Flowers produce variations in color saturation by arranging petals at oblique and varying angles.

Large color variations can be observed across the face of a flower even when individual petals are the same color. We investigated whether these color variations could be explained by a model that incorporates multiple reflections of light between petals and transmissions of light through petals before the light returns to the observer. The three flowers that we selected for the study exhibited large color variations across the face of the intact flower but had no significant observable difference in color saturation across a single petal or between petals when petals were removed from the flower. We used a spectroradiometer to measure the spectrum across the faces of intact flowers and across individual petals. The measured spectra for all of the flowers were consistent with the proposed model.

Quantitative versus subjective evaluation of mammography accreditation phantom images.

The precision of quantitative and subjective evaluations of phantom image quality has been studied. Twenty-seven images of the American College of Radiology (ACR) mammography accreditation phantom were acquired under different x-ray techniques and digitized. Several quantitative image quality measures were obtained from each image by analyzing microcalcification and nodule target objects in the phantom. All images were also scored subjectively by 8 observers, each of whom provided a count of the number of objects seen in each target class (fibrils, microcalcifications, and nodules). An analysis was performed to predict the subjective measurements from the quantitative measurements and to estimate their variabilities. It was found that the subjective measures could be well predicted by the quantitative measures and that the variance of the quantitative measures was significantly smaller than that of the subjective measure, by almost a factor of 10. The implication for the ACR accreditation program for mammography is that a substantial improvement is possible in the image quality evaluation process by performing computerized analysis of the phantom images in addition to subjective analysis.

Efficient coding of natural time varying images in the early visual system.

We investigate the hypothesis that the early visual system efficiently codes natural time varying images, first by tracking part of the image, then by matching the spatiotemporal properties of the neural pathway to those of the tracked image. A representation for the time varying image is formulated which consists of two spatiotemporal components, a velocity field component and a stationary component. We show, using digitized sequences of natural images, that the spatiotemporal spectrum and other attributes of the image markedly differ before and after tracking. The temporal frequency bandwidth and velocity distribution of the velocity field component are diminished in the region of tracking and broaden with increasing eccentricity from this region. On the other hand, the spectrum of the stationary component is unaffected by tracking. Comparison of the properties of the tracked image to those of the M and P pathways suggests that each pathway transmits different attributes of the tracked image. A retinal architecture which varies with eccentricity also matches the properties of the tracked image.