The indices of refraction of ADP, KDP, and deuterated KDP were measured at temperatures between the Curie point and room temperature. The estimated accuracy of the data is ±0.0001. The empirical equation Δ<i>n</i>= (<i>n</i><sup>2</sup>+<i>an</i>+<i>b</i>)<i>c</i>(298-<i>T</i>) where <i>n</i> is the index at room temperature, Δ<i>n</i> is the change in index, and <i>T</i> is the temperature in °K, fits the measurements to within experimental error. The values of the constants <i>a, b</i>, and <i>c</i> are tabulated for each material, permitting <i>n</i> to be calculated for any temperature.
RICHARD A. PHILLIPS, "Temperature Variation of the Index of Refraction of ADP, KDP, and Deuterated KDP," J. Opt. Soc. Am. 56, 629-632 (1966)