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A major problem a visual system faces is how to fit the large intensity variation of natural image streams into the limited dynamic range of its neurons. One of the means to accomplish this is through the use of fast light adaptation of the photoreceptors. In order to investigate this, we measured first time series of natural intensities, and second responses of fly photoreceptors to these time series. Time series representative of what each photoreceptor of a real visual system would normally receive were measured with an optical system measuring the light intensity of a spot comparable to the field of view of single human foveal cones. This system was worn on a head-band by a freely walking person. Resulting time series have a high rms-contrast in the order of 1, and power spectra behaving approximately as 1/f (f: temporal frequency). Measured time series were subsequently presented to fly photoreceptors by playing them back on an LED. The results show that fast light adaptation indeed keeps the response within the dynamic range of the cells and that a large part of this range is actually needed for packing the information in natural time series.

This paper focuses in the interpretation of material properties of reflectivity and specularity assessed by the visual system under illumination consisting of both a focal and a diffuse component (the ‘sun-and-sky’ illumination assumption). This assumption provides for four kinds of luminance gradients: gradients of incident illumination, gradients of reflectivity, gradients of secondary self-illumination and gradients of shadowing. The analysis considers the dissociation of the material properties carried by specularity from the geometric properties of object shape, taking the sinusoidal surface as a canonical shape exemplar.

11 pags., The European Metrology Research Program (EMRP) is a metrology-focused program of coordinated Research and Development (RD) funded by the European Commission and participating countries within the European Association of National Metrology Institutes (EURAMET). It supports and ensures research collaboration between them by launching and managing different types of project calls. Within the EMRP Call 2012 >Metrology for Industry>, the joint research project (JRP) entitled >Multidimensional Reflectometry for Industry> (xD-Reflect) was submitted by a consortium of 8 National Metrology Institutes (NMIs) and 2 universities and was subsequently funded. The general objective of xD-Reflect is to meet the demands from industry to describe the overall macroscopic appearance of modern surfaces by developing and improving methods for optical measurements which correlate with the visual sensation being evoked. In particular, the project deals with the >Goniochromatism>, >Gloss> and >Fluorescence> properties of dedicated artifacts, which will be investigated in three main work packages (WP). Two additional transversal WP reinforce the structure: >Modelling and Data Analysis> with the objective to give an irreducible set of calibration schemes and handling methods and >Visual Perception>, which will produce perception scales for the different visual attributes. Multidimensional reflectometry involves the enhancement of spectral and spatial resolution of reference gonioreflectometers for BRDF measurements using modern detectors, conoscopic optical designs, CCD cameras, line scan cameras, and modern light sources in order to describe new effects like sparkle and graininess/coarseness. More information and updated news concerning the project can be found on the xD-Reflect website http://www.xdreflect.eu/.