Patent Application
20080024767
If the lenses 33, 35 in the object beam path 26 and the reference beam path 27 are identical, the light in the reference beam path and the object beam path traverses exactly the same distance, through identical refractive material, so that the geometrical displacement of the measurement focus in the object beam path 27 is exactly equal to the change in optical length in the reference beam path 26. This can also be achieved with imaging lenses 33 and 35 that are different, by introducing a compensation plate 36 in one or both of the two beam paths, so that in total the same thickness and the same type of refractive material is traversed. This ensures that the geometrical displacement of the measurement focus in the object beam path 26 is exactly equal to the change in optical length in the reference beam path 27.
The problem of synchronizing the motion of object imaging lens 33 and reference mirror 34 is solved by fixing the position of the reference mirror 34, the object imaging lens 33, the reference imaging lens 35, and preferably also the compensation plate 36, in relation to each other. The resulting unit is then moved along the optical axis by one single electromechanical scanner/actuator, as illustrated by the double arrow.
In a preferred embodiment said optical elements 33, 34, 35, 36 are arranged in one single, exchangeable cartridge 32. Since all optical elements (imaging lenses 33 and 35, compensation plates 36) that need to be exchanged to obtain a different optical magnification are placed in one single cartridge 32, the optical magnification of the imaging POCT system according to the invention can be quickly and simply changed by exchanging a cartridge with a first magnification level with another cartridge with a second magnification. No other element of the optical system must be changed, and no time-consuming and complicated readjustments are necessary.
The light reflected back from the focus plane 11 into the object beam 26 and the reflected light traveling back in the reference beam 27 is subsequently recombined by beam splitter 24, and enters the detection beam path 25, where it is imaged by a detector imaging lens 29 onto the surface of the two-dimensional OCT image sensor 28. The individual pixel elements of the sensor 28 are individually capable of demodulating the received OCT signal. Such an OCT image sensor is disclosed, for example, in EP 1458087. An aperture 30 can be employed to optimize the fringe contrast in the sensor plane, as a function of wavelength, focal distance and pixel size. Depending on the reflectance of the object 31 more or less light is reflected back into the beam splitter.
To correct low reflectance from the object, a neutral density filter 38 can be arranged in the reference beam path 27, reducing the amount of light returning from the reference mirror 34, and enhancing the contrast detected in the detection beam path 25.
In a further advantageous embodiment it would also be possible to arrange a compensation plate in the object beam, which would correct for the differences of the reference imaging lens and the object imaging lens. This approach allows for the realization on an exchangeable cartridge containing only the object imaging lens, which must be exchanged anyway, and the corresponding compensation plate. Such a simplified exchangeable cartridge would be part of the optical unit that is linearly moved along the optical axis by the single electromechanical scanner/actuator.
A second embodiment of a pOCT instrument according to the present invention, with a synchronized, complementary high-resolution image acquisition system, is shown in
The functional principle of this second embodiment, particularly the whole interferometer part, can be essentially identical to the first embodiment shown in
If no additional illumination of the object other than from the low-coherence light source 41 is used, the beam splitting element 56 should be a beam splitter. If, however, an additional light source is employed for lighting the object, with a spectral range different to the low-coherence light source 41, a dichroic mirror is preferable. A suitable dichroic mirror 56 will let the low-coherence light part pass to the OCT image sensor 45, and part of the detection beam having other wavelengths will be deflected to the high-resolution image sensor 57.
The image acquisition process with the high-resolution photosensor 57 is preferably synchronized with the OCT volumetric image acquisition using the OCT image sensor 45. As a consequence it must be known for each high-resolution image taken with photo sensor 57, from which object focus plane 48 it has been taken, i.e. which object depth plane was in focus at the time of image acquisition. This allows, for example, fusing the OCT images with the high-resolution images, and forming highly resolved volumetric images with additional information such as the local color. If a particular object has been identified, for example, in the OCT depth image, then the corresponding high-resolution black-and-white or color image can be retrieved, in which this particular object can be inspected with much higher lateral resolution, and with additional information such as color.
Another preferred embodiment of the imaging pOCT apparatus according to the present invention is shown in
This embodiment essentially performs the same function as the embodiment disclosed in
As detailed above, the aperture 47 is employed to optimize the fringe contrast in the sensor plane, as a function of wavelength, focal distance and pixel size. In the shown embodiment it influences also the amount of light impinging on the high-resolution photo sensor 57.
If no additional lighting other than the low-coherence light source 41 is used, then the reflective element 60 should be a beam splitter. If an additional light source is used, emitting light in other spectral ranges than the low-coherence light source 41, then a dichroic mirror is preferable.
In yet a further embodiment a plane deflection mirror is arranged in the object path instead of the reference path, so that the object beam is deflected by 90° to a direction parallel to the reference path. It is also possible to use mirrors in both beam paths, for example deflecting both beams by 45°, in order to obtain parallel beams.
This concept can be varied in other ways. The remaining requirement is that both the reference beam and the object beam are parallel prior to focusing them on the reference mirror respectively the object focus plane, since this will allow for the synchronous linear movement of both elements with one single linear actuator.
| Number | Date | Country | |
|---|---|---|---|
| 60833810 | Jul 2006 | US |
