光応用計測研究室

　Currently, optical metrology is widely used in nondestructive monitoring, medicine, and biology. Digital holography (DH) is one of the fields of Optical Metrology. DH can be used to measure the shape, deformation, and displacement of objects. Our laboratory is studying on DH and aims to develop new measurement and data processing methods that can be used not only in the laboratory but also in the production field.

　As shown in the figure, the laser beam is divided into two optical paths by a beam splitter, one of which is irradiated to the object to be recorded, and the other is used as a reference light. The interference fringes obtained by the superposition of these two beams are recorded by a CCD, and when numerical calculations are performed on a computer, the phase and amplitude of the light reflected by the object are reconstructed. This method is called Digital Holography (DH) because it uses holograms as digital data. DH can be used to measure the shape, deformation, and displacement of objects.

　Our laboratory is conducting research on the application of Digatal Holography (DH), which enables non-contact 3D measurement of objects, to industrial measurement. Here we introduce some of the research we are studying in our laboratory.

　We have been developing and researching the method to evaluate the drying state of paints by applying DH. Reconstructed images are obtained from holograms that record coating films as measurement targets. The drying state of films can be quantitatively evaluated in a non-contact manner by monitoring the amount of displacement of films from the phase difference obtained from the reconstructed images.

　Based on this technology, we jointly developed with a company the "Cure Tester", a device that can evaluate the drying and curing state of paints and adhesives. Nowadays, we are conducting research to evaluate changes in the refractive index of paints and the misalignment of optical components bonded with UV adhesives, as well as the drying and curing states.

　In this research, we proposed a multiple-wavelength method inspecting the innner wall such as piping fixtures in factories and power plants with a digital holographic endoscope. In this method, a conical mirror is inserted into the pipe for measurement. The holographic reproduced images recorded at multiple wavelengths are used to visualize and measure the shape of the inner wall of the pipe. Furthermore, by using multiple wavelengths for recording holograms, the measurement range can be adjusted and spectral and color image information can be obtained.

　In figure, the inner wall of a 14 mm diameter copper pipe was scanned and photographed. Cracks, protrusions, aluminum foil and rust can be seen on the inner wall of the tube. Objects with a height of 0.2 mm can be identified. By decreasing the interval between scanning steps, a sharper reproduced image can be obtained. Thus, it is possible to measure the type and location of scratches, thickness of foreign matter, etc.

　In this research,enables non-contact, highly accurate measurement for safe operation of facilities and detection of foreign objects in confined space. In the future, we will reduce errors and automate color adjustment to achieve even higher accuracy in measurement.

　FMCW technology detects the beat frequency corresponding to the time delay between the reference and the object waves, which are frequency modulated, to measure the distance from the detector to the object.
　We have introduced frequency-modulated continuous wave (FMCW) technology into digital holography (DH). The technique can obtain a hologram including the amplitude and phase information of the object wave from the detected beat frequency. In addition, by multiplexing the beat frequency in the time-frequency domain, a required hologram can be selectively extracted from a single hologram set. We have applied the technique to several applications, such as selective imaging of multiple objects and polarization analysis.