Radiometry: The Measurement of Optical Radiation



Radiometric Laboratory Facilities

A general purpose prism refractometer instrument was designed to measure the thermo-optic coefficient (change of index of refraction with temperature) of optical materials. The current wavelength range is from 350 nanometers to 15 micrometers. We have measured over a temperature range from 10K to 770K but currently limit the range to ~62K and up. The nominal uncertainties in index are n/104 and the temp coefficient is within 10% of value. Precision refractometry at room temperature in the visible is done with a Wild Model 79 precision spectrometer with visible line sources. The nominal uncertainties are less than 0.001% with this instrument.   More detailed information can be found in the PDF document “Prism Refractometer at the University of Arizona“.  The procedure for sample preparation and typical measurement costs are in the PDF document “OSC Cyrogenic Refractometer – Prism Specifications.”  Equation fitting to data is also routinely done as an aid in the determination of the thermo-optic coefficients.  A brief listing of dispersion equations is available (PDF).

More on Radiometry

One topic of current interest to me involve the proper usage of our symbols, units and nomenclature. High on the list is the proper and improper usage of the term intensity.

Another personal area of interest is blackbody radiation simulation. Blackbody radiation is inherently lambertian. Some of the newer designs, taking their cue from effective light-trapping structures, sacrifice lambertian characteristics to achieve a higher effective emissivity in a preferred direction. This is typically accomplished using a specular black surface in the cavity rather than a diffuse black. Indeed, the use of a specular black in a light trap can significantly increase the absorptance for some directions at the expense of others. However the use of this design can lead to polarization in the radiated power if the trap design is “linear” (i.e., a wedge) rather than axially symmetric (i.e., a cone). Watch here for future developments!

Trap detectors are of great interest in the radiometry community. The earliest trap detector that I have seen dates back near the turn of the century, the Smithsonian water-flow pyrheliometer. In this context, I am defining a trap detector as one wherein the reflected component is returned to the active surface for another chance for absorption. A recent design I am studying is a refractive trap, accomplished by gluing three Hamamatsu S-1337 detectors to three faces of a BK7 or silica pentaprism. The positive feature is that the detector surfaces are protected, not exposed to the environment like other traps. This makes it more rugged and durable. The downside is one first-surface reflection from the entrance side of the prism. This can be characterized if the index of refraction is known. In addition, the small (4%) reflection can aid in alignment and can be easily maintained. Stay tuned for more.

Another interest, an outgrowth of the take-home lab associated with my now-defunct OPTI 350 junior-level radiometry course, is the radiometric characteristics of our nearest neighbor, the Moon.

Look to the page More radiometric links for more about radiometry.