Refractivity of lithium fluoride with application to the calibration of infrared spectrometers
Leroy W. Tilton, E. K. Plyler · Journal of research of the National Bureau of Standards · 1951
The refractivity of lithium fiuolide was measured from 0.4 to 5.9 micron s and expressed for 23.6° C by a four-con stant di spersion formula by which an interpolation table was prepared for included wavelengths.The temperat ure coefficient was determin ed a -1.63 X 10-5 (constant within ± 0.05) for the visible region and a table of in dice made for 20° to 60° C at equal intervals of A from 0.4 to 0.7 micron.Most of t he observed indices agree with the tabulated values within ± 3 X 10-6 for the visible r egion and ± 3 X 10-5 in the infrared.In calibrating an infrared spectrometer simple empirical r elationships between drum reading, D, and wavelength are not entirely reliable.Three steps are recommended: (1) t he expression of D in terms of E , the emergent angle of energy from the prism ; (2) ex pression of E in terms of refractive index; and (3) the obtaining of t he indices or their expression a s a function of A. The first step requires at least two constants, one for angular eq uivalent of the screw and another for the particular Littrow-mirror orientation for D= O.The second requires knowledge of the refracting angle of the prism and its ol•ientat ion with respect to the incident e nergy.The adequate expression of ind ex a s a function of wavelength usually r equires four constants.