GB2085611A

GB2085611A - Refractometer for Testing Gemstones

Info

Publication number: GB2085611A
Application number: GB8130013A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-10-11
Filing date: 1981-10-05
Publication date: 1982-04-28
Also published as: GB2085611B; JPS5793237A; DE3038477A1; IT1197423B; IT8149459A0

Abstract

A refractometer for measuring the refractivity of gemstones has a housing (1) containing a refraction element (3) with a support surface (30) for a gemstone (2); a light emitting diode (4), the emitting surface (41) of which contacts the element (3); and an optical system (5), which receives light reflected from the gemstone and which incorporates a scale (52) on which the shadow edge corresponding to the critical angle of reflection, and representing the refractive index of the gemstone, can be read. <IMAGE>

Description

SPECIFICATION Refractometer for Testing Gemstones The invention relates to a refractometer for measuring the optical refractivity of gemstones, the refractometer comprising a housing which houses both a refraction element with a support surface for a gemstone under test, and an optical system (including e.g., mirrors, lenses etc.) associated with a scale; and a monochromatic (e.g. of approximately 589 nm.) light source in the form of at least one light emitting diode; whereby in use light from the source is reflected off a surface of the gemstone and passes directly into the optical system where a reading corresponding to the critical angle of reflection and hence the refractive index of the gemstone, can be read off the scale. Such a refractometer is hereinafter referred to as of the kind described.

For illuminating refractometers which require monochromatic light it is known to use as the light source light emitting diodes (LED) which are arranged outside the refractometer housing containing the optical refraction element. Because of the relatively low power of the light output of such LED's it is necessary to use a number of them in order to provide aceptable light conditions for the person operating the refractometer. This is because the light has to cover a relatively long distance within the housing before it enters the optical refraction element, which may be a prism, and it is well known that the intensity of illumination decreases with the square of the distance. Because of the requirement of such a large number of LED's and/or the necessity for constructing an individually sealed light source unit outside the housing, such refractometers are of comparatively large size.

More compactly constructed apparatus is also known (DE-GM 7714524), in which a light source in the form of a strip light is arranged inside the refractometer housing, especially on the rear wall of the housing at a distance from the refraction element. This light source is not monochromatic and for producing monochromatic light a suitable filter has to be interposed, and this in turn diminishes the intensity of illumination. In any case, such colour filters do not meet the requirements of the monochromatism of the light source. International agreement stipulates that for refractometric illumination in the testing of gemstones it is necessary to use monochromatic light of 589 nm.

with a spectral band width of 10 nm.

Monochromatic light of greater band widths, semi-monochromatic light produced with colour filters or polychromatic light, for example such as that produced by incandescent lamps, as with the known type of apparatus leads to a substantially less clearly defined shadow line with less contrast and reduces the readability precision on the refractometer scale by approximately a power of 10.

It is an object of the invention to provide a small and compact refractometer with good light conditions and a high degree of precision in reading off the scale.

This object is achieved, according to the invention, with a refractometer of the kind described in which the light emitting diode is housed within the housing with the light emitting surface of the or at least one of the light emitting diodes located substantially in contact with the refraction element.

The resulting advantages are, in particular, that a considerably smaller number of light emitting diodes are needed as the wight source than was hitherto the case and it is possible to construct a refraction unit in which a light source giving off monochromatic light is completely integrated within the refractometer housing so that the handling of a number of additional light elements is unnecessary. It is thus possible to obtain an apparatus which works without any substantial loss in light output and which ensures the highest possible degree of accuracy in reading off the scale.

A particularly advantageous construction results if the light source is rigidly connected to the optical refraction element. This results in a further simplification in the construction and handling and any air gap between the light source and the optical refraction element can be eliminated. The optical refraction element may be constructed from a material making it possible for the light source to be integrated in the optical refraction element, i.e. forming a closed unit with the element, thereby fulfilling particularly well the requirement of constructing a small refractometer.

The LED-light source may be integrated with an attachment part for the refraction element thus providing a practical arrangement from the point of view of assembly and exchangeability.

Furthermore, in order to achieve a high degree of illumination, the light source may comprise a number of light emitting diodes with the individual light emitting surfaces of the light emitting diodes arranged in a tight bundle adapted to the geometry of the refraction element.

Some examples of refractometers according to the invention are illustrated schematically in the accompanying drawings, in which: Figure 1 is a view of a refractometer with an LED-light source integrated within the housing; Figure 2 shows two LED-light sources in connection with an optical refraction element; Figure 3 is a partial view of an LED-iight source integrated with an optical refraction element; and, Figure 4 shows a partial view of an optical refraction element with three LED elements.

Figure 1 shows a housing 1 of a gemstone refractometer with a gemstone 2 sitting on its support surface, which is provided with a contact liquid having a known refractive index. The surface is formed by a flat surface 30 of an optical refraction element 3, shown in the form of a prism. This refraction element 3 is a body made of transluscent material such as glass having a specific refractive index. Instead of the prism, it may also be, for example, in the shape of a hemisphere, or of any other appropriate shape.

The optical refraction element 3 has a light entry surface 31 which is contacted by a light emitting surface 41 of a light emitting diode 4. On the light emitting side of the optical refraction element 3 there is provided an optical system 5 of known construction which essentially comprises two lenses 51, a scale 52, a prism 53 and an eyepiece 54.

The chain double dotted line shows a passage of light beam through the refractometer 1. The light emitted by the LED 4 directly reaches the light input surface 31 i.e. without interruption and therefore without loss of illuminating intensity, and thus into the optical refraction element 3.

After reflection at the surface of the gemstone 2, this light then reaches the optical system 5 and, after reflection in its prism 53, reaches the eye of the observer 6 which is thus in a position to determine on the scale 52 the position of the shadow edge line corresponding to the limit of total refraction and therefore to the critical angle of incidence and reflection which is dependent upon the refractive index of the gemstrone.

It is important to use a refraction element 3 which is as small as possible so as to keep the light path as short as possible in order to avoid unnecessary light losses since the light intensity decreases in accordance with the inverse square law. In other words the more the light source 4 and scale 52 are compressed with respect to the light beam, the higher will be the degree of precision in reading the scale, as a result of the more intensive illumination of the scale thereby increasing the contrast at the shadow line.

In the example according to Figure 2 there are two LED's 4 integrated with an attachment element 32 which directly contacts the entry surface 31 of the optical refraction element 3. The LED's 4 thus form one constructional unit with the optical refraction element 3 and may be assembled as a whole and if necessary exchanged, whereby at the same time the arrangement of the light emitting surfaces 41 of the LED's 4 are secured in the attachment part 32 so as to avoid any losses in the light intensity.

As shown in Figure 3 it is even possible to integrate one or more of the LED's 4 in the optical refraction element 3 in so far as this is allowed by the manufacturing method and especially the material, so that hereto it is possible to obtain a ciosed unit.

Figure 4 shows an arrangement of several LED's 4 in tight distribution over the light entry surface 3 1 of the optical refraction element for use either when a very high illuminating intensity is required, or when individual LED's 4 of reduced individual light power are used. As a result of the multiple arrangement and the even distribution over the light entry surface 31 it is possible to obtain an increased light intensity for the observations.

Claims

1. A refractometer for measuring the optical refractivity of gemstones, the refractometer comprising a housing which houses both a refraction element with a support surface for a gemstone under test, and an optical system associated with a scale; and a monochromatic light source in the form of at least one light emitting diode, whereby, in use, light from the source is reflected at a surface of the gemstone and passes directly into the optical system where a reading corresponding to the critical angle of reflection and hence to the refractive index of the gemstone can be read off the scale; wherein the light emitting diode is housed within the housing with a light emitting surface of the or at least one of the light emitting diodes located substantially in contact with the refraction element.

2. A refractometer according to claim 1, wherein the light source is rigidly connected to the optical refraction element.

3. A refractometer according to claim 1 or claim 2, wherein the light source is integrated with the optical refraction element.

4. A refractometer according to claim 1 or claim 2, wherein the light source is integrated with an attachment part for the refraction element.

5. A refractometer according to any one of the preceding claims, wherein the light source comprises a number of light emitting diodes with the individual light emitting surfaces of the light emitting diodes arranged in a tight bundle adapted to the geometry of the refraction element.

6. A refractometer substantially as described with reference to any one of the examples illustrated in the accompanying drawings.