Apparatus for measuring light scattering

Abstract

Apparatus for measuring light scattering of a sample comprising a light beam source, means for collimating the beam and making it impinge on the sample in a perpendicular direction, at least one light sensor, and at least one spatial filter between the sample and the optical sensor, provided with two apertures, means for measuring the total power reaching the sensor and means for measuring the power of beams with a low k vector after the beam traverses the filter. The invention provides thus a simplified, portable and compact device for measuring different parameters like haze, turbidity, etc. can be built, for any sample and without the need of changing detectors.

Claims

1. An apparatus for measuring light scattering of a sample S comprising: a light beam source SO generating a light beam; means for collimating the light beam and making the light beam impinge on the sample S in a perpendicular direction; at least one light sensor; plural spatial filters F.sub.N between the sample and the at least one light sensor, each spatial filter provided with two apertures wherein a scattering angle, .sub.c, of a light beam passing through each filter is defined by: ${}_c=\arctan \left(\frac{D_1+D_2}{2.Math.L}\right)$ wherein D.sub.1 and D.sub.2 are diameters of the two apertures, respectively, of respective spatial filters and L is the total length of respective spatial filters in the direction of the light beam; means for measuring total power reaching the at least one light sensor, and means for measuring power of the scattered light beam having a k vector lower than respective scattering angles .sub.c of corresponding spatial filters amongst the plural spatial filters F.sub.N, at a plurality of different scattering angles .sub.c of corresponding spatial filters through which the scattered light beam passes.

2. The apparatus according to claim 1, wherein the means for measuring the total power are a second light sensor and a beam splitter or dielectric mirror BS.sub.0 for splitting the beam in two directions, a first direction of the two directions towards at least one spatial filter F amongst the plural spatial filters F.sub.N and the at least one light sensor and a second direction of the two directions towards the second light sensor.

3. The apparatus according to claim 2, further comprising a second beam splitter or dielectric mirror between the sample S and the light beam source SO for allowing the apparatus to work in reflection mode.

4. The apparatus according to claim 3, wherein in the plural spatial filters F.sub.N, N is greater or equal than 2, the plural spatial filters F.sub.N having different apertures.

5. The apparatus according to claim 4, wherein the at least one light sensor is a photodiode.

6. The apparatus according to claim 2, wherein in the plural spatial filters F.sub.N, N is greater or equal than 2, each of the plural spatial filters F.sub.N having different apertures.

7. The apparatus according to claim 1, wherein the at least one light sensor is a photodiode.

8. The apparatus according to claim 1, wherein the means for measuring the total power are a second spatial filter F.sub.T in parallel with at least one spatial filter F amongst the plural spatial filters F.sub.N between the sample S and the at least one light sensor, the second spatial filter F.sub.Thaving a greater aperture diameter than that of the at least one spatial filter F.

9. The apparatus according to claim 8, wherein in the plural spatial filters F.sub.n, N is greater or equal than 3, each of the plural spatial F.sub.N having different apertures.

10. The apparatus according to claim 9, further comprising a beam splitter or mirror BS.sub.1 for allowing the apparatus to work in reflection mode.

11. The apparatus according to claim 10, wherein the at least one light sensor is a CMOS camera or CCD camera.

12. The apparatus according to claim 11, where the light beam source SO is of any predetermined wavelength or white, tunable, or a light emitting diode.

13. The apparatus according to claim 8, further comprising a beam splitter or mirror BS.sub.1for allowing the apparatus to work in reflection mode.

14. The apparatus according to claim 1, wherein the at least one light sensor is a CMOS camera or CCD camera.

15. The apparatus according to claim 1, where the light beam source SO is of any predetermined wavelength or white, tunable, or a light emitting diode.

16. The apparatus according to claim 1, wherein the apparatus is a turbidimeter, hazemeter or glossmeter.

17. The apparatus according to claim 1, wherein each measured value of the power of the scattered light beam, having a k vector lower than respective scattering angles .sub.cof corresponding spatial filters amongst the plural spatial filters F.sub.N and measured at the plurality of different scattering angles .sub.c of corresponding spatial filters through which the scattered light beam passes, is associated with diameters of apertures of the corresponding spatial filter, a length of the spatial filter in the direction of the light beam of the corresponding spatial filter, and a scattering angle .sub.c of the corresponding spatial filter.

18. A scatterometer for measuring light scattering of a sample S comprising: a light beam source SO generating a light beam; means for collimating the light beam and making the light beam impinge on a sample S in a perpendicular direction; at least one light sensor; plural spatial filters F.sub.N between the sample and the at least one light sensor, each spatial filter provided with two apertures wherein a scattering angle, .sub.c, of a light beam passing through each filter is defined by: ${}_c=\arctan \left(\frac{D_1+D_2}{2.Math.L}\right)$ wherein D.sub.1 and D.sub.2 are diameters of the two apertures, respectively, of respective spatial filters and L is the total length of respective spatial filters in the direction of the light beam; means for measuring total power reaching the at least one light sensor, and means for measuring power of the scattered light beam having a k vector lower than respective scattering angles .sub.c, of corresponding spatial filters amongst the plural spatial filters F.sub.N, at a plurality of different scattering angles .sub.c, of corresponding spatial filters through which the scattered light beam passes.

19. An angle-resolved scattering analyzer, comprising: a light beam source SO generating a light beam; means for collimating the light beam and making the light beam impinge on a sample S in a perpendicular direction; at least one light sensor; plural spatial filters F.sub.N between the sample and the at least one light sensor, each spatial filter provided with two apertures wherein a scattering angle, .sub.c, of a light beam passing through each filter is defined by: ${}_c=\arctan \left(\frac{D_1+D_2}{2.Math.L}\right)$ wherein D.sub.1 and D.sub.2 are diameters of the two apertures, respectively, of respective spatial filters and L is the total length of respective spatial filters in the direction of the light beam; means for measuring total power reaching the at least one light sensor, and means for measuring power of the scattered light beam having a k vector lower than respective scattering angles .sub.c, of corresponding spatial filters amongst the plural spatial filters F.sub.N, at a plurality of different scattering angles .sub.c, of corresponding spatial filters through which the scattered light beam passes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings illustrate a preferred embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be embodied.

(2) FIGS. 1a and 1b show the technical problem related with the prior art.

(3) FIG. 2 shows a spatial filter, in the form of a cylinder or two apertures

(4) FIG. 3 is similar to FIG. 2, the filters having apertures of different diameters.

(5) FIG. 4 shows the relationship between the diameter of the beam, the field of view and the diameter(s) and length of the filter.

(6) FIG. 5 is a graph of the low k power versus scattering for various apertures.

(7) FIG. 6 is a graph of the low k power versus different apertures for three samples with different haze.

(8) FIGS. 7a-7d show embodiments of the invention using a beam splitter to measure total k power and low k power.

(9) FIGS. 8a-8d show embodiments of the invention using at least two filters, one for measuring the low k power and another one for measuring the total k power.

DETAILED DESCRIPTION OF THE INVENTION

(10) The invention filters and measures the optical power of a representative part of low k vectors (defined as low k power) and a representative part of the total k vectors (defined as a total k power) power by means of at least one filter with two apertures. The light source can be of any predetermined wavelength or white, tunable or it can be a light emitting diode. In this way a simplified, portable and compact device for measuring different parameters like haze, turbidity, etc. can be built, for any sample and without the need of changing detectors.

(11) Indicating with P.sub.LA the light power corresponding to beams (k vectors) with angles lower than 2.5, P.sub.HA the light power corresponding to beams (k vectors) with angles equal or greater than 2.5, and P.sub.T the total light power emerging from sample:
P.sub.T=P.sub.LA+P.sub.HA(2)

(12) And consequently:
P.sub.HA=P.sub.TP.sub.LA(3)

(13) The haze expression (1) as a function of low and total k vectors powers:

(14) $\begin{array}{cc}H\left[\%\right]=100*\left[\left(1-\frac{P_{\mathrm{LA}}^S}{P_T^S}\right)-\left(1-\frac{P_{\mathrm{LA}}^R}{P_T^R}\right)\right]H_S=\left(1-\frac{P_{\mathrm{LA}}^S}{P_T^S}\right)& \left(4\right)\end{array}$
is the haze from the sample and

(15) $H_R=\left(1-\frac{P_{\mathrm{LA}}^R}{P_T^R}\right)$
is the reference haze (measurement without sample).

(16) In FIG. 2 (left) the cut-off .sub.c angle of the k vectors passing through a cylinder tube is defined by the diameter D and length L of the tube.

(17) $\begin{array}{cc}{}_c=\arctan \left(\frac{D}{L}\right)& \left(5\right)\end{array}$

(18) The same cut-off .sub.c angle can be achieved with two aligned circular apertures with equal diameters and external surfaces separated at a distance L, as it is shown in FIG. 2 (right).

(19) FIG. 3 (left) shows a filter consisting in a conical tube defined by its length and two different apertures in its edges and a filter consisting in two apertures separated a distance L (right). The cut-off .sub.c angle of the k vectors passing through the apertures is defined by the diameters D.sub.1, D.sub.2 and length L.

(20) $\begin{array}{cc}{}_c=\arctan \left(\frac{D_1+D_2}{2.Math.L}\right)& \left(5\right)\end{array}$

(21) The filters could be advantageously made of a material (at least the inner walls) with high absorption over the spectrum of the light of the source. Examples of suitable materials are plastic (preferentially of black color), slate and Teflon. When the filter presents a residual reflection, it may be preferable to use the apertures design in order to avoid possible internal reflection from the inner walls of the cylindrical tube.

(22) The circular surface area of the spot illuminating the sample with diameter Dc is the field of view (FOV) of the spatial filter at a distance d. Only beamlets (k vectors) coming from points inside the FOV will reach the photodetector. D.sub.c is defined as:
D.sub.c=D.sub.1+2.Math.d.Math.tan(.sub.c)(6)

(23) As shown in FIG. 4, the circular surface area of the spot exiting the spatial filter and arriving to the sensor with diameter D.sub.r is the back field of view of the spatial filter at a distance d.sub.r. The sensor surface must be equal or larger than the spot surface D.sub.r in order to contain all the light power exiting the spatial filter. In case d.sub.r=0, sensor surface must be equal or larger than the second aperture. D.sub.r is defined as:
D.sub.r=D.sub.2+2.Math.d.sub.r.Math.tan(.sub.c)(6)

(24) The embodiment of FIG. 4 shows a collimated beam, with diameter Ds illuminating a sample and two separate apertures. A diverging beam, i.e coming from a hazy sample placed before the filter, passes through the spatial filter. The spatial filter has a particular FOV equal to D.sub.c. That means the optical sensor after the spatial filter only see D.sub.c surface of the sample. Taking this into account, it is necessary that the input collimated beam has a spot diameter D.sub.s equal or larger than D.sub.c. If D.sub.s were smaller than D.sub.c the sensor would see a non-illuminated part of the sample. The spatial filter blocks any k vector with angle higher than .sub.c.

(25) The light sensor can be placed touching the D2 aperture or at a distance. Its active area, with diameter D.sub.r, has to be equal or larger than the area of the beam spot exiting the spatial filter in order to avoid errors in power light measurement.

(26) FIG. 5 shows the calculated low k power as a function of scattering for various spatial filter apertures. Scattering here is represented as scattering percentage (the percentage of scattered light power related to total light power). The slope of the curves and the nominal value for a particular scattering, increases with D. That means the invention has better accuracy measuring low k vectors using spatial filters with higher apertures. This design condition defines the effective area of the photodiode or of the image sensor, because a sensor area equal or larger than the spatial filter area is needed in order to measure the low k power without errors.

(27) FIG. 6 shows the calculated low k power as a function of the spatial filter aperture for various samples with different scattering percentage. Low k power increases with filter diameter but the slope of the curves, and the nominal value for a particular diameter, decrease with scattering. That means for a particular D, the invention has better accuracy measuring low k vectors in samples with lower scattering.

(28) The preferred conditions to enhance the accuracy of the invention are: a short distance between sample and spatial filter, large diameters of apertures and large area of the photodetector or light sensor.

(29) In a first embodiment, total power is measured by means of a beam splitter or mirror (FIG. 7a). The scattered light is split into two beams; the low k vectors' power is measured from the first beam by means of a spatial filter F as shown in FIG. 7a, with .sub.c=2.5 (that is, in this particular example, we measure haze as in the definition given above). The total power is measured collecting the whole power of the second beam.

(30) As one possible implementation of this embodiment, FIG. 7a shows a light source SO, preferably a white light source, such as a light emitting diode LED, with a collimation system for the light source, preferably a Khler system, illuminating a hazy sample, a beam splitter BS.sub.0, preferably with 50/50 split percentages, a lens or lenses to collect the total forward scattered power and a first photodiode to measure it, a spatial filter and a second photodiode to measure the low k power.

(31) To achieve a collimated and homogeneous input beam a Khler illumination followed by a collimation lens setup is used. A white light source LED illuminates the collimation system and after a 1 mm diameter aperture, the homogeneous and collimated beam impinges on the hazy sample. A 50/50 beam splitter splits it into two beams. One beam is collected by the photodiode PD1 which measures a signal proportional to the total power transmitted. The second beam goes through two aligned 1 mm apertures filters and is then measured by the photodiode PD2. This measurement is proportional to the low k vectors power. The diameters and the separation sets the filter cut-off angle at 2.5, angle established by the haze standards EPA 180.1 and ISO 7027.

(32) The haze value in the first preferred embodiment can be calculated:
V.sub.PD1.sup.S=R.sub.1.Math.A/100.Math.P.sub.T.sup.S(7)

(33) V.sub.PD1.sup.S is the voltage in the PD1 photodiode output, R.sub.1 is its responsivity in [V/W], and A is the percentage of the light split to into the first beam.
V.sub.PD2.sup.S=R.sub.2.Math.B/100.Math.P.sub.LA.sup.S(8)

(34) V.sub.PD2.sup.S is the voltage in the PD2 photodiode output, R.sub.2 is its responsivity in [V/W], and B is the percentage of the light split into the second beam.

(35) A and B are factors which are known to the skilled person and depend on the beam splitter. If a 50/50 beam splitter is used, A=B.

(36) V.sub.PD1.sup.R and V.sub.PD2.sup.R measures P.sub.T.sup.R and P.sub.LA.sup.R, respectively, in the reference measurement without any sample.

(37) According to the previous definitions of

(38) $H_S=\left(1-\frac{P_{\mathrm{LA}}^S}{P_T^S}\right)\mathrm{and}{H}_R=\left(1-\frac{P_{\mathrm{LA}}^R}{P_T^R}\right),$
the low k power can be written as a function of haze:
P.sub.LA.sup.S=(1H.sub.S).Math.P.sub.T.sup.S(9)

(39) Substituting in equation (8):
V.sub.PD2.sup.S=R.sub.2.Math.B/100.Math.(1H.sub.S).Math.P.sub.T.sup.S(10)

(40) Dividing equation (10) by (7):

(41) $\frac{V_{\mathrm{PD}2}^S}{V_{\mathrm{PD}1}^S}=\left[\frac{R_2.Math.B/100}{R_1.Math.A/100}\right].Math.\left(1-H_S\right)$$\frac{V_{\mathrm{PD}2}^S}{V_{\mathrm{PD}1}^S}=K_{\mathrm{cal}}^{-1}.Math.\left(1-H_S\right)\mathrm{where}{K}_{\mathrm{cal}}=\left[\frac{R_1.Math.A}{R_2.Math.B}\right]$

(42) K.sub.cal is a calibration factor depending on the beam splitter and photodiodes, known by a person skilled in the art.

(43) Therefore the haze sample H.sub.s can be calculated as

(44) $\begin{array}{cc}{H}_S=1-K_{\mathrm{cal}}\frac{V_{\mathrm{PD}2}^S}{V_{\mathrm{PD}1}^S}& \left(11\right)\end{array}$

(45) Similarly for the measured reference voltages:

(46) 0$\begin{array}{cc}{H}_R=1-K_{\mathrm{cal}}\frac{V_{\mathrm{PD}2}^R}{V_{\mathrm{PD}1}^R}& \left(12\right)\end{array}$

(47) And the haze value becomes:

(48) $\begin{array}{cc}H\left[\%\right]=100*\left[\left(1-K_{\mathrm{cal}}\frac{V_{M2}^S}{V_{\mathrm{PD}1}^S}\right)-\left(1-K_{\mathrm{cal}}\frac{V_{M2}^R}{V_{\mathrm{PD}1}^R}\right)\right]& \left(13\right)\end{array}$

(49) The procedure is as follows: 1. Calculate K.sub.cal, depending on the beam splitter and photodiodes. 2. Measure without any sample the voltage values of V.sub.PD1.sup.R and V.sub.PD2.sup.R. 3. Measure without a hazy sample the voltage values of V.sub.PD1.sup.S and V.sub.PD2.sup.S. 4. Apply to the low k measurement the error correction of the equation (11) obtaining V.sub.M2.sup.R and V.sub.M2.sup.S. 5. Calculate Equation (13) with the previous calculated and measured values.

(50) There are other possible embodiments involving the use of a beam splitter or mirror for measuring the total k power in which the measurements are done in a reflection mode. As it can be seen in FIG. 7b . . . in a reflection mode, a collimated beam illuminates perpendicularly a hazy sample and is reflected by a beam splitter BS.sub.1. The reflected light is deflected by a second beam splitter BS.sub.0 in order to measure the low k power and the total k power in a different direction (perpendicular or other) of the illuminating beam in the same way as in the previous embodiment. Low k power is measured from the first split beam by means of a spatial filter F and a photodiode PD2 and total k power is measured from the second split beam collecting the whole light power by means of a lens system and another photodiode PD1.

(51) In another implementation, multiple spatial filters F.sub.N for different cut-off angles can be used.

(52) Low k power referred to different cut-off angles can be measured with different spatial filters placed inside one of the split beam exiting the beam splitter. Spatial filters can be placed preferably uniformly distributed in a plane perpendicular to the light beam direction and close, or touching the beam splitter, as it is shown in FIG. 7c.

(53) In another implementation, multiple spatial filters, for different cut-off angles in a reflection mode can be used. Low k power referred to different cut-off angles can be measured from one of the beams exiting the second beam splitter, as it is shown in FIG. 7d.

(54) In another embodiment, shown in FIG. 8a, the low k vector's power is measured by means of a spatial filter F.sub.L, with .sub.c=2.5 degree. The total power is measured by means of a second spatial filter F.sub.T with a high .sub.c. To avoid errors in the total power measurement .sub.c has to be high enough that the spatial filter does not significantly cut any k-vectors. Any haze range can be measured accurately choosing adequately the ratio D/L of the spatial filter to measure the total power, as it is shown in table.

(55) TABLE-US-00001 Measurable Haze Range [%] 0-2 0-10 0-25 0-50 0-75 0-99 0-99.99 Minimum 0.04 0.05 0.06 0.08 0.13 0.84 9.51 D/L of Total K Filter

(56) In this geometry both measurements, low k power and total power, can be measured with spatial filters with equal FOVs, i.e low k power is measured with a spatial filter at a distance 1 cm whose first aperture is 0.40 mm and total k power is measured with a spatial filter at a distance 1 cm whose first aperture is 0.40 mm too, only differing in the length of the spatial filter. Since FOV is depending on the first aperture and the distance, in this case, both power measurements are related with the same sample's area, and power ratio calculation for haze will be correct. But, both measurements, low k power and total power, can be measured too with spatial filters with different FOVs. That is different first apertures diameters at the same distance d, or equal first apertures diameters at different distances d, or combining different first apertures with different distances d. i.e low k power is measured with a spatial filter at a distance 1 cm whose first aperture is 0.40 mm and total k power is measured with a spatial filter at a distance 1 cm whose first aperture is 1.50 mm, The consequence is that both power measurements are referred to different size areas of the sample and for this reason the power ratio calculation for haze will be incorrect unless both measurements were adequately weighted with a rescaling factor according to:

(57) ${\mathrm{FOV}}_{\mathrm{CFactor}}=\frac{{\mathrm{FOV}}_T}{{\mathrm{FOV}}_{\mathrm{LA}}}=\frac{{\left[D_{c\_T}+2.Math.d.Math.\tan \left({}_{c\_T}\right)\right]}^2}{{\left[D_{c\_\mathrm{LA}}+2.Math.d.Math.\tan \left({}_{c\_\mathrm{LA}}\right)\right]}^2}$

(58) To avoid weighting errors short distance between sample and spatial filter is preferred. There are other possible embodiments involving the use of a beam splitter or mirror for measuring the total k power in which the measurements are done in a reflection mode. As it can be seen in FIG. 8b, in a reflection mode, a collimated beam illuminates perpendicularly a hazy sample and the reflected light scattering is deflected by a beam splitter in order to measure the low k power and the total power in a different direction (perpendicular or other) of the illuminating beam. Scattered light exiting the beam splitter can be measured with the same procedure described in the previous embodiment, that is, using one or two sensors SE and by means of two filters with different D.

(59) In another implementation, a plurality of spatial filters F.sub.N can be used for measure low k power at different cut-off angles, and the total k power.

(60) Low k power referred to different cut-off angles can be measured with different spatial filters placed inside one of the split beam exiting the beam splitter. Total power can be measured with one of the spatial filters of the array. Spatial filters can be placed preferably uniformly distributed in a plane perpendicular to the light beam direction and close, or touching the sample, as it is shown in FIG. 8c.

(61) In another implementation, multiple spatial filters, for different cut-off angles in a reflection mode can be used. Low k power referred to different cut-off angles can be measured from one of the beams exiting a beam splitter used in order to measure the low k power and the total power in a different direction (perpendicular or other) of the illuminating beam as it is shown in FIG. 8d.

(62) Since a plurality of spatial filters is used to measure low k power at different cut-off angles, these scattering power values can be represented in a scattering radiation diagram. An example of a plurality of spatial filters for measuring a scattering radiation diagram for angles between 1.25 and 9.31 degree is shown in the following table:

(63) TABLE-US-00002 Diameter Apertures[mm] 1.5 1.3 1 0.9 0.8 0.7 0.6 0.4 0.2 Length 9.15 9.15 9.15 9.15 9.15 9.15 9.15 9.15 9.15 Tube [mm] Cutt-off 9.31 8.09 6.24 5.62 5.00 4.37 3.75 2.50 1.25 Angle []

(64) Since an image sensor is used to measure the light power of a plurality of spatial filters, and each group of pixels is in charge of measure the light power of each spatial filter, the shadow and diffracted image of the sample provided by each group of pixels can be processed by image processing techniques in order to obtain a microscopic resolved image, with a resolution equal to the pixel size and a field of view equal to the image sensor dimensions. Image processing techniques to recover amplitude and phase from a diffracted image are well known in holographic reconstruction and lens-free microscopy.

(65) In this text, the term comprises and its derivations (such as comprising, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

(66) On the other hand, the invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.), within the general scope of the invention as defined in the claims.