Niko Penttinen

A Wide Spectral Range Reflectance and Luminescence Imaging System

In this study, we introduce a wide spectral range (200–2500 nm) imaging system with a 250 μm minimum spatial resolution, which can be freely modified for a wide range of resolutions and measurement geometries. The system has been tested for reflectance and luminescence measurements, but can also be customized for transmittance measurements. This study includes the performance results of the developed system, as well as examples of spectral images. Discussion of the system relates it to existing systems and methods. The wide range spectral imaging system that has been developed is however highly customizable and has great potential in many practical applications.

Spectral imaging using consumer-level devices and kernel-based regression

Hyperspectral reflectance factor image estimations were performed in the 400-700 nm wavelength range using a portable consumer-level laptop display as an adjustable light source for a trichromatic camera. Targets of interest were ColorChecker Classic samples, Munsell Matte samples, geometrically challenging tempera icon paintings from the turn of the 20th century, and human hands. Measurements and simulations were performed using Nikon D80 RGB camera and Dell Vostro 2520 laptop screen as a light source. Estimations were performed without spectral characteristics of the devices and by emphasizing simplicity for training sets and estimation model optimization. Spectral and color error images are shown for the estimations using line-scanned hyperspectral images as the ground truth. Estimations were performed using kernel-based regression models via a first-degree inhomogeneous polynomial kernel and a Matérn kernel, where in the latter case the median heuristic approach for model optimization and link function for bounded estimation were evaluated. Results suggest modest requirements for a training set and show that all estimation models have markedly improved accuracy with respect to the DE00 color distance (up to 99% for paintings and hands) and the Pearson distance (up to 98% for paintings and 99% for hands) from a weak training set (Digital ColorChecker SG) case when small representative training data were used in the estimation.

Color and Image Characterization of a Three CCD Seven Band Spectral Camera

In this study spectral and spatial characterization of a seven channel FluxData 1665 MS7 three-CCD spectral video camera was performed in terms of the sensor spectral sensitivity, linearity, spatial uniformity, noise and spatial alignment. The results indicate small deviation from ideal linear sensor response. Also, a small spatial non-uniformity and systematic shift exists between the channel images. However, images were observed to have high quality in term of noise. Spectral characterization showed that the sensor has good response in the 380-910 nm region with only some sensitivity limitations in the 715-740 nm range. We also evaluated the spectral reflectance estimation in 400-700 nm range using empirical regression methods and the Digital ColorChecker SG and ColorChecker charts. These experiments resulted in average ΔE00 color accuracy of 1.6 – 2.4 units, depending on the illuminant and estimation method.

Evaluation of complex gonioapparent samples using a bidirectional spectrometer.

Many applications use gonioapparent targets whose appearance depends on irradiation and viewing angles; the strongest effects are provided by light diffraction. These targets, optically variable devices (OVDs), are used in both security and authentication applications. This study introduces a bidirectional spectrometer, which enables to analyze samples with most complex angular and spectral properties. In our work, the spectrometer is evaluated with samples having very different types of reflection, concerning spectral and angular distributions. Furthermore, an OVD containing several different grating patches is evaluated. The device uses automatically adjusting exposure time to provide maximum signal dynamics and is capable of doing steps as small as 0.01°. However, even 2° steps for the detector movement showed that this device is more than capable of characterizing even the most complex reflecting surfaces. This study presents sRGB visualizations, discussion of bidirectional reflection, and accurate grating period calculations for all of the grating samples used.

Goniospectrometric space: identifiable presentation of spectral goniometric data for complex diffractive samples

Optical security takes advantage of complex gonioapparent effects of diffractive samples having strong angular and spectral dependence of reflected light. However, resolving the full angular and spectral properties of these kinds of targets might be a tremendous task. Preferably, one would like to measure a more limited number of illumination-viewing directions, using a multiangle goniometer, and still reveal the complex and unique properties of the target. In this study, we use a method for converting the full angular reflection data into reduced goniospectrometric space and further on into an xDNA graph, which we find to show good potential as a fingerprint for gonioapparent surfaces when limited measurement geometries are available. For the evaluation of the xDNA graph, we use two goniometric devices with a 45° incident angle illumination, a high-resolution bidirectional spectrometer, and a portable multiangle goniometer. This study tests the xDNA graph by evaluating the effects of geometry count and spectral resolution in goniometric measurements and further finds that the xDNA graph indeed works best with a reduced count of geometries and is not sensitive to lowered spectral resolution.

Optical analysis of adsorption sensitivity of titanium, Ti-6Al-4V, and Ti-35Nb-6Ta.

Here we report the optical analysis of protein adsorption sensitivity of titanium (Ti), Ti(6)Al(4)V, and Ti(35)Nb(6)Ta. The optical sensor used was a diffractive optical element based sensor, which analyzes magnitude and coherence of probe beam reflected from the measured surfaces. Also, the roughness and other necessary parameters were taken into account on the final verdict. The material Ti(35)Nb(6)Ta showed positive initial reaction to the human plasma fibrinogen, which was the protein used. The Ti(35)Nb(6)Ta was observed to be more active than the grade 2 titanium.

On the analysis of optical signals from Ti35Nb6Ta and Ti6Al4V surfaces

Chemical components and initial optical responses of Ti35Nb6Ta alloy are reported. Polished titanium and other titanium alloy Ti6Al4V served as reference surfaces. The chemical composition was determined with an X-ray photoelectron spectroscope (XPS) for the surfaces as well as for water, phosphate buffered saline (PBS) and for human plasma fibrinogen (HPF in PBS) exposed surfaces. The reflectance of the surfaces was modeled utilizing Bruggemans model, to evaluate the optical changes that the chemical reaction of each liquid can produce. After the model, a diffractive optical element (DOE) based sensor was used to determine the temporal optical signal from the sample surfaces. The coherent and non-coherent signals gathered with DOE sensor were compared to the reflectance model. Exposing to the liquids showed surface oxidation, which could produce lowered reflectance of the surface. The model and the initial temporal responses showed similarities in non-coherent reflectance.

Optical Detection of Protein Adsorption on Doped Titanium Surface

The frequently used biomaterial in hard tissue replacement, such as dental and orthopaedic implants, is titanium. (Ball et al., 1996; Hook et al., 2002a; Huang et al., 2003; Imamura et al., 2008; Jones et al., 2000; Walivaara et al., 1994; Yang et al., 2003) These kind of biomaterial applications made of titanium are satisfactory products, because of their ability to adsorb certain proteins. After implantation, within a few seconds, the biomaterial surface becomes coated with a film of adsorbed proteins mediating the interaction between the implant and the body environment. Since an implant is exposed to blood during implantation, the initial protein layer is mainly composed of plasma proteins. Human plasma fibrinogen (HPF) is the relevant protein, which adsorbs on biomaterial surfaces. HPF partakes in blood coagulation, facilitates adhesion and aggregation of platelets (Cacciafesta et al., 2001; 2000). The structure and composition of the adsorbed protein layer determine the type and extent of the subsequent biological reactions, such as activation of coagulation and immune response and osseointegration (Nygren et al., 1997). Thus, the initially adsorbed protein layer is a factor determining the biocompatibility , and also in recent years interest has been focused to preparation of hydrocarbons doped with Ti and used different methods to analyzing of biocompatibility for important proteins . The production and application of doped titanium surfaces are under intensive research, and the results have shown the positive views on the adaptation of these materials as a biomaterial, as equal or even better than the bulk titanium. The doping of titanium is performed typically by inserting impurities like N, Nb, Zr, Ta, Al, Cr and V.

Optical anisotropic reflectance from W720 LIPSS surface

Optical anisotropic reflectance from laser induced periodic surface structures (LIPSS) of stainless steel (W720LIPSS), which were produced by a femtosecond laser, were investigated by using polarized probe beam in a spectrophotometer. Remarkable repeatability in optical anisotropic reflectance was recognized.

Optical evaluation of ink prints made with heat-treated alder and birch

Japanese woodblock printmaking is a traditional printing method practiced for centuries. The modern challenge to this fine art is the constantly decreasing amount of available wood, wild cherry tree, which has been traditionally used as a printing block material. The increasing amount of interest focuses on finding alternative wood types, which would compare successfully with the desired quality of the wild cherry tree. Our initial research has shown that heat-treated alder and birch trees could both be considered as possible alternatives for block material. In this paper we introduce the optical characterization (CIELAB color and gloss) of black ink prints made on two different handmade papers with printing blocks made of low-temperature (120°C, 140°C) heat-treated alder and birch. Results show lowered ink transfer from wood when the heat treatment temperature rises, more transferred ink from alder, and printing paper differences due to fiber content differences.