Reflectometer employing an integrating sphere and lens-mirror concentrator

A reflectometer wherein a light having a predetermined wavelength is projected into an integrating sphere containing a sample whose reflectance is to be measured. As the projected light strikes the sample, rays are reflected back the walls of the integrating sphere; some of which strike the area within the field-of-view of a concentrator. The rays striking within the field-of-view of the concentrator are focused upon a detector element which allows one to determine the reflectance of the sample. A control means is utilized to control the wavelength of the projected light through a spectrophotometer, and calculate the reflectance of the sample.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention in general describes a reflectometer and in particular a 
reflectometer employing an integrating sphere and lens mirror 
concentrator. 
2. Description of Related Art 
Integrating spheres are used to measure diffuse reflectance and diffuse 
transmittance (also known as directional hemispherical 
reflectance/transmittance). In the visible spectrum, the integrating 
sphere coatings usually consist of a white diffuser (e.g., MgF, 
BASO.sub.4, or "Halon.RTM."). While for the infrared spectrum, an aluminum 
surface is frequently sandblasted and then plated with gold. 
One of the chief disadvantages of integrating spheres, especially in the 
infrared spectrum, is that their throughput is low. Low throughputs lead 
to unacceptable signal-to-noise ratios when measuring samples with low 
reflectances or transmittances (e.g., &lt;1%). This is also related to the 
detector field-of-view 
Recently, in Snail and Hanssen, Integrating Sphere designs with isotropic 
throughput, Appl. Opt. Vol 28, No. 10, May 15. 1989, pp. 1793-1799, a 
family of three reflectometer designs using nonimaging concentrators to 
restrict FOV of an integrating sphere's detector, with no concomitant loss 
in signal, were described. The designs exhibited a uniform throughput over 
the hemisphere above the sample, but each design had limitations. The 
first of the described designs is felt to be impractically long for small 
detector fields-of-view, whereas the second design, which utilized a 
compound elliptic concentrator (CEC) to view exactly one-half of the 
sphere, is very sensitive to misalignment errors with specular samples. 
The third design, which uses an inverted compound parabolic concentrator 
(CPC), is felt to be impracticable with dewared detectors in the infrared 
spectrum and also exhibits a nonuniform throughput in the visible light 
spectrum with a non-dewared detector if the reflectance of the CPC mirror 
is not sufficiently high (e.g., &gt;95%). 
SUMMARY OF THE INVENTION 
The object of this invention is to provide a reflectometer that is compact, 
has a small detector field-of-view and a high, uniform, throughput. 
Additional objects, advantages and novel features of the invention will be 
set forth in part in the description which follows, and in part will 
become apparent to those skilled in the art upon examination of the 
following or may be learned by practice of the invention. The objects and 
advantages may be realized and attained by means of the instrumentalities 
and combinations particularly pointed out in the appended claims. 
According to the present invention, the foregoing and other objects are 
attained by projecting a light of a predetermined wavelength into an 
integrating sphere onto a sample whose reflectance is to be measured. As 
the projected light strikes the sample, rays are reflected off the sample 
and strike the walls of the integrating sphere where they are again 
reflected. Some of the reflected rays strike the wall within the 
field-of-view of a concentrator. The concentrator collects the rays that 
enter and focuses them upon a detector which measures the amount of 
reflected light. 
A computer controls the wavelength of the light to be projected into the 
integrating sphere through a spectrophotometer and initially projects the 
light onto a standard material and then upon the sample whose reflectance 
is to be measured. The control means determines the measured reflectance 
of the reference material and computes a calibration factor to be applied 
to calculate the reflectance of the sample.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to the drawings in which like numerals represent like 
elements throughout the views, FIGS. 1, 2 and 2a represent a reflectometer 
10 employing an integrating sphere 38 and a lens-mirror concentrator 28. 
In the preferred embodiment of the reflectometer 10, FIG. 1, a light source 
12 generates a light beam 13, either an infrared (IR) (i.e.,a Nernst 
Glower or Glow Bar manufactured by Ayers Engineering and Manufacturing Co. 
of Ramona, Calif.), ultraviolet (UV) (i.e., deuterium lamp) or visible 
(i.e., Quartz-Halogen lamp) , which is projected into a spectrophotometer 
14 which measures photometrically the wavelength range of the light beam 
13 and radiates a desired wavelength 15 through the port 22 onto a sample 
26 whose reflectivity is to be measured by a detector 28 in an integrating 
sphere 36, such as, a plasma gold-type sphere, such as that manufactured 
by Labsphere, N. Sutton, N.H. 
The spectrophotometer 14 may be either a dispersive type which utilizes a 
grating that selects one specific wavelength of light to be projected , 
such as, a 0.25 meter Digikrom 240, manufactured by CVI Corp. of Tucson 
Ariz., or an interferometer capable of handling a range of wavelengths, 
the most common being a Michelsen-type interferometer manufactured by 
Nicolet of Madison, Wis. The wavelength to be projected onto the sample 26 
is selected by a control means 18, such as a computer, which analyzes the 
signal 29 from the detector 28, stores it within its memory and steps the 
spectrophotometer 14 to another wavelength to be measured, or in the case 
of the interferometer, translates the moving mirror. These steps are 
continued until a predetermined wavelength spread has been measured, 
whereupon, the control means analyzes the data obtained and displays the 
analyzed data by a display means 19, i.e., a plotter or visual display, in 
a form desired by the operator. 
Referring to FIG. 2, the beam 15 enters the integrating sphere 36 through 
the port 22, striking the sample 26 and is reflected back in all 
directions to the wall of the sphere 38 and into the FOV of the detector 
28. A typical path of the reflected light beam 15a strikes the sphere wall 
38a and is again reflected in all directions. 
The inner surface of a sphere for the visible and near infrared spectrum 
has a coating 37 of powdered Teflon.RTM.-like material, preferably 
Halon.RTM., with a Lambertian quality For the infrared spectrum, gold (Au) 
39, on top of a sandblasted aluminum (Al) or plasma-sprayed Al, is a 
reflector surface, as shown in FIG. 2a. Surfaces with a Lambertian quality 
have the directional characteristic of distributing the reflected light 
uniformly over the entire sphere's inner surface. Therefore, wherever the 
light reflected from the sample 26 strikes a point on the sphere wall it 
is evenly distributed over the entire sphere surface. However, the 
interior of the sphere 36 may be coated with any material that allows for 
near perfect diffusion in the visible spectrum, e.g., all diffuse 
coatings, such as barium sulfate (BASO.sub.4). 
The atmosphere within the integrating sphere 36 should be either a vacuum 
or an infrared inactive or non-absorbing gas 16, i.e., argon (Ar), oxygen 
(O), nitrogen (N), etc. The non-absorbing gas 16 is preferred over 
atmospheres that contain water vapor or carbon dioxide (CO.sub.2) and may 
be injected into the integrating sphere 36 either directly or through the 
spectrophotometer 14. Such a non-absorbing gas is required in the infrared 
spectrum since certain absorptive bands will be present in water vapor or 
CO.sub.2 that will produce erroneous reflectivity measurements. 
A baffle 42 shields that portion of the sphere wall viewed by a detector 28 
from direct reflections from the sample, this area of the sphere 36 having 
a higher throughput to the detector 28. If the first reflection from the 
sample 26 were to strike the sphere 36 wall in the area of the 
field-of-view of the detector 28 then this would provide an erroneous 
reading of the reflectivity of the sample 26. The portion of the sphere 36 
wall shielded from the first reflections off the sample 28 is 
characterized by the cap radius, F, and forms the field-of-view (FOV) of 
the detector 28. The baffle is normally elliptical in shape and as thin as 
possible, preferably (&lt;1 mm thick) and supported from the inner wall of 
the sphere 38 by a wire 46. However, other shapes such as circles may be 
utilized. A highly reflective material should coat the baffle 42, 
preferably one having either a specular (i.e., Au mirror) or diffused 
(i.e.,sandblasted Al) characteristic. A preferred coating would be the 
same material used to coat the interior surface of the sphere--a 
Lambertian material having a bi-directional distribution function (BRDF) 
of 1/.pi.. The design rules for the placement of the baffle 42 are (1) 
that it should not be placed so as to enter the beam path of the light 15 
entering the sphere 36 and striking the sample 26 and (2) the baffle 
should not extend into the field-of-view of the detector 28. For a given 
sphere diameter, beam port 15 size and sample port 26a, there is a maximum 
field-of-view which still permits the placement of a baffle 42 in a great 
circle according to these criteria. This is illustrated in FIG. 2. In some 
instances it may be desirable to have two baffles 42, the first as stated 
above to shield the sphere wall, F, from reflections emanating from the 
sample test 26 and a second to shield the sphere wall from reflections 
emanating from the reference sample 32. 
After striking the sphere 36 wall a secondary reflection 15b, is generated 
which again strikes the sphere wall within the cap radius, F, at point 
F.sub.v an is again reflected along a path 15c into the concentrator 45 
through a port 44. Port 44 is a double convex infrared lens, such as lens 
manufactured by Janos Technology, Inc. of Townshend, Vt. A low refractive 
index lens material, such as KCl, is preferred in order to reduce the 
interface reflectance, thereby broadening the concentrator's acceptance 
angle. The concentrator-lens 15 may be one of two types utilizing 
hyperbolic mirrors; a first type is a trumpet-type having a convex 
hyperbolic mirror. A second type is the compound hyperbolic concentrator 
(CHC)-type 45 having a concave hyperboloid mirror which provides a 
one-bounce solution for meridional rays. A meridional ray being a ray that 
is in a plane containing the optic axis of the concentrator 15. These two 
types of concentrators are commonly called non-imaging concentrators and 
both types will provide equally satisfactory results. Under certain 
conditions, a planar cone is also possible, See, M. Collares-Peireira et 
al., Lens-mirror combinations with maximal concentration, Appl. Opt. Vol. 
16, No. 10. pp. 2677-2683. Oct. 1977. 
The CHC lens and trumpet-lens restrict the detector's 28 field-of-view 
(FOV) to the cap radius, F. The non-imaging concentrators are preferably 
made by the process of diamond turning metal optics, however, they may 
also be fabricated with an electroforming process such as used by Infrared 
Laboratories, Tucson, Ariz., and others; all of which will perform 
satisfactorily. 
In the preferred embodiment 20, a CHC type concentrator 45 having a 
hyperbolic mirror, such as that manufactured by Texas Instruments, Dallas, 
Tex., is utilized. The reflected light beam 15c passing through the port 
44 strikes the concentrator 45 and a reflected meridional ray is projected 
onto the detector 28. A meridional ray after one reflection within the 
concentrator 45 strikes the detector 28. However, a second type of ray, 
the skew ray may also be present. A skew ray is a ray that is not in a 
plane containing the optic axis of the concentrator 45. With the skew ray 
there are a multiple of reflections of the beam within the concentrator 45 
prior to intercepting the detector 28. The skew rays effect decreases the 
throughput of the lens-concentrator system slightly near the acceptance 
angle. The acceptance angle in this embodiment being 10.degree.. 
The detector 28, nominally mercury-cadmium-telluride (HgCdTe), for use in 
the infrared spectrum, such as that manufactured by Belov Technology of 
New Brunswick, N.J., measures the intensity of the reflected light which 
is recorded in the control means 18. In the visible light region a 
photomultiplier tube, such as those manufactured by Hamamatsu Photonics 
Systems Corp. of Bridgewater, N.J. (e.g., Model R928), or solid state 
detectors of the silicon photodiode type, such as those manufactured by EG 
& G of Sunnyvale, Calif., may be selected for use as detectors 28. 
However, in the near infrared region, from 0.8 microns to about 2.5 
microns, a lead sulfide detector element, such as that manufactured by 
Opto Electronics-Textron of Petaluma, Calif., may be used. However, 
depending upon the wavelength to be measure, detectors 28 may be made of 
different materials. 
A standard reference material (SRM) 32 is used to calibrate reflectometers 
in the visible and near infrared spectrums. The light beam 15 entering the 
sphere 36 through port 22 is directed onto a reference sample 32 which is 
a material of known reflectivity. The SRM 32 may be either a metal (i.e., 
gold) of a specular characteristic for the near UV to about 2.5 microns, 
or ceramic (black or white) material with a diffuse characteristic for use 
from 300 nm in the UV region to 2-2 1/2 microns in the I range. By 
comparing the standard reflectance of the SAM 32 to the reading of 
reflectance obtained by the reflectometer 10, a calibration factor for the 
reflectometer is obtained. (Currently, Sam for wavelengths &gt;2.5 microns 
are available from the National Physical Laboratory in the United 
Kingdom.) 
A second correction factor may be included in the measured reflectance of 
the sample 26 to compensate for secondary reflections--those striking the 
sample 26 and being reflected off it again and striking the cap radius, F, 
a second time to be measured by the detector 28. This factor can be 
determined numerically and such computation is well known to those skilled 
in the art. However, this factor is, in practice, small and it is possible 
to obtain accurate measurements without this secondary reflection 
correction factor. A correction for light lost out the beam port 22 can 
also be made by adding a second detector (not shown) and beam splitter 
(not shown) in the beam path 15 at the beam port 
If it is desired, to shrink the cap radius, F, below that determined to be 
optimum, as shown here, the length of the concentrator (especially 
compound elliptic concentrators) extends out further and could becomes as 
long as two sphere diameters. Although this makes a very unwieldy 
structure, in some circumstances the researcher may feel that this 
disadvantage is overcome by the results received. Compound Elliptic 
Concentrators (CEC) tend to be even longer than CHCs. 
Currently, the infrared (I) limits of the CHC-lens system will be set by 
diffraction effects. Typically, as the wavelength of light starts to get 
comparable to the smaller aperture diameter of the CHC, diffraction 
effects will start to be noticeable. In the CHC-lens system shown in the 
preferred embodiment, this limit would be in the mm-wave regime. 
Practically, the technological limit of measurable wavelength for the 
infrared region is 25 microns because of the current state-of-the-art of 
the components of the invention, However, the concept of the invention may 
be extended to any wavelength, at least 100 microns, with the development 
of suitable components in the future. 
When probing reflectance measurements into the ultraviolet spectrum, the 
coating of the sphere 38 becomes more critical. In the ultraviolet 
spectrum, the limits are determined by the reflectivity of the metals 
coating the inside of the sphere 38. This reflectivity of metals in the UV 
range tends to fall off very rapidly below 4000 .ANG.. (SEE, F. A. Jenkins 
& H. E. White, Fundamentals of Optics, McGraw-Hill Book Co., New York, 
N.Y., 1976, pg. 536). Currently, Halon.RTM. meets the requirements down to 
around 200-300 nanometers. The principles set forth on the coupling of the 
detector 28 set forth in this specification would apply to wavelengths 
considerably higher in the spectrum and any limitations would depend upon 
the composition of the coating applied on the sphere 38 interior. 
An advantage of the concentrator system is that it has a very high 
throughput when compared to the existing collimator-based systems (1-50x). 
Throughput being defined as the ratio of the amount of light that actually 
makes it to the detector (detector power) to the amount of light that 
actually enters the sphere (beam power), typically less than one percent. 
The light entering the sphere has large quantities lost through the beam 
port and by absorption in the walls of the sphere and the sample. However, 
the advantages of the integrating sphere far outweigh this low value of 
throughput which in some cases lowers the signal-to-noise ratio of the 
reflectometer, a fundamental shortcoming of the integrating sphere.