WO2007053092A1

WO2007053092A1 - Computer screen photo-assisted measurement of intensity and/or polarization change of light upon interaction with a sample

Info

Publication number: WO2007053092A1
Application number: PCT/SE2006/001251
Authority: WO
Inventors: Ingemar LUNDSTRÖM; Daniel Filippini; Jim Bakker; Hans Arwin
Original assignee: Rgb Technologies Ab
Priority date: 2005-11-04
Filing date: 2006-11-03
Publication date: 2007-05-10

Abstract

A method for measuring a property of a test sample utilizing one of the test methods ellipsometry, surface plasmon resonance and nephelometry, wherein the method includes the steps; providing a test sample being an object with which an optical interaction with light takes place, illuminating said test sample using a program controlled display as a light source, which program controlled display is composed of at least one activated pixel providing an illumination from an illuminating area of said program controlled display, arranging said program controlled display to illuminate said test sample with polarized light, detecting light emerging from said test sample utilizing a detector coupled to said program and evaluating said property from signals from said detector.

Description

Computer screen photo-assisted measurement of intensity and/or polarization change of light upon interaction with a sample

Technical field

The present invention relates to a method and a system for measuring a property of a test sample utilizing any of the test methods ellipsometry, surface plasmon resonance (SPR), and nephelometry.

Background of the invention

Optical measurement techniques like ellipsometry and SPR allow non-labeled assays, very sensitive thickness measurements and many other sensing and measurement applications. These and other techniques are well established in the scientific world and are used for standard sensing and testing as well as fundamental research. However, nowadays most of these techniques are only available for well furnished labs. Cost and complexity of the necessary measurement systems hinders massive use outside research or industrial environments. The computer screen photo-assisted technique (from hereon called CSPT) uses computer screens or other controllable displays as a programmable light source. In case of an optical response, a web camera can be used for detection. The high availability of potential CSPT setups makes it a globally distributable sensing system. A CSPT platform provides a programmable planar light source with a spatially resolved spectral and intensity control. In case of LCD screens, linearly polarized light is obtained directly from the light source, which can be used for measurements requiring polarized light.

The CSPT technique has already been described in the document US 7 092 089 for detecting spectroscopic changes. It has been used for gas sensing and bioassay evaluation. The present invention deals with the use of CSPT for the detection of polarization and/or intensity changes upon interaction of light or its evanescent field with a sample. The polarization and/or intensity changes are then used to determine properties of the sample, i. e. in a sensor application. Summary of the invention

The object of the invention is achieved in a simple way by replacing complex illuminating systems by a screen or a display commanded by software.

The invention involves measurement of changes induced in light from a computer screen or other display (as described in [patent]) upon interaction with a sample. These interactions cause changes other than, but not excluding, spectroscopic changes. Non-spectroscopic changes can be intensity or polarization changes as defined below. Existing techniques based on such interactions are for example (but not limited to) ellipsometry, surface plasmon resonance, and nephelometry (here representing scattering based techniques). The possible use of these techniques is shortly outlined below.

In brief an aspect of the invention involves:

A method for measuring a property of a sample utilizing one of the test methods from the group of:

- ellipsometry,

- surface plasmon resonance,

- nephelometry, wherein the method includes the steps of:

- providing a test sample being an object with which an optical interaction with light takes place,

- illuminating said test sample using a program controlled display as a light source, which program controlled display is composed of at least one activated pixel providing an illumination from an illuminating area of said program controlled display,

- arranging said program controlled display to illuminate said test sample with polarized light,

- detecting light emerging from said test sample utilizing a detector coupled to said program and

- evaluating said property from signals from said detector. Ellipsometry

Ellipsometry [4] deals with the detection of the polarization change of light upon interaction with a sample. As a non-intrusive but powerful optical technique it has many applications involving the determination of optical and microstructural properties of a sample, especially for thin film applications. It has also been proven useful for fast and accurate sensing in a wide range of fields [5].

Generally, an ellipsometric setup consists of a (usually monochromatic) light source 1 , a i detector 2, two (linear) polarizers (first and second) 3, 4 and a wave retarder. A common, prior art, setup is the PCSA setup, which stands for Polarizer, Compensator, Sample,

Analyzer (see fig. 1a). The compensator 5 is the wave retarder and the analyzer the second polarizer 4. With the combination of the first polarizer 3 and compensator 5, any polarization state can be generated. This state will generally change upon interaction with a

! sample 6. This changed state is analyzed by the analyzer and detector.

Figure 1b shows a possible setup for performing the CSPT ellipsometry measurements according to one embodiment of the invention. The sample is inserted in a container to keep out disturbing ambient light and control the angle of incidence. Figure 1c shows a ) possible setup for a measurement in transmission mode. Note that in some literature this is called polarimetry instead of ellipsometry. A measurement can involve detection of either transmitted or reflected light, or both, at any angle of incidence fo.

If an LCD screen is used as a light source, the light from this screen is linearly polarized. ) Therefore a polarizer is not necessary for the incident light and only the second polarizer needs to be added. Other screens, like cathode ray tube (CRT) screens, emit unpolarized light, so a polarizer for the incident light may be needed when using such screens.

Possibilities with CSPT ellipsometry are (but not limited to) 3 - Imaging ellipsometry

- Reflection and/or transmission ellipsometry

- Multiple angle reflections within one measurement because of imaging

- Dynamic/in situ measurements

- Intensity can be controlled by the screen to exploit the full range of the detector - Multiple measurements can be performed simultaneously because of large area imaging

- Internal reflection ellipsometry, which may or may not involve exploiting the surface plasmon resonance effect (see below).

Internal reflection and Surface Plasmon Resonance

Internal reflection measurements can be made using a prism or other transparent object and letting the light reflect internally at the interface between the object and the ambient as shown in the insert in figure 1 b. Upon reflection of the light at the interface under conditions

) of total internal reflection (the case when the angle of incidence is larger than the critical angle), an evanescent field will penetrate through the interface and into any material present on it. An interaction will take place and the reflected light will be influenced by this interaction. If the material present on the interface is a thin layer of metal with many free electrons, a surface plasmon can be induced, causing a strong decrease in the reflected

J intensity for resonance conditions. This phenomenon is very sensitive to material present on the metal and can thus be used as a sensing technique. Note that traditionally the reflected intensity is monitored in SPR-based sensing but that recently it has been shown that also ellipsometry can be utilized [8].

) Nephelometry

Nephelometry is also referred to as Turbidimetry and is carried out by measuring the intensity changes due to scattering and to some extent absorption of light in a liquid. By measuring the intensity of the light passing through the liquid first without and secondly with the analyte present in the liquid, a differential measurement of the amount of insoluble

5 suspended aggregates can be performed. Another possibility here is to use a reference sample, i.e. the liquid without an analyte, and measure the reference and the test liquid simultaneously, by utilizing the possibility to configurate the light source in an optimal way.

The invention further includes a computer program product controlling the interaction 3 between said program controlled display and said detector for performing the measurings according to one of the test methods of claim 1. The computer program product is further programmed to perform the calculations based on the signals from said detector for deriving said property of the test sample. Definitions

CSPT

Computer Screen Photo-Assisted technique. Using a computer screen or other programmable display as a light source as described in US 7 092 089.

Intensity

The power per unit of area on an area perpendicular to the propagation direction (Poynting vector). The intensity is proportional to the square of the amplitude of the electric field associated with the light wave and can be divided in partial intensities for different polarization angles.

Polarization changes

Changes in the polarization state of the light, which can involve one ore more of the following effects:

- Attenuation (decrease in intensity) in one or more polarization directions

- Change in phase in one or more polarization directions

- Change in polarization angle of a given polarization state

- Change in the degree of polarization

Measurement

Detection of polarization and/or intensity changes in light upon interaction with a sample.

Sample

- Entity (object) with which the optical interaction with light takes place

- May include a chemical or biochemical interface intended to interact with a target environment

- May contain embedded integrated optical components (examples below) to aid the measurement

- May include parts aimed at physical interactions such as temperature, pressure, moisture, radiation

- May contain thermoelectric elements, piezoelectric elements, thixotropic materials, optically active substances, etc.

- Can consist of solid, liquid and gaseous parts - Can be porous

- May interact with a target environment that modifies its interaction with the light and thus act as a mechanical, thermal, magnetic, optical, chemical, biochemical or other sensor

The setup

- Complete arrangement including computer screen, web camera and a specially designed sample holder.

- The illuminating conditions provided by the illuminating screen as described in document US 7 092 089 can be used for aiding the measurements by defining the shape of the light source, its 2D coordinates on the screen, its intensity, polarization angle and spectral composition.

- The separate color channels in the web camera can provide additional spectral information.

- The setup may contain additional optical components (examples below)

- The setup can be optimized for a specific application. Specific example: measure at Brewster angle or angle which minimizes p-polarized reflection on the uncoated or precoated sample to optimize sensitivity to thickness changes. Another example is to utilize the SPR-phenomena in thin metal films.

Optical components

- Polarizers

- Lenses

- Mirrors

- Interference cavities

- Filters

- Retarders

- Diffractive elements

- Refractive elements

- Nanostructured antennas

- Wave guides

- Etc. Further definition of ellipsometry

To avoid wrong interpretation of the term ellipsometry and to further define the scope of the test method, the use of the term is more precisely described herein:

Polarimetry is the measurement of the polarisation of light; a polarimeter is the scientific instrument used to make these measurements. Polarimetry of thin films and surfaces is commonly known as (reflection) ellipsometry. Polarimetry when light is propagating through a sample is referred to as transmission ellipsometry. Here we use the term ellipsometry to cover both the reflection and transmission cases.

Polarimetry can be used to measure various optical properties of a material, including linear birefringence, circular birefringence (also known as optical rotation or optical rotary dispersion), linear dichroism, circular dichroism and scattering. To measure these various properties, there have been many designs of polarimeters. Some are archaic and some are in current use. The most sensitive polarimeters are based on interferometers, while more conventional polarimeters are based on arrangements of polarising filters, wave plates or other devices.

Optically active samples, such as solutions of chiral molecules, often exhibit circular birefringence. Circular birefringence causes rotation of the polarisation of plane polarised light as it passes through the sample.

A simple polarimeter to measure this rotation consists of a long tube with flat glass ends, into which the sample is placed. At each end of the tube is a Nicol prism or other polarizer. Light is shone through the tube, and the prism at the other end, attached to an eye-piece, is rotated until all light is shut off. The angle of rotation is then read off of a scale. The specific rotation of the sample may then be calculated.

CD - Circular dichroism

(The change of ellipticity is measured)

Circular dichroism (CD) spectroscopy measures differences in the absorption of left-handed polarized light versus right-handed polarized light which arise due to structural asymmetry.

The absence of regular structure results in zero CD intensity, while an ordered structure results in a spectrum which can contain both positive and negative signals. 01251

In general, this phenomenon will be exhibited in absorption bands of any optically active molecule. As a consequence, circular dichroism is exhibited by biological molecules, because of the dextrorotary (e.g. some sugars) and levorotary (e.g. some amino acids) molecules they contain. Noteworthy as well is that secondary structure will also impart a distinct CD to their respective molecules. Therefore, the alpha helix of proteins and the double helix of nucleic acids have CD spectral signatures representative of their structures.

ORD - optical rotatory dispersion

(The change of the direction of linearly polarized light is measured)

Optical rotation or optical activity is the rotation of linearly polarized light as it travels through certain materials. It occurs in solutions of chiral molecules such as sucrose (sugar). It is used in the sugar industry to measure syrup concentration, in optics to manipulate polarization, in chemistry to characterize substances in solution, and is being developed as a method to measure blood sugar concentration in diabetic people.

Simple polarimeters have been used to measure the concentrations of simple sugars, such as glucose, in solution. In fact, one name for glucose, dextrose, refers to the fact that it causes linearly polarized light to rotate to the right or dexter side. Similarly, levulose, more commonly known as fructose, causes the plane of polarization to rotate to the left. Fructose is even more strongly levorotatory than glucose is dextrorotatory. Invert sugar, formed by adding fructose to a solution of glucose, gets its name from the fact that the conversion causes the direction of rotation to "invert" from right to left.

Optical activity is a type of birefringence. Any linear polarization of light can be written as an equal combination of right-hand (RHC) and left-hand circularly (LHC) polarized light:

Eg₀ = ΕRHO + e*^{~ Q}EχHσ,

where E is the electric field of the light. The relative phase between the two circular polarizations, 2θ₀, sets the direction of the linear polarization to θ₀. In an optically active material the two circular polarizations experience different refractive indices. The difference in the indices quantifies the strength of the optical activity,

Δn = n_Rπc ~ ΠLHC- This difference is a characteristic of the material (for substances in solution it is given as the specific rotation). After travelling through length L of material the two polarizations pick up a relative phase of

_{2ΔΘ =} ΔnI2,

where λ is the wavelength of the light (in vacuum). Consequently, the final polarization is rotated to angle ΘO + Δθ.

Generally, the refractive index depends on the wavelength. The variation in rotation with the wavelength of the light is called optical rotatory dispersion (ORD).

ORD versus CD (Both being forms of transmission ellipsometry, or as stated above forms of polarimetry).

ORD spectra and circular dichroism spectra are related through the Kramers-Kronig relations. Complete knowledge of one spectrum allows the calculation of the other.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1

Experimental setup for ellipsometric measurements, a) shows a general, prior art, PCSA setup with the insert defining the parameters for a 3-phase system (substrate, layer and ambient), b) shows a possible CSPT setup in reflection mode, c) shows a possible setup in transmission mode. Note that other angles of incidence than shown are also possible.

Figure 2

CSPT measurements. A shows the measurement on sample 1 , with different shades indicating different thicknesses. The numbers indicate the thicknesses as measured by conventional spectroscopic ellipsometry. B shows similar measurements performed on sample 2, using different intensities on the screen as indicated in the left column. Figure 3

Measurement results. Intensity plotted as a function of thickness for sample 1 (A) and sample 2 (B). The * symbols indicate the measurements, the solid lines the model fit. The different colors indicate the corresponding colors (red, green and blue) in the web camera measurements.

Figure 4

Comparison of images taken with (top) and without (bottom) polarizer. Without the polarizer the different thicknesses are not visible.

EMBODIMENTS OF THE INVENTION

A number of embodiments of the invention will be described below supported by the enclosed drawings.

Off-null ellipsometry in a CSPT setup is disclosed and exemplifies a concept of the invention. An adaptation of off-null ellipsometry is demonstrated using a computer screen photo-assisted technique. Large area imaging of thin layer thickness is possible using this technique with a potential thickness resolution of less than 1 nm. This makes it suitable for biosensing applications like immunoassays and detection of DNA reactions. The computer screen serves as a homogeneous large area illumination source, which can be tuned to different intensities for different parts of the sample, to obtain a large sensitivity range without sacrificing thickness resolution. Other applications than biosensor applications are also possible and the availability of equipment for the technique, based on computer screens, opens the possibility of "ellipsometry for all". For thicker layers, color information caused by interference can also be exploited.

The computer screen photo-assisted technique (as described in document document US 7 092 089) utilizes regular computer sets and web cameras for the characterization and classification of color or fluorescent substances [1-3]. CSPT platforms are versatile devices able to serve diverse sample formats or sensing principles with one single instrument. The wide availability of personal computers makes CSPT an attractive approach for the evaluation of assays in primary health care centers or at home. Certainly, not all optical biosensing involves color or fluorescent indicators. Non-labeled assays demand more sophisticated techniques such as surface plasmon resonance or ellipsometry.

Here we describe in some detail ellipsometric measurements (measuring polarization change of light) using a CSPT setup. The linear polarization state of the light emitted can be directly utilized when using an LCD screen, for other screens an extra polarizer 3 may be needed.

Theory

Several different ways of performing ellipsometric measurements exist. The technique used here is called off-null ellipsometry. In off-null ellipsometry, the optical components are set in such a way that no (or very little) light is reaching the detector in the reference situation. Any change in optical properties of the surface (e.g. the growth of a thin film on its surface) will then lead to an increase in intensity of the reflected light. The intensity change carries information about the change on the surface, for example the thickness of a layer grown on it.

If the polarization state is set in such a way that the light is linearly polarized after reflection from the sample, it can be extinguished by the analyzer by setting its angle perpendicular to the light's angle of polarization. In some special case, e.g., if the sample is a bulk dielectric, no phase shift between the s- and p-polarized light will occur and any linearly polarized incident light will still be linearly polarized after reflection. In this case the compensator can be omitted.

The reflected intensity from any type of sample, bulk or film-coated, for a setup without a compensator (PSA) is a function of the polarizer and analyzer settings and the sample properties and is given by:

2

I = Ir R_n cosAcosP + R, siaAsinP (1 ) where P is the polarizer angle, A the analyzer angle and R_p and R_s are the Fresnel reflection coefficients for p- and s-polarized light, respectively. For a bulk sample, R_p and R_s reduce to the Fresnel reflection coefficients of the material, r_p and r_s. For a bulk sample with a single homogeneous layer on top (see insert in fig. 1a), the reflection coefficients become: ^Λ' - l _{+ r r} e^{nβ { ]} r + r e^aβ

*. <! «, ^(2b) where r_3b is the Fresnel reflection coefficient of the interface between material a and b, with the numbers representing the ambient (material 0), the layer material (material 1 ) and the substrate (material 2). β is the film phase thickness and is given by _{β =} ^_{Nι COs φι} (3)

with of the thickness of the layer, λ the wavelength of the light, /V₁ the refractive index of the layer material and φ_\ the angle of the light inside the layer, which can be calculated using Snell's law with fo, N₀ and N₁. When using a PCSA null ellipsometer setup, the intensity can be shown to vary linearly with the layer thickness of for small values of d [Q]. This is not the case for the PSA setup, but with modern computers to perform the calculations and proper calibration, this does not constitute a problem.

Figure 1 b shows the setup used for performing the CSPT measurements. The light source is a program controlled display, such as a computer screen, in the figure represented by numeral 1. The sample 6 is inserted in a container 7 to keep out disturbing ambient light and control the angle of incidence. The container 7 has inclined walls, wherein a first polarizer 3 can be attached to a first wall and a second polarizer being attached to a second wall. The angle of incidence is, in the example shown, chosen to 70° and as illustrated in the figure, the first polarizer 3 is omitted in the present example, since an LCD screen is used. The light reflected from the test sample 6 is detected utilizing a detector 2, according to the present example represented by a web camera. The setup is placed in such a way that the polarized light from the screen has a polarization angle of 45° from the plane of incidence. In case of use of the test method SPR, internal reflection measurements can be made using a prism 9 or other transparent object and letting the light reflect internally at the interface between the object and the ambient as shown in the insert 8 in figure 1b. The test sample 6 is in this case attached to said interface at the top of a flow cell 10.

Figure 1c shows how the technique may be used in transmission ellipsometry mode at normal incidence. A substance interacting with the sample may cause a polarization change detectable in the CPST setup. Also operation at oblique incidence in transmission mode is possible.

Measurements

Test samples were prepared with SiO₂ films on an Si substrate. Si was thermally oxidized and the oxide was etched away in steps with an aqueous hydrofluoric acid (HF) solution. The thickness of each step was determined by spectroscopic ellipsometry on an variable angle spectroscopic ellipsometer (VASE) from J.A. Woollam Co. Two samples are presented in this work, one with oxide thicknesses varying from 4.8 to 16.8 nm and one with thicknesses varying from 7.9 to 39.2 nm. The samples are referred to as sample 1 and sample 2, respectively.

Before a measurement, a bare Si wafer (with only native oxide) is placed in the sample container and the polarizer is rotated to obtain minimum intensity in the web camera. After this nulling procedure, the sample is inserted in the container and the system is ready for measuring by recording one or more frames with the web camera.

Figure 2a shows the recorded picture for sample 1. The picture shows varying shades for different thicknesses, the larger the thickness, the brighter the shade. The web camera sensitivity is set in such a way that the intensity range fits optimally in the camera's sensitivity range. The measurements performed on sample 2 are shown in figure 2B. These measurements were all made with a constant web camera setting, but varying screen intensity. At the highest intensity, the thickest layers become hard to distinguish because the intensity falls outside the camera's sensitivity range and it gets saturated. At low screen intensities, however, the thinner layers become harder to distinguish because the intensity is low compared to the camera range.

The obtained pictures were analyzed by calculating the average intensity in the red, green and blue color channels of the web camera for a given area on each of the steps. The results were also compared to theory by fitting the model to the calculated intensities and using the thicknesses measured by regular ellipsometry. For a correct model, equation (1) should be calculated for each wavelength in the emission spectrum of the computer screen and integrated using the screen spectrum as weighting factor. The calculated intensity spectrum should then be multiplied by the sensitivity spectra of the color filters in the web camera to obtain the final result. However, for simplicity we have chosen to use one wavelength for each primary color. This gives three functions, describing the intensity for each color as a function of thickness. These functions are fit to the measured results using a least squares fit as follows:

^C 1 ^""^{" C}2 J theory ⁼ ^ measured ^• v *7 with C₁ and c₂ as fit parameters. The multiplication factor c₂ is needed to fit the model to the scale of the web camera measurements, the term C₁ to compensate for a constant intensity offset caused by for example background illumination and black current in the camera's CCD detector.

Figure 3 shows the measurement results together with the model fit. For one single measurement with one color channel, the parameters in the model would be dependent on each other and the model fit would be meaningless. With three different color channels and several thicknesses, however, the shape of the curve and the difference between the color channels disentangles the variables, making this type of fit procedure possible. Both measurements show a qualitatively good agreement with the model. The model fit could probably be improved by integrating over the whole spectrum as suggested above. It should also be noted that the setup has an intrinsic angular spread given by the lateral size of the sample. While the model is useful for checking the validity of the measurement, it is of little importance for applications. In a sample for medical testing, one of the objectives of the CSPT applications, a built in reference can be used for calibration. This will be done in the form of a few layers with known thicknesses, to which the unknown layers, for example protein layers, can be compared. In most cases the answer will then come down to a "yes" or a "no" (e.g. a certain protein was present or not), but if necessary, even a fairly precise thickness can be obtained by interpolating between the known thicknesses of the references. (To express the results as thickness originates from the optical models used in research applications but may not be the most appropriate measure in applications. A conversion to surface mass density (μg/cm²) could be one possibility. In a sensor application a normalization to standards can be done and the sensor response can be expressed e.g. enzyme activity, protein concentration, etc.)

The measurements on sample 1 show that thickness differences of less than 2 nm can be measured using the current setup. By tuning the setup (i.e. optimizing the angle of incidence and polarizer angles for maximum sensitivity to thickness changes) and good image processing, the resolution can be expected to be below 1 nm. Provisional results E2006/001251

15

show that this is indeed possible and results with commercial ellipsometers down to 1 pm show this claim to be reasonable.

Figure 4 shows a comparison of measurements with and without the polarizer to proof that the sensitivity is actually obtained by using the polarizer.

In principle more information about the sample can be obtained from a null ellipsometry measurement than just the thickness. However, in most cases it will be just the thickness which is of interest. Then the setup can be optimized and simplified for thickness measurements by setting the angle of incidence to the angle Φ at which the intensity of the radiation reflected from the uncoated or precoated surface reaches a minimum (as described in US Patent 4,521 ,522). If the setup is then arranged in such a way that the light from an LCD screen is polarized parallel to the plane of incidence, all the light can be used for the analysis, which improves the thickness sensitivity.

Use of the technique

With potential sub nm thickness resolution, this technique is suitable for biosensor applications, for example immunoassays and DNA detection, since monolayers of biomolecules can be detected. This has already been demonstrated by Ostroff et al. using a technique described as "fixed polarizer ellipsometry" [7]. Ostroff et al. also describe how the technique can be made more versatile by tuning the optical components for a specific application. The CSPT setup has many advantages compared to the fixed polarizer ellipsometry setup. The computer screen functions as a controllable homogeneous planar light source. The intensity can easily be modulated to compensate for high reflectivity. This can be used to expand the sensitivity range of the measurements. When nulled on bulk Si, a thick layer can produce an intensity which is outside the range of the web camera. This can be compensated by either decreasing the sensitivity of the web camera or decreasing the intensity of the screen. In the latter case, the intensity can be decreased to different levels for different areas on the sample, depending on the detected intensity. This means that even large thickness variations on a sample can be measured in spite of the relatively low sensitivity range of the camera, which is an additional advantage of using the computer screen as a light source. 006/001251

16

Since the web camera and computer screen are not ideal components and can vary according to circumstances, a calibration will always be necessary. If this calibration is included in the sample as described above, it does not influence the ease of use for a home user and the setup can be developed for home testing applications. Given the sensitivity and possibility to tune CSPT based ellipsometry, it may prove a powerful tool for the home testing environment.

Other applications than biosensor applications are of course also possible. This technique opens the perspective of "ellipsometry for all", where fast and easy measurements can be made in a low cost setup, e.g. spreading the principle of ellipsometry to schools in general and making it available for home use applications.

When measuring thicker layers (typically a few hundred nanometers or more), interference effects become visible. The strong wavelength dependence of these effects gives rise to extra color information which, because of the nature of the setup, can easily be measured. This can give high thickness precision even for thicker layers, which is an additional advantage of the CSPT setup.

Regarding the detectors 2, these can be of different types, like large area metal-oxide- semiconductor devices, digital or video cameras, polymer photo-detectors or conductive photo- sensitive sensors which depending on the particular application can be patterned on a flexible or rigid substrate.

The light source can be reduced to an individual pixel, still preserving the light properties already described, with sizes around 250 μm by side and scanning pitch in the same range. In these conditions this tiny light source (or also other composed by several illuminated pixels) can be spatially scanned over a large area sensing device.

In the examples, the computer screen can be simultaneously used to display test results or for analysis of these results via software on the computer or via internet, since only a part of the screen is used as a light source in a multitasking computer platform.

It should also be pointed out that the large area light source can be focussed to a small area sample, increasing in this way the light intensity on the sample. References

[1] D. Filippini and I. Lundstrόm, Appl. Phys. Lett. 81, 3891 (2002)

[2] Filippini, D., Svensson, S.P.S., Lundstrόm, I., 2003. Chem. Commun., 240-241

[3] Daniel Filippini, Jimmy Bakker and lngemar Lundstrom, Sensors and Actuators B: Chemical, 106:1 , 302-310 (2005)

[4] R.M.A. Azzam and N. M. Bashara, Ellipsometry and polarized light, North-Holland (1987)

[5] Arwin, H., Ellipsometry-based Sensor Systems, Chapter in "Encyclopedia of Sensors", (Eds C A Grimes, E Dickey and M V Pisko) VoI 3, pp 329-358, American Scientific Publishers (2006)

[6] Hans Arwin, Stefan Welin-Klintstrom and Roger Jansson, Journal of Colloid and Interface Science 156:2 , 377-382 (1993)

[7] Rachel M. Ostroff, Diana Maul, Gregory R. Bogart, Shao Yang, Jennifer Christian, Deborah Hopkins, David Clark, Brian Trotter, and Garret Moddel, Clinical Chemistry 44:9 2031-2035 (1998)

[8] Hans Arwin, Michal Poksinski and Knut Johansen, Applied Optics, 43:15, 3028-3036 (2004)

Claims

E2006/00125118CLAIMS

1. Method for measuring a property of a sample utilizing one of the test methods from the group of:

- ellipsometry,

- surface plasmon resonance,

- nephelometry, the method including the steps of:

- providing a test sample (6) being an object with which an optical interaction with light takes place,

- illuminating said test sample (6) using a program controlled display (1 ) as a light source, which program controlled display (1 ) is composed of at least one activated pixel providing an illumination from an illuminating area of said program controlled display (1),

- arranging said program controlled display (1 ) to illuminate said test sample (6) with polarized light,

- detecting light emerging from said test sample (6) utilizing a detector (2) coupled to said program and

- deriving said property from signals from said detector (2).

2. The method according to claim 1 , further including the step of:

- including in said test sample (6) a chemical or biochemical interface designed to interact with a target environment.

3. The method according to claim 2, further including the step of:

- arranging said test sample (6) to interact with said target environment for modifying its interaction with the light.

4. The method according to any one of the preceding claims, further including at least one from the steps of:

- embedding integrated optical components in the test sample (6),

- including in the test sample (6) parts interacting with a physical quantity, such as temperature, pressure, moisture, radiation,

- adding to the test sample (6) at least one from the group of: a thermoelectric element, a piezoelectric element, a thixotropic material, an optically active substance.

5. The method according to claim 1 , further comprising the step of:

- using a computer screen as said program controlled display (1 ).

6. The method according to claim 1 , further comprising the step of: providing said derived property for any one from the group of:

- displaying the derived property on a display,

- displaying the derived property on said program controlled display (1 ),

- storing the derived property in a memory,

- using said derived property for performing calculations.

7. The method according to claim 1 , further comprising individually modulating the colour of each individual pixel in the display by software.

8. The method according to claim 1 , further comprising scanning the color of each individual pixel within the visible range by software.

9. The method according to claim 1 , wherein the colour, size, shape, modulation and background colour of said illuminating area is configured through a user interface.

10. The method according to claim 1, further comprising displacing said illuminating area of said program controlled display (1 ) over time.

11. The method according to claim 1 , wherein the step of displaying further comprises displaying said derived property on a part of said program controlled display (1 ) that is not used for illumination.

12. The method according to claim 1, further comprising utilizing a polarizer (4) between said sample and said detector.

13. The method according to claim 1 , further comprising a step of evaluating said signals from said detector (2) by software coupled to said program controlled display (1 ).

14. The method according to claim 1 , further comprising a step of evaluating said signals from said detector (2) through an on-line analysis by an expert or an expert system.

15. The method according to claim 1 , further comprising a step controlling said program controlled display (1) and said detector (2) utilizing a computer.

16. The method according to claim 1 , further comprising

- determining thickness variations of said test sample (6) as one property of the sample.

17. The method according to claim 1 further comprising any one from the step of:

- defining the shape of the light source,

- determining the two dimensional coordinates of said illuminating area,

- determining the intensity of the light from said illuminating area,

- determining the intensity of light from different parts of said illuminating area,

- determining the polarization angle of the light emitted from the illuminating area,

- determining the spectral composition of the light emitted from the illuminating area.

18. A system for measuring a property of a sample utilizing one of the test methods from the group of:

- ellipsometry,

- surface plasmon resonance,

- nephelometry, characterized in that said system comprises:

- a test sample (6) being an object with which an optical interaction takes place with light illuminating the test sample,

- a program controlled display (1 ) arranged to be used as a light source for illuminating said test sample,

- polarization means (3) for providing light from said light source to be polarized light,

- a detector (2) coupled to said program controlled display (1) and arranged to detect light emerging from said test sample (6),

- a polarizer (4) arranged between said test sample (6) and said detector (2).

19. The system according to claim 18, wherein said program controlled display (1 ) is a computer screen.

20. The system according to claim 18, wherein said test sample (6) has at least one embedded optical component from the group of:

- a polarizer,

- a lens,

- a mirror,

- an interference cavity,

- a filter,

- a retarder,

- a diffractive element,

- a refractive element,

- a nanostructured antenna,

- a wave guide.

21. The system according to claim 18, wherein an optical component from the group of:

- a polarizer,

- a lens,

- a mirror,

- an interference cavity,

- a filter,

- a retarder,

- a diffractive element,

- a refractive element,

- a nanostructured antenna,

- a wave guide. is positioned between said light source (1) and said detector (2).

22. The system according to claim 18, wherein said detector (2) is one of: a web camera, a digital camera, a video camera.

23. The system according to claim 18, wherein said detector (2) is one of: a semiconductor device, a photo sensitive detector, a polymer detector, an ion- sensitive device.

24. The system according to claim 18, wherein the system comprises a sample holder (7) for holding said test sample (6) at a predetermined position and in a predetermined orientation in relation to said display (1), further to hold said polarization means (3), if any, in a predetermined angle in relation to the light illuminating the test sample (6), and to hold said polarizer (4) in a predetermined angle in relation to light reflected or transmitted from the test sample (6).

25. Use of the system according to claim 18 as a bio sensor, such as for immunoassays and DNA detection.

26. Use of the system according to claim 18 as a sensor for detecting thickness variation of one or several layers of the test sample (6).

27. A computer program product controlling an interaction between said program controlled display (1 ) and said detector (2) for performing the measurings according to one of the test methods of claim 1.

28. The computer program product according to claim 27 performing calculations based on the signals from said detector (2) for deriving said property of the test sample.