The general term for optical surfaces better than the lowest level of the scratch-dig standards has become "supersmooth", and is seen in technical literature as well as in advertising pieces. A performance number, such as BRDF, which can be measured from the uncoated optical surface by equipment such as the Variable Angle Scatterometer at the Air Force Wright Aeronautical Laborataories at Wright-Patterson Air Force Base is proposed as a method of generating better optical surface specifications. Data show that surfaces of average BRDF values near 10 parts per billion per steradian (0.010 PPM/Sr) for β-βo = 0.5, are now possible and measurable.
Ultra-black etched electroless nickel plate was developed by the National Bureau of Standards as a high absorption solar collector surface finish. This etched electroless nickel was plated on different surfaces and substrate materials for which such quantities as bi-directional reflectance, total integrated scatter, absorptivity and emissivity were measured. Temperature cycling, outgassing and adherence were investigated. These experimental data were used to determine the suitability of this etched nickel plate as a baffle coating. This finish is unsuitable for infrared applications. Investigations at BASD are being conducted to extend the suitability of this surface finish to the infrared. Investigations are not considered complete; however, major improvements in the etched nickel process indicate potential for a suitable baffle coating for a majority of applications. The solar emissivity has been preserved, while greatly enhancing the room temperature emissivity.
The total reflectance of certain black coatings which might be used as optical blacks has been measured at near normal incidence in integrating sphere spectrophotometers from 0.2 to 20.0 micrometers. Scanning Electron Microscope (SEM) pictures of some of the surfaces have been made. Some of the surfaces exhibit relatively constant spectral reflectance, but others show extensive spectral selectivity. Surface topography shows wide variations in structure and appears to correlate with reflectance properties. Renewed interest in the emissivity and absorptance of surfaces for thermal radiative transfer in cryogenics, space, solar applications, etc., has brought to bear technologies for measurement which were not practical in the early days of the space program. Two such technologies are SEM and integrating sphere spectrophotometers in the 2.5 to 20.0 micrometer range.
Vector theory has been used to obtain the power spectral density of the surface height distribution at several different wavelengths, conditions of polarization and geometries. The surface height was then determined both by integration and by curve fitting.
Normal incidence, specular reflectance spectra of three samples of optical black coatings have been measured in 10 filter passbands at effective wavelengths between 12 and 700 μm. Strong absorption, due to amorphous silicate material in the coatings, is found in the 15-50 μm region of each spectrum. Reflecting-layer model calculations are fitted by a non-linear least-squares routine to the spectra at wavelengths beyond the absorption band to determine far-infrared optical parameters. It is found that, with a similar wavelength dependence, the absorption coefficient of a new black coating (ECP 2200) made by the Minnesota Mining and Manufacturing Company (3M) is less than 1/10 that of the discontinued coating, 3M Black Velvet Nextel. It is also shown that the extinction effect of a very rough substrate is negligible compared with the effects of increased upper surface roughness and coating thickness attendant upon fully covering a very rough substrate.
Kirchoff's scalar theory must be modified when applied to diffraction from three-dimensional objects. Different approaches are presented and their predictions for wide-angle diffraction from rounded edges are compared to brute-force angular spectrum calculations.
The results of experiments designed to study the relationship between the visible scatter from mirrors and their particulate contamination are described. Data for both "clean" and deliberately contaminated samples is presented. The results reveal some of the features of surface contaminant scatter and indicate that this scattering mechanism could account for the discrepancies that have been observed between measured scattered light profiles and profiles predicted by theories based on random surface microstructure.
Then a isolated particle is hang up by optical levitation, the sampling of his scattering diagram give the possibility to determine the infrared optical characteristics of powders, atmospheric dursts... The inversion of Mie formalism, carried out by solving a system of non-linear equations according to the Newton-Paphson method, leads to the determination of the complex refractive index. A basic system for studying the 10.6 pm scattering has been made and the first results are given.
The surface quality of optics used in an extremely sensitive laser instrument, such as a Ring Laser Gyro (RLG), is critical. Experimental evidence is presented on the damage to this class of optics that results from typical processing and handling. Also presented is a possible solution to this problem.
Proc. SPIE 0384, Principles Of Stray Light Suppression And Conceptual Application To The Design Of The Diffuse Infrared Background Experiment For NASA's Cosmic Background Explorer., 0000 (4 October 1983); doi: 10.1117/12.934947
The Diffuse Infrared Background Experiment (DIRBE) is a 10 band filter photometer that will operate at superfluid helium temperatures. Diffuse galactic and extragalactic infrared radiation in the 1-300 micrometer wavelength region will be measured by the instrument. Polarization measurements will be made for 3 bands in the 1-4 micrometer spectral region. The main sources of unwanted radiation are the sun, earth, thermal radiation from an external sun shield, the moon, the brighter planets and stars, and sky light itself from outside the instrument's nominal one degree square field of view. The system level engineering concepts and the principles of stray light suppression that resulted in the instrument design are presented.
During the design of a stray light suppression system that requires a great amount of stray light attenuation, someone inevitably will ask or consider a specular black coating on the sections where the system is most sensitive. Although the initial attraction is obviously due to the very low large angle scatter, about 100 times lower than a typical diffuse black scatterer, the choice is seldom helpful. The first argument against specular black cavities is the propagating specular beam. Usually this is so obvious a consideration that the designer immediately folds it into consideration. What is seldom considered is the several ways that the near specular paths, which are almost always present, can contribute significantly higher levels of scattered light than diffusely coated surfaces. This paper highlights some of the second level considerations that should be made when specular coatings are to be used.
The APART/PADE stray radiation analysis program uses paraxial image theory to transfer radiation through lenses or to reflect it from mirror surfaces. However, this theory is inadequate in analyzing systems having specular nonimaging surfaces for the rejection of aperture heat loads. This paper describes two methods by which the capabilities of APART/ PADE can be extended by means of real-ray-based algorithms. The first method is to use ray tracing to calculate geometrical configuration factors (GCFs) when nonimaging specular reflections are involved. The GCFs can then be appended to the files used in Program 3 of APART/PADE. This approach can be easily extended by ray-splitting techniques to include the effects of surface scatter from the specular baffles. The second method is to use ray tracing to define a synthetic BRDF for a complex reflective baffle. The synthetic BRDF can be input to APART/PADE in the form of a lookup table, and it can accurately model the distribution of radiation emerging from the baffles. An example of this method is the analysis of an off-axis rejection test in which light from the radiation source is reflected from the baffle into the test chamber and thereby significantly increases the chamber background radiation.
A stray radiation analysis program (nicknamed MINI-APART after its namesake: APART) suitable for use on the HP-41C/CV calculator is described. The program is ideally suited for quick estimates of stray light performance in well-baffled optical systems, which are limited by scatter from the first optical element. Critical path models are described, including single scatter, double scatter, diffraction-scatter, and thermal emission-scatter. Program use is illustrated, and several comparisons are made with the results obtained by the large stray radiation programs, GUERAP-3 and APART/PADE.
Recent re-measurement of the Out-of Filed-of View-Rejection (OFVR) characteristics of the ZIP telescope #2 indicated that the Point Source Power Transmittance (PSPT) was increased by as much as a factor of 400 at one off-axis source orientation in comparison with the OFVR measurement of the identical telescope three years ago. The OFVR degradation was caused by particulate contamination of the telescope optical surfaces. After cleaning of the primary mirror, the OFVR performance was restored for off-axis angles of 2 degrees to 7 degrees, but the OFVR was still degraded by a factor of 2 to 20 for off-axis angles between 8 degrees to 15 degrees. This remaining degradation was also restored after cleaning of the secondary mirror. Since the original OFVR measurement, the telescope package was opened about 10 times in a Class 100 clean room environment for intergration and testing and was exposed to Class 100 environment 3 to 4 days at a time. Subsequently, the telescope was sealed and stored in an evacuated vessel at room temperature for two years.
The problem of light scatter from optical surfaces is amplified to a critical level for the optics used in the Ring Laser Gyro (RLG). This has led to the development of a scatterometer at the RLG Lab, Wright-Patterson AFB, which can detect low level light scatter from the high quality optics used in RLG's, without first overcoating with metals. A 5 mw Helium-Neon laser is used to illuminate a 0.5 Lam diameter spot on the surface of the test part. The incident angle can be varied from 0° to 90°, and the test piece can be maneuvered with five degrees of freedom, four of which are computer controlled. Scatter measurements are made with a photomultiplier detection system which can easily measure down to 10 parts per billion per steradian. The computer can analyze and plot the results very quickly and accurately for further interpretation.