Brian W. Keelan

ISO 20462: a psychophysical image quality measurement standard

ISO 20462, a three-part standard entitled “Psychophysical experimental methods to estimate image quality,” is being developed by WG18 (Electronic Still Picture Imaging) of TC42 (Photography). As of late 2003, all three parts were in the Draft International Standard (DIS) ballot stage, with publication likely during 2004. This standard describes two novel perceptual methods, the triplet comparison technique and the quality ruler, that yield results calibrated in just noticeable differences (JNDs). Part 1, “Overview of psychophysical elements,” discusses specifications regarding observers, test stimuli, instructions, viewing conditions, data analysis, and reporting of results. Part 2, “Triplet comparison method,” describes a technique involving simultaneous five-point scaling of sets of three stimuli at a time, arranged so that all possible pairs of stimuli are compared exactly once. Part 3, “Quality ruler method,” describes a real-time technique optimized for obtaining assessments over a wider range of image quality. A single ruler is a series of ordered reference stimuli depicting a common scene but differing in a single perceptual attribute. Methods for generating quality ruler stimuli of known JND separation through modulation transfer function (MTF) variation are provided. Part 3 also defines a unique absolute Standard Quality Scale (SQS) of quality with one unit equal to one JND. Standard Reference Stimuli (SRS) prints calibrated against this new scale will be made available through the International Imaging Industry Association.

Effect of grayscale resolution on the performance of lung nodule detection on a softcopy display

A four-alternative forced-choice experiment was carried out to examine the effect of 8-bit versus 10-bit grayscale resolution on the detection of subtle lung nodules on a medical grayscale liquid crystal display (LCD). Sets of four independent backgrounds from each of three regions were derived from a very low-noise X-ray acquisition of a chest-phantom with an amorphous selenium radiographic detector. Simulated nodules of fixed diameter (10 mm) and varying contrast were digitally added to the centers of selected background images. Subsequently, multifrequency image processing was performed to enhance the image structures, followed by a tonescaling procedure that resulted in pixel values being specified as p-values, according to DICOM Part 14: The Grayscale Display Function. To investigate the effect that grayscale resolution may have upon softcopy detectability, each set of four images in the experiment was quantized to both 8-bit and 10-bit resolution. The resulting images were displayed on a DICOM-calibrated LCD display supporting up to 10 bits of grayscale input. Twenty observers with imaging expertise performed the nodule detection task for which the signal and location were known exactly. Results from all readers, chest regions, and backgrounds were pooled, and statistical significance between fractions of correct responses between 8-bit and 10-bit resolution was tested. Experimental results do not demonstrate a statistically significant difference in the fraction of correct answers between these two input grayscale resolutions.

ISO 12232 revision: determination of chrominance noise weights for noise-based ISO calculation

Three ISO speeds for digital cameras, yielding the minimum, typical, and maximum exposures recommended for use, are defined in International Standard 12232, which is under revision. The typical and minimum acceptable exposures are based upon signal-to-noise criteria, described in ISO 12232, in which visual (perceptually relevant) noise is computed as a weighted sum of variances from a luminance (Y) and two chrominance (R-Y, B-Y) channels. The weights of the two chrominance variances, C1 (R-Y) and C2 (B-Y), are in need of reevaluation because of: (1) changes in linearization procedures being introduced in the revision of ISO 12232; (2) the limited nature of the original experiment to determine C1 and C2 and (3) suspicion that the initial C1 and C2 values were too high, overemphasizing the contributions of chrominance noise to perception. This paper describes the image simulations, psychophysical experiment, and analyses conducted to determine new values for the chrominance weights to be used in the revised standard. The values obtained, C1 = 0.279 (standard error = SE = 0.025) and C2 = 0.088 (SE = 0.017), are approximately one-half as large as those in the original version of ISO 12232. Systematic variation of the weights with the color of noise-sensitive uniform areas in the scenes is observed, but the effect is small and does not have a practical impact on the standard.