Huw Owens

Colour statistics of natural and man‐made surfaces

The reflectance spectra of natural and man‐made surfaces are highly constrained. Statistical analyses have been conducted that confirm that the surface reflectance spectra form a set of band‐limited functions with a frequency limit of approximately 0.02 cycles/nm. The reflectance spectra can be represented by a linear‐model framework and are adequately described by 6‐12 basis functions. However, the spectral properties of surfaces are not so constrained as to allow the human visual system to recover the surface properties from cone excitations. Furthermore, trichromatic colour devices such as scanners and cameras can only capture illumination‐specific colour information.

Blur tolerance for luminance and chromatic stimuli

We investigated the blur tolerance of human observers for stimuli modulated along the isoluminant red-green, the isoluminant yellow-blue, and the luminance (black-white) direction in color space. We report the following results: (i) Blur difference thresholds for red-green and luminance stimuli (of equal cone contrast) are very similar and as low as 0.5 min of visual angle; for yellow-blue the lowest blur thresholds are much higher (1.5 min of visual angle). (ii) The smallest blur thresholds are found for slightly blurred square waves (reference blur of 1 arc min) and not for sharp edges. (iii) Blur thresholds for red-green and black-white follow a Weber law for reference (pedestal) blurs greater than the optimum blur. (iv) Using the model proposed by Watt and Morgan [Vision Res. 24, 1387 (1984)] we estimated the internal blur of the visual system for the black-white and the red-green color directions and arrived at the following estimates: 1.2 arc min for black-white stimuli at 10% contrast and 0.9 arc min for red-green stimuli at 10% cone contrast. Blur tolerance for yellow-blue is independent of external blur and cannot be predicted by the model. (v) The contrast dependence of blur sensitivity is similar for red-green and luminance modulations (slopes of -0.15 and -0.16 in log-log coordinates, respectively) and slightly stronger for yellow-blue (slope = -0.75). Blur discrimination thresholds are not predicted by the contrast sensitivity function of the visual system. Our findings are useful for predicting blur tolerance for complex images and provide a spatial frequency cutoff point when Gaussian low-pass filters are used for noise removal in colored images. They are also useful as a baseline for the study of visual disorders such as amblyopia.

Optical properties of cotton and nylon fabrics coated with silica photonic crystals

In this work, colourant-free coloured fabrics have been produced by the self-assembly of silica nanoparticles (SNPs) using a natural sedimentation method. The optical properties of the SNP-coated fabrics were investigated and the spectral reflectance, chromaticity coordinates, and CIE L* a* b* values are reported. The overall colour effect on the fabric can be described by Bragg’s diffraction of the ordered SNP photonic crystal and the Mie scattering of the disordered SNP arrangement. It was found that the structural colour on the tight black woven cotton fabric was more uniform than that from the loose black knitted nylon fabric. Although the structural colour was not easily perceived on a white fabric due to the strong reflection of the white background, the SEM images show that an ordered photonic crystal was formed on the fabric and a small colour difference by the introduction of SNPs was measured using instrumental colour measurement.

The spatio-chromatic sensitivity of the human visual system

The response of the human visual system depends on a multitude of image features, such as the wavelength (colour) of the visual stimulus and its spatial frequency content. Hence we need to take into account the spatial and chromatic sensitivity as well as spatio-chromatic interactions to properly characterize visual sensitivity. In this paper we report two experiments that further characterize the spatio-chromatic sensitivity of the human visual system for stationary stimuli, namely the detection of small visual orientation differences and the detection of blur. In both cases we find that the visual system is equally sensitive to red-green and to black-white modulations for a wide range of spatial parameters. Furthermore, the contrast dependence for red green and black white modulations is identical, suggesting that the same mechanism mediates both types of stimulus. Our results are in accordance with the hypothesis that both tasks are mediated by the parvocellular as opposed to the magnocellular pathway.

A method to improve the quality of silica nanoparticles (SNPs) over increasing storage durations

AbstractThe solvent varying technique (SVT) provides a simple method for the production of uniform batches of silica nanoparticles (SNPs) of a target average diameter. SNPs synthesized using the SVT have been observed to agglomerate over increasing storage times leading to an increase in average particle diameter. Since the particle diameters of the SNPs produced using the SVT may vary over increasing storage durations, the previous model, suggested by Gao et al., which is based on the diameter of the original SNPs, is unreliable when predicting a target particle diameter using the initial volume of ethanol. A centrifuge and replacement of solvent method has been applied in this investigation to the SNP solutions created using the SV technique. This reduces the amount of unused reactants in the centrifuged colloidal suspensions, which further improves the quality of the SNPs and hence any subsequent photonic crystals. Post centrifuge and replace, the morphology of the centrifuged particles is more uniform than that of the original particles, which has been evaluated using SEM micrographs. The face-centered cubic (FCC) structures observed on the surface of the photonic crystal films have also been imaged using a SEM. A linear equation for the prediction of the SNP diameters for a given initial amount of ethanol is proposed based on the centrifuged SNP diameters. The particle diameter measurements for the new equation were recorded using a DLS instrument. The dispersion of the SNPs was also recorded using DLS. The morphology of the surface of the particles has been confirmed using TEM micrographs. Graphical abstractᅟ