Fluorescent nanoparticles (NPs) such as KYb<sub>2</sub>F<sub>7</sub>:Tm<sup>3+</sup> potential in biomedical applications due to their ability to absorb and emit within the biological window, where near infrared light is less attenuated by soft tissue. This results in less tissue damage and deeper tissue penetration making it a viable candidate in biological imaging. Another big factor in determining their ability to perform in a biological setting is the surface chemistry. Biocompatible coatings, including polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), pluronic and folic acid are commonly used because they pose several advantages such as ease of functionalization, better dispersion, and higher cellular uptake. To study the effects of the NP surface chemistry, KYb<sub>2</sub>F<sub>7</sub>:Tm<sup>3+</sup> a solvothermal method using PEG, PVP, pluronic acid, and folic acid as a capping agent, followed by thorough optical characterizations. Optical changes were thoroughly studied and compared using absorption, emission, and quantum yield data. Cell viability was obtained by treating Rhesus Monkey Retinal Endothelial cells (RhREC) with KYb<sub>2</sub>F<sub>7</sub>:Tm<sup>3+</sup> and counting viable cells following a 24 hour uptake period. The work presented will compare the optical properties and toxicity dependency on the surface chemistry on KYb<sub>2</sub>F<sub>7</sub>:Tm<sup>3+</sup>. The results will also indicate that KYb<sub>2</sub>F<sub>7</sub>:Tm<sub>3+</sub> nanoparticles are viable candidates for various biomedical applications.
Nanosecond pulsed electric fields (nsPEF) cause the formation of small pores, termed nanopores, in the membrane of cells. Current nanoporation models treat nsPEF exposure as a purely electromagnetic phenomenon, but recent publications showing pressure transients, ROS production, temperature gradients, and pH waves suggest the stimulus may be physically and chemically multifactorial causing elicitation of diverse biological conditions and stressors. Our research group's goal is to quantify the breadth and participation of these stressors generated during nsPEF exposure and determine their relative importance to the observed cellular response. In this paper, we used advanced imaging techniques to identify a possible source of nsPEF-induced acoustic shock waves. nsPEFs were delivered in an aqueous media via a pair of 125 μm tungsten electrodes separated by 100 μm, mirroring our previously published cellular exposure experiments. To visualize any pressure transients emanating from the electrodes or surrounding medium, we used the Schlieren imaging technique. Resulting images and measurements confirmed that mechanical pressure waves and electrode-based stresses are formed during nsPEF, resulting in a clearer understanding of the whole exposure dosimetry. This information will be used to better quantify the impact of nsPEF-induced acoustic shock waves on cells, and has provided further evidence of non-electrical-field induced exposures for elicitation of bioieffects.
Until recently, many contrast agents widely used in biological imaging have absorbed and emitted in the visible region, limiting their usefulness for deeper tissue imaging. In order to push the boundaries of deep tissue imaging with non-ionizing radiation, contrast agents in the near infrared (NIR) regime, which is not strongly absorbed or scattered by most tissues, are being sought after. Upconverting nanoparticles (UCNPs) are attractive candidates since their upconversion emission is tunable with a very narrow bandwidth and they do not photobleach or blink. The upconversion produced by the nanoparticles can be tailored for NIR to NIR by carefully choosing the lanthanide dopants and dopant ratios such as KYb<sub>2</sub>F<sub>7</sub>: RE<sup>3+</sup> (RE = Tm, Er). Spectroscopic characterization was done by analyzing absorption, fluorescence, and quantum yield data. In order to study the toxicity of the nanoparticles Monkey Retinal Endothelial Cells (MREC) were cultivated in 24 well plates and then treated with nanoparticles at different concentrations in triplicate to obtain the optimal concentration for in vivo experiments. It will be shown that these UCNPs do not elicit a strong toxic response such as quantum dots and some noble metal nanoparticles. 3-D optical slices of nanoparticle treated fibroblast cells were imaged using a confocal microscope where the nucleus and cytoplasm were stained with DAPI and Alexa Fluor respectively. These results presented support the initial assumption, which suggests that KYb2F7: RE<sup>3+</sup> would be excellent candidates for NIR contrast agents.
Recently, there has been a great amount of interest in nanoparticles which are able to provide a platform with high contrast for multiple imaging modalities in order to advance the tools available to biomedical researchers and physicians. However, many nanoparticles do not have ideal properties to provide high contrast in different imaging modes. In order to address this, ultrasmall lanthanide doped oxide and fluoride nanoparticles with strong NIR to NIR upconversion fluorescence and a strong magnetic response for magnetic resonance imaging (MRI) have been developed. Specifically, these nanoparticles incorporate gadolinium, dysprosium, or a combination of both into the nano-crystalline host to achieve the magnetic properties. Thulium, erbium, and neodymium codopants provide the strong NIR absorption and emission lines that allow for deeper tissue imaging since near infrared light is not strongly absorbed or scattered by most tissues within this region. This also leads to better image quality and lower necessary excitation intensities. As a part of the one pot synthesis, these nanoparticles are coated with peg, pmao, or d-glucuronic acid to make them water soluble, biocompatible, and bioconjugable due to the available carboxyl or amine groups. Here, the synthesis, morphological characterization, magnetic response, NIR emission, and the quantum yield will be discussed. Cytotoxicity tested through cell viability at varying concentrations of nanoparticles in growth media will also be discussed.
Optoacoustic microscopy (OAM) is an emerging technology combining the beneficial features of optical contrast and ultrasound resolution, to form a hybrid imaging technique capable of multi-scale, high-contrast and high-resolution imaging through optically scattering biological tissues. In the past 15 years, two system modifications have been developed for optoacoustic / photoacoustic microscopy: acoustic-resolution AR-OAM and optical-resolution OR-OAM. Typically, acoustic resolution systems can image deeper tissues structures, however, with resolution at least an order of magnitude worse than the systems of optical-resolution. It would be attractive for variety of biomedical applications to attain high (submicron) resolution at a depth exceeding the present limit of the optical resolution optoacoustic microscopy. Here we introduce a novel, all-optical method for OAM, in which not only thermal energy deposition, but also optoacoustic signal detection is achieved optically. In our design the probe laser beam was used as an ultrawide-band ultrasonic transducer. In this method the acoustic pressure wave amplitude is proportional to the angle of deflection of the probing CW laser beam incident on a balanced dual photodiode. Such laser beam deflection (LBD) method overcomes the limitations of conventional piezoelectric ultrasound transducers and optical interferometers. LBD method allows one to use high numerical aperture objectives for better focusing, avoid distortions associated with the system elements that separate optical and acoustic paths, and provides better sensitivity than any optical interferometer. It also provides a non-contact method that is insensitive to optical and acoustic artifacts typical of backward mode of optoacoustic imaging. The LBD sensitivity depends on a large number of system parameters such as probe beam power, spot size, interaction length, optical refraction index of the coupling medium, laser wavelength, photodiode sensitivity, proximity to the optoacoustic source, and thus, can be optimized. The basic setup of OR-LBD-OAM shows high sensitivity competitive with commercial ultrasonic transducers. We report first images of biological cells and tissues obtained using this technique.
In this paper, we study the computational modeling of the localized surface plasmonic and scattering field
effect arising from of gold nanorods. We also report the synthesis and optical characterization of core-spacer-shell
nanocomposites composed of gold nanorods coated with SiO2 and finally coated with Y2O3:Er3+/Yb3+
(Aunanorods@mSiO2@Y2O3:Er3+/Yb) through a layer-by-layer method. Preliminary upconversion analysis of singly
(Aunanorods@mSiO2@Y2O3:Er3+/Yb) at 980 excitation indicates that the composition has to be optimized to
understand the role of silica as a spacer and near field enhancer (gold nanorod) in the system.
Nanoparticles doped with rare earth ions for biomedical imaging and infrared photodynamic therapy (IRPDT) have been
synthesized, characterized, and compared. Specifically, these nanoparticles utilize two primary modalities: near infrared
excitation and emission for imaging, and near infrared upconversion for photodynamic therapy. These nanoparticles are
optimized for both their infrared emission and upconversion energy transfer to a photoactive agent conjugated to the
surface. Finally, these nanoparticles are tested for toxicity, imaged in cells using the near infrared emission pathway, and
used for selective killing of cells through the upconversion driven IRPDT.
In this paper, we discuss the concept of an efficient infrared upconverting phosphor as an energy converting material that
could potentially improve the efficiency of Si solar cells in bifacial configuration. Basic spectroscopic studies of Yb and
Er-doped La<sub>2</sub>O<sub>2</sub>S phosphor was reported with particular attention to its upconversion properties under 1550 nm
excitation. Different concentrations of phosphors were synthesized by solid state flux fusion method. The phosphor
powders were well crystallized in a hexagonal shape with an average size 300-400 nm. The most efficient upconverting
sample (1%Yb: 9% Er doped La<sub>2</sub>O<sub>2</sub>S) was also studied under the illumination with infrared (IR) broad band spectrum
above 1000 nm. Our measurements show that even with an excitation power density of 0.159 W/cm2 using a tungsten
halogen lamp the material shows efficient upconversion corroborating the fact that the present phosphors could be
potential candidates for improving the efficiency of the present Si solar cells.
Yb and Er-doped Y<sub>2</sub>O<sub>2</sub>S phosphor was synthesized by solid state flux fusion method and their upconversion spectral
properties were studied as a function of different Yb concentrations. The solid state flux fusion results in well
crystallized hexagonal shaped phosphor particles of average size 3.8 μm. The detailed optical characterizations such as
absorption, emission, and fluorescence decay were performed to explore the emission processes in the UV-VIS-NIR as
well as to quantitatively estimate the fluorescence quantum yield. Upconversion spectral studies show that for all the
compositions, green emissions are stronger, particularly; the green emission intensity is 1.7 times stronger than the red
one with composition of 8 mol% Yb and 1 mol% Er. Mechanisms of upconversion by two photon and energy transfer
processes are interpreted and explained. The color coordinates are measured and the color tunability was analyzed as a
function of the 980 nm excitation power. Results show that the Y<sub>2</sub>O<sub>2</sub>S:Yb,Er phosphor offers power dependent color
tuning properties where the emission color can be tuned from 490 to 550 nm by simply changing the 980 nm excitation
power from 10 to 50 mW.
Highly efficient upconverting phosphors (NaYF4) doped with erbium ions are bio-conjugated and used for cancer
imaging and photodynamic therapy. Once they are conjugated, the particles are injected into mice to demonstrate that
cancer imaging with a near-infrared excitation source is possible. Finally, the particles are also conjugated with a
photosensitive molecule with strong absorption near the upconversion emission peak (~ 550nm). The upconversion
energy causes the photosensitive molecule to create highly reactive oxidative species, which puncture and kill the cell to
which it is attached. These particles are then used in a mouse model, and the size of the tumors is modeled as a function
of the dosage and duration of the photodynamic therapy.
Nonlinear optical properties of barium titanate (BaTiO<sub>3</sub>) nanoparticles are investigated as a function of size and shape.
BaTiO<sub>3</sub> is an attractive option as a nonlinear material because it can exhibit a high second and third order electronic
susceptibility even at the nanoscale. These particles are employed as contrast agents/biomarkers and phase conjugate
nanomirrors in imaging, utilizing second harmonic generation for two-photon microscopy and four-wave mixing for
three-photon microscopy and scattering reversal image enhancement. Silver is also used to create a shell around the
BaTiO<sub>3</sub> nanoparticle to see if a core/shell structure enhances any of the nonlinear effects.
Barium titanate (BaTiO<sub>3</sub>) is a good candidate for phase conjugation on the nano-scale, as four-wave mixing has been
shown in nanoparticles of this type. Also, the ability to dope this material with rare earth elements, with strong
absorption and emission lines, makes it possible to use these as multi-functional, multi-modal probes for biomedical
applications. BaTiO<sub>3</sub> nanoparticles are synthesized using a precipitation method and fully characterized. These particles
are used in a four wave mixing setup to optically conjugate scattered light traveling through turbid media, such as tissue,
to re-obtain lost image information due to the scattering process.
The near-infrared (NIR) optical properties of human retinal pigmented epithelial (RPE) cells and rare earth nanopowders
were studied using a double-integrating sphere setup. The Kubelka-Munk and Inverse Adding-Doubling techniques were
applied to obtain absorption and scattering coefficients. These are compared with the coefficients obtained through the
Representative Layer Theory described by the Dahm equation. Retinal pigmented epithelial monolayers were cultured
from an ARPE19 line in thin cell culture windows, and the nanopowders were pressed into samples of varying thickness.
Samples were optically characterized as a function of wavelength. A brief discussion of the shortcomings of existing
techniques for computing optical properties when applied to physically thin samples is provided, followed by a
comparison between the optical properties of the samples returned by the different techniques.
Studies of bioluminescence in living animals, such as cell-based biosensor applications, require measurement of light at different wavelengths, but accurate light measurement is impeded by absorption by tissues at wavelengths <600 nm. We present a novel approach to this problem—the use of a plastic window in the skin/body wall of mice—that permits measurements of light produced by bioluminescent cells transplanted into the kidney. The cells coexpressed firefly luciferase (FLuc), a vasopressin receptor—Renilla luciferase (RLuc) fusion protein, and a GFP2--arrestin2 fusion protein. Following coadministration of two luciferase substrates, native coelenterazine and luciferin, bioluminescence is measured via the window using fiber optics and a photon counter. Light emission from the two different luciferases, FLuc and RLuc, is readily distinguishable using appropriate optical filters. When coelenterazine 400a is administered, bioluminescence resonance energy transfer (BRET) occurs between the RLuc and GFP2 fusion proteins and is detected by the use of suitable filters. Following intraperitoneal injection of vasopressin, there is a marked increase in BRET. When rapid and accurate measurement of light from internal organs is required, rather than spatial imaging of bioluminescence, the combination of skin/body wall window and fiber optic light measurement will be advantageous.
With recently developed diode-lasers to resonantly pump solid-state crystalline lasers, new opportunities arise for
systems such as Tb<sup>3+</sup> as an activator ion in different host matrices. For example the observed fluorescence from <sup>5</sup>D<sub>4</sub>→
<sup>7</sup>F<sub>5</sub> transition (540 to 560 nm) of Tb<sup>3+</sup> in TbAlO<sub>3</sub> represents such a possibility. There is little fluorescence quenching in
this crystal involving this transition, and the measured lifetime is approximately 2 ms, long enough to sustain sufficient
population for stimulated emission. The quantum efficiency is better than 50 % as measured in this material. For this
same transition, others have reported room-temperature pulsed laser operation at 544 nm for Tb:YLF, where the lifetime
is comparable. Mid- and long wavelength infrared laser emission has been observed for Tb<sup>3+</sup> in chalcogenide glass fibers
that complement our spectroscopic findings for Tb<sup>3+</sup> in pedestal-grown Y<sub>2</sub>O<sub>3</sub> and YAG fibers. We have identified the
infrared transitions that may lase at transitions between different manifolds within the <sup>7</sup>F<sub>J</sub> multiplet. In the present study
we first evaluate the various visible and infrared experimental findings with a Judd-Ofelt analysis of Tb<sup>3+</sup> in TbAlO<sub>3</sub>. We
predict a radiative lifetime of 3.5 ms for the excited <sup>5</sup>D<sub>4</sub> manifold to the <sup>7</sup>F<sub>J</sub> manifolds with more than 50% of the
emission represented by the <sup>5</sup>D<sub>4</sub>→ <sup>7</sup>F<sub>5</sub> transition. To account for the visible stimulated emission, we report transition
probabilities for <sup>5</sup>D<sub>4</sub>→ <sup>7</sup>F<sub>J</sub> transitions and for diode-pumped infrared transitions we report similar spectroscopic
properties for transitions within the <sup>7</sup>F<sub>J</sub> multiplet.
Highly luminescent nanoparticles, such as, trivalent erbium-doped yttrium-oxide, Er<sup>3+</sup>:Y<sub>2</sub>O<sub>3</sub>, are expected to have a wide
range of applications, including imaging, range finding, flash lidar, and other remote-sensing possibilities as well as
medical applications. These particles are synthesized by the precipitation from a homogeneous solution of the metal ions
and urea at elevated temperatures. The morphology of the calcinated materials, revealed through SEM, shows uniformly
spherical aggregates 200 nm or less depending on the ratio of the metal ions in the initial solution. Room temperature
optical absorption and emission spectra show that the trivalent erbium ions in Er<sup>3+</sup>:Y<sub>2</sub>O<sub>3</sub> nanocrystals possess sharp
absorption lines and strong emission in near infrared region that are characteristic of Er <sup>3+</sup>:Y<sub>2</sub>O<sub>3</sub> grown as large single
crystals. Low temperature (8 K) spectra obtained from these particles were analyzed in detail for the crystal-field
splitting of the <sup>2S+1</sup>L<sub>J</sub> multiplet manifolds of Er<sup>3+</sup>(4f<sup>11</sup>) including the ground-state manifold <sup>4</sup>I<sub>15/2</sub>, and excited manifolds
<sup>4</sup>I<sub>9/2</sub>, <sup>4</sup>F<sub>9/2</sub>, <sup>4</sup>S<sub>3/2</sub>, <sup>2</sup>H<sub>11/2</sub>, <sup>4</sup>F<sub>7/2</sub>, <sup>4</sup>F<sub>5/2</sub>, and <sup>4</sup>F<sub>3/2</sub>. Fluorescence lifetimes and results from an analysis of the intensities of
manifold-to-manifold transitions are also reported. Similarity of the nanocrystalline and large single crystal Er<sup>3+</sup>:Y<sub>2</sub>O<sub>3</sub>,
we propose that the simple, inexpensive method described in this study will lead to further investigation of these
nanocrystals for their optical properties, especially in the near infrared region of the spectrum.
Near infrared characterization of optical properties of various tissue components of healthy human and bovine eyes has been performed. The indices of refraction (<i>n</i>) of these ocular tissues were determined using a Michelson interferometer. The total diffuse reflection (<i>R<sub>d</sub></i>) and total transmission (<i>T<sub>t</sub></i>) measurements have been taken for individual ocular tissue by using double-integrating spheres and infrared laser diodes. The Inverse Adding Doubling computational method based on the diffusion approximation and radiative transport theory is applied to the measured values of <i>n</i>, <i>R<sub>d</sub></i>, and <i>T<sub>t</sub></i> to calculate the optical absorption and scattering coefficients of the human and bovine ocular tissues. The scattering anisotropy value was determined by iteratively running the inverse adding doubling method program and a Monte Carlo simulation of light-tissue interaction until the minimum difference in experimental and computed values for <i>R<sub>d</sub></i> and <i>T<sub>t</sub></i> were realized. A comparison between the optical characterization of human and bovine ocular samples is also
The optical scattering, absorption, and polarization properties of human retinal tissues are investigated for a number of laser wavelengths in the visible range. The indices of refraction of these tissues are determined by applying Brewster's law. The inverse adding doubling method based on the diffusion approximation and radiative transport theory is applied to the measured values of total diffuse transmission, total diffuse reflection, and index of refraction to determine the optical absorption, scattering, and scattering anisotropy coefficients of the intact retinal tissues from healthy and diseased (neovascularized) human eyes. The polarization studies show that the retinal tissues possess significant intrinsic polarization characteristics, that are more pronounced in diseased tissues than in healthy tissues.
Recent advances in the growth of rare earths doped into ceramic (poly-crystalline) materials have generated considerable interest for the next generation of tactical laser systems mainly because ceramics provide larger size, greater strength and lower cost factors in design than their single-crystalline counterparts. For many years, Nd:YAG has been the laser material choice for stability and high power Er has been an ion laser source of interest for defense due to its eye-safe emission at 1.5 μm and has applications in infrared counter-measures, illumination detection, remote sensing and communication technologies.
A model Hamiltonian including atomic and crystal-field terms is diagonalized within the complete 4f<sup>11</sup> SLJMJ basis set which includes 364 states. Within the standard deviation obtained between 117 comparable calculated-to-observed Stark levels, one set of atomic and crystal-field parameters describes the splitting of the Nd<sup>3+</sup> and Er<sup>3+</sup> energy levels in either the ceramic or single-crystal host.
We report a detailed crystal-field splitting analysis for a number of multiplet manifolds of Nd<sup>3+</sup> and Er<sup>3+</sup> in both the ceramic and single-crystal form of YAG (Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub>). With few exceptions, analysis shows that the energy-level structure of Nd<sup>3+</sup> and Er<sup>3+</sup> is similar in the ceramic and single-crystal laser rods.
An in vivo investigation to determine instrinsic differences in optical activity of tissues of a three-day-old chicken embryo was performed. A homogenous flux of light at 514.4 nm from an argon ion laser was used to develop 13 of the 16 Mueller matrix components of backscattered light. The results show that all tissues backscatter light predominately in the same polarization state as that of the incident light. This effect manifests itself as large non-zero intensities in the diagonal terms of the Mueller matrix. Any change in polarization is distributed unbiased to the other polarization states as the off diagonal Mueller matrix elements each consists of a low intensity image. Depolarization by birefringent tissue structures such as collagen or actin-myosin fibers which would lead to non-zero intensities in the off diagonal Mueller matrix elements, is not observed. This may be due to the lack of such structures given the early age of the embryo.
An in-depth characterization of optical properties of human retinal and retinal pigment epithelium (RPE)/choroidal tissues has been performed. The indices of refraction of these ocular tissues were determined by applying Brewster's law. The inverse adding doubling method, based on the diffusion approximation and radiative transport theory is applied to the measured values of the total diffuse transmission, total diffuse reflection, and collimated transmission to calculate the optical absorption, and scattering of the human retinal and retinal pigment epithelium/choroidal tissues. The scattering anisotropy coefficients were calculated using an independent method which relates scattering angles and intensities. The resulting values have been analyzed using appropriate statistical methods.
A spectroscopic analysis is performed on Er3+ (4f11) ions doped in order to assess this material for its potential as a near infrared laser. The Judd-Ofelt model is applied to the room temperature absorption intensities of Er<sup>3+ </sup>(4f<sup>11</sup>) in NaBi(WO<sub>4</sub>)<sub>2</sub> to obtain the three phenomenological intensity parameters: Ω<sub>2</sub> = 5.50 x 10<sup>-20 </sup>cm<sup>2</sup>, Ω<sub>4</sub> = 1.00 x 10<sup>-20 </sup>cm<sup>2</sup>, and Ω6 = 0.71 x 10<sup>-20 </sup>cm<sup>2</sup>. The intensity parameters are then used to determine the radiative decay rates (emission probabilities), radiative lifetimes, and branching ratios for the Er<sup>3+ </sup>transitions from the excited state multiplet manifolds to the lower-lying manifold states. Using the radiative decay rates for the Er<sup>3+ </sup>(4f<sup>11</sup>) transitions between the corresponding excited states and the lower-lying states, the radiative lifetimes of eight excited states of Er<sup>3+ </sup>are determined in this host. Using the room temperature fluorescence lifetime and the radiative lifetime of the <sup>4</sup>I<sub>13/2</sub>→<sup>4</sup>I<sub>15/2 </sub>(1.52 µm) transition of Er<sup>3+ </sup>in NaBi(WO<sub>4</sub>)<sub>2</sub>, the quantum efficiency is determined to be 84% for this laser material.
An in-depth characterization of the optical properties of bovine retinal and retinal pigment epithelium-choroidal tissues has been performed. The indices of refraction of these ocular tissues were determined by applying Brewster's law. The inverse adding doubling method based on the diffusion approximation and radiative transport theory is applied to the measured values of the total diffuse transmission, total diffuse reflection, and collimated transmission to calculate the optical absorption, scattering, and scattering anisotropy coefficients of the bovine retinal and retinal pigment epithelium-choroidal tissues. The values of the optical properties obtained from the inverse adding doubling method are compared with those generated by the Monte Carlo simulation technique. Optical polarization measurements are also performed on bovine retinal tissues. Our studies show that both retina and retinal pigment epithelium-choroid possess strong polarization characteristics.
Optical polarization study is performed on the healthy and neovascularized human retinal and retinal pigment epithelium (RPE)/choroidal tissues. Linear polarizer and polarizing analyzer have been employed to determine the intensity changes as well as polarization shifts of the polarized laser light scattered off these tissues from both the left and right eyes. Our studies show that both retinal and choroidal tissues possess strong polarization properties. The polarization shift is found to be higher in the neovascularized tissues than in the healthy tissues. It is also observed that the greater the polarization shift, the lower the intensity of the scattered light.
Recent advances in the current state of knowledge about the properties of physiological and synthetic melanin are reviewed in the context of that pigment's optical properties, the physical structure that confers some of its unusual properties, its supportive role in the visual process (by absorbing excess light and reducing intraocular light scatter), its role in some (but not all) laser interactions with ocular tissue, its protective properties (by absorbing potentially phototoxic short-wavelength visible light), and its photoinducible free radical properties. The ability of melanin to form a long-lived radical during visible light irradiation may serve as a protective mechanism against light damage by transforming optical energy into chemical potential energy that can be dissipated in a chain of coupled redox reactions. If a cellular event such as antioxidant depletion occurs that disrupts this chain, however, the melanin radical may promote photo-oxidative damage in ocular tissue. Thus melanin may play two opposing roles in the eye: one protective and the other potentially damaging.
The separation of on-axis scattered and unscattered transmission through turbid media has been a difficult experimental task in recent years. This study suggests the use of a polarimeter to filter out the contribution of scattered light to the net on-axis transmission. Red blood cells (RBC) were used to produce the scattering effect. The scattering level was varied by: (1) altering the distance of the detector from the sample, (2) using erythrocytes from three different species, e.g., the dog, goat, and human, which are know to have different RBC sizes, and (3) allowing the RBCs from each species to shrink and swell osmotically. An He-Ne laser was used as the source of the radiation so that data were obtained at a wavelength in the spectral region used in oximetry and hemoglobinometry. In each case, the difference in the scattering cross sections obtained for each sample, with and without polarization filtering, gave us a measure of the filtered scattered light. The results obtained were in close agreement with the expected contribution of scattered radiation to the net axial transmission. This method may be used effectively for all studies involving measurements of on-axis transmission through turbid media, such as biological tissue.