Proceedings Volume 3428

Illumination and Source Engineering

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Proceedings Volume 3428

Illumination and Source Engineering

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Volume Details

Date Published: 20 October 1998
Contents: 4 Sessions, 18 Papers, 0 Presentations
Conference: SPIE's International Symposium on Optical Science, Engineering, and Instrumentation 1998
Volume Number: 3428

Table of Contents

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Table of Contents

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  • Special Purpose Illumination Sources
  • Fibers and Light Pipes
  • LED Spectral Issues
  • Design of Illumination Optics
  • Fibers and Light Pipes
Special Purpose Illumination Sources
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Spectral radiance and temporal stability of a uniform radiance source integrating sphere with enhanced blue performance
A uniform source sphere was designed and built that included both tungsten-halogen and xenon arc lamps. The spectral radiance of the system and arc lamp stability were measured. Results show significant spectral radiance in the blue region of the spectrum, which is greatly improved over conventional systems that use only tungsten-halogen lamps. Short-term stability measurements of the radiance due to one arc lamp show stability of 0.26%. This value is more than ten times greater than that of a tungsten-halogen lamp, which is inherently more stable, but the xenon lamp is sufficiently stable for many applications. Considering the improved blue performance, xenon lamps offer possibilities for uniform source spheres used to calibrate cameras that operate in the blue region of the spectrum, such as remote sensing systems.
Vacuum-compatible standard diffuse source, manufacture, and calibration
Los Alamos National Laboratories has completed the design, manufacture and calibration of a vacuum-compatible, tungsten lamp, integrating sphere. The light source has been calibrated at the National Institute of Standards and Technology and is intended for use as a calibration standard for remote sensing instrumentation. Calibration 2(sigma) uncertainty varied with wavelength from 1.21% at 400 nm and 0.73% at 900 nm, to 3.95% at 2400 nm. The inner radius of the Spectralon-coated sphere is 21.2 cm with a 7.4 cm square exit aperture. A small satellite sphere is attached to the main sphere and its output coupled through a stepper motor driven aperture. The variable aperture allows a constant radiance without effecting the color temperature output from the main sphere. The sphere's output is transmitted into a vacuum test environment through a fused silica window that is an integral part of the outer housing of the vacuum shell assembly. The atmosphere within this outer housing is composed of 240 degree(s)K nitrogen gas, provided by a custom LN2 vaporizer unit. Use of the nitrogen gas maintains the internal temperature of the sphere at a nominal 300 degree(s)K +/- 10 degree(s). The calibrated spectral range of the source is 0.4 micrometers through 2.4 micrometers . Three, color temperature matched, 20 W bulbs together with a 10 W bulb are within the main integrating sphere. Two 20 W bulbs, also color temperature matched, reside in the satellite integrating sphere. A silicon and a germanium broadband detector are situated within the inner surface of the main sphere. Their purpose is for the measurement of the internal broadband irradiance. A fiber-optic-coupled spectrometer measures the internal color temperature that is maintained by current control on the lamps. Each lamp is independently operated allowing for radiances with common color temperatures ranging from near 0.026 W/cm2/sr to about 0.1 W/cm2/sr at a wavelength of 0.9 micrometers (the location of the peak spectral radiance).
Modeling of constant irradiance illumination system
The optical system discusses is an integrating sphere based uniform source. The system consists of an integrating sphere illuminated with a tungsten halogen lamp system. The output of the integrating sphere is projected using a large single- element collimating lens. Such a system is advantageous in the testing of optical detectors since in a specific region of the projected beam the irradiance is constant in a volume around the optical axis and for a significant distance along the optical axis. This property eases the positioning requirements of devices under test relative to the lens. Relative irradiance predictions of a plane in the collimated beam made using ZEMAX Optical Design Program are presented. Measured data of the relative irradiance in this specific plane achieved with the implemented optical system are presented. Comparison between the predictions and measured data are made. Predictions agreed with the measured data to within 5%.
Color-shadow TV filming of nonstationary gas flow
Igor Victorovic Ershov, Vladimir N. Otmennikov
The color shadow method with video taping for the gasdynamic nonstationary process investigation of wind tunnel is described. Different schemes of this method realization using shadow devices IAB-451, IZK-462, IAB-459 and others are presented. Great potential of this method using the Central Research Institute of Machine Building wind tunnels is demonstrated on specific study of gas flow pattern and its uniformity intricate shape model gas stream flow around, interaction of jet streams and other are shown.
Fibers and Light Pipes
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Design methods for illumination light pipes
Juan Manuel Teijido, Hans Peter Herzig, Claude Fuhrer, et al.
Light pipes are useful for non conventional illumination problems in cars, for displays, etc. The design of such devices is a challenging problem because of the large number of parameters. Depending on the application, the light pipes have various shapes and dimensions. This paper investigates different design strategies for light pipe illumination devices. A new generation of commercially available non- sequential ray-tracing programs allow the analysis of scattering light pipes. However, the modeling of complex optical systems requires the knowledge of the physical behavior and the related numerical algorithms. For the design optimization, ray-tracing is too cumbersome because of the large number of necessary rays. Therefore, we introduce a finite element approach to describe the power transfer through appropriately chosen volume elements. This method gives a good estimation of the macroscopic behavior of the light pipe. The fine tuning of the different pipe parameters is made easier. In addition, the possibility to simulate the light pipe in its surrounding environment by means of global illumination methods is discussed.
Characterizing fiber optic illuminators for lighting applications
Nadarajah Narendran, Nishantha Maliyagoda
This manuscript describes methods for evaluating fiber-optic illuminators. In an initial study, several 150-watt commercial fiber-optic illuminators were tested for system efficacy. The illuminators were tested with a 1-foot reference fiber optimized for the particular illuminator as supplied by the respective manufacturer. The variations in fiber sizes and configurations made comparative evaluation between systems more difficult. Furthermore, one data point per system doesn't provide sufficient information about the illuminator. Alternatively, two sets of standard fibers were utilized to evaluate fiber-optic illuminators that had a common single port configuration and size. The data generated with the fiber sets was useful for developing a family of plots showing the flux, flux density, and color variation as a function of fiber diameter. This method eliminates premature judgment of illuminator performance on only one measurement with one fiber diameter. Finally, the use of a CCD camera was evaluated for characterizing the beam of a reflectorized lamp, typically used in a fiber- optic illuminator. A low-wattage reflectorized metal halide lamp was used in this section of the study. The amount of flux available for coupling into a given size fiber and the fiber location for optimum light coupling were estimated using the CCD camera. The results obtained thus far show that the CCD camera is a useful tool for characterizing fiber-optic illuminators.
How to report light loss values for optical fibers used in fiber optic lighting applications
Andrew Bierman, Nadarajah Narendran, Nishantha Maliyagoda
This manuscript attempts to explain why reporting a single value for light loss or a single wavelength-dependent attenuation graph is of minimum use to the lighting specifying community. Experimentally it was found that the light loss for different diameter fibers of the same type with the same illuminator varied from 1.1 to 1.5 percent per foot. This is mainly attributed to the higher number of reflections at the core-cladding interface when light travels through a smaller diameter fiber compared to a larger diameter fiber. In a similar experiment it was found that the light loss for the same fiber, size and material, on three different commercial illuminators with the same 150-watt metal halide lamp varied from 1.4 to 2.4 percent per foot. The numerical aperture through which light is coupled into a fiber and the spectral uniformity at the common-end of the fiber can affect fiber attenuation. It is shown here that the color separation of a metal halide lamp has minimal impact on the light attenuation and thus the greatest light loss effect is due to the launch angle. Reporting a family of attenuation plots, as function of fiber diameter and launch angle would be more useful for predicting light loss and also for developing high efficiency fiber optic lighting systems.
LED Spectral Issues
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Colorimetry, chromaticity space, and LEDs
Carolyn F. Jones
LEDs are narrow-band emission sources and present special problems in colorimetric characterization. Chromaticity space is mapped using Gaussian spectral models to represent narrow-band emission sources. The Gaussian maps show that the spectral regions are distorted and non-uniform, with a curious `warp' in the yellow. These maps do not resemble the traditional Kelley chart. The Gaussian models are used to understand effects of spectral shape on colorimetric values. The models predict the sensitivity of dominant wavelength to spectral width and spectral peak. The predictions are compared to LED measurement data. The predictions have practical implications for LED colorimetry, including measurement conditions and equipment resolution.
Blue LED photometry
Blue LEDs with very high luminous intensity and shorter peak wavelength create new problems in their evaluation. Specially constructed photometers are needed for accurate evaluation.
Novel method to correct inaccuracies of photometer heads for the measurement of LEDs
Gyorgy Czibula, Janos P. Makai
LED's are radiators with a spectrally narrow emission band generally less than 50 nm. Compared to broadband light sources, like lamps, their spectral power distribution can be classified as quasi-monochromatic. Photometer heads integrate the spectral emission of a light source for the whole visible spectral range, this generally averages out a great part of the error caused by the local spectral mismatch of the V((lambda) ) detector. On the contrary, if LED's are measured this local mismatch error appears entirely in the measurement results. Measuring red LED's with a photometer head of f1' <3% the absolute error can be even greater than 5%. The same head produces for commercial broadband light sources absolute errors less than 0.8% (in many cases less than 0.3%). Concerning quality industrial measurements this level of high error is not acceptable. A simple method will be shown how to increase the measurement accuracy of existing photometer heads. A computer simulation using generated and real LED spectral power distributions as well as spectral responsivity curves of a large number of real photometer heads took place. The results show that this method provides much higher absolute accuracy than the classical one.
Centroid wavelength of LEDs determined by broadband measurement
Carolyn F. Jones
The centroid wavelength of a spectrum can be determined directly from the ratio of quantum and radiant flux values. This method is a fundamental spectral measurement since it satisfies the mathematical definition of centroid wavelength. Using broad-band methods for measuring quantum and radiant flux, the centroid wavelength can be determined from a ratio of photocurrents. This simple technique yields spectral information, which is normally available only by spectroradiometry. The technique is extendable beyond the visible spectrum, and has useful applications beyond LED measurements. LEDs are used to demonstrate this method for determining centroid wavelength. The LEDs are characterized by both broad band methods and spectroradiometry. The results are discussed and evaluated. The measured LEDs range from red to blue and also include white. The broad-band measurements are made without prior knowledge of the spectral distribution. For LEDs, the accuracy can be improved by using correction techniques. The broad-band centroid wavelength is within +/- 5 nm of the value determined by spectroradiometry.
Design of Illumination Optics
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Uniform LED illuminator for miniature displays
Vladimir Medvedev, David G. Pelka, William A. Parkyn
The Total Internally Reflecting (TIR) lens is a faceted structure composed of prismatic elements that collect a source's light over a much larger angular range than a conventional Fresnel lens. It has been successfully applied to the efficient collimation of light from incandescent and fluorescent lamps, and from light-emitting diodes (LEDs). A novel LED-powered collimating backlight is presented here, for uniformly illuminating 0.25'-diagonal miniature liquid- crystal displays, which are a burgeoning market for pagers, cellular phones, digital cameras, camcorders, and virtual- reality displays. The backlight lens consists of a central dual-asphere refracting section and an outer TIR section, properly curved with a curved exit face.
Design of illumination lenses via extrinsic differential geometry
William A. Parkyn
A novel method is described of designing lenses for general illumination tasks that are not circularly symmetric. The crux of the method is the specification of the illumination task by a grid on the unit sphere of directions. This grid, or tessellation, has cells which vary in solid angle such that each encompasses the same luminous flux: high intensity corresponds to small cells, and vice versa. Another grid, having the same topology and number of cells, is formed according to the intensity distribution of the source. An illumination lens must then transform the source distribution into the task distribution, via one or more refractions. Thus the direction vector of a cell in the source grid must be redirected into that of the corresponding cell in the task grid. Snell's law in vector form enables the derivation of a corresponding surface normal vector, or sequence of normal vectors, that will accomplish this redirection. Extrinsic differential geometry is then used to generate a lens surface having, as closely as possible, this distribution of surface normals, which must be irrotational to generate a smooth surface. This class of lenses has only recently become producible due to the advent of electric-discharge machining for the shaping of non-rotationally symmetric injection molds for plastic lenses.
New tool for determining diode laser beam intensity profiles in military weapon simulators
Jack L. Arnold, James C. Solinsky
The paper presents recently developed CAD capabilities which allow the precision design of the kill zones in MILES lasers used for force-on-force training. Effects of dynamic atmospherics are quantitatively addressed. A graphical user interface is presented. The system designer is allowed to do `what if' laser and detector designs with the goal of accurately emulating combat outcomes. Algorithms are described for laser radiation field calculations, target detector geometry, and the modeling of dynamic atmospheric effects. Kill zone plots are presented. Use of the software to explore design options for area effect weapons is described, as is use of the software to explore the effects of recoil.
Nonrotationally symmetric reflectors for efficient and uniform illumination of rectangular apertures
A requirement for a uniformly illuminated rectangular aperture is common in optical design, particularly for projection applications. The naive approach to producing uniform illumination over a rectangular aperture is to uniformly fill a circular zone which circumscribes the desired rectangular region. Unfortunately, this technique is wasteful of flux, e.g. a rectangular aperture having as aspect ratio of 1.5:1 collects only 59% of the flux incident on the circumscribing circle. Other approaches such as rectangular light pipes and mosaic lens arrays dilute source etendue, increase the length of the optical train, and can suffer from losses due to multiple reflections. We have previously discovered that the performance limit due to skewness conservation for collecting light from a 3D source and projecting it into a beam can be overcome by a numerically optimized reflector with a nonrotationally symmetric star-like cross-section. In this paper we present preliminary results of our research into designing high- efficiency, single-element nonrotationally symmetric reflectors which provide uniform flux delivery into rectangular apertures.
Fibers and Light Pipes
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Aberration effects on illumination beams focused by lens systems
John F. Forkner
In previous papers the author has shown that lens systems focusing wide-angle, large diameter illumination beams do not focus the output beam-waist at the location predicted by the standard paraxial lens imaging equations. The paraxial equations fail because they are meant to apply to rays that lie in the plane of the lens system axis of symmetry- meridional rays, whereas illumination beams are composed of skew or out-of-plane rays. The skew-ray equations derived in the author's earlier papers turn out to provide the correct description of the non-imaging optics of focused illumination beams. The skew-ray beam equations allow the basic lens system parameters; focal length and conjugate distances to be determined from the input beam diameter and divergence angle. The lens system parameters calculated from the skew-ray beam equations are optimal in the sense that they define an optical system that theoretically satisfies the fundamental requirement of conservation of E'tendue. Therefore if the appropriate aberrations of this theoretically optimum illumination beam optical system can be corrected the system should approach 100 percent geometric throughput. This paper discusses how certain types of imaging optical systems can be used as models for the design of equivalent non-imaging illumination beam- transformer optical systems. The use of these image system models permits a direct assessment of the importance of the classical image aberrations in the design of highly efficient illumination beam optics. The paper also outlines design methods for optimizing illumination beam optics using existing imaging optics software. The conclusion is that practical illumination optics beam-transformers can be designed with throughputs above 99 percent, even using a single lens.
Test methods for optical fiber
Mary Poppendieck
This paper proposes a simplified method for measuring the transmission and attenuation of optical fiber used for illumination. Use of an integrating sphere, in combination with the traditional fiber optic cut-back measurement approach, is described. Use of color filters to measure specific sets of wavelengths is also proposed. A `blue filter' method of measuring yellow color shift is described. Tests run using a lumen meter and a blue filter are summarized.
System approach to remote illumination design
Alexander Rizkin
System concept implementation for the remote illumination system (RIS) design has been discussed. The relationship between application criteria and system specifications is formulated based on the RIS configuration. An engineering approach to RIS design, evaluation and optimization using system efficiency calculation is described.