Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1998-08-27
2001-02-06
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S339070, C250S339090
Reexamination Certificate
active
06184528
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to methods of evaluating substrate surface conditions of a coated substrate, and more particularly to a method of collecting and comparing infrared reflectance signatures within a spectral wavelength range of substrates having coatings having a non-zero transmittance within spectral wavelength range.
BACKGROUND OF THE INVENTION
Scatterometers provide a simple, nondestructive monitoring and evaluation technique to determine the surface microstructure of a sample. Generally, a scatterometer facilitates measurement of the directional energy distribution of radiation reflected from a sample surface. For example, if the sample surface is perfectly mat then the reflected radiation is diffuse, i.e., equal in all directions, whereas if the sample surface is not perfectly mat then the reflected radiation has a specular component, i.e., more concentrated in certain directions. This technique is useful in many areas of technology where observation of surface characteristics is evaluated.
Substrate materials having visually opaque coatings are often subject to physical/chemical changes. For example, substrate material may be susceptible to corrosion or heat damage which alter the substrate surface characteristic. The substrate coating, however, prevents a simple visual inspection of the nature of the underlying substrate surface.
It is therefore evident that there exists a need in the art for a method to evaluate substrate surface conditions of coated substrates where such method is nondestructive and is relatively simple to perform.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a spectral nondestructive method for evaluating substrate surface characteristics of a sample substrate. The sample substrate has a sample substrate surface and a generally visually nontransmissive sample coating disposed on the sample substrate surface. The sample coating is transmissive within a first infrared spectral wavelength range and the sample substrate is reflective within the first infrared spectral wavelength range. The method begins with directing infrared radiation from an infrared radiation source towards the coated sample substrate. Specular and diffuse infrared radiation reflected from the coated sample substrate is collected. The reflected radiation is measured as a function of wavelength in the first infrared spectral wavelength range to obtain measured reflectance data representative of the reflectance of the coated sample substrate. The measured reflectance data is compared to reference reflectance data representative of a sample substrate surface having a known physical characteristic within the first wavelength range to obtain differential data. The differential data is correlated to physical characteristics of the sample substrate surface.
In the preferred embodiment of the present invention, the first infrared spectral wavelength range is from 2 to 7 microns. The collection of the reflected infrared radiation may be facilitated by the use of an integrating sphere. In addition, a portable Fourier transform infrared spectrometer may be used to measure the reflectance of the coated sample. The reference reflectance data is representative of a coated reference substrate which is formed of similar material as that of the sample substrate and coating. Additional data may be collected and measured in a second infrared spectral wavelength range to account for differences in the reflectance characteristics of the sample coating in comparison the reference coating. In addition, the physical characteristics may correspond to corrosion or heat damage.
In another embodiment of the present invention, the reflectance of the sample substrate surface is calculated within the first infrared spectral wavelength range using the measured reflectance data to obtain measured substrate data. Such measured substrate data is compared with reference substrate data within the first infrared spectral wavelength range. The reference substrate data may be representative of a reference substrate formed of similar material as that of the sample substrate.
As such, based on the foregoing, the present invention mitigates the inefficiencies and limitations associated with prior art methods evaluating substrate surface conditions. The present invention is particularly adapted to facilitate evaluation and testing of subsurface characteristics of coated substrates. For example, a painted metal substrate is susceptible to corrosion (e.g., rust). It is recognized that corrosion changes the substrate surface reflectance characteristics. While the paint prevents light transmission in the visible range, the present method facilitates inspection substrate surface through the paint within a first infrared spectral wavelength range (i.e., an infrared evaluation window).
Importantly, the present method is nondestructive and non-contacting. In this respect, the method is purely spectral in nature and does not alter the physical integrity of the sample being evaluated. Advantageously, the present method may utilize commercially available devices, e.g., integrating spheres and Fourier transform infrared spectrometers.
Accordingly, the present invention represents a significant advance in the art.
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Casagrande Louis Gregory
Clarke James A.
DiMarzio Don
Silberstein Robert P.
Gabor Otilia
Hannaher Constantine
Stetina Brunda Garred & Brucker
Vought Aircraft Industries, Inc.
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