The commercial clocks are also subject to error because the atoms are only measured travelling in one direction. Primary atomic clocks reverse the direction of the beam so the atoms travel in one direction for a specified period before the direction is reversed and the atoms travel the opposite way. Measurement is taken by comparing the frequency of the clock in each direction, allowing resolution of one of the microwave problems associated with the interrogation of the atoms. The double beam design reduces some uncertainties associated with the atoms' trajectories through the microwave cavity Fifteen years ago, the most critical source of uncertainty was the Time Dilation effect. Also known as the relativity effect or Second Order Doppler shift, the effect is due to the relative motion between the clock's microwave cavity and the atoms passing through it with a finite velocity.
Correct measurements rely on the knowledge of the velocity of the atoms.
Dr. Boulanger had found a precise way to measure the velocity distribution of the atoms in the beam making it possible to evaluate the correction needed for the Time Dilation effect very accurately. Dr. Boulanger's discovery allowed scientists to measure other frequency uncertainties more accurately. Another factor limiting accuracy for atomic clocks was the black-body radiation. It isn't visible to the eye. The infrared radiation emitted by the object is not equal to that emitted by your hand.