Abstract: The expressions of transmisivity in the method of light cut-off (for short and long wave-length oases respectively) were dirived:
f(ν_c)=1+15/(π⁴){x_c³ln(1-e^-x_c)-3∑_n=0^∞ e^-(n+1)x_c[2/((n+1)⁴)+(2x_c)/((n+1)³)+(x_c²)/((n+1)²)]} (x_c=(hν_c)/kT ≥ 2)
f(ν_c)=15/(π⁴)x_c³[1/(3)-(x_c)/8+(x_c²)/60-0.0001984x_c⁴+0.0000037x_c⁶-7.5156325×10^-8x_c⁸+1.6059044×10^-9x_c¹⁰-……] (x_c ≤ 2)
These expressions were used in the experimental measurements. It was proved that the expressions were useful in improving the accuracy and measuring speed. According to these expressions a table (with 3000 data and an accuracy of 10^-5) and a useful expression (with a calculating accuracy of 2×10^-4) of x (the composition of Hg_1-xCd_xTe versus F (transmisivity) were obtained:
x=0.1334+0.8240F-2.140F₂+4.840F³ -5.450F⁴+2.550F⁵ (0.17 < x < 0.443)