Imaging an object may be considered as a collection of points from the plane of the object that are being focused by an optical system onto a collection of points on the plane of an image sensor. When there is a need to obtain spectral information as well as spatial information of the object, there is a fundamental problem, since this task is essentially the need to simultaneously capture a two-dimensional image of the object plane together with the color of each point of the object plane, this being essentially a “third dimension” of the object plane, and to record these three dimensions of information on the two-dimensional plane of the image sensor. A number of proposed solutions have been suggested in the prior art to solve this problem.
One of the possible optical systems may be one that includes an array of pinholes that may be positioned at the focal plane of the light reflected off the object, while the image sensor is located beyond the focal point such that the image acquired is defocused (and would be later focused by appropriate software). The pinhole array is used to differentiate between points from the plane of the object, such that there won't be any overlap of points in the plane of the image sensor. Without the pinholes there is overlap between points on the imager's plane, which would make it practically impossible to correlate between points on the imager's plane to points on the object's plane and thus practically impossible to restore the spatial information of the object.
A filter array comprising sub-filters may be added to the system and may be positioned at the aperture stop, such that spectral information may be acquired by the optical system as well as spatial information. That is, every pixel at the imager's plane has two “coordinates”; one for the angle at which light was reflected off the object, and a second for the sub-filter which the light reflected off the object passed through. However, the main disadvantages of using a pinhole array is losing spatial information, and losing light when collecting the light reflected off the object, since the pinhole array blocks some of the light reflected off the object from being projected onto the imager. One of the groups implementing such an optical system is, for example, the MITRE Corporation, Mclean, Va. (Horstmeyer R., Athale R. and Euliss G. (2009) “Modified light field architecture for reconfigurable multimode imaging”. Proc. of SPIE, Vol. 7468, 746804).
Another possible optical system that may be used to create an image of an object while providing spatial and spectral information is one where instead of a filter array located at the aperture stop, a mask is located at the aperture stop. The mask, according to Wagadarikar et al. a group from Duke University, Durham, N.C., USA, is called a ‘coded aperture’ (A. A. Wagadarikar, N. P. Pitsianis, X. B. Sun and D. J. Brady (2009). “Video rate spectral imaging using a coded aperture snapshot spectral imager.” Optics Express 17(8): 6368-6388). The optical system described in this article does not comprise a pinhole array so there is an overlap between pixels of the image sensor. The mask is random with the requirement of being 50% open for passage of light that is reflected off the imaged object. With this optical system there is minimal loss of spatial resolution, since the scenes that are being imaged do not consist of dramatic spectral changes, and the objects are relatively large so it is not difficult to distinguish between areas of the same spectra.
The mask, according to the above optical system, provides combinations of spatial and spectral “coordinates” that may describe the object. (The “coordinates” are acquired by the imager followed by software reconstruction, in order to focus the acquired images). In areas of the object where the spectrum is substantially similar, only the spatial data is missing. The mask is then used to separate between close points with similar spectrum on the imager's plane, so it would be easier to correlate those points to points on the object's plane. However, when close points on the object have different spectrum (e.g., along the edges of the object) it is more difficult to distinguish between the points projected onto the imager.
Images that provide spatial as well as spectral information may be important within small scale in-vivo imaging devices, e.g., endoscopes and capsule endoscopes. Spatial information is needed in order to determine the in-vivo location of the device, and spectral information of in-vivo tissue is important for determining various diseases at early stages that may be expressed in changes in spectra of various in-vivo particles, e.g., hemoglobin. There is therefore a need for a new optical system that may be implemented into devices that are to be inserted in-vivo, in order to acquire images that contain both spatial and spectral information.