EP2235503A1

EP2235503A1 - Arrangement for determining the reflectivity of a sample

Info

Publication number: EP2235503A1
Application number: EP08862939A
Authority: EP
Inventors: Nico Correns; Werner Hoyme; Felix Kerstan; Thomas Keune; Wilhelm Schebesta
Original assignee: Carl Zeiss MicroImaging GmbH
Current assignee: Carl Zeiss Microscopy GmbH
Priority date: 2007-12-19
Filing date: 2008-12-10
Publication date: 2010-10-06
Also published as: DE102007061213A1; WO2009077110A1; US20110007319A1

Abstract

The invention relates to an arrangement for measuring the reflectivity of the direct or scattered reflection of a sample (8), having a light source for separately lighting the sample (8) and of comparative surfaces. The arrangement comprises, in addition to the light source, preferably a reflector lamp (2), - a white standard (6), a black standard (7), and the surface of the sample (8) for embodying a measurement surface, wherein the exchange of the white standard (6), the black standard (7) and the sample (8) is provided in a prescribed sequence relative to each other, - means for measuring the intensity of the light reflected from an internal white surface (10) and for measuring the intensity of the light reflected from each measuring surface, and - an evaluation circuit designed for registering the measured intensity values and for linking the same mathematically to the reflectivity.

Description

Arrangement for determining the reflectance of a sample

The invention relates to an arrangement for measuring the reflectance of the direct or scattered reflection of a sample, with a light source for separate illumination of the sample on the one hand and of comparison surfaces on the other hand, as well as with an evaluation circuit for computational linkage of the determined from the comparison surfaces intensity values to the reflectance.

Arrangements for determining the reflectance of sample surfaces are known in the prior art in which a white standard area and a black standard area are used as comparison areas. In each case at the beginning of the measurements, a measuring scale is determined with these two standards and the measuring instrument is calibrated with this scale. Between the two standard values formed by the white standard and the black standard, both measured at the same place as the sample, the reflectance determined by the sample surface is arranged.

However, in the measuring devices used for reflection measurement, the system parameters change because, for example, the intensity of the light source decreases or the sensitivity of sensors used for the optoelectronic conversion of received signals changes. To compensate for these changes, recalibrating the scale is required for repeated measurements.

In the simplest case, the sample is removed from the sample plane, in its place first the white standard area and then the black standard area or vice versa positioned and thus redefined the measuring scale. This procedure can only be used in the laboratory because of the relatively large time required for the calibration. In the process measuring, however, these interruptions are annoying and in many cases not possible.

Although systems are known in which the duration of the interruption is reduced by the system is calibrated only once at the beginning of a measurement process with the two standards and then after a predetermined period of time, for example one hour, changes in the system parameters are compensated by that within the measuring device additional surfaces, one each for black and one for white, are temporally successively pivoted into the Meßlichtstrahlengang and serve as reference surfaces.

However, measuring processes must also be interrupted for the swiveling in and out of the white and black areas and thus can not run continuously.

In addition, it can not be ruled out in the measuring devices known from the prior art that light reflected from the sample surface irradiates the sensors, thereby preventing exact calibration.

On this basis, the invention has the object, an arrangement of the type mentioned in such a way that a more efficient determination of the reflectance of samples or sample surfaces is guaranteed than in the prior art. This object is achieved with an arrangement comprising: a) a light source for separate illumination of both a white surface and a measuring surface, wherein

a white standard, a black standard and the sample surface are provided for the embodiment of the measuring surface, and

b) means for measuring the intensity of the light reflected from the white surface and for measuring the intensity of the light reflected from the measuring surface, c) an evaluation circuit, configured to register the measured intensity values and their computational link to the reflectance.

This is advantageously achieved that the calibration during a measuring process as often as desired and in any predetermined periods of time can be made without significantly disturbing the measurement process, as will be explained below with reference to exemplary embodiments.

Advantageously, the white surface is formed diffusely reflecting, and the light source, the white surface and the means for intensity measurement are enclosed on all sides by the housing of a measuring head, whereas the measuring surface is arranged outside the measuring head. In this regard, the housing of the measuring head has an area which is transparent to the light emanating from the light source and to the light reflected from the measuring area. Because of the integration of the white surface in the housing enclosed by the measuring head is below also the term internal white area used.

At least one optoelectronic transducer is provided as a means for measuring the intensity, which is referred to hereinafter as a detector in connection with the invention described herein, and are optical fibers each with upstream coupling optics for detecting and transmitting light which is reflected at the internal white area, and of light reflected at the measuring surface to the detector.

In a preferred refinement of the arrangement according to the invention, a plurality of coupling-in optics with a downstream optical waveguide are arranged radially symmetrically with respect to the measuring surface for detecting light reflected at the measuring surface. Thus, the influence of structures on the result of the intensity measurement is reduced because the measurement is not only based on the light that passes from only one direction of reflection to the detector. The influence of structures is better compensated the more coupling positions are positioned around the measuring surface.

A first shutter is arranged in the transmission path of the light which is reflected from the measuring surface and passes to the detector, and a second shutter is arranged in the transmission path of the light which is reflected by the internal white surface and reaches the detector. The two shutters are provided for blocking or release of the respective transmission path and designed accordingly. The measurement of intensity values depending on the blocking or release of the transmission paths is provided as follows:

With :

Iw the intensity of the light reflected by the white standard,

Iwi the intensity of the light reflected from the internal white area,

I _D intensity with unlit detector surface,

I ₃ the intensity of the light reflected by the black standard, and

Ip the intensity of the light reflected from the sample.

From the intensity values thus determined, the determination of a corrected reflectance R _{P is provided} in the manner explained below on an example.

It is particularly advantageous if the first white standard area is inclined to the direction of propagation of the light reflected by the measuring area in such a way that this light does not strike the internal white area. This ensures that the result of the measurement of the intensity I _w of light reflected from the internal white surface can not be falsified by light reflected from the measurement surface.

Furthermore, it is advantageous if the internal white surface is of annular design and a plurality of coupling optics are positioned on a centrally arranged circumference, which are each connected via optical waveguides and a shutter to a detector, the detectors are sensitive to different wavelength ranges.

With this embodiment, it is possible to use the arrangement according to the invention for an extremely broad wavelength range of the illuminating light directed at a sample. For example, three detectors may be provided, one of which is sensitive to the visible light wavelength range (VIS), a second to the near infrared wavelength range (NIR) and the third to the ultraviolet light wavelength range (UV).

It is also particularly advantageous to provide a light source which emits light with a spectral-isotropic intensity distribution. This can be formed, for example, as a reflector lamp.

In the simple case, the detector may be designed as a photodiode and the evaluation circuit for registering and linking integral intensity values.

Higher accuracy can be achieved, however, if the detector is part of a spectrometer and a having spatially resolving receiving surface. In this case, the spectrometer can be equipped with two light entry gaps, wherein the transmission of light reflected at the internal white surface to a first entrance slit of the spectrometer and the transmission of light reflected at the measuring surface to the other entrance slit of the spectrometer.

Within the spectrometer, the light incident through the two entrance slits is directed onto the receiving surface, and the evaluation circuit is designed to register and link spectrally resolved intensity values.

In a likewise preferred embodiment of the arrangement according to the invention, the propagation direction of the incident of the light source on the measuring surface light with the normal N _{M of} the measuring surface forms an angle α> 0, and the propagation direction of the light coming from the measuring surface for coupling optics closes with the normal of Measuring surface an angle γ = α + ß with the angle ß> 0 a.

Thus, the spatial beam intensity distribution, for example, a reflector lamp is exploited in such a way that the axial and radial dependencies on the sample or sample surface resulting irradiance in the largest possible range of the working distance, that is the distance between the light source and the measuring surface, each other compensate, and so a reflection measured value is obtained, which is largely independent of the working distance. The invention will be explained in more detail with reference to an embodiment. In the accompanying drawings show

Fig.l is an illustration of the basic structure of the inventive arrangement,

2 shows a plan view of the arrangement according to Fig.l,

3 shows the representation of advantageous propagation directions of the light striking the measuring surface from the light source and the light reaching from the measuring surface to the coupling optical light relative to the normal of the measuring surface,

4 shows an example of the arrangement of a shutter in a realized by means of optical waveguide transmission path of the measuring or reference light to the spectrometer,

5 shows an example of the arrangement of several coupling optics radially symmetrical to the measuring surface,

6 shows a timing diagram for some measured quantities to illustrate the principle of internal referencing.

Fig.l shows a built-in a measuring head 1 reflector lamp 2, of which a first radiation component 3 is directed through a Meßkopffenster 4 on a sample holder 5.

The sample holder 5 is provided and designed to receive a white standard 6, a black standard 7 and a sample 8 for which the reflectance R _{P is to be} determined. White standard 6, black standard 7 and sample 8 can be positioned on the sample holder 5 and are interchangeable there in a given sequence. Within the measuring head 1, a second radiation component 9 of the light coming from the reflector lamp 2 is directed to a scattered reflective surface designed as a measuring standard of another white standard, referred to below as an internal white surface 10.

Furthermore, 1 optical waveguide 11, 12, 13 and 14 are provided within the measuring head. The optical waveguide 11 is preceded by a coupling-in optical system 15 which is positioned so that it receives the radiation reflected by the internal white surface 10 and couples it into the optical waveguide 11. The light coupled into the optical waveguide 11 by means of the coupling optical system 15 passes to the light entry side of a shutter 16, the light exit side of which is optically connected to the optical waveguide 12. The optical waveguide 12 is in communication with a first entrance slit 17 of a spectrometer 18.

The optical waveguide 13 is preceded by a coupling optics 19, which is provided for collecting light and is formed, which is reflected by a measuring surface, either from the located on the sample holder 5 white standard 6, black standard 7 or the surface of the sample 8 and by the Meßkopffenster 4 enters the measuring head 1 inside.

The coupled from the coupling optics 19 in the optical waveguide 13 light is propagated within the optical waveguide 13 to the light entrance side of a shutter 20 and passes from the light exit side of the shutter 20 in the optical waveguide 14. The optical waveguide 14 terminates in a second entrance slit 21 of the spectrometer 18th Thus, the coupling optics 15, optical fibers 11, shutter 16 and optical fibers 12 form a transmission path for light to the spectrometer 18, which is reflected by the internal white surface 10, and the coupling optics 19, the optical waveguide 13, the shutter 12 and the optical waveguide 14 form a transmission path for reflected light from the measuring surface to the spectrometer 18th

Within the spectrometer 18 is the spatially resolving receiving surface 22 of a detector to which both the spectrum of light entering through the entrance slit 17 and the spectrum of the light entering through the entrance slit 21 strikes.

The signal outputs of the receiving surface 22 and the control inputs of the shutters 16 and 20 are in communication with an evaluation circuit (not shown in the drawing) for registering intensity values for light reflected at the internal white area 10 and at the measurement area, i. is formed on the white standard 6, on the black standard 7 or on the sample 8 reflected light and the mathematical combination of these intensity values.

The measurement of the intensity values is carried out as a function of the blocking or release of the transmission paths as follows, wherein in the exemplary embodiment of spectrally measured, dependent on the wavelength intensities is to be assumed:

With :

I _w the intensity of the light reflected by the white standard 6,

I _Wl the intensity of the light reflected from the internal white surface 10,

I _D intensity with unlit detector surface,

I ₃ the intensity of the light reflected by the black standard 7, and

Ip the intensity of the light reflected from the sample 8.

The following applies to the reflected intensities:

I _W = I- (R _{F +} R _W. [\ - R _F ] ² ) ₊ I _D

I _S = I- (R _F + R _S - [IR _F ] ² ) + I _D

I _P = I- (R _{F +} R _P - [\ - R _F ] ² ) ₊ I _D

With

I: intensity of the radiation component 3 to the external

Measuring surface, I ₂ : intensity of the radiation component 9 to the internal

White surface 10,

R _N : reflectance of white standard 6, R _Wl : reflectance of internal white 10, R ₃ : reflectance of black standard 7, Rp: reflectance of the sample 8,

R _F : reflectance of Meßkopffensters 4,

Measurement sequence and determination of the reflectance are provided as an example as follows:

At a time tθ at the beginning of a measuring process an output calibration is first carried out based on the measurement surface as used in a predetermined sequence white standard 6 and black standards 7 by the intensity values I _w, I _Wl, _s are measured I ₀ and I.

The measurements take place at time tθ and at all subsequent times t> tθ uniformly with the integration time it = min (it _e , It ₁ ), where it _e and iti the integration times for fully controlled signals I _w and I _Wl at time tθ the output calibration are.

The intensity values at time tθ of the output calibration are calculated together as follows:

Calculation of a difference D _ws (t0):

D _ws (t θ) = I _w (t θ) -l _s (t θ) = I (to) - [I _{R F} ] ² - (R _w - R _s ) Calculate a difference D _Wl (t0):

Dw, (O) = l _Wl (f0) - I _D (tθ) = I ₁ (tθ) ^• R _Wl Calculation of a difference D _s (t0):

D _s (t θ) = I _s (t θ) - I _D (t θ) = I (to) ■ (R _F + R _s ^■ [l - R _F ] ² )

Due to the difference formation, the intensity I _{0 of} the unlit detector surface is calculated out of the measured intensities I _w , Is and I _Wl . The calculated differences D _ws , D ₅ and D _Wl o are retained until the next external calibration.

The output calibration is successfully completed when the intensities remitted by the corresponding external standards 6 and 7 and by the internal white area 10 have been measured as intensity values I _W1 , I _D , Is and I _w with the integration time it.

In the course of the subsequent long-term measurement of the sample material, an internal referencing for the purpose of recalibration is carried out at predetermined time intervals Δt in order to compensate for the change in system parameters and thereby obtain the long-term stability.

For this purpose, only the intensity values I _W1 (t) and I _D (t) and the intensity value I _P (t) at the times t = t0 + Δt are measured on the basis of the sample 8.

The intensity values are calculated as follows: Calculation of a difference D _W i (t):

D _W , (0 = i _m (0-Λ> (0 = IM) Rw,

Calculation of a difference D _P (t): D _P (t) = I _P (t) -I _D (t) = W- (R _F + R _P (t) - [\ -R _F ] ² )

While the calculated differences D _ws , D ₅ and D _Wl o are maintained until the next external calibration, the calculated differences D _Wl (t) and D _P (t) are updated at each time t> tθ.

After each internal referencing, the quotient _{display is} updated.

It describes the relative change in sensitivity and measurement intensity, which is the same at the internal and external measurement location.

The recalibration is successfully completed when the current values I _W1 (t), I0 (t) have been measured and introduced into the calculation of the result value R _P (t) according to the formula below:

The differences thus determined and the quotient q (t) of the internal referencing are computed with one another at each time t> tθ in the quotient Q (t): _Dp (t) q (t) - D _SQ

Q (O =

D WSO

The reflectance R _P (t) of the sample 8 or sample surface at time t results from the quotient of the measured values Q (t) and the certified values of the white or black standards R _w and Rs used in the initial calibration:

If non-certified standards are used, R _w = 1 and R ₃ = 0 must be assumed. The measured reflectivities Rp (t) are then valid only with respect to the specific instances of the standards R _w and R _s and not independent of them.

To illustrate the principle of internal referencing, FIG. 6 shows by way of example a time diagram for a few values.

From Fig.l is further seen that the internal white surface 10 with the measuring surface forms an angle δ, which ensures that the internal white surface is inclined to the propagation direction of the reflected light from the measuring surface so that this light does not hit this internal white surface can. In Fig.l this fact is indicated by a dashed line 23.

The internal white surface 10 is formed annularly on the inside of a truncated cone, which is centered to the propagation direction of the reflector lamp 2 for measuring head window 4 or to the sample holder 5 is directed. This can be seen in connection with Figure 2, a plan view in the direction D from Fig.l on the truncated cone.

2 further shows a circumference 24 arranged concentrically thereon, on which - optionally in a special embodiment of the arrangement according to the invention - in addition to the already described coupling optics 15 and 19, to the optical waveguides 11, 12, 13, 14, to the shutters 16 and 20 and to the spectrometer 18 further, not provided with separate reference numerals coupling optical waveguide and shutter and connected in the same manner as already described with spectrometers, the additional spectrometers are sensitive to different wavelength ranges.

As already explained, it is possible with this embodiment of the arrangement according to the invention to determine the reflectance for a very broad wavelength range of the illuminating light directed onto the sample 8, such as VIS, NIR or UV.

Reference is made to FIG. 3 to a likewise advantageous embodiment of the arrangement according to the invention. This embodiment has the purpose of exploiting the spatial beam intensity distribution of the reflector lamp 2 used here, for example, in such a way that compensate the axial and radial dependencies of the irradiation intensity resulting on the surface of the sample 8 in the largest possible range of the working distance z and so the to be determined reflection measurement R _P from the working distance z is largely independent. This is achieved by the propagation direction of the incident of the reflector lamp 2 to the respective measuring surface light with the normal N _{M of} the measuring surface forms an angle α> 16 °, and the propagation direction of the light coming from the measuring surface for coupling light with the normal N _M of Measuring surface an angle γ = α + ß with the

Angle β> 4 °, where the apex of the angle γ is at z = 100 mm.

As a coupling optics 19, for example, a lens with a focal length of f = 5 mm can be provided.

An integration of the shutter 16, 20 in the optical waveguide 11, 12 and 13, 14 is shown in Figure 4 by way of example with reference to the shutter 16. In this case, from the coupling optics 15 to the shutter 16, an optical waveguide bundle with a diameter of 1 mm and a numerical aperture NA = 0.22, and from the shutter 16 to the entrance slit 17 an optical waveguide 12 with a diameter of 0.6 mm and a numerical aperture NA = 0.37.

5 shows, with a view from the reflector lamp 2 on the circular Meßkopffenster 4 an example of the arrangement of several coupling optics radially symmetrical to the direction of irradiation of the light on the measuring surface. The other coupling optics, like the coupling optics 19, are each followed by an optical waveguide in which the measuring surface is reflected by the measuring head window 4 into the measuring head 1 and deflected by the coupling optics collected light is first forwarded to the shutter 20, from where it via the common optical waveguide 14 and the entrance slit 21 on the receiving surface 22 passes.

Of course, embodiments are included in the inventive concept in which the spectrometer has only one entrance slit, the light paths are brought together in front of the spectrometer and then the light passes through the one entrance slit and the respective spectrum is imaged on the receiving surface 22.

A particular advantage of the arrangement according to the invention is that it can be used both for measuring the direct reflection from a sample surface and for measuring the scattered reflection of a sample.

LIST OF REFERENCE NUMBERS

1 measuring head

2 reflector lamp

3 radiation component

4 measuring heads

5 sample holder

6 white standard

7 black standard

8 sample

9 radiation component

10 internal white area

11, 12,

13, 14 optical fibers

15 Einkoppeloptik

16 shutters

17 entrance gap

18 spectrometers

19 coupling optics

20 shutters

21 entrance gap

22 reception area

23 dashed line

24 perimeter

working distance

Claims

claims

An arrangement for measuring the reflectance on a sample (8), comprising, a) a light source for separately illuminating both a white area (10) and a measuring area, wherein

- For the embodiment of the measuring surface, a white standard (6), a black standard (7) and the sample (8) are provided, and

in a predetermined sequence the exchange of the white standard (6), the black standard (7) and the sample (8) is provided for each other, b) means for measuring the intensity of the light reflected from the white surface (10) and for measuring the intensity of the white each reflected light from the measuring surface, and c) an evaluation circuit, which is designed to register the measured intensity values and their computational link to the reflectance.

2. Arrangement according to claim 1, wherein at least the white surface (10) is formed diffusely reflecting.

3. Arrangement according to claim 1 or 2, wherein the light source, the white surface and the intensity measuring means are enclosed on all sides by the housing of a measuring head (1), whereas the measuring surface outside the measuring head (1) is arranged.

4. Arrangement according to claim 3, wherein the housing of the measuring head (1) has a region in the form of a Meßkopffen- Sters (4), which is transparent to the emanating from the light source and the reflected light from the measuring surface.

5. Arrangement according to one of the preceding claims, wherein

- Is provided as means for measuring the intensity of at least one optoelectronic detector, and

- Optical waveguide (11, 13), each with upstream coupling optics (15, 19) for detecting and transmitting light reflected at the white surface (10) and light reflected at the measuring surface to the detector are present.

6. Arrangement according to claim 5, wherein a plurality of coupling optics are arranged with a downstream optical waveguide radially symmetrical to the measuring surface for the detection of light reflected on the measuring surface.

7. Arrangement according to claim 5 or 6, wherein

- In the transmission of reflected light on the measuring surface to the detector, a first shutter (20) is arranged, and

- In the transmission of light reflected at the white surface (10) light to the detector, a second shutter (16) is arranged, and

- The two shutters (16, 20) are designed to block or release the respective transmission path.

8. Arrangement according to claim 7, wherein the measurement of intensity values in dependence on the blocking or release of the transmission paths is provided as follows:

With :

I _w the intensity of the light reflected from the white standard (6),

Iwi the intensity of the light reflected from the white surface (10),

I ₀ intensity with unlit detector surface,

I ₃ the intensity of the light reflected from the black level (7), and

Ip the intensity of the light reflected from the sample (8).

Arrangement according to claim 7, wherein on the basis of the intensity values I _w , Iwi, I _D and I ₃ an initial calibration and then at predetermined time intervals tΔ based on the intensity values Iwi, ID and the intensity value I _P based on a sample measurement internal referencing is provided for the purpose the recalibration of the arrangement and thereby to compensate for changes in system parameters.

10. Arrangement according to one of claims 5 to 9, wherein from the respective coupling optics (15, 19) to the shutter (16, 20) an optical waveguide bundle (11, 13) with a diameter of 1 mm and a numerical aperture NA = 0 , 22 is provided, and

- From the respective shutter (16, 20) to the detector, an optical waveguide (12, 14) with a diameter of 0.6 mm and a numerical aperture NA = 0.37 is provided.

11. Arrangement according to one of the preceding claims, wherein the white surface (10) to the propagation direction of the reflected light from the measuring surface is inclined so that this light does not hit the white surface (10).

12. Arrangement according to one of the preceding claims, wherein

- The white surface (10) is annular, and

- On a centrally arranged circumferentially (24) a plurality of coupling optics are positioned, which are each connected via optical waveguides and a shutter to a detector, wherein

- The detectors are sensitive to different wavelength ranges.

13. Arrangement according to claim 12, with three detectors, one for the wavelength range of the visually perceptible light (VIS), a second for the near infrared wavelength range (NIR) and the third for the wavelength range of ultraviolet light (UV) is sensitive.

14. Arrangement according to one of the preceding claims with a light source which emits light with spectral-isotropic intensity distribution, preferably designed as a reflector lamp (2).

15. Arrangement according to one of the preceding claims, wherein the detector is designed as a photodiode and the evaluation circuit for registering and linking integral intensity values.

16. Arrangement according to one of claims 1 to 14, wherein

- The detector is part of a spectrometer (18) and a spatially resolving receiving surface (22), the transmission of the white surface (10) reflected light to a first entrance slit (17) of the spectrometer (18), and the transmission of reflected light from the measuring surface to a second entrance slit (21) of the spectrometer (18) is provided, wherein within the spectrometer (18) through the two entrance slits (17, 21) incident light is directed to the receiving surface (22), and the evaluation circuit for registration and linkage spectrally resolved intensity values is formed.

17. Arrangement according to one of the preceding claims, wherein the propagation direction of the light striking the measuring surface from the light source with the normal N _{M of} the measuring surface encloses an angle α> 0, and

- The propagation direction of the light reaching from the measuring surface to the coupling optics with the normal of the measuring surface an angle γ = α + ß with the angle ß> 0 includes.

18. Arrangement according to claim 17, wherein

the distance z between the light source and the measuring surface is in the range z = 100 mm to 200 mm,

- the angle α = 16 °,

the angle β = 4 °,

the vertex of the angle γ is z = 100 mm, and

- Is provided as a coupling lens, a lens with a focal length of f = 5 mm.