Investigating the optical translucency of Spectralon using BSSRDF measurements.

Translucent materials have the property of reflecting light beyond the illumination point due to subsurface light propagation in the material. These reflectance properties can be characterized using the bidirectional scattering-surface reflectance distribution function (BSSRDF), a radiometric quantity that is a function of spatial, angular, spectral, and polarization parameters. At very small scales, we have observed that Spectralon, a commercial material widely used as a diffuse reflectance calibration standard, can be regarded as translucent. This can generate measurement errors and limit Spectralon's reliability as a calibration artefact for instruments that measure optical quantities on very small surfaces. To characterize the translucent properties of Spectralon, we have measured its BSSRDF using an experimental setup based on a goniospectrophotometer with a spatial scanning system for detection. In the present study, we show that Spectralon cannot be considered an opaque material at small scales (below 1 mm). For instrument measuring on small areas, Spectralon can be used for calibration only when the illumination area and the observation area differ by more than 1 mm in radius.

Real-time skin chromophore estimation from hyperspectral images using a neural network.

BACKGROUND: Hyperspectral imaging for in vivo human skin study has shown great potential by providing non-invasive measurement from which information usually invisible to the human eye can be revealed. In particular, maps of skin parameters including oxygen rate, blood volume fraction, and melanin concentration can be estimated from a hyperspectral image by using an optical model and an optimization algorithm. These applications, relying on hyperspectral images acquired with a high-resolution camera especially dedicated to skin measurement, have yielded promising results. However, the data analysis process is relatively expensive in terms of computation cost, with calculation of full-face skin property maps requiring up to 5 hours for 3-megapixels hyperspectral images. Such a computation time prevents punctual previewing and quality assessment of the maps immediately after acquisition. METHODS: To address this issue, we have implemented a neural network that models the optimization-based analysis algorithm. This neural network has been trained on a set of hyperspectral images, acquired from 204 patients and their corresponding skin parameter maps, which were calculated by optimization. RESULTS: The neural network is able to generate skin parameter maps that are visually very faithful to the reference maps much more quickly than the optimization-based algorithm, with computation times as short as 2 seconds for a 3-megapixel image representing a full face and 0.5 seconds for a 1-megapixel image representing a smaller area of skin. The average deviation calculated on selected areas shows the network's promising generalization ability, even on wide-field full-face images. CONCLUSION: Currently, the network is adequate for preview purposes, providing relatively accurate results in a few seconds.

Evaluating edge loss in the reflectance measurement of translucent materials.

In many commercial instruments for measuring reflectance, the area illuminated on the measured object is identical to the area from which light is collected. This configuration is suitable for strongly scattering materials such as paper, but issues arise with translucent materials, because a portion of the incident light spreads around the illuminated area by subsurface transport and escapes the detection system. This phenomenon, referred to as edge loss, yields erroneous, underestimated reflectance measurements. In the case of colored and opalescent materials, the impact of edge loss on the measured reflectance varies with the wavelength, which is a significant issue for spectrophotometer and colorimeter users. In the present study, we investigate the edge-loss phenomenon with an emphasis on human skin measurement. In particular, we use a mathematical model to estimate the PSF of translucent materials, relying on the diffusion approximation of the radiative transfer theory, to predict edge-loss measurement error. We use this model to discuss the suitability of several commercial spectrophotometers to accurately measure the translucent materials of various optical properties and show that not all devices can adapt to all translucent materials.

Bidirectional reflectance measurements over a micrometric surface area using a goniospectrophotometer.

The increasing use of a spatially varying bidirectional reflectance distribution function (svBRDF) to describe the appearance of an object raises the important question of how BRDF values change when measured on a small scale. For this reason, we present a new goniospectrophotometer with the ability to measure the BRDF at the micrometer scale (µBRDF). The instrument produces BRDF measurements with a measurement surface diameter of 31 µm. This device is designed to aid in the extension of the BRDF metrological scale from centimeter to micrometer size. We support the credibility of our µBRDF measurements using a specially made test sample with uniform diffuse white dots on a uniform black background, measuring its bidirectional reflectance in one geometrical configuration at many spatial locations. This sample can easily be modeled using a few unknown parameters. The agreement between our measurements and the model demonstrates the credibility of the measurement technique.

Three-dimensional maps of human skin properties on full face with shadows using 3-D hyperspectral imaging.

Hyperspectral imaging has shown great potential for optical skin analysis by providing noninvasive, pixel-by-pixel surface measurements from which, applying an optical model, information such as melanin concentration and total blood volume fraction can be mapped. Such applications have been successfully performed on small flat skin areas, but existing methods are not suited to large areas such as an organ or a face, due to the difficulty of ensuring homogeneous illumination on complex three-dimensional (3-D) objects, which leads to errors in the maps. We investigate two methods to account for these irradiance variations on a face. The first one relies on a radiometric correction of the irradiance, using 3-D information on the face's shape acquired by combining the hyperspectral camera with a 3-D scanner; the second relies on an optimization metric used in the map computation, which is invariant to irradiance. We discuss the advantages and drawbacks of the two methods, after having presented in detail the whole acquisition setup, which has been designed to provide high-resolution images with a short acquisition time, as required for live surface measurements of complex 3-D objects such as the face.