Variable Interference Filter Research Articles

Design of a Variable-Interference Filter (VIF) for Compact High Resolution Wavelength Detectors

A high-resolution and compact wavelength detector capable of resolving very small changes in the spectral information of the incident light is highly desirable for many applications such as Lab-on-Chip systems, biosensors, chip-sized detectors, on-chip fluorescence spectrometry, wavelength shift detectors, interrogations system, etc. In this work we present the simulation and design of a variable interference filter (VIF) constituted by a wedge-shaped Fabry-Perot cavity placed in between two stacks of thin films, forming dielectric reflectors. This device constitutes an optical filter that converts the spectral information in a position dependent signal that can be measured by a photo detector, a PDA or a CCD. We have investigated the performance of the designed VIF depending on its optical properties in order to optimize its geometrical configuration. With the results obtained we intend to fabricate a multi-channel spectral detector that can be used in environmental analysis.

Masking mechanisms applied to thin-film coatings for the manufacturing of linear variable filters for two-dimensional array detectors

We propose a method for manufacturing linear variable interference filters for two-dimensional (2D) array detectors, based on the use of correcting masks combining both rotation and translation movements of the masks and substrates. The major advantage of this method is its capability to produce several identical filters in a single run. 20 mm x 20 mm samples were manufactured with a wavelength ratio almost equal to 2 along the thickness gradient direction. In agreement with calculations, the measured uniformity perpendicular to the gradient is about 99.8% along 20 mm.

Mapping the Polarization Pattern of Plasmon Modes Reveals Nanoparticle Symmetry

Mapping the Polarization Pattern of Plasmon Modes Reveals Nanoparticle Symmetry

How Far Can Ki‐Energy Reach?—A Hypothetical Mechanism for the Generation and Transmission of Ki‐Energy

‘Ki-energy’, which can be enhanced through the practice of Nishino Breathing Method, was reported to have beneficial health effects. Although Ki-energy can play an important role in complementary and alternative medicine (CAM), as yet it is unknown how Ki-energy is generated, transmitted through air and received by another individual. We previously proposed that Ki-energy may include near-infrared radiation, and that the wavelength was between 800 and 2700 nm. Since Ki-energy is reflected by a mirror, we believe that the ‘Ki-beam’ has a small divergence angle. It can also be guided in a desired direction. The acrylic mirror reflection experiment suggests that the wavelength may be between 800 and 1600 nm. Using a linear variable interference filter, we found that Ki-energy may have a peak around 1000 nm. We have also observed that ‘sensitive’ practitioners responded to Ki sent from a distance of 100 m. All of these results suggest that (i) Ki-energy can be guided as a directional ‘beam’ with a small divergence angle; (ii) the beam can be reflected by a mirror and (iii) Ki-energy may have a specific wavelength. Since these properties are characteristics of the laser radiation, we propose a quantum physics-based mechanism of ‘Light Amplification by the Stimulated Emission of Radiation’ (i.e. LASER) for the generation of Ki-energy. Volunteers responded to Ki even with a blindfold. This suggests that the skin must be detecting Ki-energy. We propose that the detector at the skin level may also have the stimulated emission mechanism, which amplifies the weak incident infrared radiation.

The automatic alignment and mosaic of video frames from the variable interference filter imaging spectrometer

The variable interference filter imaging spectrometer (VIFIS), is a modified video‐based remote sensing instrument. The creation of a hyperspectral cube for a flight line from the VIFIS instrument relies on the successful alignment and mosaic of sequential video frames. A number of techniques have been tested to assess which is the most reliable at finding accurately matched positions between sequential frames. Results show correlation techniques to be more successful than ordinal measures and invariant moments. Once a list of matching points had been created the transform between frames can be calculated and a mosaic generated. The application of an affine transform between every frame led to severe distortions in the imagery and rendered the final product useless. Experiments have shown that an affine transform was not required in each case and should only be applied when it provides an improvement over the mapping provided by a lower level transform.

Absorption Spectra of Dye Solutions Measured Using a White Light Thermal Lens Spectrophotometer

We describe a two-beam thermal lens spectrophotometer that uses a xenon lamp as the excitation source and a low power diode pumped Nd-YAG laser as the probe light. The white light from the xenon source is filtered using a variable interference filter producing a partial monochromatic light within the spectral range 400-700 nm and with a spectral resolution of 10 nm. We measure the thermal lens spectrum of a nonfluorescent dye (Malachite Green) and show that this spectrum reproduces its absorbance spectra measured by the usual transmission method. A comparison of the thermal lens and the absorbance spectra of a fluorescent dye (Rhodamine B) reveals substantial differences. These differences can yield important applications of the device for the characterization of fluorescent materials.

The design of broadband, wide-angle interference filters for solar concentrating systems

The optical performance of spectrally selective interference filters for wide-angle solar applications is addressed. A radially variable interference filter is proposed that can maintain acceptable efficiency over its entire surface in solar concentrating systems with a wide field of view. By applying ray tracing methods and a mapping technique, the angle at which the greatest proportion of energy is incident can be determined at as a function of position on the filter surface. The resulting angular distribution is used to design the interference filter with a tapered thickness profile for optimum performance according to the changing mean angle. The method has been applied for the optimisation of a spectral beam splitter in a hybrid photovoltaic/thermal central receiver system. Two types of interference filters are compared: the discrete multilayer and the rugate filter. Results for bandpass and bandstop configurations are presented, and the performance and practical issues associated with each solution are discussed.

The use of the VIFIS (variable interference filter imaging spectrometer) to obtain information on vegetation properties using multiangular data

High resolution multispectral images obtained from different view angles at the field level provide a potentially valuable new approach to the remote sensing of canopy structure as an indicator of water or nutrient stress to the vegetation. Extraction of frequency distributions of radiance from such images gives more information than does the simple bi‐directional reflectance factor which averages the responses for all the surfaces in a large‐area pixel. We describe here a preliminary analysis of this novel use of the Dundee low‐cost Variable Interference Filter Imaging Spectrometer (VIFIS) for this type of study.

Recuperation of infrared stimulated luminescence of feldspars

Abstract After bleaching the optically stimulated luminescence (OSL) signal to a low residual level, the signal has been found to increase during subsequent storage or preheating. This effect is well known in quartz as “recuperation of OSL after bleaching” (Aitken, M.J., Smith, B.W., 1988. Optical dating: recuperation after bleaching. Quat. Sci. Rev. 7. 387–393.). A better understanding of recuperation in feldspars could help the dating specialist, because this process might be different from the recuperation observed in quartz. This paper highlights a few examples of a larger study, which cannot be shown here in complete detail. We carried out a recuperation study of infrared-stimulated luminescence (IRSL) of different feldspars from a mineral collection, mainly alkali feldspars and one albite. The samples were irradiated with doses of 200, 1000 and 2500 Gy in a 60 Co gamma cell. Subsequently, the samples were stored in the dark at room temperature (3 weeks up to 6 months, depending on the applied dose), so that the very intense irradiation-induced phosphorescence can decrease for many orders of magnitude. The emitted OSL was measured through detection filters also used in dating (Schott UG 11, Hoya U 340 for detection of near UV-emissions and Schott BG 39 for detection in the visible range). Recuperation times up to 100 days were used. The recuperated-OSL emissions were measured either with the optical filters mentioned above or with a modified experimental set-up using a variable interference filter with a continuous detection range from 400 to 700 nm. In some feldspars very intense recuperation signals (up to 100% and more of the initial signal) were observed when optical stimulation was performed with IR and broadband detection using the BG 39 or when detection was carried out in the near ultraviolet region. The IRSL emissions at 410 and 560 nm, measured with the interference filter, showed no recuperation despite a clearly detectable first shine-down.

Shallow water bathymetry using integrated airborne multi-spectral remote sensing

An integrated low cost airborne multi-spectral remote sensing system is described and evaluated for remote sensing for shallow water bathymetry. The system consists of: two 35mm motor driven reconnaissance cameras using colour and colour infrared film. Three optically filtered (including removable internal IR cut-off filters), electronically shuttered CCD progressive scan cameras (Sony XC-7500) integrated into an airborne direct digital recording system using a PC processor, a 32-bit RGB analogue to digital conversion card and Zip disk storage. Two CCD based imaging spectrometers providing approximately 10nm bandwidth spectral data across the CCD spectrum (400nm to 1000nm). These CCD cameras were used with a variable interference filter fixed in front of the sensor surface. This provided a 'rainbow' image of the ground varying across the image from 400nm to 700nm (visible) and 700nm to 1000nm. Field studies were undertaken to evaluate the performance of digital multi-spectral (DMV) imagery, supplementary reconnaissance photography (SRP) and VIFIS imaging spectrometry for mapping shallow water bathymetry. The results indicate good performance in shallow water and suggest that with further refinement the system could be used to give a quick comprehensive estimate of shallow water depths

Evaluation of the application potential of a variable interference filter imaging spectrometer for the water environment

Evaluation of the application potential of a variable interference filter imaging spectrometer for the water environment

Multichannel chromatography and on-line spectra from a flame photometric detector

A rotating variable interference filter has been incorporated into a flame photometric detector to acquire and simultaneously display low-resolution spectra that are diagnostic for eluting peaks. In its present form this approach yields ten chromatograms (for ten different wavelength segments) that are stored in the computer for subsequent data manipulation such as baseline correction and curve smoothing; three-dimensional display; peak and noise diagnostics; and magnification, subtraction, or elemental correlation of chromatograms. A third, spectral dimension is thus added to the conventional two, chromatographic dimensions of retention time and analyte intensity. The three-dimensional performance is at present accompanied by an order-of-magnitude loss in detection limit and linear range. Typical one-segment (i.e. 5% dwell time) elemental detectabilities, at S/Np-p = 2, are 7 · 10−13 g P/s (via HPO*), 3 · 10−11 g S/s (S2*), 7 · 10−12 g Ge/s (GeH*) and 4 · 10−12 g Ru/s.

New Instrumentation for Use in Excitation-Emission Fluorescence-Polarization Measurements

A fluorescence excitation-emission matrix (EEM) is an excellent way to determine a specific substance in a multicomponent sample. In the present investigation, a novel, rapid-scanning spectrofluorometer has been developed for measuring an EEM; the instrument employs a 0.35-meter monochromator for choosing the excitation wavelength and a continuously variable interference filter for selecting the emission wavelength. With the system, a 51 × 93 excitation-emission matrix can be collected in 3.2 min, and detection limits of 500 ng/L are obtained for fluorescein dye in ethanol. Fluorescence polarization measurements in the plane parallel and perpendicular to the polarized excitation light adds another dimension to the instrument for characterizing a sample. The instrument is controlled by an IBM XT computer although data analysis is performed on a Macintosh computer with Spyglass software. A series of two-dimensional contour plots and multidimensional excitation-emission-polarization plots can be rapidly generated.

Hadamard Spectrometry for Thermoluminescence Investigations

A Hadamard encoded spectrometer with a special variable interference filter allows attaining high throughput and sensitivity. In our instrument the interference frlter is frxed between condenser lenses. Tle transmitted wavelength changes continuously along the length of the frlter. The multislit cyclic Hadamard mask moves step by step behind the filter to multiplex signal. Some details concerning the data recording, handling and decoding are specific for this technique of investigation of thermoluminescence spectrum. PACS numbers: 07.65.—b, 78.60.Kn

Excitation-Emission Matrix Scanning Spectrofluorometer

A compact rapid-scanning spectrofluorometer specifically designed to acquire an excitation-emission matrix (EEM) has been developed. A conventional grating monochromator is used for selecting the excitation wavelength. The induced fluorescence is monitored with the use of a variable circular interference filter. The EEM is acquired through rapid scanning of the fluorescent wavelength achieved via rotation of the filter in conjunction with slow scanning of the excitation wavelength using a computer. A Xe-arc lamp modulated at 540 Hz is used as an excitation source. The fluorescent signal is then detected by a photomultiplier tube whose output is integrated by a lock-in amplifier. The excitation flux is simultaneously monitored with the use of a quantum counter to compensate for the fluctuation of the lamp intensity. A 26 × 31 element EEM can be acquired within a minimum time of 4 min at a detection limit of 2 μg/L with the use of fluoresceine in ethanol as sample. The performance of our EEM scanning spectrofluorometer is further evaluated for other samples of different components.

Fast, self-compensating spectral-scanning ellipsometer

The advantages of rapid spectral scanning have been combined with the inherent accuracy of the compensating ellipsometer operated in the polarizer-compensator–sample-analyzer configuration. Wavelength is scanned over the visible–UV (370–720 nm) at a maximum rate of 114 nm/s by rotating a continuously variable interference filter. A three-reflection Fresnel rhomb serves as the achromatic quarter-wave compensator. A microcomputer is used to collect spectroscopic measurements, perform instrument calibrations, digital filtering, and data interpretation.

FAST, SELF-NULLING SPECTROSCOPIC ELLIPSOMETER : INSTRUMENTATION AND APPLICATION

The advantages of rapid spectral scanning have been combined with the inherent accuracy of a compensating ellipsometer operated in the polarizer- compensator-sample-analyzer configuration. Wavelength is varied over the visible-UV (370-720 nm) at a maximum rate of 114 nm/s by rotating a continuously- variable interference filter. A three-reflection Fresnel rhomb serves as the achromatic quarter-wave compensator. A microcomputer is used to collect spectroscopic measurements, perform instrument calibrations, digital filtering and interpret data. Wavelength-independent parameters of multiple-film optical models have been determined by treating measurements of delta and psi at different wavelengths as independent measurements. Experimental and predicted ellipsometer measurements are compared by use of statistical techniques for the determination of optimum values and confidence limits of model parameters.

Fiber Optic Spectrophotometry of Color Changes in Cooked Chicken Muscles

Abstract A fiber optic spectrophotometer with a continuously variable interference filter monochromator was used to measure the absorbance spectra of breast and leg meat from chickens. At less than 450 nm, absorbance was very high in both red and white muscles. In each spectrum, an absorbance peak at 560 nm due to myoglobin was detected. These peaks were of greater magnitude in red muscles than in white muscles. Thorough cooking by baking and pressure frying caused a marked drop in absorbance at all visible wavelengths above 500 nm. The absorbance peak of myoglobin was greatly reduced after cooking.

A Visible and Near-Infrared Scanning Photometer for Field Measurements of Spectral Albedo and Irradiance under Polar Conditions

AbstractThe design and operating characteristics are presented for a visible and near-infrared scanning photometer which can measure incident and reflected spectral irradiance from 400 nm to 2 450 nm. The instrument is designed to record over 98% of the solar radiation incident upon and reflected from the Earth’s surface using a turret-type cosine collector and a circular variable interference filter. The apparatus has been tested successfully on Arctic sea ice at temperatures down to –20°C. It is self-contained and easily portable, and its mass is less than 16 kg. Some preliminary results are presented.

A Visible and Near-Infrared Scanning Photometer for Field Measurements of Spectral Albedo and Irradiance under Polar Conditions

AbstractThe design and operating characteristics are presented for a visible and near-infrared scanning photometer which can measure incident and reflected spectral irradiance from 400 nm to 2 450 nm. The instrument is designed to record over 98% of the solar radiation incident upon and reflected from the Earth’s surface using a turret-type cosine collector and a circular variable interference filter. The apparatus has been tested successfully on Arctic sea ice at temperatures down to –20°C. It is self-contained and easily portable, and its mass is less than 16 kg. Some preliminary results are presented.