in other cases, such as those of refractometers used for indicating the state of charge of a battery as a function of the refractive index of the electrolyte of the battery.
A refractometer of this type has already been disclosed in British Pat. No. 2 008 793, having one face formed by a cylindrical portion receiving an incident light beam, in contact with the said liquid, and one exit face for light rays refracted by the receiving face and transmitted through the prism, forming along the exit face on opposite sides of a transition line an illuminated and a non-illuminated zone, the position of the transition line being a function of the refractive index of the liquid.
As explained in Optica acta 1970, vol. 17, No. 5, pp 363-380, it is possible, by means of a prism having a surface that receives a light beam comprises a cylinder portion immersed in the liquid, the refractive index of which is required to obtain on a face where the light rays emerge a clear transition line between an illuminated zone formed by the refracted part of the beam passing through the two different refractive index media, and a non-illuminated zone. The face where the light rays converge is within a plane parallel to one diameter of the cylinder perpendicular to the general direction of the refracted rays, and the position of the transition line between the illuminated and non-illuminated zones is a practically linear function of the refractive index of the liquid. This refractometer operates based on the fact that the angle of incidence of the rays of the light beam increases progressively, due to the curvature of the receiving surface, so that beyond a critical angle, the rays of the light beam are no longer refracted through the prism. This critical angle of incidence is a function of the relationship between the refractive index of the liquid and that of the prism.
All the refracted rays reach the plane of convergence to the right of a certain point which forms the boundary between the illuminated and non-illuminated zones, and the position of the line of contrast between the zones is a function of the relationship between the refractive indices of the liquid and that of the probe.
These considerations apply so long as the temperature of the liquid and the prism is known, or remains constant. When a refractometer of this type is used to determine the refractive index of the electrolyte of an electrochemical battery and in this way the degree of charge of the battery, the temperature of the electrolyte, which can vary by several tens of degrees when a car battery is involved, is not known. Consequently, it is not possible to establish a direct correlation between the refractive index measured and the state of charge of the battery.
Refractometers are known which are provided with means for compensating the temperature variations, in such a way as to measure the index of a liquid as a function of a reference temperature.
Apparatus of this type forms notably the subject of French Patent 2 159 771 and U.S. Pat. No. 3 625 620. In these two solutions, use is made of a correction mechanism using a bimetal compensation strip. Although a mechanism of this type is simple, its presence leads to complications, presupposes prior adjustment and is affected by vibrations.
The aim of the present invention is to provide a solution which is substantially free of the above-mentioned disadvantages.
To this end, the s
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Polymer Handbook, by J. Bandrup and E. H. Immergut, Title page only.
Plastic Optics: Reclaiming a Fine Technology, by Peter Lahaye, pp. 27-29, Optical Spectra, Jul. 1974.
Evans F. L.
Stanley Electric Co. Ltd.
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