Juha Saarela

Refractive index matching improves optical object detection in paper

The demand for high-quality recycled pulp products has increased the need for an efficient deinking process. Assessing process efficiency via residual ink on test sheets has so far been limited to the sheet surface due to the poor transparency of paper. A refractive index matching method was studied to obtain a quantitative measure of particles within the volume of a paper sheet. In actual measurements a glass plate with etched lines from 8.5 ?m to 281.1 ?m wide was placed beneath the layers of cleared paper, and visible lines were counted with a microscope. Three different paper grades were tested with transparentizing agents. A diffusion theory-based regression model was used to find a correlation between transparency, paper grammage and paper thickness. These equations enable the determination of the size of an object detectable from a paper with a certain transparentizing agent or the parameters of a test sheet needed to detect objects of a known size. Anise oil was found to be the better of the two agents used, and they both had better transparentizing ability than air or water. The transparent paper grammage of the paper grades was determined for all the tested media. Papers transparency was found to depend more on papers thickness than grammage.

Electrical heating synchronized with IR imaging to determine thin film defects

Measuring conductive thin film properties during production and in end products is a challenge. The main demands for the measurements are: production control, reliability and functionality in final applications. There are several ways to measure thin film quality in a laboratory environment, however these methods are poorly applicable for production facilities. In order to bypass the limitations of existing methods, a simple synchronized heating and IR-imaging based system was implemented. To demonstrate the proposed method, Indium Tin Oxide (ITO) was selected as an example of conductive thin films. PET-ITO films were bent to obtain samples with defects. The proposed method was used and automated signal processing was developed. The results show that the system developed here is suitable for defining breakage types and localizing defects.

Measuring pulp consistency and fines content with a streak camera

The problem of determining the fines content of pulp in cases where consistency is not known remains to be solved. We explored the hypothesis that this problem may be solved by studying shape changes in a laser pulse after it has travelled through the pulp.A matrix was constructed of pulp samples with consistencies varying from 0 to 1.5% by increments of 0.2% and fines contents varying from 0 to 50% by 10% increments. A streak camera was used to record three pulses simultaneously. The first was a reference pulse, which was used to calibrate the measurement pulses. The second was a pulse measured at an angle of 90° to the straight light path. The third was the straight path pulse.Different fines contents form their own lines on consistency–maximum power graphs and consistency–time of 50% power fall graphs. When transmitted power is plotted against time of 50% power fall the lines representing different fines contents cross each other. These results indicate that the fines content and consistency can be measured in some cases with a single measurement. Also, if water is added in a controlled manner, measurement of the lowering in consistency allows the original consistency and fines content to be determined.

Determination of the refractive index of paper with clearing agents

The refractive index of paper was determined by measuring the propagation delay of photons in optically cleared paper boards. The determination was based on the assumption that photon propagation delay achieves minimum value as the paper is optimally cleared. The measured paper sheets was made from elemental chlorine-free market pulp, i.e. fully bleached, unbeaten, softwood kraft pulp. Nine different clearing agents with a refraction index between 1.329 and 1.741 were eLuperimented with. According to the streakmem measurements, the refractive index of the test paper was 1.557.

Synchronized thermography for multi-layer thin film characterization

Organic solar cells and organic LEDs are typically made of conductive and semi-conductive thin films. The uniformity requirement for these films is exceptionally high. In the case of multi-layer structures, surface characterization based methods (e.g. profilometer, atomic force microscope, scanning electron microscope) encounter certain challenges when attempting to detect the defects inside the structure. One way to overcome this drawback is by using synchronized thermography (ST). In this work ST is used to study multi-layered thin film structures. Indium Tin Oxide (ITO) was used as an example of conductive thin film and poly(3,4-ethylenedioxy-thiopene):poly(styrene-sulfonate) (PEDOT:PSS) was used as an example of a hole transporting layer. Uniformity differences were generated in these layers and ST was used to detect them. The results show that ST is capable of localizing small defects in the stack using a single infrared (IR) image. It can often be deduced from the same image in which layer the defect is located. This shows that ST is capable of profiling the structures of multi-layer thin films.

IR-imaging based system for detecting the defects of conductive materials

Uniformity of conductive materials is an important property which is measured during manufacturing and in finished products, especially in electronics applications such as organic solar cells. Differences in uniformity are often very small, invisible or below the surface of the sample. Therefore, they are not always detectable even by high-resolution imaging systems. Respectively, electrical conductivity measurements are limited to those mainly between the measuring probes. Uniformity difference measurements are time-consuming in the case of a large area characterization. To bypass the described limitations, a simple heating and IR-imaging based system was designed and demonstrated with conductive materials. Samples with different defects were used to investigate the correlation of conductance and defect positioning. By making punched holes in the samples, it was possible to demonstrate how the local resistances of thin films have functions to each other and how this may be observed on an IR-figure. Thermographs of punched thin films confirm that those areas where the holes prevented the current flow have lower heat emissions. Therefore, it can also be concluded that, generally, the temperature is highest at the areas where current density is highest. When comparing the defects of bent samples to these punctured ones, the correlations of resistance and breakage areas were comparable. The applied system is capable of localizing small defects in large-area samples using a single IR-image. This is a significant advantage from the manufacturing process measurement point of view.

Detecting Defects in Photovoltaic Cells and Panels and Evaluating the Impact on Output Performances

This paper investigates the ways to detect defects in photovoltaic (PV) cells and panels. Here, two different methods have been used. First, the output behavior was characterized by measuring the amount of current at different voltage levels to obtain the current-voltage and power-voltage curves. Second, infrared emissions of forward-biased nonilluminated PV cells and panels were measured by the use of synchronized thermography. From these measurements, temperature maps can be derived, which indicate that the temperature within a given PV cell unevenly rises due to the defects in the cell. Uneven temperature distribution indicates defects and reduced output power.

Detecting defects in photovoltaic modules with the help of experimental verification and synchronized thermography

In this paper we investigate defects in photovoltaic modules which cause variations in output performances. Here, we concentrate on two possible ways to verify potential output power levels. Firstly, we characterise the output behaviour by measuring the amount of current at different voltage levels to obtain the I-V (Current-Voltage) and P-V (Power-Voltage) curves. Secondly, we measured the infrared (IR) emissions of photovoltaic modules by the use of synchronized thermography. From those measurements, temperature maps can be derived which indicate that the temperature rises differently in photovoltaic modules due to defects. As a result, we are able to establish quantitative and qualitative verifications of photovoltaic modules.

Estimating the impact of defects in photovoltaic cells and panels

This paper investigates defects in photovoltaic cells and panels which cause notable losses in output performances. Here, the focus lies on the impact of hairline cracks which result in a remarkable drop of the available output current and, thus, the available output power. Firstly, samples were characterised with the help of synchronized thermography (ST) in order to localise and analyse the defects. Secondly, samples were measured with the help of electrical verification to obtain the characteristic I-V (Current-Voltage) curve. Finally, the geometric area of PV cells was calculated which corresponds to the effective area for energy production due to the presence of a defect. Results show the correlation between the available power of PV cells with temperature variations in IR-emissions. Proposed methods are capable of detecting defects in PV cells and quantise the impact on output performances.

Detection of laser induced dielectric breakdown in water using a laser doppler vibrometer

This study is focused on exploring the feasibility of an all-optic surface scanning method in determining the size and position of a submerged, laser generated, optoacoustic (OA) source. The optoacoustic effect in this case was generated when the absorption of a short electromagnetic pulse in matter caused a dielectric breakdown, a plasma emission flash and a subsequent acoustic wave. In the experiment, a laser pulse with λ = 1064 nm and 12 ns pulse length was aimed at a volume of deionized water. When the laser beam was focused by a f = 16 mm lens, a single dielectric breakdown spot occurred. When a f = 40 mm was used several breakdowns in a row were induced. The breakdowns were photographed using a double shutter camera. The acoustic wave generated by the dielectric breakdowns were detected at a point on the water surface using a laser Doppler vibrometer (LDV). First, the LDV signal was used to calculate the speed of sound with an accuracy of 10 m/s. Secondly, the location and length of the dielectric breakdown was calculated with an accuracy of 1 mm. The calculated position matched the breakdown location recorded by a camera. The results show that it is possible to use LDV surface measurements from a single spot to determine both the position and length of the OA source as well as the speed of sound in the medium. Furthermore, the LDV measurements also show a secondary peak that originates from the OA source. To unravel the origin and properties of this interesting feature, further investigations are necessary