This study is concerned with the interpretation of the ionization gauge detector signal in a molecular beam experiment as a particular functional of the reflected molecule distribution function, called the nominal value of the detector signal. The detector configurations examined are those used to measure number density and time‐of‐flight velocity distributions. Expressions for the actual detector signals are established on the basis of the apparatus geometry; the nominal expressions are derived as approximations to the actual signal. Differences between these actual and nominal functionals are examined quantitatively using the Nocilla reflection model. It is found that in the density case the significant apparatus parameter is the solid angle subtended at the detector by the illuminated target area. If this solid angle is large, the error introduced by assuming the detector signal to be represented by the nominal expression becomes significant when the reflection is highly anisotropic. In the velocity distribution measurement, the significant factor is the shutter function. For slow shutter speeds, differences between the actual and nominal expressions for the detector signal increase rapidly as the error in the time the shutter is open increases.