Title: Method and apparatus for analysis of schlieren
Abstract: The method for evaluating schlieren in glassy or crystalline optical materials includes irradiating a test sample of the optical material with light and producing a shadow image of the test sample on a projection screen. The shadow image of the test sample is received in an electronic image receiving device, such as a digital camera, and is compared with another shadow image of schlieren obtained with a comparison sample by means of interferometry. Then the optical material of the test sample is evaluated with the help of the comparison results.
Patent Number: 6,891,980 Issued on 05/10/2005 to Gerhard, et al.
| Inventors:
|
Gerhard; Michael (Aalen, DE);
Lentes; Frank-Thomas (Bingen, DE);
Kusch; Christian (Jena, DE);
Singer; Wolfgang (Aalen, DE);
Moersen; Ewald (Mainz, DE)
|
| Assignee:
|
Schott Glas (Mainz, DE);
Carl Zeiss SMT AG (Oberkochen, DE)
|
| Appl. No.:
|
090975 |
| Filed:
|
March 5, 2002 |
Foreign Application Priority Data
| Mar 09, 2001[DE] | 101 11 450 |
| Current U.S. Class: |
382/321; 382/312 |
| Intern'l Class: |
G06K 007/10 |
| Field of Search: |
382/141,312,321
315/383,382
348/744,810
356/129,237.2
313/452,453
|
References Cited [Referenced By]
U.S. Patent Documents
| 4121247 | Oct., 1978 | Henry.
| |
| 5291102 | Mar., 1994 | Washburn.
| |
| 5399947 | Mar., 1995 | Washburn.
| |
| 5583632 | Dec., 1996 | Haga.
| |
| 5585691 | Dec., 1996 | Washburn.
| |
| 5694479 | Dec., 1997 | Guering et al.
| |
| 5764345 | Jun., 1998 | Fladd et al.
| |
| 5988645 | Nov., 1999 | Downing.
| |
| Foreign Patent Documents |
| 405181166 | Jul., 1993 | JP.
| |
| WO0091/02429 | Feb., 1991 | WO.
| |
Primary Examiner: Do; Anh Hong
Attorney, Agent or Firm: Striker; Michael J.
Claims
1. A method of analysis of schlieren, said method comprising the steps of:
a) irradiating a test sample with light from a light source;
b) producing a shadow image of the test sample on a projection screen;
c) receiving the shadow image of the test sample projected on the projection
screen in an electronic image receiving device;
d) processing the shadow image received in the electronic image receiving device
to measure schlieren image contrast; and
e) comparing said schlieren image contrast measured in the shadow image of the
test sample with schlieren image contrast of a shadow image of a schlieren pattern
of a comparison sample and evaluating the schlieren of the test sample by means
of the comparing.
2. The method as defined in claim 1, wherein said schlieren pattern of said comparison
sample is measured interferometrically to obtain interferometric measurements and
further comprising calibrating said schlieren image contrast of the test sample
with said interferometric measurements.
3. The method as defined in claim 2, wherein the comparison sample and the test
sample are made from identical optical materials.
4. The method as defined in claim 1, wherein the comparison sample is made from
a different material than that of the test sample and the comparison sample made
from said different material is calibrated with another comparison sample made
of another material that is identical to said optical material of the test sample.
5. The method as defined in claim 1, further comprising calibrating a synthetic
schlieren plate comprising said comparison sample, which has an artificial schlieren
with a positive phase shift in stages of about 5 nm to about 50 nm and widths of
about 0.1 mm to about 0.5 mm at a wavelength of 550 nm.
6. The method as defined in claim 1, further comprising tilting and rotating
said test sample in a plurality of directions in relation to an optical axis of
a measurement device for the processing of the shadow image.
7. The method as defined in claim 3, wherein said optical materials each comprise
a crystalline or a glassy material.
8. The method as defined in claim 4, wherein said different material and said
optical material each comprise crystalline or glassy material.
9. The method as defined in 4, wherein said optical materials each or said different
material consists of crystalline material and said crystalline material is calcium
fluoride or barium fluoride.
10. The method as defined in claim 1, wherein the electronic image receiving
device is a digital camera.
11. An apparatus of analysis of schlieren, said apparatus comprising:
means for irradiating a test sample with light from a light source, said means
for irradiating including a sample holder and the light source;
means for producing a shadow image of the test sample on a projection screen,
said means for producing the shadow image including the projection screen;
means for receiving the shadow image of the test sample projected on the projection
screen in an electronic image receiving device;
means for processing the shadow image received in the electronic image receiving
device to measure schlieren image contrast, said means for processing the shadow
image being connected electronically with said means for receiving the shadow image;
means for comparing the measured schlieren image contrast measured in the shadow
image of the test sample with schlieren image contrast of a shadow image of a schlieren
pattern of a comparison sample and evaluating the schlieren of the test sample
by means of the comparing.
12. The apparatus as defined in claim 11, wherein the shadow image of the comparison
sample or a synthetic schlieren plate comprising the comparison sample is stored
in the electronic image processing device.
13. The apparatus as defined in claim 11, wherein the electronic image receiving
device comprises a digital camera.
14. A process for evaluating an optical material for the manufacture of lenses,
prisms, light conductive rods, optical windows and optical components for DUV photolithography,
steppers, lasers including Excimer lasers, wafers, computer chips, integrated circuits
and electronic devices including integrated circuits, said process comprising a
method of analysis of schlieren of said optical material, said method comprising
the steps of:
a) irradiating a test sample of said optical material with light from a light
source;
b) producing a shadow image of the test sample on a projection screen;
c) receiving the shadow image of the test sample projected on the projection
screen in an electronic image receiving device;
d) processing the shadow image received in the electronic image receiving device
to measure schlieren image contrast; and
e) comparing the measured schlieren image contrast measured in the shadow image
of the test sample with schlieren image contrast of a shadow image of a schlieren
pattern of a comparison sample and evaluating the schlieren of the test sample
by means of the comparing.
15. The process as defined in claim 14, wherein the electronic image receiving
device is a digital camera.
16. The process as defined in claim 14, wherein the comparison sample is made
from a different material than that of the test sample and the comparison sample
made from said different material is calibrated with another comparison sample
made of another material that is identical to that of the test sample.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for evaluation of schlieren
in optical materials by means of shadow methods.
2. Description of the Related Art
Schlieren, according to conventional speech usage, are bounded regions
in optical materials, which act optically because of local changes in refractive
index and are mostly visible in an image or picture in the form of filaments, strips
and bands. Nearly all optical materials do not have an index of refraction that
is exactly constant throughout, but instead it varies within a certain range. Schlieren
are defined as optical inhomogeneities of small structural width and a high refractive
index gradient.
A variety of different methods are known for detection of schlieren in optical
materials. Practically all these methods are based on detecting changes of the
optical wave front after passing through the sample to be tested and analyzed.
Interferometry, Töpler methods and shadow methods are the most widely used methods.
Changes in the wave front are detected directly by means of interferometry
and Töpler methods; however surface imperfections and faults on the test sample,
such as surfaces that are not completely planar, effect the measurement directly.
These measurements involve great effort and expense because of the high requirements
for surface uniformity of the test sample.
Usually commercially obtained interferometers are not ready for schlieren
measurements; i.e. if schlieren must be measured with an interferometer, it must
be constructed for that purpose. Thus it is important that spatial resolution of
the interferometer is sufficient so that fine schlieren structures can be detected,
whose size is in the micron range.
To detect schlieren it is preferably to use shadow methods, because they are
comparatively
sensitive and can be performed when the surface quality of the samples is similar
to that of typical optical materials.
In shadow methods the optical material is either between a light source and the
eye of an observer and the shadow casting schlierens are established by moving
and tilting the sample (MIL-G-174A and similar standards), or the sample is irradiated
with light and the schlieren contained in the sample are projected as shadows (DDR
Professional Standard TGL 21790, similar ISO standard is widely distributed).
Also DIN 3140, Part 3, concerns schlieren, however in practice has only a limited significance.
Since schlieren are spatial formations, the methods described in Standard TGL
21790 and also in DIN 3140 attempt to characterize the extent of the schlieren
by an effective schlieren surface or area. Known procedures have the disadvantage
that they depend strongly on the subjective evaluation of the observer. In Standard
TGL 21790 definite schlieren comparisons are derived from measurement of visibility
threshold. However the comparison of the schlieren image with a test pattern occurs
in the known methods only by eye and thus depends on the subjective observation
powers of the observer to some extent.
In the conventional procedure the shadow image of the respective test sample
is
copied onto a piece of paper, the width and the intensity of the individual shadow
lines is subjectively evaluated and the evaluation or analysis is registered on
the sketch.
Furthermore the above-described standard is exclusively related to glass
and glass-like substances. However increasingly crystalline materials are used
for optical components, especially for wavelengths, which are outside of the visible
range, which means substantially below about 400 nm and above about 800 nm. Thus,
for example, there is an increasing demand for monocrystalline materials made of
alkali and alkaline earth fluorides (CaF
2, BaF
2, SrF
2,
among others) for UV applications, such as UV lithography or lenses and widows
for irradiation and imaging apparatuses. Crystals provide the basis for many optical
elements in the IR spectral range.
Glasses and crystals differ by their respective disorderly and orderly structures.
Schlieren in crystals can have entirely different causes than schlieren in glass.
The activity of schlieren in crystals depends, among other things, very strongly
on the position and orientation of the inhomogeneities producing the schlieren
(e.g. grain boundaries). In crystals schlieren can be produced by band-shaped structures
of greater width, but of only reduced thickness (e.g. displacements, small angle
grain boundaries). The methods developed for glasses are not suitable to analyze
substrates with schlieren of this type permeating them with the required accuracy.
Also the test or comparison schlieren samples used for glasses are not usable
for crystals because of the completely different mechanisms for producing schlieren
in crystals. A coating in the known comparative schlieren test plate developed
for testing optical glass simulates phase discontinuities of different thicknesses
and widths. However one such two-dimensional schlieren test plate is not suitable
for representing the action of schlieren in crystals.
Especially during testing for the present invention the phase deviation
or shift has the opposite sign in single crystalline materials from that in glasses.
The schlieren test and comparison plate known up to now however usually is made
from an insensitive material, for example quartz or quartz glass. Different structures
are provided in this plate, e.g. by means of a thin coating and a photo mask. Usually
cavities or openings of e.g. 0.2 mm width and 1 cm length are provided in a 10
nm coating, whereby a phase shift is produced when a wave front passes through.
In principle, it is also possible to produce a phase shift by application of raised
regions instead of cavities or openings, whereby the sign of the shift changes.
The subjective intensity and width of the lines in the shadow image evaluated with
the help of the comparison or test plate however has no reliable significance for
test samples made from crystalline material, since a change of the light wave front
is caused by a change in the index of refraction. It has been found in the scope
of the invention that the results obtained by comparison with known schlieren test
plates with the shadow methods for crystals are mostly unusable and unreliable.
In practice they produce no relevant conclusions regarding the quality of the crystalline
optical elements.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and an apparatus
for objective analysis and evaluation of schlieren by means of shadow methods,
which is suitable not only for optical glass materials but also for crystalline materials.
Furthermore it is another object of the present invention to provide
a method, which is independent of the currently used subjective methods for analysis
and evaluation of schlieren by means of shadow methods.
It is also an object of the present invention to provide an apparatus for performing
these methods for analysis and evaluation of schlieren.
The method for analysis or evaluation of schlieren according to the invention
comprises the steps of:
a) irradiating a test sample with light from a light source;
b) producing a shadow image of the test sample on a projection screen;
c) receiving the shadow image of the test sample projected on the projection
screen
in an electronic image receiving device, preferably a digital camera;
d) processing the shadow image of the test sample received in the electronic
image
receiving device to measure the image contrast of the schlieren of the test sample; and
e) comparing the measured image contrast of the shadow image of the test sample
with the image contrasts of a shadow image of the schlieren of a comparison sample
and analyzing or evaluating the schlieren of the test sample according to the comparison results.
The apparatus for performing this method according to the invention includes
the light source for irradiating the test sample, a sample holder for holding the
test sample or the comparison sample, a projection screen and of course the electronic
image receiving device, preferably a digital camera.
According to the invention the shadow formed by the schlieren in a sample
is acquired or received with the help of the digital camera and the contrast of
the shadow image is determined by image processing. Since a direct measurement
of the wave front delay in the material by changing the optical wavelength is not
possible with shadow methods, the image obtained with the shadow methods is standardized
or calibrated by means of structures with known wave front delays. The schlieren
of an arbitrary sample (comparison sample) is measured interferometrically and
correlated with the shadow image or contrast of the respective schlieren. In this
way a simple shadow contrast to be measured may be correlated to the interferometrically
measured schlieren type. This correlation is also designated the primary or original
calibration. As comparison sample preferably pieces of a crystal are used, which
comprise the same material as te optical material of the test sample to be evaluated.
Test samples are especially preferred, which have different schlieren patterns,
such as strong schlieren or weak schlieren.
In other words, the shadow methods may be used in practice to determine the optical
quality of materials, especially crystalline materials, by the calibration performed
by connecting the values obtained with the shadow methods with interferometrically
obtained values.
The present invention thus is based essentially on shadow methods in combination
with a new comparison process in which the comparison does not occur visually,
but by means of electronic image formation.
The schlieren pattern received by the camera is processed digitally and the obtained
digital image is compared with the schlieren pattern from the original or primary
calibration, so that the average wave front deviation can be accurately determined
with interferometric precision. Detection limits of less than 5 nm phase shift
are possible at a wavelength of about 550 nm without more. A digital CCD camera
can be used as the electronic image processing device according to the invention.
The required resolution of the camera for the image processing however depends
on the respective materials to be tested, also on the interferometer. Preferably
it amounts to at least 10 micron/pixel, preferably at least 15 microns/pixel. For
example, 20 microns/pixel are attained with calcium fluoride.
The samples to be evaluated by the method according to the invention and the
apparatus according to the invention do not require expensive interferometric measurements.
An interferometric calibration of the measuring arrangement for shadow images of
the comparison sample made of the same material is required only once. In this
calibration the calibration values obtained by it can be compared later to the
values obtained from the test sample, since it has been shown that the product
of the phase shift squared times the phase width always leads to the same contrast.
Preferably the calibration values of the original or primary calibration are stored
directly in the processor or processing means, so that the calibration data (virtual
schlieren plate) arises, which can be retrieved at any time.
According to the invention the calibration values of the original or primary
calibration are still compared with the shadow cast by a conventional synthetic
schlieren plate. Since the contrast or the established contrast with the camera
also depends on the screen, on which the shadow is produced, and on the focusing
and brightness of the light source, it is possible in a simple manner to test whether
the calibration is always still valid by means of a synthetic schlieren plate.
The electronic image processing of the test sample shadow image and the comparison
sample image permits a quantitative classification of the occurring schlieren,
which does not depend on subjective decisions of an observer. The determination
of the schlieren by means of shadow formation in this way is fast, simple and economical.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
The objects, features and advantages of the invention will now be illustrated
in more detail with the aid of the following description of the preferred embodiments,
with reference to the accompanying figures in which:
FIG. 1 is schematic cross-sectional view through an apparatus for measurement
of schlieren of a test sample by the shadow method according to the invention;
FIG. 2 is a plan view of a shadow image of a synthetic schlieren plate comprising
a comparison sample with artificial schlieren according to the invention; and
FIG. 3 is a plan view of a typical schlieren pattern in an optical crystal.
DETAILED DESCRIPTION OF THE INVENTION
The measurement process according to the shadow method described in the following
detects and evaluates schlieren with interferometric precision in glassy and crystalline
optial materials in the sense of an RMS wave front deformation w
rms.
To classify the sample the value w
rms2 is derived, which
is proportional to the sum of the square of phase shifts times the widths and times
the lengths of the respective individual schlieren, summed over all schlieren,
the sum being divided by the total surface area of the sample, i.e. the value w
rms2
and which is given by formula (1):
wherein κ=proportionality factor;
F=total area of sample,
w=a phase shift,
L=length of the schlieren (in the material),
B=width of the schlieren (in the material).
The image contrast ΔT, which a given optical system provides, is connected
with the RMS wave front deformation w
rms at a given wavelength λ
by the following relationship in the form of formula (2):
Schlieren is required to be detected and evaluated, for example, at a wavelength
of 550 nm with phase shifts of 5 nm or less, especially in crystals used in the
UV spectral range.
The measurement process is designed so that it is suitable for continuous testing
of a production process. Especially an optical material can be tested in the production
process without additional finishing disregarding the required polishing. In order
to detect and evaluate the schlieren the sample of the optical material to be tested
may be rotated and tipped.
An apparatus for detection and evaluation of schlieren is described in the following paragraphs.
The apparatus substantially comprises the optical device shown in FIG. 1 with
a point light source
10, a sample holder
12, a test sample
14
to be tested, a projection screen
16 and a preferably digital camera
18.
Although in the simplified embodiment described in FIG. 1 divergent light is used,
the method and apparatus of the invention also operate with parallel light. In
the measurement process a comparison sample
40 is also arranged in the sample
holder
12. The phase shift pattern measured interferometrically from the
schlieren of known phase shift and known width of a comparison sample is used for
calibration of the shadow image and for comparison with the shadow image of the
test sample
14. A synthetic schlieren plate can be used in place of the
comparison sample
40. The synthetic schlieren plate includes preferably
raised elements or surface portions (bumps) with a positive phase shift in stages
of about 5 nm to about 50 nm and widths of about 0.1 mm to about 0.5 mm at a wavelength
of, for example, 550 nm. The substrate material and the raised elements or surface
portions (synthetic or artificial schlieren) are made from stable material with
the same index of refraction as the bulk material.
Since the phase shift in the comparison sample
40 also depends on the
material, comparison samples which are made of the same material as the test sample
14 are used. Alternatively it is always possible to use other comparison
samples made of different material and to calibrate them with a comparison sample
made from the same optical material as the sample that is tested. The schlieren
pattern of a calibrated schlieren plate calibrated in this manner can be correlated
at any time to the schlieren pattern of a plate or a sample of another material,
such as calcium fluoride or sodium fluoride.
Quartz or quartz glass is preferably used for the schlieren plate, since it
is an insensitive material.
In the calibration the shadow formation and thus the contrast to the synthetic
schlieren plate is determined and subsequently or before that the phase shifts
and phase widths of the respective shadow image of the test sample are measured
by means of interferometry. Since the square of the phase shifts multiplied with
the schlieren widths is proportional to contrast, in this way the contrast may
be correlated with the product comprising the phase shifts and widths. The spatial
resolution of the interferometer required depends on the schlieren widths to be
measured. For example, for schlieren with a width of 0.1 mm a spatial resolution
of 0.01 mm/pixel has proven sufficient. Preferably the spatial resolution per pixel
is at least 5%, preferably at least 8% and especially preferably at least 10% of
the schlieren width per pixel. In many cases an interferometer with a spatial resolution
of 200 nm per pixel or less are used in the method.
The rms values to be measured may then be determined by the sum of the respective
products, each of which is a square of the respective individual phase shift times
the width and times the length of the individual schlieren, the sum being divided
by the total surface area of the sample.
Of course in the method it is not possible to determine the phase shift or the
width of a schlieren independently from the shadow image. However specification
of the optical material is not required. The correlation between contrast and rms
value is only slightly dependent on the width of the schlieren, which is thus sufficient
for a definite schlieren classification.
The schlieren should be measured at the same incidence angle that is used in
the application of the materials measured. The same is true of the angle of emergence.
The visibility of the schlieren in the test sample depends on the angle of incidence.
A change of the incidence angle of the light of about one or two degrees can lead
already to disappearance or conspicuous changes in the shadow image.
The light source
10, the test sample
14 and the projection screen
16 are arranged along the optic axis
20 of the measurement apparatus.
The optic axis
20 extends through the light source
10 and the central
point of the sample
14. The projection screen
16 is perpendicular
to the optic axis
20. The distance d between the light source
10
and the sample
14 amounts to about 2 m and the distance between the test
sample
14 and the projection screen
16 is about 1 m.
The camera
18 is arranged somewhat laterally or to the side of the optic
axis
20, so that the shadow image of the test sample
14 on the projection
screen
16 can be received without the test sample
14 or another element
of the apparatus blocking or extending into the image field of the camera
18.
So that the perspective deformation remains small, the angle between the optic
axis
20 of the measurement apparatus and the optic axis of the camera
18
is kept as small as possible. Also the projection screen
16 can be inclined
to the camera
18 according to choice or arbitrarily.
The sample holder
12 receives the usually disk-shaped sample
14
made of glassy or crystalline optical material. The sample
14 is gripped
in the sample holder
12 so that the surfaces of the sample
14, through
which the optic axis
20 of the measurement apparatus extends, extend perpendicular
to this optic axis. The optical center point of the sample
14 is located
always on the optic axis
20 of the measurement apparatus. The sample
14
can be tilted about ±50 degrees about a perpendicular tilting axis, which
intersects the optical axis
20 at a right angle. Furthermore the sample
14 can be rotated around its center point at least about 90 degrees about
a rotation axis intersecting the optic axis
20.
The surfaces of the test sample
14, through which the optical axis
20
extends, are parallel to each other and polished in the manner usual for optical elements.
The pattern of the synthetic schlieren plate
40 serves for comparison
of the schlieren image of the test sample
14 obtained with the shadow methods
with schlieren of known phase shift and known width.
The shadows of the schlieren of the test sample
14 or the synthetic schlieren
plate
40 are projected on the projection screen
16 in the measurement
apparatus. The sample
14 in the sample holder
12 is illuminated with
a predetermined light cone by a point source
10 of high illumination power.
The illumination intensity on the projection screen
16 similarly amounts
to preferably at least 100 lux. The light cone is preferably formed as a divergent
light beam. Additional optional optical elements, such as a UV blocking filter,
may be arranged in the light path only in the immediate vicinity of the light source
or contingent real image.
After the light beam passes through the sample the shadow image arises as a
phase dependent superposition of the light detracted by the schlieren with the
primary light cone from the light source. The schlieren information is primarily
contained in the first approximately in the phases, whereby only a small component
is amplitude modulated and only this portion is detected without further effort
as an intensity difference.
This very contrast-poor amplitude image is now received as interference-free
as possible with a high signal-to-noise ratio. Moreover it is advantageous to detect
the actual image produced on the projection screen
16, not an image produced
in the air with the help of optics. The projection screen
16 must provide
diffuse light scattering without light-directly elements and must have no detectable
surface structure. The diffuse reflectivity may not vary from place to place on
the screen.
Since the screen image has less than 1% contrast, it is necessary to amplified
the contrast already in the digital camera
18 so that a subsequent digitization
can take place in order to store the image in digital form in a data bank. Furthermore
for this purpose a CCD camera with the highest possible pixel count (for example
1300×1030 pixels) and at least one 12 bit digitization means must be used.
The raw data received with the camera must be processed in order to eliminate
local sensitivity differences of the camera
18 and contingent local variations
on the projection screen
16. It also must be processed to obtain a representation
of the schlieren with high contrast, which is possible by means of conventional
computer programs, like that which is obtained by means of image processing software
with contrast and intensity control or detection together with commercial/CCD cameras.
The processing of the raw data takes places simply and easily by software, which
must be prepared. The schlieren image detection should be able to distinguish between
scratches, naps, surface defects and poor cleaning of the of the disk/optics. Also
schlieren that are smaller than the noise can be detected with suitable contrast.
Principally the image detection is a copy of the cooperation of the eye and the
brain, which means that the schlieren are distinguished as in the case of human
detection, by means of contrast variations. Accordingly a schlieren filter searches
along a line for points or regions with average, dark or bright light intensity.
As shown in FIG. 3, a schlieren is characterized by an elongated shape with a bright
central region and dark edges. In contrast a schlieren-free region is characterized
by an average brightness. A cross-section through a region, which contains schlieren,
thus shows a transition from average brightness (schlieren-free region) to dark
schlieren edge, then to bright schlieren center and again to dark schlieren edge
opposite the first schlieren edge and finally to average brightness. When neighboring
regions that follow each other in a longitudinal direction have this sort of brightness
signature, it can be concluded that a schlieren has been detected. This sort of
software can be optimized in a known manner by means of a plurality of known parameters.
That means that the contrast thresholds between average-dark-bright-dark-average
signatures must be set up and the symmetry of the brightness signatures must be
established. Moreover the number of neighbors having the same signature that establishes
the presence of a schlieren structure must be determined. Similarly the widths
of these type of schlieren varies. A typical schlieren image is shown in FIG.
3.
To control the reception conditions, for calibration and for correlation of the
image contrast of schlieren from a test sample
14 of the optical material
to be tested with schlieren of a predetermined widths and phase shifts the artificial
schlieren plate
40 is put in the sample holder
12. The image of the
schlieren pattern on the projection screen
16 is received in the camera
18. Then the dependence of the schlieren contrast on the phase shift is
measured for the same image scale, offset and magnification or amplification of
the camera.
FIG. 2 shows the shadow image of an artificial schlieren plate
40 obtained
with the above-described measurement apparatus. FIG. 2 also includes the numerical
values of the phase shifts and schlieren widths. The image contrast measured on
the synthetic schlieren plate
40 may be represented as a function of the
phase shifts and the schlieren widths and compared with the image contrast of the
test sample
14 made from the material to be tested.
The comparison occurs in an electronic image processing device connected to the
camera
18. The gray scale values for the measured image contrasts are compared
with each other and a quantitative evaluation of the schlieren in the test sample
14 is derived from that comparison.
It is not necessary to take a new shadow image of the artificial schlieren plate
40 for the same type of test sample
14. The shadow image of the artificial
schlieren plate
40 can be stored electronically in the image processing
device in an already more or less prepared form.
The calibration with the synthetic schlieren plate
40 is thus in principle
only required once, since all further comparisons can be performed in the computer
or processing means. The reason is because the image intensities or contrasts produced
with the artificial schlieren plate are stored in the computer or processing means
as the basis for the comparison. In spite of that it can be important from time
to time to test whether the contrast values or intensities of the image obtained
by the camera with the artificial schlieren plate have changed. The reason for
that can be, for example, a change in the camera focusing, dust and dirt on the
lens of the camera or also an engineering defect. The same goes of course for the
light source, with which the schlieren is produced. Aging of the light source,
the lens and the other components can also cause such changes in the illumination
intensities. In later measurement the artificial schlieren plate
40 is exclusively
adjusted to guarantee that the reference values stored in the computer or processing
means always are valid.
The above-described process and the above-described apparatus are suitable not
only for determination of schlieren in materials for artificial lenses, but also
in materials for other optical elements, for example prisms, cubes, light guides
and the like. Use of calcium fluoride, barium fluoride, strontium fluoride and
sodium fluoride crystals, as well as other crystals, especially large-sized crystals,
is especially preferred.
The invention of course also concerns the use of the apparatus and method according
to the invention for testing materials used to manufacture lenses, prisms, light
guide rods, optical windows and other optical components, such as those for DUV
photolithography, steppers, lasers, especially excimer lasers, wafers, computer
chips, as well as integrated circuits and electronic devices, which have circuitry
and chips.
The disclosure in German Patent Application 101 11 450.8 of Mar. 9, 2001 is incorporated
here by reference. This German Patent Application describes the invention described
hereinabove and claimed in the claims appended hereinbelow and provides the basis
for a claim of priority for the instant invention under 35 U.S.C. 119.
While the invention has been illustrated and described as embodied in a method
and apparatus for analysis of schlieren, it is not intended to be limited to the
details shown, since various modifications and changes may be made without departing
in any way from the spirit of the present invention.
Without further analysis, the foregoing will so fully reveal the gist of
the present invention that others can, by applying current knowledge, readily adapt
it for various applications without omitting features that, from the standpoint
of prior art, fairly constitute essential characteristics of the generic or specific
aspects of this invention.
What is claimed is new and is set forth in the following appended claims.
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