Method and apparatus for inspecting cigarette density

A cigarette density inspection method, comprising the steps of:

• sampling, at intervals of a predetermined period, a density signal of a predetermined cigarette zone of a reference cigarette including a local low-density portion, while translating the reference cigarette in a direction aligned with a longitudinal axis of the reference cigarette;
• determining a reference density signal using the sampled reference cigarette density signal, the reference density signal indicating a density of the reference cigarette, the predetermined cigarette zone being shorter in length than the reference cigarette and including an indication of the local low-density portion;
• sampling, at intervals having the same length as the predetermined period for the sampling associated with the reference cigarette, a density signal associated with plural zones of an inspected cigarette while translating the inspected cigarette in a direction aligned with a longitudinal axis of the inspected cigarette;
• determining an actual density signal indicating a density of the inspected cigarette using the sampled inspected cigarette density signal, the plural zones of an inspected cigarette partially overlapping one another and each having substantially the same length as that of the predetermined cigarette zone in the reference cigarette;
• determining a correlation between a changing pattern of the reference density signal in the predetermined cigarette zone of the reference cigarette and a changing pattern of the actual density signal at lengthwise portions of the inspected cigarette, on the basis of a sum of squares or absolute values of differences between actual density signals and reference density signals;
• and
• determining whether the inspected cigarette includes a local low-density portion based on the correlation determined in the correlation determination step.