Comparison of two methods for accurate measurement of modulation transfer functions of screen-film systems

The modulation transfer function (MTF) of a screen-film system can be measured by two methods, i.e., a slit method with Fourier transform on the line spread function and a square-wave response function (SWRF) method. However, it is still uncertain whether MTFs obtained by the two methods are identical. In this study, MTFs of relatively sharp and unsharp screen-film systems were measured by using the two methods. The slit method provided slightly greater MTF for the relatively sharp system than the SWRF method. However, MTFs of the unsharp system obtained with the two methods were comparable. Generally, the slit method tends to provide reliable results for unsharp systems, whereas the SWRF method is favorable for sharp systems. Accuracy and consistency of these measurements were examined by comparison of experimental and theoretical edge responses derived from the measured MTFs. However, the difference in edge responses obtained by the two methods was relatively small compared with the variation of the measured edge responses, and thus results were considered inconclusive as to whether either of the methods can provide more accurate MTFs. International interlaboratory comparison indicated that the variation in the measured MTFs at six different institutions was relatively large for both methods. However, the MTFs of two screen-film systems measured by the slit method appear to agree with those by the SWRF method within the variation expected from the interlaboratory comparison.