The integrating sphere is the base for a physical model of the wireless optical indoor communication channel. The author has found that the approximate expression for the channel transfer function for the integrating sphere used in that model can be replaced by an exact expression. A comparison shows that the approximation is valid for a wide range of interest, however there are some differences for high frequency values. For the impulse response (obtained by the inverse Fourier transform), the author observes significant deviations in the time region where first order reflections contribute significantly to the curve shape.