Search

We have developed optimization methods for generating free-form surfaces. The main applications are in the industrial on-line spectroscopic and machine visions measurements, where spectral and spatial uniformity, good efficiency and pass-line stability are required parameters. We used commercial software for ray-tracing engine. We have developed in-house optimization routines, but used also build-in functions. In this work we have designed a speckle free alternative for laser line generation based on the LED sources. The optical system is based on the collimation part and a free-form mirror. Simulation results are presented.

A viewing system was designed and a prototype realized for the in-vessel inspection of the International Thermonuclear Experimental Reactor. The viewing is based on the line scanning principle, and the system consists of 10 identical units installed on top of the reactor at 36 deg intervals. Each device contains a laser, beam steering mirrors, and viewing probe with insertion mechanics. The probe has an outside diameter of 150 mm and a length of 14 m. The illumination design applies frequency-doubled Nd:YAG lasers whose beams are guided through hermetically sealed windows into the vacuum vessel. The diffuser optics creates a vertically oriented light stripe onto the vessel surface that is viewed by the imaging optics, consisting of 16 modules altogether covering horizontal and vertical field-of-views of 2 and 162 degrees. The optical images are transferred to CCD cameras via coherent fiber arrays. The multi-focus design uses stacked fiber rows whose ends are assembled into different axial positions. The viewing probes rotate at a constant angular speed of 1deg/s and pictures are taken at 0.01 deg intervals. The complete picture of the vessel interior is generated in 6 minutes producing 5.8 *10 exp(9) image pixels. The image processing and analysis of possible defects in the vessel surfaces are performed off-line after the viewing procedure. A full-scale prototype of the viewing probe was constructed to demonstrate the feasibility of the design. Its illumination optics utilizes a diffractive optics element that transforms the collimated input beam into a rectangular output lobe with uniform intensity. The prototype has horizontal and vertical imaging optics field-of-views of 2 and 12 degrees. The test results showed that the prototype can take pictures of good quality applying a continuously rotating probe having an angular speed of 0.08 deg/s. In optimum conditions, the minimum resolvable feature size at 3-m distance is smaller than 1 mm, which satisfies the requirement specification. Further development is needed to increase the illumination power to improve the imaging speed and to develop linear fiber arrays that are compatible with the vacuum and high-flux radiation environment of the primary vacuum vessel.

A CMOS imager-based spectrometer is used to interrogate a network containing a large number of Bragg grating sensors on multiple fibers as part of a proprietary structural health monitoring system. The spectrometer uses a Prism-Grating-Prism (PGP) to spectrally separate serially multiplexed Bragg reflections on a single fiber. As a result, each Bragg grating produces a discrete spot on the CMOS imager that shifts horizontally as the Bragg grating experiences changes in strain or temperature. The reflected wavelength of the Bragg grating can be determined by finding the center of the spot produced. The use of a random addressing CMOS imager enables a flexible sampling rate. Some fibers can be interrogated at a high sampling rate while others can be interrogated at a lower sampling rate. However, the use of a CMOS camera brings several specific problems in terms of signal processing. These include a logarithmic pixel response, a low signal-to-noise ratio, the long pixel time constant, obtaining sufficient process priority for the control program, and proper selection of the window of interest. In this paper we investigate computer algorithms and hardware solutions to address these problems. We also present experimental data to validate these solutions including calibration data and initial field-testing data with 24 sensors on 4 fibers.

Imaging spectrographs have traditionally been utilized in aerial and remote sensing applications. A novel, compact and inexpensive imaging spectrograph developed by VTT Electronics is now available. It contains a multichannel fiber optic sensor head, a dispersive prism-grating-prism (PGP) component and digital CCD matrix camera capable of area integration. In rolled steel manufacturing, a protective oil film is applied on steel to resist corrosion while in transport and storage. The main problems in the oiling machine are film thickness control and jet failures. In this application, the spectrum of fluorescence of an oil film was measured simultaneously with parallel fibers. A relatively simple calibration and analysis procedure was used to calculate the oil film thickness. On-line color control for color reproduction is essential in both consumer and industrial products. The instrument was tested and analyzed for measuring differences in color by multivariate analysis of the spectra and by color space coordinate estimation. In general, a continuous spectrum is not absolute requirement. In these two examples, filter-based measurement would probably cost less thana PGP spectrograph solution. On the other hand, by measuring the spectrum and using an advanced signal processing algorithm one production version will cover all installations in both applications. In practice, only the fiber sensor mechanics need to be modified.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The direct vision, dispersing prism-grating-prism (PGP) component has been invented in VTT Electronics by the author (Finnish Patent No. 90289, EP 93907891.1). The optical construction of the component is illustrated in Figure 1. The PGP is composed of a specially designed volume transmission grating cemented between two symmetrical or nearly symmetrical prisms. Between the grating and the prisms there are optical filters cutting off unwanted wavebands and changing the spectral response. The PGP often contains the aperture stop of the spectrograph optics. The operating principle of the PGP can be seen also in Figure 1. A collimated light beam is dispersed in the PGP so that the central wavelength of the spectrum passes axially through the PGP and the spectrum is formed perpendicular to the optical axis with good spectral linearity. The straight optical axis is the most important advance of the PGP.

This paper presents an imaging spectrometer principle based on a novel prism-grating-prism (POP) element as thedispersive component and advanced camera solutions for on-line applications. The PGP element uses a volume typeholographic plane transmission grating made of dichromated gelatin (DCG). Currently, spectrographs have been realized for 400-1050 nm region but applicable spectral region of the PGP is 380 - 1800 nm. Spectral resolution is typically between1.5 and 5 nm.The on-axis optical configuration and simple rugged tubular optomechanical construction of the spectrograph provide a good image quality and resistance to harsh environmental conditions. Spectrograph optics is designed to beinterfaced to any standard CCD camera. Special camera structures and operating modes can be used for applications requiring on-line data interpretation and process control. 1. INTRODUCTIONImaging spectroscopy is a new versatile and powerful method for simultaneous, efficient measurement of spatial andspectral maps of various two dimensional objects13. In the advanced form of this technology a dispersive stationaryspectrograph module is connected to a matrix detector to form a fixed imaging spectrometer. One dimension of the arraydetector constitutes a spatial line image through the entrance slit and the other dimension is used to register the spectrumfor every line element. In many applications the movement of the object (process stream, web) forms the other spatialdimension of the image automatically. Imaging spectroscopy has emerged mainly due to a rapid evolution of charge-cou-pled devices (CCD), digital signal processing (DSP) components and new imaging spectrographs. By the use of imagingfiber bundles or multiple, distributed fiber systems, multipoint spectroscopy can also be applied to many hazardous or not

This paper describes two new spectroscopic techniques which are utilizing hybrid integrated optoelectronics particularly suitable for field and hand-held use. First, the LED module is based on a linear array of light emitting diodes and a fixed monochromator, and provides a solid-state electrically scanned source for pre-dispersive spectrometers. A prototype module operating from 810 to 1060 nm with resolution of 10 nm scans one spectrum in 19 ms and has a solid glass construction with dimensions of 4 X 4 X 7 cm. Potential applications include miniature, rugged and low cost instruments for transcutaneous blood and tissue spectroscopy in the near infrared (NIR) region.

During the year 1992 a prototype airborne imaging spectrometer is developed in Finland. The instrument is in the first place developed for technology demonstration, performance verification and algorithm development. The first tests of the AISA (Airborne Imaging Spectrometer for different Applications) are performed during the end of 1992 and the beginning of 1993. This paper describes the instrument design concept and will list the first test results. The instrument has 288 spectral channels, a spatial resolution of 384 pixels across track and is through software flexible programmable. Targets during the development have been simplicity and robustness. Applications can be found in forestry, ecology, hydrology, geology, agriculture etc.