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.

Picture archiving and communication systems (PACS) use large numbers of video monitors distributed throughout a hospital to display images to physicians. While luminance transfer characteristics of monitors may be measured in some institutions, resolution and other image quality factors are frequently ignored, except in research settings. This work centered around developing a quality control tool and identifying a set of measurements with which to measure display image quality. The tool consisted of an EG&G gamma scientific telemicroscope on a translation table, controlled by a portable computer and positioned on a movable cart. With this tool, we were able to make highly accurate measurements on monitors at multiple locations in the hospital. We settled on measurements consisting of spatial resolution, luminance uniformity, and stability of the display function under different conditions. We concentrated on three pairs of high quality monitors, each pair used together in a clinical or research setting.

The purpose of our experiments was to estimate basic sensitivity to motion gradients and to evaluate the evidence for second-order integration and differentiation of motion signals. We measured sensitivity to spatially sinusoidal contrast modulation between two oppositely moving bandpass-filtered noise images. The motion-contrast sensitivity function, defined as the inverse of threshold modulation amplitude as a function of modulation spatial frequency, was bandpass in shape with declines at both highest and lowest frequencies. The functions for three noise spatial frequencies had approximately the same shape when modulation frequency was expressed as a fraction of noise frequency. We compared the data with a model in which linear motion filters, whose outputs are squared or rectified, are followed by a second stage of excitatory or inhibitory pooling. The data are consistent with a model in which (1) all excitatory pooling occurs at the linear stage and (2) the second stage contains a large inhibitory pooling area, with a radius approximately eight times that of the linear receptive field.

Contrast energy thresholds were measured for discriminating the direction of a drifting sinusoidal grating multiplied by an independently drifting space-time Gaussian (a generalized Gabor). We argue that the stimulus with the lowest contrast energy threshold identifies the receptive field of the most efficient linear motion filter. This optimal motion stimulus is found to be at 3 c/deg and 5 Hz, with a width and height of 0.44 deg and a duration of 0.133 sec, corresponding to spatial and temporal bandwidths of 1.1 and 2.5 octaves, respectively. The spectral receptive field is aligned more nearly to the Cartesian axes than to the velocity contour.