Light sources in digital projection experience a transition from Xenon to laser pumped phosphor and pure RGB laser sources with constantly growing light flux. Today’s state of the art cinema projectors with laser sources are achieving a light flux of up to 75000 lumens to illuminate screens of 30 m diagonal and more. With increasing light flux the thermal load on the optical components increases, limiting the quality of the projection. The optical system of high-end cinema projectors usually consist of a large volume prism assembly and a high-end projection lens system. Especially the prism assembly experiences a high thermal load due to long light paths. The requirements on the performance of the optical glass in terms of maximum transmittance therefore constantly increases. Proper material characterization and selection helps to enable future projection requirements.
Laser phosphor projection sources enable a cost effective way to generate high light flux, since they take advantage of the recent cost down of blue laser diodes and do not need to use expensive green and red lasers. The primary colors are e.g. generated by blue laser diode illuminating a phosphor wheel to create yellow light. The yellow light is subsequently split into green and red light by means of e.g. a dichroic filter. With increasing light flux standard phosphor wheels that are based on a phosphor embedded in silicone, degrade at high peak temperatures. Wheels with a ceramic phosphor have a much higher temperature stability and offer a significant improvement. This paper discusses the requirements on optical materials used for digital projection.
The refractive index, Abbe number and transmittance are the most important properties of optical glass. Nevertheless, in many applications the spatial refractive index variation - called homogeneity – is of highest priority. The index homogeneity of optical glass is a key property in industrial metrology devices and professional movie cameras for example. These applications often use small to medium sized lenses produced from cold cuts or hot pressings. On the other side of the range, scientific instruments of large astronomical telescopes require optical glasses with excellent index homogeneities on large apertures of several 100 mm up to even meter sized lenses e.g. for atmospheric dispersion correction. The challenge of enabling highest refractive index homogeneities in small and large dimensions requires tight control of all production steps from melting to hot forming and fine annealing. This paper gives an insight overview on the process of generating high quality glass part from small glass pressings to meter sized glass blanks. It gives suitable help for the interpretation of refractive index homogeneity of optical glass in relation to their dimensions. Latest results added to this overview reflect the current state of this topic at the optical glass manufacturer SCHOTT.
Optical glasses with certain inner quality e.g. low striae content are essential for good optical systems. A stria is a small local change in the refractive index inside the glass due to small local changes of the glass composition. A stria results in a wave front distortion that can cause a blurring of the image. The effect of striae inside optical glasses on different optical systems are simulated in order to value its significance. Such striae simulations turned out to be difficult and bringing the design software to its limits. Some pitfalls are discussed leading to criteria for trustworthy (reliable) simulation results. These criteria are shown and finally reliable simulation results for an eye piece used in a microscope are shown. Furthermore the results obtained in this publication lead to additional work on striae simulations.
More and more applications utilize the short wave infrared (SWIR) spectral range. The SWIR range is defined from about 0.9 to 3 μm. SWIR applications can be found for example in inspection processes of circuit boards, solar cells, bottles and food. The SWIR range is used in identification, sorting, surveillance, inspection and more. With SWIR applications characteristics can be visualized that normally would not be detectable with visible light, like rotten fruits in fruit sorting, fakes in paintings, content levels in visually non-transmitting bottles. For all these machine vision applications specific optics are used that in the ideal case have transmittance in the visual spectral range and in the SWIR range. Optical designs require materials that are transmittance in the visible and the SWIR range, sometimes even up to 4 μm. Most optical glasses have a good transmittance up to 2 μm, but at 2.5 μm the transmittance strongly decreases. There is also not much available information on the details of the transmittance curve in the SWIR range available of optical glass. This presentation will demonstrate SCHOTT optical glasses with good transmittance even up to 4 μm. If applications require transmittance at even larger wavelengths, it is possible to utilize IRG infrared material with transmittance up to 8 µm, that still can be used in the visible down to 0.6 μm. Other critical information for the optical designs are the dispersion and index change with temperature (dn/dT) characteristics in the SWIR range of these materials. This paper will discuss the availability of such data.
Optical systems in space environment have to withstand harsh radiation. Radiation in space usually comes from three main sources: the Van Allen radiation belts (mainly electrons and protons); solar proton events and solar energetic particles (heavier ions); and galactic cosmic rays (gamma- or x-rays). Other heavy environmental effects include short wavelength radiation (UV) and extreme temperatures (cold and hot). Radiation can damage optical glasses and effect their optical properties. The most common effect is solarization, the decrease in transmittance by radiation. This effect can be observed for UV radiation and for gamma or electron radiation. Optical glasses can be stabilized against many radiation effects. SCHOTT offers radiation resistant glasses that do not show solarization effects for gamma or electron radiation. A review of SCHOTT optical glasses in space missions shows, that not only radiation resistant glasses are used in the optical designs, but also standard optical glasses. This publication finishes with a selection of space missions using SCHOTT optical glass over the last decades.
The fluorescence of optical glasses is a property that needs to be taken into account in optical designs for life science applications. Many optical glasses from SCHOTT show a very low intrinsic or auto-fluorescence. The fluorescence depends mainly on the applied excitation wavelength and the optical glass type. The fluorescence of optical glasses is usually defined as the quotient of the integral of the emission spectrum with the integral of the emission spectrum of a reference glass. This definition does not give any information about the actual quantum efficiency of the fluorescence. In this presentation recent data on the integral fluorescence of SCHOTT optical glasses are presented. Additionally, first measurements of the quantum efficiency of SCHOTT optical glasses are presented and compared to the standard method.
Optical glasses with certain inner quality e.g. low striae content are essential for good optical systems. A stria is a small
local change in the refractive index inside the glass resulting in a wave front distortion that can cause blurring of the image.
During the production process of optical glass, striae are observed by measuring it with the so-called shadow graph method.
This simple measurement displays a stria as a shadow on an observation screen. A human operator evaluates the contrast
by comparing it with references. The new proposed approach uses a digital camera and image processing to measure human
independent the stria level. A first repeatability measurement shows wave front deviation (maximum deviation, peak-topeak)
of less than +/- 8 nm.
Femtosecond lasers are more and more used for material processing and lithography. Femtosecond laser help to generate three dimensional structures in photoresists without using masks in micro lithography. This technology is of growing importance for the field of backend lithography or advanced packaging. Optical glasses used for beam shaping and inspection tools need to withstand high laser pulse energies.
Femtosecond laser radiation in the near UV wavelength range generates solarization effects in optical glasses. In this paper results are shown of femtosecond laser solarization experiments on a broad range of optical glasses from SCHOTT. The measurements have been performed by the Laser Zentrum Hannover in Germany. The results and their impact are discussed in comparison to traditional HOK-4 and UVA-B solarization measurements of the same materials. The target is to provide material selection guidance to the optical designer of beam shaping lens systems.
The upcoming extremely large telescope projects like the E-ELT, TMT or GMT telescopes require not only large amount of mirror blank substrates but have also sophisticated instrument setups. Common instrument components are atmospheric dispersion correctors that compensate for the varying atmospheric path length depending on the telescope inclination angle. These elements consist usually of optical glass blanks that have to be large due to the increased size of the focal beam of the extremely large telescopes.
SCHOTT has a long experience in producing and delivering large optical glass blanks for astronomical applications up to 1 m and in homogeneity grades up to H3 quality in the past.
The most common optical glass available in large formats is SCHOTT N-BK7. But other glass types like F2 or LLF1 can also be produced in formats up to 1 m. The extremely large telescope projects partly demand atmospheric dispersion components even in sizes beyond 1m up to a range of 1.5 m diameter. The production of such large homogeneous optical glass banks requires tight control of all process steps.
To cover this demand in the future SCHOTT initiated a research project to improve the large optical blank production process steps from melting to annealing and measurement. Large optical glass blanks are measured in several sub-apertures that cover the total clear aperture of the application. With SCHOTT's new stitching software it is now possible to combine individual sub-aperture measurements to a total homogeneity map of the blank. In this presentation first results will be demonstrated.
Highly chromatic corrected optical systems rely on optical glasses with precise optical positions represented by refractive index and Abbe number. A modern production of optical glasses requires an economical, fast and accurate way of monitoring its fabrication. We demonstrate that an automated Hilger-Chance type refractometer fulfills all these needs. Therefore the uncertainty of a set of optical glasses is analyzed on the basis of a high number and long time reproducibility measurements. It turns out that the standard deviations after several hundreds of measurements taken over almost an decade in refraction is better than 10-5 in refraction and 0.02% in dispersion.
Many laser applications need a homogeneous - so called flat hat - light distribution in the application area. However,
many laser emit Gaussian shaped light. The technology of diffractive optical elements (DOE) can be used to shape the
Gaussian beam into a flat hat beam at a compact length. SCHOTT presents a DOE design of a flat hat DOE beam shaper
made out of optical glass. Here the material glass has the significant advantage of high laser durability, low scattering
losses, high resistance to temperature, moisture, and chemicals compared to polymer DOEs. Simulations and
measurements on different DOEs for different wavelength, laser beam width, and laser divergence are presented.
Surprisingly the flat hat DOE beam shaper depends only weakly on wavelength and beam width but strongly on laser
divergence. Based on the good agreement between simulation and measurement an improved flat hat DOE beam shaper
is also presented.