Refractometer with an index-dependent aperture division.

p0001The present invention relates to a refractometer comprising a measuring prism with a measurement surface which is in optical contact with the sample to be measured, a lighting device with a light beam, which falls under such an angle range on the measurement surface that even the critical angle for Toltalreflexion in him is included, and a photoelectric receiver.

p0002Such a device is known from DE-AS 1,266,016. This illuminates the light reflected from the measurement surface light beam whose angular limit depends as light-dark boundary of the refractive index-dependent critical angle of total reflection, two photocells that are differentially exposed depending on the position of these light-dark boundary. A comparison photocell is always taken by the reflected light and another comparison photocell always receives only scattered light.

p0003Such a refractometer only has a sharp light-dark boundary, when measured with monochromatic light, or if - is used a so-called compensator for compensating the dispersion of white light (Abbe refractometer) - when illuminated with white light.. To measure the refractive index curve (refractive index as a function of wavelength) of such a refractometer is not suitable. It also has the disadvantage that all the components necessary for the measurement are arranged in the immediate vicinity of the measuring prism.

p0004The object of the present invention is therefore to provide a refractometer, which is simple in construction as possible, must be at the smallest possible number of parts in the vicinity of the measuring prism and which is also suitable for measuring the refractive index curve.

p0005The stated object is starting from a given in the preamble of claim 1 structure solved in the present invention that the light reflected by the measurement surface is focused by optical means to a receiver or a receiver surface.

p0006In contrast to the known determination of the refractive index by measuring the critical angle of total internal reflection or the local position of the light-dark boundary of the light reflected from the measurement surface radiation, in the inventive solution, the refractive index-dependent aperture division is by focusing the reflected radiation on a receiver or a receiver surface measured as change in intensity and evaluated. Since for each wavelength of the radiation corresponding to the belonging to their refractive index an aperture division takes place, which is independent of the aperture division of other wavelengths may be established by a spectrometer and the refractive index profile measurement.

p0007It is advantageous to use a diode array spectrometer which allows simultaneous measurement of the refractive index course in a large spectral range.

p0008to combine the spectrometer via a light conductor to the measuring head, wherein the entry surface of the light guide serves as a receiving surface on which the reflected light is focused by the measuring prism is particularly advantageous. In this case it is expedient to separate the serving for the illumination light source from the measuring head and to connect via a light guide with him.

p0009Further advantageous embodiments of the invention will become apparent from the dependent claims and an explanation of the figures.

p0010Particular advantages of the invention are that the measuring head can be very compact, that no moving parts are necessary and that the measuring head is explosion-proof because no electric voltages and no large optical powers are necessary in it.

p0011The invention is illustrated below with reference to the Figures 1 to 3 illustrated embodiments. show case<ul><li>1 shows an arrangement with a divergent incident on the measurement surface beam path;</li><li>Figure 2 shows an arrangement with a convergent incident on the measurement surface and the beam path</li><li>3 shows an exemplary embodiment in which is an observation window parts of the measuring prism.</li></ul>

p0012In figure 1 is provided with (11) a measuring prism referred to the measuring surface (11m), the sample (12) is applied, whose refractive index is measured. The measurement surface (11m) is emitted from the light source (13) divergent outgoing radiation beam (13s) illuminated. The geometric dimensions are selected so that in the angular range (a) of the divergent beam (13s) and the critical angle (g) is included for total reflection, resulting from the relationship sin (g) = n / N results, where N is the refractive index of the measuring prism (11) and n is the refractive index of the sample (12). Of the measuring surface (11m) reflected radiation is focused to the outlet of the measuring prism (11) through the lens (14) to the receiver (15).

p0013Those rays of the light source (13) acting on the measuring surface (11m) impinge at an angle which is greater than the n of the refractive index of the sample (12) dependent limit angle (g) of the total reflection to be totally reflected at the measurement surface (11m), while enter the other beams in the sample (12) with low Reflexionsverslusten accordance with the Fresnel formulas and thus do not reach the receiver (15). It occurs as an aperture division. In the figure, 1 is the part of the aperture in which the rays are totally reflected, filled dots.

p0014In accordance with the respective refractive index ratio n / N is thus a charakterististischer part (t) emanating from the light source aperture (a) is reflected at the measurement surface (11m) and focused on the receiver (15). The incident on the receiver refractive index dependent Aperturteil and thus the intensity measured by the receiver is therefore a direct measure for the refractive index n of the sample (12). If one uses as a light source (13) a continuum source and a receiver, for example a diode array spectrometer, then can be simultaneously the refractive index ratio n (λ) / N (λ) determined in a broad spectral range by the wavelength-dependent reflectivity R (λ) is measured the measurement surface, the over a calibration curve in the dispersion curve n (λ) can be converted.

p0015The arrangement shown in Figure 1 is, however - like the arrangements shown in Figures 2 and 3 - also suitable for measurement in only one or a few wavelengths. In this case can be used as a light source such as a light emitting diode or a line source. Here, a filter or a changing device with a plurality of filters can be arranged before or after the measuring prism. In these cases, can receiver a photodiode can be used.

p0016If a diode-array spectrometer is used as a receiver to receive the refractive index profile, then at the location of the receiver (15) in Figure 1, either the entrance slit of the spectrometer, or - in a particularly advantageous On the design of the invention - the input face of a light guide of the spectrometer leads are arranged. The latter embodiment of the invention is shown in Figure 2, is shown in the same time that the light incident on the measurement surface light beam may also be convergent. In addition, the light source via a light conductor (27) is connected to the μeßprisma in FIG. 2

p0017The measuring prism shown in Figure 2 (22) has the shape of a hemisphere; therefore no further optical components are required. With (22m), the plane measuring surface of the measuring prism is referred to, which is mounted in a measuring head (26) at its flange-like rim with bores (26b) is formed for attachment.

p0018The light guide (27) leads to a non-illustrated lamp housing in which, for example, a xenon, halogen, deuterium or metal halide lamp is included. The end face (27e) of the light guide, the core for example has a diameter of 200 .mu.m, is used as cold point light source. It is arranged at a distance from the spherical surface (22k) which is greater than its focal length, so that the beam path in the interior of the hemisphere is convergent. Through the spherical surface (22k) of the measuring prism (22m), the end face (27e) of the light guide (27) is displayed in the receiver surface (25). There, the top surface (28a) of the light guide (28) is arranged, which has a core diameter of 600 microns, for example, has not shown and leads to a diode array spectrometer. The terminals of the light guide (27) and (28) on the lamp housing and diode array spectrometer made in the example, from DE-OS 3701721 the known type.

p0019For the measurement liquid, pasty or solid substances can be used, the latter must have a flat surface, which, with a liquid whose refractive index is equal to or greater than the refractive index N of the measuring prism are brought into optical contact with the measuring surface

p0020In the figure 3 an embodiment for the measurement of the refractive index or the refractive index curve is shown attached to a substance that is located in a reaction vessel. In this case, ball portion coupled to the measuring head (36) firmly connected (32k) together with the observation window (39) of the reaction vessel, the hemisphere of the measuring prism (32), so that the measuring surface (32m) is located at the ball portion side facing away from the observation window. Ball portion (32k) and observation window (39) are suitably made from the same type of glass so that they have the same refractive index and refractive index profile. They are brought into contact by a holder (39h) which presses the measuring head (36) with the ball portion (32k) against the observation window (39). The optical contact is achieved for example by a suitable liquid having a sufficiently high refractive index or by Ankitten.

p0021In the measurement, it is advisable to first against air or a control sample with a known refractive index and a known refractive index profile and thus to detect all device-specific features that are included in the measurement. Subsequently, several substances or substance mixtures are measured, their refractive indices and refractive index profiles are known and distributed low over the interest range of refractive indices. The intensity values ​​obtained are - used to calibrate the device - taking into account the values ​​for air or for the comparative sample by forming a quotient. Such calibration of the refractive index scale where appropriate, in their wavelength dependency is required only at long intervals, during the measurement against air or a comparative sample is expediently often performed.