Image unsharpness: two methods to express image unsharpness in one value

Two methods to express image unsharpness in one value are compared. The first method is commonly used and uses the noise-equivalent passband or its reciprocal value the equivalent blur (1/Ne) determined from the modulation transfer function (MTF). A radiographic image of a sharp edge is made and scanned by a microdensitometer and the film density is transferred into energy values with the use of the characteristic curve. After differentiation, a Fourier transform results in an MTF curve. A single value (Ne) substituting the complete MTF curve can be derived by integrating the squared MTF values over frequency. This noise-equivalent passband is expressed in 1p mm-1. 1/Ne is the equivalent blur (or sampling aperture) expressed in millimetres. The second method for determining the unsharpness derives a value from the energy-related edge response function, which will be called the equivalent unsharpness. Both methods use the same reference unsharpness, which indicates that the ''equivalent blur'' and the ''equivalent unsharpness'' are comparable values qualifying image unsharpness. Results of practical tests using both methods show that the values are practically the same. However, the second method is relatively simple and therefore an attractive tool for determining image unsharpness. (author)