Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
  • G01N21/4133—Refractometers, e.g. differential
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
  • G01N21/45—Refractivity; Phase-affecting properties, e.g. optical path length using interferometric methods; using Schlieren methods
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/47—Scattering, i.e. diffuse reflection
  • G01N21/49—Scattering, i.e. diffuse reflection within a body or fluid
  • G01N21/51—Scattering, i.e. diffuse reflection within a body or fluid inside a container, e.g. in an ampoule

Definitions

• the refractive index is the speed of light travelling in vacuum proportional to the speed of light in the substance in question, i.e. c/v.
• the refractive index partially depends on the frequency of the light, which is why lights of different colours refract differently (dispersion phenomenon).
• the refracted light is on the same level as the incoming light and the normal of the surface.
• the light emanating from a light source disperses evenly in all directions.
• the light waves spread from a straight travelling direction.
• the deviation from the straight travelling direction is called bending of the light i.e. diffraction.
• This kind of deviation occurs when light must travel through a hole or for instance travel over the edge of an obstacle.
• the phenomenon can only be detected well if the size of the hole or obstacle does not deviate much from the frequency of the light.
• An opening of a suitable size creates a series of light and dark streaks on a screen, which consist of primary and secondary maxima since waves from different directions bend differently.
• the diffraction phenomenon i.e. the bending of the light, causes light beams from different openings to interfere.
• the beams bend in the same direction and strengthen each other the said diffraction maxima are created, which look like bent beams. This means that when a laser is targeted perpendicularly to a grid, one can observe that the light disperses in the grid creating light maxima in different directions.
• the diffraction grid phenomenon is based on a diffraction figure created on a screen.
• the position of the intensity minima in the figure depends on wavelength and the width of the openings. If there are two openings, an interference figure is created. If openings are added the opening system forr ⁇ s a grid, and as a result of the interference the definition of the primary maxima grows. In a grid of many openings the light beams interfere in which case an interference figure, in which minima and maxima alternate, is created on the screen placed behind the grid.
• the position of the primary maxima depends on the distance between the openings, the refractive angle and the wavelength of the light.
• the most common methods for determining the refractive index are based on determining the critical angle of the reflection of light targeted on a sample on a certain wavelength.
• Other methods besides the reflection method are e.g. the surface plasmon resonance method, spectrophotometry and diffraction grid methods.
• a well known diffraction grid method is based on the use of a transmission grid, in which method the grid is situated in the dish containing the measured sample.
• the grid is illuminated from behind the grid with a laser beam, which the grid then divides into several beams (different orders) thus creating an interference figure (a row of dots or circles within one another) which can be captured on the screen.
• This method is based on determining the place of a certain point (which is captured on the screen) in the interference figure.
• the substance surrounding the grid in this case the sample to be measured
• has a certain refractive index n The functioning of the grid depends on the wavelength of the laser sent through it (i.e.
• the places of the orders on the screen depend on the wavelength of the light) and the alteration of the wavelength of the laser depends on the refractive index of the sample (the substance surrounding the grid) to be measured.
• the refractive index thus alters the wavelength of the light and thus the places of the different orders on the screen change. A point is visible on the screen which is followed.
• Another well known diffraction grid method is based on the use of a reflection grid, in which method the grid is situated after the sample to be measured and a reference sample.
• both the sample and the reference are illuminated with the laser beam, in which case the first "light beam” travels through the reference water and the other through the sample.
• the beams that have gone through are reflected via the diffraction grid to a CCD camera.
• the camera sees the interference figure, which consists of streaks, which is obtained by the shape of the openings in the grid.
• the optical range of the reference beam and the sample beam differ due to the refractive index of the sample. The larger the refractive index, the larger the difference in optical range. Due to the optical range difference a phase difference is also created between the beams. This results in an interference figure.
• the reflective grid thus combines the reference beam and the sample beam and the interference streaks and the angles of them are calculated in the method.
• the problem with using the well known diffraction grid methods is the accurate determination of the light spot in the interference figure, since the shape and therefore location of the spot in question is not accurately definable.
• a diffraction grid is used to measure the refractive index (in another way than the methods mentioned above) by dividing the light beam, with the grid, into several beams which are directed by lenses through sample and reference dishes, and thus utilising all these beams for the measuring of the refractive index by collecting the light beams onto a cell in streak figures and measuring the height, width and intensity of the streaks, which depend on the refractive index.
• the grid has not, however, been shaped to accomplish an interference phenomenon as the mentioned diffraction grid methods have.
• an apparatus is used to measure the refractive index, which comprises a light source, a sample dish for the substance to be measured and a detector, for instance a camera, which is used to store the light which has passed the sample.
• the light from the light source [which can be a LED (Light Emitting Diode) or an LD (Laser Diode)] travels through the sample in a prism and the refractive index is determined on the basis of which place the light that has been received by the detector, for instance a camera, which is shown on a screen. On the basis of this place, the change in angle of the light and the refractive index can be determined.
• the apparatus can also be used to determine the turbidity on the basis of the amount of light that has been received. Neither does this method provide an especially accurate result as the shape of the point is inaccurate, whereby the place of the light on the screen cannot be determined very accurately.
• Turbidity describes the penetration of the light through the liquid. In a pure liquid, in which solid matter has not been mixed, the light travels quite linearly. The solid matter that has not dissolved in the liquid causes the light to scatter and absorb. The turbidity increases as the solid matter increases. The size, shape and dispersion of the particles of the substance also affect the turbidity. In addition to determining the refractive index, the impurities in the liquid can be determined by determining the turbidity.
• Turbidity measurements are based on the identification of the light that has been dispersed by the solid matter in the liquid. Turbidity can also be measured by examining the penetration of the light i.e. by measuring the transmittance. This method is best suited for high values of turbidity because small concentrations do not alter the transmittance significantly enough. The most common uses for turbidity measurement include the purity monitoring of drinking water and waste water as well as the monitoring of industrial processes.
• the object of the invention is therefore to develop a method and apparatus, with which the same or higher level of definition of the refractive index can be achieved than with the known methods based on spectrophotometry or total reflection.
• the object of the invention is an apparatus for the determination of the refractive index and/or turbidity, which comprises a light source, a sample dish which contains the liquid to be measured, and a light sensitive sensor, with which the light that has passed the sample, is stored in a cell for analysis. It is mainly characterized in that the apparatus additionally comprises a diffraction grid element, which bends the light between the light source and the sample dish, for the collection of the diffracted light that has passed through the sample dish onto a cell in a point figure as a result of interference.
• the light from the light source is directed via the diffraction grid element which bends the light between the light source and the sample dish.
• the light beams which have been bent into different angles by the sample are stored in the form of a spot matrix figure into the cell and the refractive index of the liquid is determined by the location of the spots.
• the turbidity of the liquid is determined by the intensity of the spots.
• the invention makes innovative use of the interference phenomenon.
• the openings of the diffraction grid have been form in such a way that light interferes when it passes through the grid. Since the interference figure obtained by the grid is brighter than the figure obtained by one opening, and since more light waves participate in the formation of the maximum of each spot in the grid, the maxima can be determined with greater accuracy.
• the invention especially uses location data of more than one, preferably several or all, of the spots in the measurement. This occurs practically with the aid of a digital camera and computer in the measurement. The camera works by drawing the received light on a level of film, in which it is stored on film or a cell as a point figure.
• a 4 X 4 spot matrix can be produced from a laser beam using a binary amplitude hologram, which location spots can be determined more accurately than in the background art.
• the detection is done with a CCD camera which produces data which is then analysed with a computer.
• FIGURES Figure 1 presents a principal image of the apparatus of the invention.
• Figure 2 presents the aperture of the diffractive optical element utilised in the apparatus used in the example.
• Figure 3 presents a photo of an interference figure formed by a diffraction grid element, taken by a CCD camera in the example.
• Figure 4 presents the measurement results achieved by the invention in the example.
• the laser source for instance a laser lamp
• reference number 1 the laser source
• the laser is directed through the lens 2, via the light bending element 3, into parts 4 and 5 of the sample dish, for instance a double prism, and then onto the light sensitive sensor 6 of an electro optical camera.
• the first part of the dish contains a reference substance (solid matter, liquid or gas) and the second part contains the substance to be examined (solid matter, liquid or gas).
• a plastic tube can also be used as a sample dish, the wall of which acts as a reference, or, alternatively the apparatus can be calibrated with water. In neither case, there is not needed any two part sample dish.
• the bending element 3 is some kind of a diffractive lens, which focuses the light beam into one level.
• the distance of the focal plane from the distance of the bending has been organised behind the prism thus allowing for the determination of the sent light points in the focal point.
• the element causes a multi spot figure on the light sensitive sensor 6 of the electro optical camera, which location coordinates depend on the refractive index of the reference substance and the examined substance.
• the turbidity of the liquid is determined from the ratio between the reference signal and the measurement signal.
• An apparatus was used in the method, which comprises a light source, a prism containing the sample to be examined, through which the light from the light source is directed via a lens, and a CCD camera, with which the light that has passed through the sample is stored on a cell for analysis.
• the lens of the apparatus was also a commercial product and its object is to make the laser light parallel.
• the apparatus comprises a diffractive optical element. It consists of chrome manufactured on a glass plate, the opening structure of which has been formed using chrome streaks, has been presented magnified in figure 2. The width of the chrome streak was ca 5 ⁇ m, the thickness 200 nanometres.
• This focusing diffractive optical element produces a spot figure in its focal level, which contains 16 light spots.
• the physical size of the aperture (the part through which the light passes) of the diffractive optical element is 4mm x 4mm. The smaller details are ca 5 ⁇ m in size.
• the focal distance is 100mm.
• the light sensitive cell of the CCD camera in the apparatus consists of 640x480 pixels.
• the camera used was a Logitech webcam quickcam.
• the figure formed on the focal level of the diffractive element is captured on the cell of the camera.
• the light from the light source is thus directed through the prism via the lens and further through the diffraction grid element bending the light between the light source and the sample dish, in which case a spot matrix figure was formed on the cell of the camera.
• the refractive index between the reference water and the sample is detected with the cell of the CCD camera as a change in figure location.
• the calculations of the results are done with a computer.
• the refractive index of the liquid is determined by the location of the spots in such a way that the pixel size of the cell is 8.3 ⁇ m and the size of the spot matrix is about 0.4 mm.
• Concentration c weight promille informs how many promille food colouring is in the sample water.
• the samples have been made by measuring water and food colouring on an analysis scale.
• the concentration is derived from the ratio between the masses of water and food colouring.
• the intensity of the DOE image of figure 3 is calculated with the formula
• N and M are the dimensions of the point matrix and Iy is the intensity of the pixel.
• Iy is the intensity of the pixel.
• the area from which the average of the total intensity has been calcualted has been marked with a dashed line in figure 3.
• Information about the anisotropy of the liquid is derived from the visiblity parameter V, which describes the visiblity of the spot matrix. The visiblity is determined by the formula
• Values S, Vx and Vy are calculated from figure 3. Location information is obtained by calculating the location coordinates (x,y) and taking their average.
• the numerical values of S, Vx and Vy are obtained by calculation from this DOE image.
• Vx describes the visibility in the x direction and Vy describes the visibility in the y direction and D calculated from the ration of these describes the anisotropy.
• the refractive index is calculated from the location of the figure on the cell.
• the program seeks the image from the cell of the camera.
• the program defines the coordinates for the location of the image, which are compared to the reference coordinates obtained from the pure water.
• the refractive index is calculated with the formula
• ⁇ is the bending angle of the prism (90 degrees) and ⁇ is the deviation angle calculated from the cell.
• the turbidity of the liquid is calculated with the formula
• the anisotropy of the liquid has been determined, in the invention, by determining the parameter D which describes the asymmetry of the point matrix, which can be calculated from the spot figure.
• the intensity of the spot figure is calculated only from the area of the light spots, thus not from the entire area of the image.
• the logical result is that, as turbidity increases the turbidity value S increases.
• T the transmission of the light that has penetrated the reference liquid and, correspondingly, I is the intensity of the light that has penetrated the sample.
• ⁇ x is the transition in the direction x and ⁇ y is the transition in the direction y.
• ⁇ refers to the change in Euclidean distance in the figure compared to the reference location.
• the determination of the location of the figure has been performed very accurately in the invention, since the average of the (x,y)-coordinates of the light points are calculated in the invention, in which case the result is more reliable than if only one point would be acknowledged or if the location of the figure would be observed manually and seen as a whole.

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Citations (3)

• 2006
  • 2006-05-08 FI FI20060439A patent/FI20060439L/fi not_active Application Discontinuation
• 2007
  • 2007-05-08 WO PCT/FI2007/000123 patent/WO2007128865A1/en active Application Filing

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