Spectroradiometry by Means of Modified Spectrophotometers Harry K. Hammond, III

National Bureau of Standards, Washington, D.C. Received 25 June 1963.

Spectroradiometric data on light sources have been obtained in relatively small quantities by relatively few laboratories with a variety of equipment designed or assembled locally for this ex­plicit purpose. The author knows only one instrument of Ameri­can manufacture that was marketed as a spectroradiometer for light sources, and it is no longer available. The small number of instruments required per year and the specialized use discourage instrument manufacturers from attempting to design spectro-

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radiometers. The requirements for this type of instrumentation are conveniently met by modification of spectrophotometers de­signed for reflectance and transmittance measurements. Two papers reporting such modifications have appeared recently in Applied Optics. Brown1 describes the modification of a General Electric (GE) recording spectrophotometer. Grum2 describes a modified Beckman DK-2R recording spectrophotometer. These papers are welcome additions to the instrumentation literature, but like other recent papers on spectroradiometry they fail to deal with the evaluation of the intensity of spectral lines and tend to mislead the reader to conclude that accurate spectroradiometry of complex light sources is much more simple than is actually the case.

There is no disputing the fact that spectrophotometers can be modified to draw a curve that resembles the spectral irradiance distribution one would expect to obtain from a source such as a fluorescent lamp. Difficulties arise, however, when one asks such questions as: What does this curve mean? What is the ir­radiance from each of the various mercury lines relative to that from the continuum at the corresponding wavelengths? These are not new questions, but the papers cited above fail to discuss these important points. The GE spectrophotometer is designed with cams to provide an approximately constant passband. The Beckman spectrophotometer, however, is designed to adjust the slits automatically while scanning the spectrum so as to provide constant photomultiplier signals for the reference beam. This means that the slit width, and thus the passband, is a function of the spectral irradiance from the reference source. This method of operation is acceptable in spectrophotometry, where the light source, which is part of the instrument, emits a continuum and where the spectral curves are similarly continuous. Accurate spectroradiometry of sources containing spectrum lines, however, requires an accurate evaluation of the passband at the wavelength of the line. Accurate evaluation is more difficult when the pass-band is a function of the irradiance from the standard. For this reason constant mechanical slit widths are often preferred in spectroradiometry even in the case of prism instruments.

One purpose of this Letter is to suggest that each experimenter record slit settings and include the passband of the mono-chromator when reporting spectroradiometric data. The prob­lem of accurately assessing the intensities of spectral lines relative to continua is not an easy one, and a full discussion is beyond the scope of this Letter. Lord3 discussed the problem from a theo­retical point of view; Jerome4 and Henderson and Hallstead5

discussed this problem and others from the experimental point of view. Suffice it to say here that each experimenter must tackle the problem and that the accuracy of his solution should be veri­fied experimentally by taking data with widely different slit settings. Furthermore, he should not be content to apply with­out verification the dispersion curve of the instrument supplied by the manufacturer. Different slit configurations for double monochromators are advocated by various experimenters. In the one most frequently used by the author, the entrance and exit slits are set equal and the middle slit is set for a greater width. Under these conditions the recording of the line intensity is es­sentially triangular, and the intensity is proportional to the area and is computed from the product of the measured height of the line above the continuum and the width at half height. The width must be evaluated accurately, ideally on an expanded wavelength scale. When integration is required as in the de­termination of luminous flux or chromaticity coordinates, it is customary to obtain an equivalent intensity of the line by dividing the area of the triangle computed above by the integra­tion bandwidth, usually 10 nm. If the lines are sufficiently spaced or the dispersion sufficiently high so that all lines are com­pletely resolved, there is no great problem in their evaluation. The difficulty arises in the case of high-pressure arcs such as mercury and xenon when lines are broad and overlap. Here

again, the simplest solution appears to be to take data with equal entrance and exit slits and then to use a very fine integration bandwidth, say 1 nm, and simply treat the lines like rapidly varying continua.

Most experimenters have realized that the solid angle subtended at the entrance slit by the collimator lens or mirror of the spec-troradiometer must be filled with light from the test lamp and from the standard lamp in turn. To fill the collimator, a diffuser is frequently employed. At first thought, any diffuser, reflecting or transmitting, would seem to be satisfactory. Transmitting diffusers appear to offer an advantage because they can be made to pass a maximum of flux in the near rectilinear direction. However, experimenters who have made careful measurements with transmitting diffusers of ground glass or quartz have dis­covered that the tiny prismatic facets of these diffusers can cause systematic error. When a ground glass diffuser is irradiated by a lamp of known color temperature, the light transmitted by the diffuser in a rectilinear direction has a lower color temperature. This reduction in color temperature is caused by the fact that, in the process of transmission through the diffuser, blue light is preferentially deviated from the rectilinear direction to a larger extent than is red light. This phenomenon is caused in ground diffusers by the dispersion of the prism facets. In opal glass it is due to the fact that the opacifying particles are of the same order of size as the wavelength of the light.

Middleton and Smith6 made an extensive investigation of the diffusion of light by ground glass. Their data show that for crown glass and perpendicularly incident incandescent-lamp light of color temperature 2850°K, the light transmitted in the rectilinear direction is lowered in color temperature by a minimum of 150°K for a coarse-ground surface to a maximum of 550°K for the finest grind investigated. With the finest grind and an angle of view above 45°, the color temperature was raised by as much as 150°K. If the same diffuser is used with standard and test lamps, this effective change in color temperature, really a change in spectral distribution of the transmitted light, is not important as long as the two sources subtend the same angle at the diffuser. When a tubular fluorescent lamp, or even a short section of such a lamp, is measured relative to an incandescent lamp, ground glass diffusers should be avoided because of the spectral distribution error introduced by them with the change in angle of the source subtended at the diffuser. Reflecting dif­fusers are therefore preferred for comparing sources of different areas or geometric distribution of emitted flux. Plane surfaces of any diffusing material may be used, but the use of a small hollow sphere, with suitable openings, coated on the inside with a highly diffusing material (such as barium sulfate for the visible spectrum) has the great advantage of providing a mixing mech­anism, thereby permitting the diffuser to be irradiated from one direction by a nonhomogeneous beam and viewed by the mono-chromator from another direction, usually at right angles to that from which the sphere is irradiated, without any spatial inhomogeneity. If it is desired to compare two sources wave­length by wavelength, the sphere can be mounted so that it can be rotated about the viewing direction and the two sources can be mounted so that their irradiating directions make 90 degrees with the viewing direction and 90 or 180 degrees with each other.

References 1. W. J. Brown, Appl. Opt. 1, 227 (1962). 2. F. Grum, Appl. Opt. 2, 237 (1963). 3. M. P. Lord, Proc. Phys. Soc. 58, 477 (1946). 4. C. W. Jerome, Ilium. Eng. 45, 225 (1950). 5. S. T. Henderson and M. B. Hallstead, Brit. J. Appl. Phys. 3,

255 (1952). 6. W. E. K. Middleton and F. D. Smith, Can. J. Research F27,

151 (1949).

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