differential stress refractometer LDSR description
The LDSR Refractometer is designed for the measurement of compression and tensional stresses in commercial float, plate, and sheet glass. Stress measurements made with this instrument are non-destructive and are limited to transparent flat glass.

The LDSR Refractometer consists of a total internal reflection prism, a calibrated telescope, a light source and a mirror, all held securely in a metal housing. The prism is fabricated from optical glass and finished to optical tolerances. It is used for measuring stress in glass with refractive index in the range of 1.516 to 1.566. Standard index range is 1.516 to 1.526. For glass out of this range of refractive index, optional prisms covering other index ranges can be installed in the refractometer.

The viewing portion of the instrument is equipped with a 10x filar micrometer eyepiece with focusable eye lens. In use, a split image border will be observed in the field of view. An incremental micrometer adjustment dial moves a fiducial hairline. This gives an accurate measurement of the split border observed in the eyepiece.

The light source is an incandescent lamp (battery included) which is controlled by a split aperture. Light penetrates at the refracting entrance face of the prism. Illumination is at the proper angle to insure total internal reflection in the prism.  Tables are supplied with the instrument to provide instant conversion from micrometer eyepiece reading to stress values for plate, sheet, and float glass.  The LDSR instrument package includes: LDSR Refractometer, illuminator battery, carrying case, prism contact fluid, a calibration glass, and instruction manual complete with stress tables.

theory
To measure the surface stress in glass, light must first enter the glass, then emerge in a manner allowing measurement of the birefringence in the glass surface. (Birefringence is the difference in refractive index for light rays oscillating perpendicular and parallel to the plane of incidence.) The measurement is accomplished thru the technique of total internal reflection made possible by a prism that is placed in optical contact with the glass surface.

The magnitude of the surface stress is directly proportional to the birefringence of the glass surface. This relationship is expressed by the stress optic law, S = C(µ perp.- µ ||), where S is the stress in the surface perpendicular to the plane of incidence, C is a stress-optic coefficient, and µ perp. and µ || are the refractive indices for light vibrating perpendicular and parallel, respectively, to the plane of incidence. C is a constant for glass of a given composition.

differential stress refractometer LDSR specifications

Usable Sample Index Range: 1.516 to 1.566 with optional prisms.
Standard Sample Index Range: 1.516 to 1.526
Prism Index: 1.541 Standard
Eyepiece Magnification: 10x
Stress Range, per supplied tables, for: Float Glass 0-46,600 psi Plate Glass 0-39,700 psi Sheet Glass 0-52,200 psi
Weight of Refractometer: 1 pound, 10 ounces
Size of Refractometer: Height 191 mm, Width 57mm, Length 152 mm
Power Supply: 9 volt Battery. Optional power supply with Input 110/120V, 220/240V and Output 3 to 9 volts (also available)
Contact Fluid: Approx. 2 ounces provided Index (25°C) = 1.527
Calibration Glass: Factory calibration provided

warning
This instrument is not capable of determining whether or not glazing materials comply with a standard, nor is it capable of accurately making measurements of the compressive stress in the surface layer of thermally tempered glass below 10,000 PSI. Only recognized destructive testing should be used in determining impact resistance, break characteristics, and whether or not thermally tempered glass complies with standards such as, but not limited to, 16 CFR 1201, ANSI Z 97.1, ANSI Z 26.1, and the requirements of the Federal Railway Administration and Federal Specification, DD-G-451d.

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