John U. White

Very long optical paths in air

Two different multiple traversal optical systems are described; one gives the longest paths, the other the best compensation for vibration and misalignment problems. In the first, seven mirrors in a near confocal arrangement permit a large aperture beam of light to pass through a restricted volume for a discrete and very large number of times. A rectangular array of images corresponding to different numbers of passes appears on four mirrors at one end of the system. At the other end, three mirrors form the array and illuminate each image in it from one or more different directions. The possible numbers of passes are (4mn − 2)k + 2, where m and n are any integers representing, respectively, the number of columns and half the number of rows in the array. k is the number of different directions from which the array is illuminated. Geometrically, the beam may be isolated after thousands of passes; practically, the number is limited by reflection losses. In the second system the addition of four diagonal mirrors to a White cell converts the two lines of images on the single mirror to a rectangular array of images, almost squaring the maximum possible number of passes. With multiples of four rows of images in the array, the position of the output image is invariant to small errors in alignment of the mirrors.

Photoelectric Raman Spectrometer

A photoelectric Raman spectrometer has been built for the measurement of small liquid and gas samples. Employing a 1200-line per millimeter replica grating, it can record a complete Raman spectrum in six minutes and a more accurate one in an hour. The precision of repeat readings is ±1 percent. Five-milliliter liquid samples are normally used, and methods are described for reducing the size to less than one milliliter.

New Method for Measuring Diffuse Reflectance in the Infrared

In a new method of measuring diffuse reflectance in the infrared, the sample is irradiated from all directions with chopped light. The source and sample are placed at conjugate foci of a hemispherical mirror with the chopper between the source and mirror, making the component reflected by the sample distinguishable from the one emitted by it. Advantages are that reflectance may be measured over a very wide range of sample temperatures and that heating of the sample is relatively small. By adding another chopper after the sample, the emitted component may be recorded as an indication of sample temperature under the conditions of measurement. When calibrated against an aluminum mirror, the reflectance of a thin layer of MgO smoke on a mirror is measured as 99% at 2.5 μ. The performance of the system is illustrated with a number of spectra.

A Long Path Gas Absorption Cell

A long path gas absorption cell especially suited for infrared analyses of components of extremely low concentrations and absorption coefficients is described. The cell can be evacuated or pressurized. With aluminized mirrors and KBr field lenses, the evacuated cell performs with a peak efficiency of 45 percent of incident energy at 10 meters path length. Path length is adjustable by 1.25-meter increments from 1.25 meters to 10 meters. Adjustments are simple and can be made in a matter of a few minutes with good reproducibility. The cell will attach to the Perkin-Elmer Models 12, 112, and 21 spectrometers with compatibility to other attachments. The optical path can be altered quickly so that the usual sampling space of the spectrometer is available.

Infrared Grating Spectrophotometer

An instrument is described that makes convenient the use of high resolution in infrared spectroscopy. While the actual resolution is energy limited, the use of mosaics of gratings up to 1012×1412 inches in area makes it possible to resolve close to the theoretical for the largest gratings obtainable. The spectra are recorded directly as graphs of transmittance vs wavelength on scales that are accurate enough to eliminate the need for point by point recalculation. An internal mercury arc gives absolute checks on the accuracy of the sine bar mechanism that generates the wavelength scale. The transmittance scale is based on the optical null double beam principle described by N. Wright and L. W. Herscher [ J. Opt. Soc. Am.37, 211 ( 1947)] with the added freedom from scattered light that results from a double monochromator.The spectrometer may be adapted to a wide range of problems by changing detectors, sources, gratings, prisms, and windows. It is usable from the near ultraviolet to as far into the infrared as available prism materials and gratings permit. The grating may be used in any order with either single or double pass of the grating monochromator. The slit and prism drive mechanism may be automatically and continuously synchronized with any of these orders and with any of the different interchangeable components.The range of operating conditions is very large. The instrument may be operated as a double beam null indicator or as a single beam energy recording instrument. Its scanning time may be adjusted from about 10 minutes to 3 months for the full range of grating angles. Response time may be varied from one second to 100 seconds full scale. The slits may be varied automatically according to any preselected program.

Wide Range Infrared Absorption Cell

A multiple traversal gas absorption cell has been built for the measurement of strong and weak absorption bands in the same sample. By means of external adjustments its path length may be varied from ten centimeters to one meter, ten meters and intermediate values, with transmissions of 80%, 55%, and 32%, respectively. It has a volume of six liters and may be pressurized to 150 psi.