Effectiveness of interference filter for photoluminescence observations: comparison with absorption filters

We report a lens-free fluorescence imaging device using a composite filter composed of an interference filter and an absorption filter, each applied to one side of a fiber optic plate (FOP). The transmission of angled excitation light through the interference filter is absorbed by the absorption filter. The auto-fluorescence of the absorption filter is reduced by the reflection from the interference filter of normally incident excitation light. As a result, high-performance rejection of excitation light is achieved in a lens-free device. The FOP provides a flat, hard imaging device surface that does not degrade the spatial resolution. We demonstrate excitation rejection of approximately 108:1 at a wavelength of 450 nm in a fabricated lens-free device. The resolution of fluorescence imaging is approximately 12 µm. Time-lapse imaging of cells containing green fluorescent protein was performed in a 5-µm thin-film chamber. The small dimensions of the device allow observation of cell culturing in a CO2 incubator. We also demonstrate that the proposed lens-free filter is compatible with super-resolution bright-field imaging techniques. These features open a way to develop a high-performance, dual-mode, lens-free imaging device that is expected to be a powerful tool for many applications, such as imaging of labeled cells and point-of-care assay.

Green fluorescent materials such as Green Fluorescence Protein (GFP) and fluorescein are often used for observing neural activities. Thus, it is important to observe the fluorescence in a freely moving state in order to understand neural activities corresponding to behaviors. In this work, we developed an implantable CMOS imaging device for in-vivo green fluorescence imaging with efficient excitation light rejection using a combination of absorption filters. An interference filter is usually used for a fluorescence microscope in order to achieve high fluorescence imaging sensitivity. However, in the case of the implantable device, interference filters are not suitable because their transmission spectra depend on incident angle. To solve this problem we used two kinds of absorption filters that do not have angle dependence. An absorption filter consisting of yellow dye (VARYFAST YELLOW 3150) was coated on the pixel array of an image sensor. The rejection ratio of ideal excitation light (490 nm) against green fluorescence (510 nm) was 99.66%. However, the blue LED as an excitation light source has a broad emission spectrum and its intensity at 510 nm is 2.2 x 10^-2 times the emission peak intensity. By coating LEDs with the emission absorption filters, the intensity of the unwanted component of the excitation light was reduced to 1.4 x 10^-4. Using the combination of absorption filters, we achieved excitation light transmittance of 10^-5 onto the image sensor. It is expected that high-sensitivity green fluorescence imaging of neural activities in a freely moving mouse will be possible by using this technology.

Filtering strategies are a crucial aspect for signal detection in many fluorescence based systems such as chemical and/or biochemical sensors. The design, fabrication and characterization of a new waveguide absorption filter for the optimization of the fluorescence signal collection, thanks to its high numerical aperture, is here presented. The absorption filter is designed to work as an optical waveguide in order to increase the optical path and, consequently, the absorption of the excitation light. A comparison of the performances of the absorption filter and a conventional interference filter, with particular emphasis on the angular dependence of the spectral features, is also reported. We experimentally demonstrate, for what regards the attenuation capability of the excitation signal, the failure of the interference filter for incidence angles greater than 15° and the validity of the absorbing waveguide filter for large incidence angles. Finally, preliminary results performed in fluorescence on an IgG labelled/ anti-IgG assay show the improvement in detected fluorescence intensity collected by means of the proposed absorption filter compared to that measured with the interference filter. This suggests that the filtering strategy based on the waveguide absorption filter can greatly simplify fluorescence detection systems and find interesting applications in different areas of sensing, from Point of Care Testing (POCT) to environmental monitoring.

Visual perception is initiated by the eye’s optics in the sense that the neural aspects of the visual process can only operate on the information that is presented to the nervous system. The varied ways in which this information is presented in the eyes of vertebrates and invertebrates raise questions about the consequences of the function of optical structures for vision. The purpose of this chapter is to review the effects of optical filtering on eye function emphasizing such unresolved issues, for example, as the function of optical filtering in the eyes of animals with high (suprahuman) visual acuity. Aspects of optical filtering will be reviewed commencing with the physical basis of transparency of the ocular media. The effects of interference and absorption filters will be examined. Polarization filters, which are covered by Waterman (this volume, Part B), and visual pigments, which have been recently reviewed by several authors in the Handbook of Sensory Physiology Vol. VII/I and Hamdorf (this volume), will be omitted. Filtering that results from the spatial arrangements and the physical properties of the photoreceptor cells will also be discussed. The unifying nature of this topic lends itself to a consideration of these factors independent of phylogenetic classification, and for this reason examples will be drawn freely from both the vertebrates and invertebrates.

A spectrally splitting concentrating PV/T system using combined absorption optical filter and linear Fresnel reflector concentrator

Objective:The increased risk of infection for patients caused by construction and renovation near hematology inpatient clinics is a major concern. The use of high-efficiency particulate absorption (HEPA) filters can reduce the risk of infection. However, there is no standard protocol indicating the use of HEPA filters for patients with hematological malignancies, except for those who have undergone allogeneic hematopoietic stem cell transplantation. This quasi-experimental study was designed to measure the efficacy of HEPA filters in preventing infections during construction.Materials and Methods:Portable HEPA filters were placed in the rooms of patients undergoing treatment for hematological malignancies because of large-scale construction taking place near the hematology clinic. The rates of infection during the 6 months before and after the installation of the portable HEPA filters were compared. A total of 413 patients were treated during this 1-year period.Results:There were no significant differences in the antifungal prophylaxis and treatment regimens between the groups. The rates of infections, clinically documented infections, and invasive fungal infections decreased in all of the patients following the installation of the HEPA filters. When analyzed separately, the rates of invasive fungal infections were similar before and after the installation of HEPA filters in patients who had no neutropenia or long neutropenia duration. HEPA filters were significantly protective against infection when installed in the rooms of patients with acute lymphocytic leukemia, patients who were undergoing consolidation treatment, and patients who were neutropenic for 1-14 days.Conclusion:Despite the advent of construction and the summer season, during which environmental Aspergillus contamination is more prevalent, no patient or patient subgroup experienced an increase in fungal infections following the installation of HEPA filters. The protective effect of HEPA filters against infection was more pronounced in patients with acute lymphocytic leukemia, patients undergoing consolidation therapy, and patients with moderate neutropenia.

Monolithically integrated optical interference and absorption filters on thin film amorphous silicon photosensors for biological detection

In this study, a dual-band hybrid filter that achieves high excitation light rejection performance in a lensless imaging system was fabricated and incorporated into an imaging device. The hybrid filter consisted of interference and absorption filters, and a fiber optic plate (FOP). The interference filters were attached to both sides of the FOP, which was placed on top of the absorption filter to suppress the decrease in spatial resolution. In addition, the lamination order was optimized to achieve a high fluorescence observation performance. The fabricated hybrid filter was mounted on an image sensor and had the ability to indicate the green and red fluorescence components.

One look at the absorption curve for light in water suggests that the best possible window for underwater optical transmission is at the blue-green end of the visible light spectrum, centered at roughly 450 nm for clear, deep water, and shifted more towards green for coastal water [1]. While this holds true for signal transmission in the optical communications channel, it is also true for the solar spectrum, which presents as in-band white noise near the ocean surface. The greatest signal loss and interference is in the 0 to 40 meter depth range, but solar irradiance is still present to a depth of 300 meters and in some cases to a depth of500 meters. Because solar radiation shares the same spectral band as the communications signal, solar light is very difficult to reject without the loss of transmitted communications signal. For wide-angle transmitters and receivers, dielectric, thin-film (TF) filters are not effective due to the wavelength dependence on angle of incidence, thus making narrow-angle band pass filters for underwater use very difficult to design. Absorptive filters can be used to reject large portions of the visible spectrum, but the problem of in-band noise still remains since absorptive filters do not exhibit the sharp cutoff of interference filters. Sources in the range of 350 to 400 nm present a viable means to work at, or just beyond, the edge of the visible spectrum where a majority of sunlight can be rejected. Until recently, efficient, inexpensive, near ultraviolet (NUV) sources have not been readily available; but the demand for new, high power NUV light-emitting diodes (LEDs) for commercial applications has made their use in underwater applications possible. These wavelengths are not visible to the human eye thus making them good candidates for non-visible communications. The Woods Hole Oceanographic Institution's (WHOI's) Optical Communications Group has performed many field and lab experiments with short wavelengths for the purpose of wavelength division multiplexing (WDM), enhanced daylight performance, and non-visible underwater communications. Initially 405 nm LEDs were used for WDM in shallow water and then for long-range transmission tests in deep water at the Axial Seamount of the Juan de Fuca Ridge (JDR). Additional tests were performed using 405 nm and 385 nm near the surface at JDR to assess ambient daylight rejection. Recently, work has been done in very shallow water in Cape Cod Bay to assess range with high ambient light in very turbid water.

We demonstrate a dual-color lensless fluorescence imaging setup with a composite filter with a notch filter and absorption filters. By controlling the incident angle, high detection performance was achieved for green and red fluorescence.

A Dy(III) coordination polymer (CP), [Dy(spasds)(H2O)2]n (1) (Na2Hspasds = 5-(4-sulfophenylazo)salicylic disodium salt), has been synthesized using a hydrothermal method and characterized. 1 features a 2D layered structure, where the spasda3− anions act as pentadentate ligands, adopting carboxylate, sulfonate and phenolate groups to bridge with four Dy centers in η3-μ1: μ2, η2-μ1: μ1, and monodentate coordination modes, respectively. It possesses a unique (4,4)-connected net with a Schläfli symbol of {44·62}{4}2. The luminescence study revealed that 1 exhibited a broad fluorescent emission band at 392 nm. Moreover, the visual blue color has been confirmed by the CIE plot. 1 can serve as a dual-functional luminescent sensor toward Fe3+ and MnO4− through the luminescence quenching effect, with limits of detection (LODs) of 9.30 × 10−7 and 1.19 × 10−6 M, respectively. The LODs are relatively low in comparison with those of the reported CP-based sensors for Fe3+ and MnO4−. In addition, 1 also has high selectivity and remarkable anti-interference ability, as well as good recyclability for at least five cycles. Furthermore, the potential application of the sensor for the detection of Fe3+ and MnO4− was studied through simulated wastewater samples with different concentrations. The possible sensing mechanisms were investigated using Ultraviolet-Visible (UV-Vis) absorption spectroscopy and density functional theory (DFT) calculations. The results revealed that the luminescence turn-off effects toward Fe3+ and MnO4− were caused by competitive absorption and photoinduced electron transfer (PET), and competitive absorption and inner filter effect (IFE), respectively.

The emission filter plays a key role in resolving a modest-quality image of the lens-free fluorescent imager. The complementary structure of an interference filter and absorption filters exhibits a high-rejection ratio, corresponding to the lens-based fluorescence device. However, existing fabrication methods are facing challenges to reach a reasonable filter thickness for low invasiveness. It is difficult to deposit interference filters on the polymer-based absorption filter and CMOS die directly. Conversely, the interference is fragile and easy to crack so that transferring from its substrate to the image sensor is a fatiguing task. Here we report composite filter fabrication using laser lift-off (LLO) and silicon plasma etching. The LLO utilized high energy laser to separate the interference filter from the glass substrate, whereas the plasma etching tailored SiF6 gas to completely annihilate silicon-substrate whereby the filter was deposited beforehand. As a result, a narrow-size filter is successfully fabricated by LLO, yet a crack issue for a larger sensor size remains unsolved. On the other hand, the plasma etching produced large-size and spotless filters with relatively high reproducibility. Additionally, this method offers multiple device fabrication in a single process, which, we expect, could intensify largescale lens-free fluorescent imager applications in the future.

Chapter 12 - SOLAR RADIATION MEASURING INSTRUMENTS

Numerous approaches have been taken to miniaturizing fluorescence sensing, which is a key capability for micro-total-analysis systems. This critical, comprehensive review focuses on the optical hardware required to attenuate excitation light while transmitting fluorescence. It summarizes, evaluates, and compares the various technologies, including filtering approaches such as interference filters and absorption filters and filterless approaches such as multicolor sensors and light-guiding elements. It presents the physical principles behind the different architectures, the state-of-the-art micro-fluorometers and how they were microfabricated, and their performance metrics. Promising technologies that have not yet been integrated are also described. This information will permit the identification of methods that meet particular design requirements, from both performance and integration perspectives, and the recognition of the remaining technological challenges. Finally, a set of performance metrics are proposed for evaluating and reporting spectral discrimination characteristics of integrated devices in order to promote side-by-side comparisons among diverse technologies and, ultimately, to facilitate optimized designs of micro-fluorometers for specific applications.

This laboratory experiment is the first in a semester-long instrumental analysis course. Students measure the emission profiles of various light sources including light-emitting diodes (LEDs), the transmission of interference and absorption filters, and the absorptivity of colored complexes of Fe2+. They use this information to model the effect of polychromatic radiation on the shape of the calibration curve and to assess possible optical configurations for a field spectrophotometer used to determine the concentration of soluble iron in environmental systems. This experiment serves as a hands-on examination of many of the fundamental concepts in electronic absorbance spectroscopy. The typical components of a spectrophotometer and their functions are clearly illustrated (e.g., continuum vs line sources, filters vs monochromators). Topics in quantitative absorbance spectroscopy including Beer's law, deviations from linear behavior, and figures of merit are also covered. The students gain experience with the use of spreadsheets for data analysis and macro programming in Excel. The data collected in this lab are applied to current topics in analytical research including environmental analysis and the miniaturization of analytical instruments.