Publications and other reference materials referred to herein are numerically referenced in the following text and respectively grouped in the appended Bibliography which immediately precedes the claims.
In many biomedical diagnostics and industrial inspection methodologies there is a need for a number of separated narrow spectral bands of the illuminating light and several polarization orientations. This first methodology is called multispectral imaging or multispectral diagnostics technique. The second methodology is called polarimetric imaging. The combination of the two methodologies is called by the inventor multi-spectral polarimetric imaging (MSPI).
An important application of MSPI is in optical spectropolarimetric scatterometry (OSIS) where the object is scattering and the scattered light is monitored rather than the specular reflection. In the semiconductor industry, periodic patterns are produced on the wafer as marks for monitoring the fabrication process. One of the techniques being used to measure the parameters of this periodic structure is called optical scatterometry. Optical scatterometry is used in the inspection of the fabrication processes of optical surfaces because light scattered from the surface is an indication of the degree of roughness of the surface. One of the inventions on the subject was by the present inventor [1]. Since the sample is periodic, in these patents the diffracted beams are collected and measured versus the wavelength using a spectrometer, more specifically the zero order diffraction is collected. Two measurement modes were proposed, one that uses fixed polarizers, while the other uses a rotating polarizer in order to extract the ellipsometric parameters of the scattered light.
Another important application of MSPI is in the field of biomedical imaging. Bio-tissue is usually scattering with strong polarization and wavelength dependence. For example when linearly polarized light illuminates the skin, part of it is backscattered by dermal layers and rapidly depolarized by birefringent collagen fibers [2]. The birefringence produces phase retardation between light polarized along the collagen fibers and the light polarized perpendicular to their long axis. The properties of the scattered light are therefore polarization dependent and as the scattering increases the chances that a photon loses its original polarization state are high. As the photon penetrates deeper into the tissue it will experience more and more scattering events, hence the depolarization depends on the penetration depth. The penetration depth and amount of depolarization depends on the wavelength of the incident light. Hence, there is strong polarization and wavelength dependence of light polarized from the skin. It is possible to distinguish such backscattered light from the total diffusely reflected light that is dominated by light penetrating deeply into the dermis by means of the different polarimetric spectral imaging and polarized spectroscopy.
In order to apply the MSPI method to industrial and biological applications such as those described above it is necessary to employ a compact tunable element that can select each wavelength in a narrowband (<20 nm full width at half maximum (FWHM)) and fast speed (<30 msec) sequentially and to be able to control its polarization state.
Mechanisms of polarized light scattering from different tissues and tissue phantoms are well established now, based on in vitro studies. Parameters such as depolarization depth (DD), retardance, and birefringence have been studied both theoretically and experimentally. Polarized light traveling through different tissues (skin, muscle, and liver) depolarizes after a few hundred microns. Highly birefringent materials such as skin (DD=300 μm at 696 nm) and muscle (DD=370 μm at 696 nm) depolarizes light faster than less birefringent materials such as liver tissue (DD=700 μm at 696 nm).
In a simplified manner one can distinguish between two components of linearly polarized light scattered from the skin. The first, which maintains the polarization of the incident light, is the regular (specular) reflection that comes predominately from the surface of the skin. The second component undergoes multiple scattering from the various skin layers, and is therefore depolarized. Hence, using polarizers, one of the components can be eliminated, and consequently enhances either superficial topography (wrinkles, fine lines, pores) or subsurface structures (pigmentation, erythema, infiltrates, vessels) of the skin. The wavelength dependence of the DD provides another degree of freedom to modify and study. The research group of the present inventor has built a spectropolarimetric module that either uses two wavelengths with two polarization states or multiple incident linear polarizations at two different wavelengths [3,4,5,6]. However in that module the analyzer was fixed and the illumination was hitting the tissue asymmetrically.
It is therefore a purpose of the present invention to provide symmetric illumination, a compact tunable element that can select each wavelength in a narrowband and fast speed sequentially and to control its polarization state both in the input and at the output.
It is another purpose of the present invention to provide imaging apparatus that comprises the tunable element.
It is another purpose of the present invention to provide diagnostic and inspection methods of using the apparatus comprising the compact tunable element in industrial and biological applications.
Further purposes and advantages of this invention will appear as the description proceeds.