Broadband Radiometric LED Measurements.

At present, broadband radiometric measurements of LEDs with uniform and low-uncertainty results are not available. Currently, either complicated and expensive spectral radiometric measurements or broadband photometric LED measurements are used. The broadband photometric measurements are based on the CIE standardized V(λ) function, which cannot be used in the UV range and leads to large errors when blue or red LEDs are measured in its wings, where the realization is always poor. Reference irradiance meters with spectrally constant response and high-intensity LED irradiance sources were developed here to implement the previously suggested broadband radiometric LED measurement procedure [1, 2]. Using a detector with spectrally constant response, the broadband radiometric quantities of any LEDs or LED groups can be simply measured with low uncertainty without using any source standard. The spectral flatness of filtered-Si detectors and low-noise pyroelectric radiometers are compared. Examples are given for integrated irradiance measurement of UV and blue LED sources using the here introduced reference (standard) pyroelectric irradiance meters. For validation, the broadband measured integrated irradiance of several LED-365 sources were compared with the spectrally determined integrated irradiance derived from an FEL spectral irradiance lamp-standard. Integrated responsivity transfer from the reference irradiance meter to transfer standard and field UV irradiance meters is discussed.

Atomic clock with 1×10(-18) room-temperature blackbody Stark uncertainty.

The Stark shift due to blackbody radiation (BBR) is the key factor limiting the performance of many atomic frequency standards, with the BBR environment inside the clock apparatus being difficult to characterize at a high level of precision. Here we demonstrate an in-vacuum radiation shield that furnishes a uniform, well-characterized BBR environment for the atoms in an ytterbium optical lattice clock. Operated at room temperature, this shield enables specification of the BBR environment to a corresponding fractional clock uncertainty contribution of 5.5×10(-19). Combined with uncertainty in the atomic response, the total uncertainty of the BBR Stark shift is now 1×10(-18). Further operation of the shield at elevated temperatures enables a direct measure of the BBR shift temperature dependence and demonstrates consistency between our evaluated BBR environment and the expected atomic response.

High-Quality Brewster's Angle Polarizer for Broadband Infrared Application.

We have designed and constructed a linear polarizer for use with visible and infrared radiation. The broadband polarizer consists of four germanium plates arranged in a chevron geometry. Input radiation is incident near Brewster's angle for the first plate such that the reflected beam is preferentially s-wave polarized. This reflected beam is steered subsequently to the successive plates, always intersecting near Brewster's angle. The beam polarization at the output of the device is almost completely s-wave polarized. The ratio of the paraxial flux of the nearly extinguished p-wave polarized light to the s-wave polarized light transmitted through the device is found to be less than 10(-5) for laser illumination at wavelengths of 0.633, 1.32, 3.39, and 10.6 mum. Calculations predict that extinction ratios less than 10(-5) are achievable over the wavelength range from 0.4 mum to beyond 500 mum. Alternative design geometries involving fewer plates are also described along with their advantages and disadvantages.

Magnification of conic mirror reflectometers.

Conic mirror reflectometers are used to measure the diffuse reflectance and total integrated scatter of surfaces. In spite of the long history of using conic mirrors for these purposes, the maximum magnification of the three primary types of conic mirror (hemisphere, hemiellipsoid, and dual paraboloid) had not been compared quantitatively. To our knowledge, an exact magnification formula has not been published for any of the three primary conic mirrors. The maximum magnification is needed for proper sizing of detectors and radiation sources used with reflectometers. Exact analytical expressions for the maximum magnification of a Coblentz hemisphere, a hemiellipsoid, and a dual-paraboloid mirror system are derived and compared.

Testing the radiometric accuracy of Fourier transform infrared transmittance measurements.

We have investigated the ordinate scale accuracy of ambient temperature transmittance measurements made with a Fourier transform infrared (FT-IR) spectrophotometer over the wavelength range of 2-10 mum. Two approaches are used: (1) measurements of Si wafers whose index of refraction are well known from 2 to 5 mum, in which case the FT-IR result is compared with calculated values; (2) comparison of FT-IR and laser transmittance measurements at 3.39 and 10.6 mum on nominally neutral-density filters that are free of etaloning effects. Various schemes are employed to estimate and reduce systematic error sources in both the FT-IR and laser measurements, and quantitative uncertainty analyses are performed.

Effects of non-Lambertian surfaces on integrating sphere measurements.

The effects of non-Lambertian scattering of the interior wall of an integrating sphere are examined through a sphere simulation model. The model employs Monte Carlo techniques. A sphere used for measurement of directional-hemispherical reflectance is modeled. The simulation allows sphere wall scattering to vary from perfectly Lambertian to perfectly specular in steps. The results demonstrate that significant measurement error can result as the scattering deviates from the Lambertian ideal. The error is found to be a strong function of the wall reflectance value as well: it is minimized for reflectances approaching 1.0 and increases as the reflectance value decreases to the minimum value examined of 0.5. The magnitudes of the errors associated with non-Lambertian scattering are also shown to be relatively independent of the specific field of view of the detector used in the measurement.

Improved integrating-sphere throughput with a lens and nonimaging concentrator.

A reflectometer design utilizing an integrating sphere with a lens and nonimaging concentrator is described. Compared with previous designs where a collimator was used to restrict the detector field of view, the concentrator-lens combination significantly increases the throughput of the reflectometer. A procedure for designing lens-concentrators is given along with the results of parametric studies. The measured angular response of a lens-concentrator system is compared with ray-trace predictions and with the response of an ideal system.

Integrating sphere designs with isotropic throughput.

An ideal diffuse reflectometer can be defined as a reflectometer with a throughput which is independent of the angle of reflected radiation, as measured at the sample. For integrating spheres, effects related to the detector's field of view (FOV), the beam port, and internal baffles can result in a throughput which is nonisotropic. This paper analyzes these three sources of nonideal behavior and suggests three sphere designs using nonimaging concentrators which minimize FOV related errors. A technique for measuring the error due to the beam port is also discussed as well as ways of minimizing perturbations caused by baffles.

Effects of restricting the detector field of view when using integrating spheres.

Integrating sphere theory is developed for restricted field of view (FOV) detectors using a simple series solution technique. The sphere throughput, sample reflectance, and sphere wall reflectance are calculated. The effects of the sample's scattering characteristics on sphere measurements are determined. It is shown that although the generalized equations incorporating detector FOV dependence reduce to the hemispherical FOV equations in some cases, in general integrating sphere behavior is altered through restriction of the detector FOV.

Infrared surface-wave interferometry on W(100).

An IR grating on a clean W(100) surface is shown to generate both homogeneous and inhomogeneous surface electromagnetic waves. An observed interference between these two components, which can be described in terms of a two-beam interferometer with variable arm amplitude and fixed optical path, is used to measure the plasma frequency accurately in the IR.