Several near-infrared instruments on upcoming extremely large telescopes use a combination of OHARA S-FPL51 and S-LAH71 glass prisms to correct for the atmospheric differential refraction. Because large parts of these instruments are contained within a vessel cooled to liquid nitrogen temperatures, the changes in optical and mechanical properties should be considered during the design phase.
In this report, we provide an update to our continuing efforts to characterize the S-FPL51 and S-LAH71 glass under cryogenic conditions. These efforts include a measurement of the refractive index (293 K, 100 K and 80 K), transmission measurements (293 K, 77 K and 43 K) and a determination of the coefficient of thermal expansion (between 293 K and 45 K). We will describe the cryogenic test setups that we have used to obtain our results, and provide summarizing equations of these material properties.
This characterization will prove useful for conceptual design trade-offs, mechanical interface solutions and detailed optical design for future near-infrared instrumentation on large telescopes.
The angular dependent optical properties of low-e and solar control glazings have been investigated in a European project, ADOPT, within the Standards Measurements and Testing programme. Two new predictive algorithms have been proposed and tested. One algorithm is based on an empirical formula and the other is based on a hybrid equivalent model, which approximates the actual coated glass. The validation procedure has included measurements as well as Fresnel calculations. The algorithms allow the user to calculate the solar heat gain coefficient versus angle of incidence for any transparent window combination without having to perform time-consuming measurements. For the tested window configurations the precision is mostly within one percentage point as compared to measured results.
KEYWORDS: Thermal modeling, Glasses, Mathematical modeling, Optical coatings, Transmittance, Data modeling, Solar radiation models, Solar energy, Thin film coatings, Thermography
Mathematical modelling can be a powerful tool in the design and optimalization of glazing. By calculation, the specifications of a glazing design and the optimal design parameters can be predicted without building costly prototypes first. Furthermore, properties which are difficult to measure, like for instance solar and visible light properties for oblique or diffuse irradiation, can be determined accurately by calculation. At the TNO Institute of Applied Physics, a glazing model for simulation and optimalization is being developed, which allows to determine according to international standards, glazing specifications, ranging from optical properties (solar energy, visible light, color appearance) to thermal properties (U-value, thermal shock resistance, fire control properties). The model is connected to a data base containing optical constants and other properties of glasses, coatings and other materials, used in glazing. The paper discusses the Glazing Model which consists of an Optical Transfer Model simulating the optical behavior, and a Thermal Transfer Model which simulates the thermal behavior. The paper gives an overall description of the Glazing Model. A detailed description is given of the matrix formulation which forms the basis for the optical transfer model. An example of a spectrally selective double glazing illustrates how the model works. Present and future developments are also discussed.
Directional emittance plays an important role in the calculation of radiative heat exchange. It partly determines the thermal insulation of single and multiple glazing and the efficiency of solar collectors. An emissiometer has been designed and built, capable for measurements of the directional emittance at angles up to 85 degree(s). The emissiometer can be used for absolute measurements, with a black body radiator as reference, and for relative measurements using a known reference sample. In the case of low-emissivity materials the directional emittance usually has a maximum at an angle between 80 degree(s) and 90 degree(s) (pseudo Brewster angle). The emissiometer is, therefore, especially useful for the accurate characterization of low-emissivity materials like spectral selective coated glazing and solar absorbers. The paper gives a description of the instrument and results are discussed for three coated glass samples with low, medium, and high emissivity, respectively, which were measured for a temperature of 283 K. Results are given of the calculation of the thermal transmittance (or U-value) and of three types of double glazing in which the low, medium, and high emissivity coating was applied.
Solar and visual light transmittance, color appearance, thermal emissivity, and other optical properties of architectural glazing are in general angular dependent. Realistic computation of solar properties, therefore, requires the angular behavior to be known. Determination of these properties for normal or near-normal incidence according to the standards is part of a standard procedure for most laboratories, while experimental data on their angular behavior can, in general, only be obtained with great difficulty. An alternative way of determining angular properties is by calculation. This is possible using a model based upon Fresnel's equations and the optical constants of all optical media involved (glass and coatings). The optical constants (spectral complex refractive index) can be derived from the transmittance and reflectance spectra which are measured for normal or near-normal incidence. The paper discusses in detail both the measurement methods and the computer model for the determination of directional and hemispherical solar energy transmittance of single and double glazing. Results are given for single and double glazing with and without spectrally selective coating (SnO2).
Michael Hutchins, P. Dolley, Karlhanns Gindele, Michael Koehl, Ulrich Frei, Bo Carlsson, Sakae Tanemura, K. Hollands, Peter van Nijnatten, J. Havinga, A. Gonzalea, E. Mezquida
Results are presented of a round-robin programme of optical properties measurements
carried out on a commercial chemically oxidised stainless steel spectrally selective solar absorber
surface and two commonly employed reference calibration standards. Measurements of solar
absorptance and thermal emittance were obtained using a number of different spectrophotometric,
broad-band and radiometric techniques. The results are analysed for each technique employed and
comparisons are also made of results obtained from different methods. The results highlight
differences which can occur when supposedly identical methods are employed using different
commercially available instruments and reflectance attachments. The importance of understanding
calibration techniques, the use of diffuse and specular reference standards, calculation methods
for optical properties determinations and sample dependent observable effects are clearly
demonstrated. Lessons have been learned regarding 'routine' measurements which will assist in
developing better practices and refining recommended procedures for commonly quoted surface
optical properties.
As part of Task X in the Solar Heating and Cooling Programme of the International Energy Agency,
methods for the lifetime assessment of solar absorber coatings have been investigated. In a first step, the
degradation caused by the individual contributing factors (e.g. temperature, moisture, 502) was tested
for each factor separately.
The results of the temperature tests in particular could be used for a lifetime estimation with respect
to this single degradation factor. The errors in the measured degradation of the coatings, as detected by
the changes of the solar absorptance and the thermal emittance, and their influence on the estimated
lifetime, were investigated empirically in a Round Robin Test. This was performed on the same black
chrome coating by seven different laboratories following the same test procedures.
The results agree surprisingly well and are very encouraging for the definition of a recommended
standard procedure for the assessment of the thermal stability of solar absorber coatings in the future.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.