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Photometric Standards

This encompasses the realization and maintenance of scales of measurement for luminous flux, illuminance, luminous intensity (the candela), and luminance. A key element for each of these scales is the use of detectors whose spectral responsivity closely matches the CIE-defined standard photopic observer function. Recent and ongoing research includes:

  • development of design and fabrication techniques for multilayer filters for photopic and bandpass applications
  • development of photopic detectors and their characterization
  • realization of an illuminance responsivity scale with improved uncertainty
  • development of trap-detector based photometers

Spectrophotometry

This is the characterization of materials by determining their optical properties as a function of wavelength; the principal properties involved being reflectance, both diffuse and specular, and transmittance, both regular and diffuse. Recent and ongoing work includes:

  • Theory of the integrating sphere. This an old subject but the errors and lack of understanding apparent in recent papers, particularly in relation to the sphere with a flat port, created a need for rationalization and removal of the confusion.
  • Improvement in techniques for transmittance measurement with particular attention to the determination of nonlinearity in the UV, visible and IR bands.
  • Special techniques for reflectance: the development of a true-normal-incidence reflectometer.

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Colour

  • Development of colour rendering indices for specific applications such as the sorting of fruit, assessment of foodstuffs, etc.
  • Improved techniques for the measurement of colour coordinates of high-visibility fabrics.

Optical Fabrication and Instrument Metrology

  • Flatness. An automated system based on computer analysis of video-camera recordings of interferograms in the flat-Fizeau interferometer is being developed to replace the existing manual measurement system. This will remove subjectivity in the analysis of films and should lead to improved uncertainties.
  • Refractive index standard.
  • Development of techniques for characterizing photographic filters and lenses using a Twyman-Green interferometer.
  • Development of techniques for optical instrument metrology: aperture ratio (f/no.), field brightness, contrast, resolution, and wavefront distortion of aerial survey cameras.
  • Applications of thin films hardened by ion-beam-assisted deposition

Feedback-controlled electrical substitution measurement

A fundamental problem in the theory of measurement is the development of new ways to improve resolution and reduce measurement time in the measurement of physical quantities.

The practice of substituting an electrical quantity for the quantity of interest, using a sensor to observe the balance between the two quantities, and using feedback to maintain the equivalence of the substitution ¾ a technique known as feedback-controlled electrical substitution measurement (FCESM) ¾ is being applied to an increasing number of physical quantities.

The importance of a greater understanding of this technique became apparent during the realization of the candela in 1988 using an photometer that was based on a Hengstberger room-temperature radiometer. Because measurement times for the desired resolution were such that the lamps were being run for a time that was longer than was desirable, a study was made of how the feedback system should be designed to obtain an optimal compromise between resolution and measurement time. More recently, papers have appeared showing the application of FCESM to the measurement of acceleration, mass, sound pressure, flow, and the gravitational constant G as well as to radiant power.

Noise or fluctuations in the measurement system limit the resolution and measurement speed that are possible. At MSL, with the Hengstberger radiometer in mind, we developed measurement strategies for FCESM radiometers and other first-order transducers under the assumption of a noise spectrum dominated by white noise from the transducer . It has, however, become apparent in the recent development of very sensitive radiometers, in which the white-noise fluctuations have been greatly reduced, that the dominant noise has a 1/f spectral distribution and may include 1/f2 and 1/f3 components. This has created a need to review the technique and develop appropriate strategies for FCESM in these circumstances: we have been invited to work on this in collaboration with a group at NIST, Boulder.

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