The concept of an AlGaInP thin-film light emitting diode includes a structure of semiconductor layers with low optical absorption on which a highly reflective mirror is applied. After bonding this wafer to a suitable carrier, the absorbing GaAs substrate is removed. Subsequently, electrical contacts and an efficient light scattering mechanism for rays propagating within the chip is provided. To achieve high efficiency operation it is crucial to optimize all functional parts of the device, such as the mirror, contacts, and active layer. Different mirrors consisting of combinations of dielectrics and metals have been tested. New chip designs have been evaluated to reduce the absorption at the ohmic contacts of the device. For efficient light scattering, the surface roughness of the at the emission window has to be optimized.
Using these structures, and a thin active layer consisting of five compressively strained quantum wells, an external quantum efficiency of 40% is demonstrated at 650 nm. Further improvement is expected.
Since the AlGaInP material system can provide only poor carrier confinement for active layers emitting in the yellow wavelength regime, the internal efficiency of these LEDs is comparably low. In order to reduce the problem of carrier leakage, a yellow active region usually consists of some hundred nanometers of active material. To circumvent the problem of this highly absorbing active layer, a separation of the light generation and the area of light extraction is suggested for yellow thin-film LEDs. First results are presented in this paper.
The absorption of lateral guided modes in light emitting diodes is determined by the photocurrent measurement method. A theory for waveguide dispersion is presented and extended by ray-tracing simulations. Absorption coefficients of InGaN-on-sapphire and AlGaInP-based structures is evaluated by comparison with simulation curves. For nitride-based samples with emission wavelengths of 415 nm and 441 nm an absorption of 7 cm-1 is obtained. It is found that scattering is present in the buffer layer and influences the lateral intensity distribution. The investigated AlGaInP-based sample exhibits an absorption of α = 30 cm-1 at 650 nm emission wavelength.
The combination of wafer soldering using metal layers and the introduction of buried micro-reflector structures has proven to be a promising approach to fabricate high brightness, substrate-less LEDs in the AlGaInP material system. In addition to the enhanced light output, the scalability of this approach has been predicted as a major advantage. In contrast to other approaches, larger area LEDs can be fabricated without altering the epitaxial structure and thickness of layers simply by offering a larger area for light generation. First samples of amber (λ = 615 nm) buried micro-reflector LEDs with side-length up to 1000 μm have been realized. Devices mounted in packages with improved heat sinks are capable of low voltage CW operation with currents as high as 600 mA (Vfw≤ 2,8 V) without significant thermal flattening of the light-current characteristics. The maximum luminous flux achieved at these oeprating conditions is 46 lumen. Already these first experiments demonstrate the potential of the concept of buried micro-reflector LEDs not only for high-brightness but also for high-current operation. The results are among the best values of high-flux LEDs in this wavelength range.
A novel AlGaInP light-emitting diode (LED) is presented that employs high-reflectivity omni-directional reflector (ODR) submounts. It is shown that the reflective-submount (RS) LED has a higher light-extraction efficiency than conventional LEDs. Red AlGaInP RS-LEDs bonded to Si-substrates are demonstrated using a silver-based ODR. The ODR is perforated by an array of small-area low-resistance ohmic contacts. The optical and electrical characteristics of the RS-LEDs are presented and compared to conventional AlGaInP absorbing substrates (AS) LEDs with distributed Bragg reflectors (DBR). It is shown that the light output from the RS-LED exceeds that of AS-LEDs by about a factor of two.
We propose a new process for thin-film surface-textured LEDs that provides uniform current injection for both top and bottom contacts. The structure uses a partially conductive mirror. This eliminates the need for thick epitaxial layers and makes it possible to fabricate very large LEDs. Furthermore, the new process allows to obtain both high external quantum efficiency and high wallplug efficiency. 400 x 400 μm GaInP/AlGaInP LEDs reach maximum external quantum efficiencies of 35% at 12 mA without encapsulation. The wallplug efficiency reaches 34% at 2.6 mA. At an operating current of 60 mA, the devices emit 30 mW of light.
To extend the usage of silicon as light emitter in optoelectronics, two ways are exploited to overcome its indirect bangap obstacle. Metal-oxide-semiconductor structures with silicon dioxide (SiO2) nanoparticles as oxide layer exhibits electroluminescence with 1.5 x 10-4 external efficiency at Si bandgap energy. The enhancement in light emission is attributed to carrier concentration due to non-uniformity of oxide thickness. Another approach is to take advantage of direct bandgap materials. Chemically synthesized cadmium sulfide (CdS) nanoparticles are deposited on Si substrate and exhibits electroluminescence corresponding to different process treatment.
GaN-based optoelectronics allow covering the spectral range from green to UV. Silicon (Si) is an alternative substrate to the commonly used sapphire and silicon carbide (SiC) but requires sophisticated buffer structures. In this work, two high-temperature (HT) layer stacks and two low-temperature (LT) AlN layers were used for the growth of GaN buffers for optoelectronic devices on (111)-oriented Si substrates using AIXTRON metalorganic vapor phase epitaxy (MOVPE) reactors. AlN, AlGaN and GaN were grown as HT layer stack to form stress reduction layers. GaInN MQW (multi quantum wells), electroluminescence test structures (ELT) and AlN/GaN DBR (distributed Bragg reflectors) were deposited on these buffer structures on Si. The growth process was monitored by in-situ reflectivity measurements. Photoluminescence (PL), electroluminescence and the luminescence under high optical excitation of the samples on Si have been studied. Laser action at optical excitation was obtained in the MQW with a room temperature (RT) laser threshold of Ithr = 40 - 80 kW/cm2. Laser action was achieved up to 350°C. Electroluminescence emission from the ELT InGaN/GaN heterostructures was observed and measured under a minimal DC voltage of about 4 V. AlN/GaN DBR with ten periods showed reflectivities of 60% for wavelengths of 436 nm and 537 nm, respectively.
Vertical hexagonal GaN nanorods are grown on (111)Si substrates by plasma-assisted molecular beam epitaxy. No extra catalyst is used to assist the GaN nanorods growth. Nanorods top surfaces are hexagons with diameter ≤10-200 nm by field emission scanning electron microscopy. The image of high-resolution transmission electron microscopy (HR-TEM) shows that the nanorods are single crystal without dislocations. Diffraction pattern of TEM also shows that the nanorods are wurtzite GaN with direction  along the length direction. The temperature dependent photoluminescence (PL) spectroscopy shows only one peak at 3.405 eV at room temperature but two peaks at 3.467 eV and 3.433 eV at 66 K. After ammonia sulfur [(NH4)2S] treatment, the low energy peak disappears. The PL spectra are also compared to the ones of epitaxial GaN thin film on (111)Si and it concludes that the low energy peak is from the nanorods contribution. The micro-Raman spectroscopy shows Stokes scattering lines at 532.7 cm-1, 558.3 cm-1, 567.1 cm-1, and 736.1 cm-1 with 532 nm laser focused on the rod lateral surface and at 558.7 cm-1, 567.8 cm-1, and 736.4 cm-1 focused on the film from the top. The width and the length of the nanorods vary with the growth time and the nanorods growth rate keeps ~20 % higher than the film. The growth mechanisms will be discussed.
An overview of planar resonant-cavity light-emitting diodes is presented. Letting spontaneous emission happen in a planar cavity will in the first place affect the extraction efficiency. The internal intensity distribution is not longer isotropic due to interference effects (or density of states effects). The basics of dipole emission in planar cavities will be shortly reviewed using a classical approach valid in the so called weak-coupling regime. The total emission enhancement or Purcell factor, although small in planar cavities, will be explained. The design of a GaAs/AlGaAs RCLED is discussed. We review the state-of-the-art devices in different semiconductor material systems and at different wavelengths. Some advanced techniques based on gratings or photonic crystals to improve the efficiency of these devices are discussed. RCLEDs are not the only candidates that can be used as high-efficiency light sources in communication and non-communication applications. They compete with other high-efficiency LEDs and with VCSELs. The future prospects of RCLEDs are discussed in view of this competition.
III-V nitride semiconductors are suitable for LEDs having color range from blue to green. Luminous intensity and color purity of these LEDs are so high that they have been used for many applications for full color display and LCD backlight and so on.
In addition to natural colored LEDs, we have developed short wavelength LED, named TG Purple, which wavelength is typically around 380nm. TG Purple has been realized by controlling Indium composition in GaInN well layers. It can activate photo-catalysts such as TiOx, and therefore, the air purifier for automotive has been developed by combining TG Purple and TiOx photo-catalyst. The short wavelength LED is now the best light source for automotive air purifier using photo-catalyst, because LEDs fulfill the mercury-less requirement for an environmental issue and don't need special circuit like conventional UV lamps such as black light lamps and cold cathode lamps. Furthermore this short wavelength LED is used for phosphor excitation that generates many colors like blue, green, red, etc. It is likely that, with this technology, LEDs will take some part in illumination market as one of primary light sources like incandescent lamps and fluorescent lamps.
The light extraction process in GaN-based light emitting diodes (LEDs) is studied in this paper. In order to increase the light extraction efficiency of large area LEDs, several novel LED geometries are discussed. The light propagation in the LEDs is simulated numerically by using the finite-difference time-domain (FDTD) method. It is shown that the following improvements in the GaN-based LEDs are very effective for increasing the light extraction: (1) To fabricate GaN micro-pyramid array on the surface of the LED, which guides the generated light to the surface; (2) To make inverted V-shaped groove formation on the GaN layer, which restricts the average length of ray path in the LEDs and refracts the waveguide-mode light to the surface; (3) To separate the LED epilayer from its substrate and then mount it on a metal mirror base, which is used to reflect the backside light to the LED surface. The FDTD simulation results show clearly that these improved geometries guide most of the internal luminescence to escape from the LED, and increase greatly the external light-extraction efficiency of GaN-based LEDs.
Effect of degradation processes on transient currents in LEDs has been studied. It has been found that transient currents are several orders of magnitude higher than steady-state currents. The transient current time dependencies are non-exponential and show a distribution of relaxation times in the range of 1-100 microseconds. The charge associated with the transient currents is Q ~3x10-10 C which corresponds to high number of carrier traps Nt ~ 2x109 in the investigated chips. For one-year old chips an increase of charge and trap number by ~ 25% has been found compared to the fresh chips. Two probable reasons have been suggested to explain the observed increase of number of carrier traps: first one is related to increase of the number of trap sites at dislocations, and second one is a gradual phase separation process in quantum wells resulting in degradation of their quality.
A few yield loss issues in AlInGaN-based LED manufacturing are addressed in this paper. V-defects initiated from N-type GaN, multiple quantum well region, and P-type GaN are classified by their size and depth. Their impacts on device performance are discussed and the effective ways to eliminate or reduce V-defects are presented in detail. An approach using multiple composite inter-layers in a highly doped Si-GaN layer to reduce cracks is proposed. Each individual composite inter-layer reduces the stress accumulated from the layer underneath and thus keeping the N-GaN layer free from crack. The composite inter-layer is a pair of InxGa1-xN/GaN thin layers grown at low temperature (LT). Design rules and growth conditions are also discussed. Other issues which may cause yield loss or troublesome in AlInGaN LED manufacturing are briefly touched such as wafer color non-uniformity, bad cut in chip dicing, wavelength and light output correlation at wafer, chip and lamp level.
Data are presented for an GaInN based thinfilm LED. The LED is fabricated by transferring the epilayers with laser lift off from sapphire to a GaAs host substrate. In combination with efficient surface roughening and highly reflective p-mirror metallisation an extraction efficiency of 70% and wall plug efficiency of 24% at 460nm have been shown. The chips showed 12mW @ 20mA with a Voltage of 3.2V. The technology is scalable from small size LEDs to high current Chips and is being transferred to mass production.
A high-reflectivity omni directional reflector (ODR) has been incorporated into a GaInN light-emitting diode (LED) structure. The ODR comprises a transparent, electrically conductive quarter-wave layer of indium tin oxide clad by silver and serves as an ohmic contact to p-type GaN. It is shown that ODR-LEDs have low optical losses and high extraction efficiency. Mesa-structure GaInN/GaN ODR-LEDs emitting in the blue wavelength range are demonstrated and compared to GaInN/GaN LEDs with semitransparent Ni/Au top contacts. The extraction efficiency of ODR-LEDs is higher as compared to conventional LEDs with Ni/Au contacts.
Data links incorporating a green light source at 520nm are required for use with polymethyl methacrylate (PMMA) plastic optical fiber (POF) systems because they have a lower attenuation coefficient compared with conventional red light sources at 650nm. Recently, green LEDs have been developed based on Gallium Nitride (GaN) materials, and high optical output power GaN green LED lamps are now commercially available for general use in display applications. In this paper, we describe in detail the fundamental characteristics of these GaN green LEDs that are due to be employed in POF data links. We evaluate the temperature coefficients of the optical output power and the center wavelength shift and also demonstrate a green LED POF data link that complies with IEEE 1394 S100 operation. GaN green LEDs seem to be promising candidates as light sources for the next generation of POF data links for automotive applications or for long distance In-house multimedia networks. This is because, as we will show, they can operate both at high temperatures and with reduced temperature sensitivity compared with red LEDs fabricated from AlGaInP materials.
High-brightness InGaN light emitting diodes (LEDs) with an output power of 2.7, 2.3, and 1.8 mW at a driving current of 20 mA for the emitting wavelength of 470, 505, and 525 nm, respectively, were realized using metalorganic vapor phase epitaxy. The I-V characteristic of these devices experiences a reverse-bias voltage higher than 60 V for a leak current of 10 μA. We find out that the dislocation density in the n-GaN layer is crucial to achieve such high breakdown voltage. By varying growth parameters, we can tune the breakdown voltage from 10 V to 60 V.
High-efficient light emitting diodes (LEDs) emitting red, amber, green, blue and ultraviolet light have been obtained through the use of an InGaN active layers. The localized energy states caused by In composition fluctuation in the InGaN active layer seem to be related to the high efficiency of the InGaN-based emitting devices in spite of having a large number of threading dislocations (TDs). InGaN single-quantum-well-structure blue LEDs were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. The characteristics of both LEDs was almost same. These results indicate that the dislocation doesn't affect the efficiency practically.
Recently, the development of high-power light source using GaN-based LEDs has become active. In such high-power LEDs, the density of forward current is much higher than that of past LEDs. Therefore, an advantage of carrier localization in InGaN active layer becomes small, because of band filling under high injection level. This means that reducing the density of TDs becomes important, just like GaN-based laser diodes. Also, we show recent results of GaN-based LEDs.
We have performed theoretical studies on the luminous characeristics of white LED light source which composed of multi phosphors and near ultraviolet (UV) LED for general lighting. White LED source for general lighting applications requires the conditions that have high-flux, high luminous efficacy of radiation (> 100 lm/W) in addition to high color rendering index (Ra > 90) and variable color temperatures. Recently, we have proposed a novel type white LED based on multi phosphors and near UV LED system in order to high-Ra (>93). We will describe the excellent luminescence properties of white LED consisting of orange (O), yellow (Y), green (G) and blue (B) phosphor materials, and near UV LED. The color spectral contributions of individual phosphor-coated LED are theoretically analyzed using our multi LED lighting theory calculated the maximum luminous efficacy can be estimated to be approximately 300 lm/W having a high Ra of about 90 taking into account individual radiation spectrum. Illuminance distribution of white LED is in fairly good agreement with the experimental data.
Everywhere in the world, the highest quality and quantity of lighting is required during the surgical operations. However, the surgical approach has had many types and various angles, common ceiling surgical halogen shadow less lighting system cannot provide an adequate amount of beams because the surgeons' heads hinder the illuminations from reaching the operation field. Therefore, we have designed surgical lighting system composed of white LEDs equipped on both sides of goggles. In fact, we succeeded in the first internal shunt operation in the left forearm using the surgical LED lighting sytem on 11th Sept 2000. In the operation with sitting position, it was about 34 cm from the operation field to the surgeon's eye point. Therefore, in the next approach, we have to try the operations with usual standing position. To get the more powerful LED light source, we have tried to make "power white LED module" composed with Nichia white LEDs (NCCx002) on AlN plate. Then we have tried the general thoracic operation with LED goggles composed "power white LED modules" on 9th December 2002.
Light emitting diodes (LEDs) operated at relatively low currents when first used for automotive exterior signal lighting. These devices were generally the 3 mm or 5 mm LEDs that were lighted with an input current of no more than 30 milliamperers per LED. Generally these LEDs were used in center high mounted stop lamps or side marker lamps. In most cases the center high mounted stop lamps required 30 to 70 of these low powered devices depending on LED type, location on the vehicle and customer requirements. Today LEDs are being used for not only the center high mounted stop lamp, but literally all exterior automotive signal lamps. These LEDs are exceedingly brighter than the 3 mm or the 5 mm devices were. These brighter devices require the use of fewer LEDs per lamp function, but at the expense of requiring higher drive currents thus generating more heat. It is this heat generation that creates an issue that must be addressed by the lamp designer. The heat being generated by the latest generation of LED devices must be removed to ensure that the device maintains it capabilities to efficiently generate light. To enable the LED to operate efficiently certain factors of the LED must remain unchanged as it is operated over the life of the lamp in an automotive application. Two factors that must experience little or no change are the lumen output of the LED and the emitted color. The heat issue must be analyzed and eliminated or controlled when utilizing the newest "high power," high current devices in an automotive exterior signal lamp. The scope of this paper will identify the parameters that must be controlled to minimize the heat generated by the chip such that the light output of these latest "high power" LEDs is maximized and maintained.
High power deep ultraviolet (UV) light emitting diodes (LEDs) are good candidates for solid-state lighting, bio-chemical detection and short-range communications. In this paper we describe the progress from our and other research groups towards fabricating 340-280 nm LEDs. In past we have reported on deep UV LEDs on sapphire substrates with active region comprised of pulsed atomic layer epitaxy (PALE) deposited quaternary AlInGaN and ternary AlGaN multiple quantum wells (MQWs). These studies indicated a key role played by current crowding (thermal effects), active region design (polarization effects) and the base material quality (active region defects originating from buffer AlGaN layers) in controlling the emitted powers. Now using a unique AlGaN/AlN superlattice to control strain we have deposited Si-doped high Al-content n+-AlGaN layers over sapphire with thickness in excess of 2 μm. These layers and a new active layer design have yielded high power deep UV LEDs with emission wavelength from 280-340 nm. For 325 nm emission devices powers as high as 10 mW for 1 A pulsed pump current and 1 mW for 100 mA dc pump current were measured. For 280 nm emissions a power of 0.47 mW for 260 mA dc and 3 mW for 1 A of pulsed pump current was measured. For III-N deep UV LEDs, these values to date represent devices with the highest powers for the shortest wavelengths. In this paper we present the details of material and device fabrication and characterization. Initial data on device life-tests are also presented.
The requirements for maximizing the external quantum efficiency of UV nitride LEDs are discussed. It is shown that as the chip wavelength progressively decreases, nitride epi growth on a sapphire substrate becomes advantageous in terms of light extraction. The epilayer requirements for UV LEDs dictate the growth of n-AlGaN, with increasing Al contents, and the growth of UV-transparent p-GaN. It is shown that MOCVD growth in a Emcore D-180 or Ganzilla reactor is ideal for meeting the stringent epilayer requirements. Increasing light extraction efficiency and wall-plug efficiency also requires optimization of the reflecting P-contact. The relative merits of Al- and Ag-based reflecting contacts are discussed. Performance data for UV LEDs on sapphire, for drive currents up to 700 mA is shown. Finally, a practical high power UV-based white lamp is demonstrated.
There have been many innovations and technological advancements in balloon angioplasty since its introduction in the late 1970's, but percutaneous intervention on a totally occluded artery is still a challenge to the vascular interventionalist. Catheter-based intervention that avoids an invasive surgical procedure is a clear and desired advantage for the patient. A total occlusion challenges the interventionalist because the path of the artery can not be seen in the occluded vessel since the flow of the radiopaque contrast media is blocked. Optical coherence reflectometry techniques have been shown to be able to differentiate between artery wall and occlusive materials allowing the lumen of the blocked artery to be seen inside the occlusion. Light emitting diodes are a critical component of these systems making them technologically possible and economically feasible.
Laser diodes are used in high radiance applications, requiring spatially and angularly localized optical power, in order to minimize system size, cost, and power requirements. For applications requiring visible wavelengths (400nm to 700nm), AlInGaP laser diodes are utilized for wavelengths between 630nm and 650nm, and recently InGaN-based laser diodes spanning the violet to blue spectral range (400nm to 450nm) have become available, with longer wavelength devices being actively developed. It is not clear, however, when reliable laser diodes emitting in the green to yellow portion of the visible spectrum (>500nm) will be realized. In addition, single mode laser diodes required for high radiance applications typically exhibit speckle effects, are temperature sensitive, require non-linear drive electronics, and can be effected by optical feedback. High radiance LEDs with appropriate emitter dimensions can be used in place of laser diodes provided that moderate coherence and power levels are acceptable. This article examines the general dependencies of beam focusing, collimation, and coherence, on LED emitter size and radiance. Specific model examples are considered, using parameters consistent with typical laser diode collimating lenses. Edge emitting LEDs or EELEDs used for miniature scanned displays are described as high radiance LEDs suitable for other applications requiring collimation, partial coherence, or localized irradiance.
We report the performance and reliability of a 1500 nm superluminescent diode fabricated using a post-growth quantum well intermixing technique. The one-step impurity induced quantum well intermixing technique incorporating ion implantation through graded-thickness implant mask pattern is utilized here. By thermally diffusing the vacancies through the structure to the QW region, we can obtain a differential bandgap energy shift across the wafer by an amount directly related to the implant mask thickness. We use this effect to broaden the full-width half maximum of the superluminescent diode. Output powers of multiple milliwatts with full-width half maximum larger than 90 nm and spectral modulation better than 0.2 dB have been achieved from ridge waveguide multiple quantum well structure. The superluminescent diode is able to operate up to 85°C showing good uncooled operation. The true inherent superluminescent mode operation of the superluminescent diode with full-width half maximum increases along with the increment of the current injection is also discussed. Accelerated aging at continuous constant current has been carried out at 70°C, 85°C and 100°C. The life test shows a very positive result, demonstrating that this QWI technique is reliable for fabricating active devices.
Light Sciences Corporation has developed a novel LED array that was designed and manufactured to treat large bulky tumors. We describe our LED design process, culminating in the manufacture of a flexible silicone catheter currently under investigation in a Phase 1 clinical trial. The performance characteristics of the wire-bonded die to a flexible polyimide substrate forming a linear array are discussed. The LED array consists of 100 die arranged asymmetrically on the substrate with 50 LED's on either side producing up to 60mW total optical power at 38°C (500mA) over a spectral bandwidth 645-670nm FWHM. The LED's are encapsulated within biocompatible silicon for interstitial placement within the treatment tissue. The effect of time, temperature and humidity on the device performance was investigated. Optical power ranged from −2.5% to +0.5% of the normalized original power over 50 hours in 100% RH within the control group. Over a temperature range of 35°C to 50°C the optical power decreased at a rate of 0.56% per °C. Preliminary non-clinical experiments carried out in normal swine muscle demonstrate a significant treatment zone and are consistent with threshold models for photodynamic effect.
A speckle free laser probe beam is desirable for applications where a laser probe beam is preferred but laser speckles are troublesome. For example, it is an ideal replacement of laser or SLD probe beam for ophthalmic wavefront instrument that employs a Hartmann-Shack wavefront sensor. The speckle free laser probe beam employs a rotating disk of holographic phase plate in a laser probe beam path to modify the relative phase across the beam and to randomize the modified relative phase rapidly in a time sequence. The holographic phase plate is designed to produce a small but well-defined diffused angle such that the beam quality of the modified probe beam can remain close to diffraction limit. Our experiment shows that spot image with such a modified laser pobe beam is substantially free of speckles. Design considerations and parameters, as well as potential applications, of the speckle free laser probe beam are presented.
Image resolution, tissue penetration, and scan speed are among the most important parameters when designing an OCT system for ophthalmic use. Human retinal tissue is highly reflective in the near infrared spectrum range. A SLD at 820nm with 25nm FWHM spectral bandwidth provides 10μm coherence length in retinal tissue. Its appropriate power level, simplicity of use, high resolution, and relatively low cost, make the 820nm SLD the best choice light source for retinal OCT. A 1300nm SLD can penetrate deeper into the sclera tissue and since the 1300nm wavelength is highly absorbed in the vitreous, the ANSI laser safety standard allows higher maximum permissible power to the human eye. Higher scan speed can also be achieved. In this paper, we report two OCT systems that are designed specifically for retinal and anterior segment imaging of the human eye. Retinal OCT scans 400 A-scans per second, 2mm depth in tissue, and 10 μm image resolution with an 820nm SLD. Anterior segment OCT (AC-OCT) scans 2000 A-scans per second, 6mm depth in tissue, and 16μm image resolution with a 1300nm SLD. Benefits of suitable wavelength selection in scanning different tissue are clearly seen in the OCT images. Retinal OCT (OCT3) demonstrates significant improvement over the previous generation (OCT1/OCT2) from both a technical and cost point of view. AC-OCT performs 8 frames of 256 A-scans per second and is capable of imaging the human eye in vivo with minimum eye motion artifacts. It has potential use in refractive surgery, angle-closure glaucoma, and cataract surgery.
Wavefront technology promises to change the way vision care will be conducted. Wavefront-sensing optometers provide instant, accurate measurement of the total wave aberration of the eye, and that one measurement contains all the information needed for refractive vision diagnosis and refractive vision correction. Wavefront optometers are now being used to create individualized laser vision correction based, not on the coventional sphero-cylindrical correction, but rather on the total wavefront errors in patients' eyes. Comprehensive vision diagnosis based on the wave aberration and the image quality derived from it is far different from the conventional test of visual acuity. Wavefront technology has made it possible to image microscopic features as small as the photoreceptors and to improve resolution of retinal imaging techniques for early diagnosis of retinal diseases. This article is a review covering the hsitory and progress being made in the development of the wavefront technologies.
Animas Corporation is developing a long-term (>5 years), implantable optical, blood glucose sensor based on near-infrared absorption spectroscopy. While the glucose sensing R & D community tends to over promise results to the general public without convincing scientific evidence due to the business potential for the sensing market, Animas would like to present solid data showing robust glucose calibration and prediction. In vitro data from whole blood in more than 500 patients, with various medical histories, shows an excellent correlation (R2=0.94) between glucose concentrations determined using sensor prediction and traditional fingerstick measurement. The numbers of outliers identified in the thousands of measurements from the 500 patient population are less than 1%. Better than 13 mg/dl accuracy was achieved in dogs in in-vivo testing. Glucose data showing excellent tracking between measurements with the Animas sensor and a Hemocue glucometer will be presented. One of the technical challenges of developing the Animas implantable sensor is to fabricate laser diodes that have stable emission spectra, especially for those that emit in wavelength above 2 μm. Requirements for various kinds of diode light sources will be discussed.
Recently new phosphors from various material classes have been developed for LED applications by Osram OS and partners. Excitation wavelengths of these phosphors range from below 400 nm to 470 nm, enabling the creation of purple and unsaturated LED colors and even the efficient conversion of near UV-radiation into white light. By addition of red and green phosphors to white LEDs, a warm white color impression can be achieved. These LEDs are suitable for all purposes of general lighting, where a high color rendering is required. An outlook to new applications with unsaturated and warm white LEDs will be given.