In order to discriminate either isolated defects or streaks in product webs, an inspection system should first have constant sensitivity to defect conditions which may be encountered at any point in a full scan across the web. Methods of compensating for scan-to-scan sensitivity variations have hitherto assumed that the optical and electrical efficiencies of the scanning system remain constant over each scan length. However, in order to meet high precision inspection requirements, these assumptions do not apply and it is necessary to use a system that corrects for variations in optical/electrical efficiencies throughout each product scan (i.e., across the normally generated scan signal).
This invention is applicable to repetitive scan signals generally, particularly where the magnitudes of signal variation are not large; however, it is equally applicable to pedestal signals displaying relatively large signal variations, in which case logarithmic processing of the scan signals before input to the apparatus of this invention is advantageous.
Optical-electrical photographic film inspection generates non-flat pedestal signals, the compensating of which, according to this invention, requires two operations: (1) storing a logarithmically processed characteristic irregular pedestal reference signal R in a first digital memory circuit and (2) differencing each new log-processed scan A with the stored reference signal to derive the requisite log ratio result, log A/R. The equalized (flattened) instantaneous difference log ratio signal thereby obtained effectively places in true perspective the amplitudes of defect-related signals previously buried in the uneven repetitive portions of the pedestal and thereby facilitates their detection by bipolar amplitude discrimination means.
As a further operation, streak detection is achieved by subsequently accumulating the sums of the amplitudes of the instantaneous difference signals over several scans in a second memory. This procedure is defined as "coherent adding". Summing these difference signals limits the build-up of uncorrelated noise appearing on the pedestals, whereas persistent signal level shifts, such as caused by machine direction streaks, will accumulate rapidly. The second memory is reset automatically when the sum includes a sufficient number of scans to effect a good time average of the difference signals.
An alternate scheme may be used to accumulate the instantaneous difference signals when low frequency machine direction level shifts occur as a result of the product web passing over off-center rolls or from other causes, such as web flutter, which might otherwise be detected as defective product. In these situations, a digital differentiation scheme, effected by inverting and summing the alternate scan instantaneous difference signals with the address shifted accumulated partial sum values of the instantaneous difference signal waveforms stored in the second memory effectively nullifies the accumulation of these periodic bias level changes.
In general, the complete equalizer and streak detection apparatus of this invention for photographic film inspection comprises an A/D converter, two random access memories, two D/A converters and various digital control and arithmetic circuitry arranged in two adjoining loops. Each loop is provided with a random access memory and an adder-subtractor, and functions independently of the other in two operating modes.
The first mode establishes a mean pedestal waveform obtained by averaging the amplitude samples within each sampling interval of each scan in a series of 2.sup.n scans over a streak-free product. The elements in the first loop function first to accumulate a sum of the sampled amplitude values over the predetermined sequence of scans, then to derive, by means of a shifter, the average value of each sample by shifting each of the accumulated binary values stored in the first random access memory a predetermined n places in a known manner and finally to initiate the second operating mode. Although the stored standard waveform amplitude values can be maintained indefinitely in memory, for varying product roll conditions it has been found desirable to compute and store a new reference for each product run (or for each product roll).
The second mode first obtains the differences between the sampled amplitude values of the mean pedestal waveform developed in the first mode and the corresponding sample values of the instant product scan and then accumulates these instantaneous difference signals in one of two selectable ways in the second loop random access memory for later discrimination. In this manner, where, on the one hand, uncorrelated product noise will be summed to zero over the series of P product scans (where P = number of scans accumulated in the second loop before resetting), on the other, machine direction streaks having correlated signal amplitude values will rapidly build in value for easy defect discrimination. The rate at which the defect signal accumulates in memory is dependent on the severity, persistence and orientation of the streak with respect to the machine direction. Reset of the second random access memory occurs automatically when a sufficiently long time average has been obtained, e.g., on the order of P = 300 scans.
A variation of the straight additive accumulation scheme that is useful when low frequency machine direction noise is present includes inverting the instantaneous difference signals and shifting the address locations of the stored partial sum values in the second memory on the even (or odd) scans. This is equivalent to digital differentiation, since small constant changes in signal level are essentially nullified, whereas subtle streak amplitudes are allowed to build up, but at half the rate of the straight additive accumulation procedure hereinbefore described.