Radiometry. Photometry

The diffuse and regular reflectances of five optically absorbing coatings frequently used in optical systems, were measured over the 0.32 μm to 14.3 μm wavelength range, before and after exposure to heat and intense optical radiation. The measured coatings included Nextel Velvet Black, an anodised coating and NPL Super Black. The anodised coating exhibited substantial variations in its diffuse and regular reflectance values after thermal and simulated solar ageing. Solar and thermal ageing of the Nextel Velvet Black resulted in increases of its reflectance. However, thermal ageing tended to decrease the reflectance of the other paint samples examined. Thermal and solar ageing of the NPL Super Black resulted in only minor changes in its reflectance characteristics. All measurements are traceable to the UK National primary standards.

All the articles discussed & analysed here for calculating the important parameters such as solid angle subtended by the beam at the point-source, total area intercepted by the beam with a spherical surface & cone angle of equivalent beam with circular section. These formula are very useful for replacing the rectangular profile by circular profile of a beam emitted by a uniform point-source & vice versa without any change in the total radiation energy/luminous flux associated with the original beam (having either rectangular or circular profile). Thus the articles discussed & analysed here are very useful for the analysis of radiation energy & determining the intensity of (uniform) point-source in a given direction in Radiometry & for the analysis of luminous flux & luminous intensity of (uniform) point-source in a certain direction in Photometry. These are also very useful in replacing the rectangular aperture by a circular aperture & vice versa without any change in the total radiation energy/luminous flux passing through an original aperture & thus useful for the analysis & the designing of optical\beam emitting (like laser) devices based on (uniform) point-sources.

This study, presenting a history of the measurement of light intensity from its
first hesitant emergence to its gradual definition as a scientific subject, explores two
major themes. The first concerns the adoption by the evolving physics and
engineering communities of quantitative measures of light intensity around the turn of
the twentieth century. The mathematisation of light measurement was a contentious
process that hinged on finding an acceptable relationship between the mutable
response of the human eye and the more easily stabilised, but less encompassing,
techniques of physical measurement.
A second theme is the exploration of light measurement as an example of
‘peripheral science’. Among the characteristics of such a science, I identify the lack
of a coherent research tradition and the persistent partitioning of the subject between
disparate groups of practitioners. Light measurement straddled the conventional
categories of ‘science’ and ‘technology’, and was influenced by such distinct factors
as utilitarian requirements, technological innovation, human perception and
bureaucratisation. Peripheral fields such as this, which may be typical of much of
modern science and technology, have hitherto received little attention from historians.
These themes are pursued with reference to the social and technological
factors which were combined inextricably in the development of the subject. The
intensity of light gained only sporadic attention until the late nineteenth century.
Measured for the utilitarian needs of the gas lighting industry from the second half of
the century, light intensity was appropriated by members of the nascent electric
lighting industry, too, in their search for a standard of illumination. By the turn of the
century the ‘illuminating engineering movement’ was becoming an organised, if
eclectic, community which promoted research into and standards for the measurement of light intensity.
The twentieth-century development of the subject was moulded by
organisation and institutionalisation. Between 1900 and 1920, the new national and
industrial laboratories in Britain, America and Germany were crucial in stabilising the
subject. In the inter-war period, committees and international commissions sought to
standardise light measurement and to promote research. Such government- and
industry-supported delegations, rather than academic institutions, were primarily
responsible for the ‘construction’ of the subject. Practitioners increasingly came to
interpret the three topics of photometry (visible light measurement), colorimetry (the
measurement of perceived colour) and radiometry (the measurement of invisible radiations) as aspects of a broader study, and enthusiastically applied them to industrial and scientific problems.
From the 1920s, the long-established visual methods of observation were
increasingly replaced by physical means of light measurement, a process initially
contingent on scientific fashion more than demonstrated superiority. New
photoelectric techniques for measuring light intensity engendered new commercial
instruments, a trend which accelerated in the following decade when photometric
measurement was applied with limited success to a range of industrial problems.
Seeds sowed in the 1920s – namely commercialisation and industrial application, the
transition from visual to ‘physical’ methods, and the search for fundamental
limitations in light measurement – gave the subject substantially the form it was to
retain over the next half-century.

When performing high accuracy radiation thermometry, the size‐of‐source effect (SSE) of a radiometer can provide a significant contribution to the uncertainties associated with the measurements. During the development of a new radiometer designed specifically to measure the melting points of High Temperature Fixed Point cells, indirect SSE measurements were performed on a prototype instrument to aid selection of optical components and their optimum positions with the aim of minimising its SSE. As the radiometer’s objective lens can produce much of the scattered light that contributes to the SSE, a set of objective lenses were compared and found to have SSEs of between 7 × 10‐ 4 and < 6 × 10‐ 5. Further improvements were found by controlling the positioning and size of the stray light reducing Lyot stop. The diameter of the Lyot stop had to be set carefully: too small and it provides a low SSE but reduces the instrument’s signal from the source; too large a diameter and it provides little or no reduction in SSE. The sensitivities in the Lyot stop and collimating lens positions were tested and the instrument’s SSE was found to be tolerant of small displacements of either the lens or Lyot stop, however larger movements yielded an increase in SSE. The extremes in position increased the SSE to 3 × 10‐ 4 for the collimating lens and 4 × 10‐ 4 for the Lyot stop.

The demand for improved radiometric accuracy of the remote sensing instrumentation used for diagnostic applications involving hot gas emission spectroscopy requires regular “in-field” recalibration. The most convenient calibration source for such applications is a large emitting area blackbody capable of operating at temperatures approaching 1000 K which is also compact and portable. NPL in collaboration with the University of Reading have designed and assembled a large emitting area blackbody that meets these requirements. The blackbody design is based on a grooved base that is electrically heated to temperatures up to 1000 K. The base is coated with a high emissivity coating, which does not deteriorate during prolonged heating under atmospheric conditions. This base is enclosed by a specularly reflecting cavity that is water-cooled. Monte Carlo calculations were used to design the shapes of the base and reflective cavity to ensure that despite a cavity depth of 203 mm and a black body aperture diameter of 102 mm, the spectral radiance of the blackbody is known with a 1% uncertainty in the 2.5 μm to 14 μm wavelength range. The presentation will describe the design of the blackbody and the processes used for selecting the black coating of the base and the reflective coating of the specularly reflecting cavity.

Abstract White-light dispersion experiments performed by Newton with prisms remain among the most beautiful in optics and are frequently replicated in schools and universities. But there is an aspect that is little considered, namely that the light initially refracted within the equilateral prism, in addition to being further refracted and giving out the well-known spectrally dispersed bundle, is also internally reflected an indefinite number of times and, in correspondence with each reflection, produces an external refractive bundle that can be white or dispersed depending on the parity of the number of reflections previously undergone. In this paper, I analyze, both theoretically and experimentally, the properties of the main bundles coming out of the faces of the equilateral prism, when it is illuminated by p- or s-polarized white light, or by a laser beam.

The effect of the cooling rate on hardness and thermal conductivity in a metallurgical Jominy bar made of AISI 1018 steel, by means of a water end-quenched heat treatment process without diffusion-controlled case depth, is studied with photothermal radiometry (PTR). It is concluded that our two PTR techniques, common-mode rejection demodulation (CMRD) and conventional 50% duty-cycle square-wave frequency scan, are sensitive to low hardness values and gradients, unlike the high values all previous photothermal studies have dealt with to-date. Both PTR methods have yielded an anti-correlation between thermal conductivity and microhardness in this case as in previous cases with heat-treated and diffusion-controlled case depth profiles. It is shown that the cooling rate strongly affects both hardness and thermal conductivity in the Jominy-bar heat-treating process. © EDP Sciences.

White-light dispersion experiments performed by Newton with prisms remain among the most beautiful in
optics and are frequently replicated in schools and universities. But there is an aspect that is little considered, namely that the light initially refracted within the equilateral prism, in addition to being further refracted and giving out the well-known spectrally dispersed bundle, is also internally reflected an indefinite number of times and, in correspondence with each reflection, produces an external refractive bundle that can be white or dispersed depending on the parity of the number of reflections previously undergone. In this paper, I analyze, both theoretically and experimentally, the properties of the main bundles coming out of the faces of the equilateral prism, when it is illuminated by p- or s-polarized white light, or by a laser beam.