This work describes the development of silicone-based evaluation phantoms for biomedical optics in the wavelength range from 400 to 1550 nm. The absorption coefficient μ_a and the reduced scattering coefficient μ_s^′ were determined using an integrating sphere setup. Zirconium dioxide pigments were used as scatterers and carbon black as absorbers. We developed an in-house manufacturing process using a Hauschild SpeedMixer to ensure reproducibility. A set of nine cubic phantoms with three different reduced scattering and absorption coefficients was produced. Prediction of the μ_a and μ_s^′ was done by using the weighted mass concentrations of the used materials. The average prediction accuracy over all wavelengths and phantoms is 1.0% for the reduced scattering coefficient and 3.5% for the absorption coefficient.

Integrating sphere measurements are a well-established method for determining the optical properties of planar samples. In this study, the approach was expanded from slab geometry to cylindrical geometry illuminating the cylinder barrel, thereby demonstrating its applicability for determining the optical properties of human fingers. By adapting existing integrating sphere theory to cylindrical samples, the method was systematically validated using phantoms and subsequently applied to human fingers. It has been demonstrated that the absorption coefficient ${\mu }_{\mathrm{a}}$ and the reduced scattering coefficient ${\mu }_{\mathrm{s}}^{\prime }$ of cylindrical and 3D finger phantoms can be determined with a high degree of agreement to those of slab phantoms. Moreover, this approach facilitates the quantification of tissue components in human fingers, including fat, water and collagen content, total haemoglobin concentration and tissue oxygenation.

Existing optical phantoms often do not represent realistic optical and geometrical properties. This study aimed to fabricate a homogeneous silicone finger phantom that closely mimics the reflectance and transmittance characteristics of a human finger by precisely adjusting the absorption and reduced scattering coefficients in the visible wavelength range. The absorption and reduced scattering coefficients of a human finger were determined using a custom inverse model tailored for an integrating sphere system designed for cylindrical media illuminated along the barrel. To reproduce the retrieved optical properties in silicone, a reference database was created by characterizing the absorption spectra of 15 pigments dispersed in a silicone matrix. An automated fitting algorithm identified five suitable absorbing pigments, and their required concentrations were calculated to match the target absorption spectrum. The reduced scattering coefficient was independently controlled by varying the concentration of zirconium dioxide particles. An alginate mould was used to capture the finger geometry, ensuring anatomical accuracy of the phantom. The fabricated silicone finger phantom closely matched the human finger in both transmittance and reflectance, as well as in its anatomical shape. The Δ E ₂₀₀₀ value between the reflectance spectra of the human and silicone fingers was found to be 0.85. Under transmittance-mode illumination, light propagation within the silicone phantom agreed well with that of a human finger, both in visual appearance and in spatial light distribution. A method was developed to fabricate silicone finger phantoms with accurately matched optical and anatomical properties.

This work presents an experimental validation of a physics-based inverse rendering method for determining the reduced scattering and absorption coefficients of turbid materials in arbitrary shape from a single image per wavelength. Based on our previously published theoretical inverse rendering framework, we constructed and experimentally characterised a wavelength-selective measurement setup to realise and validate the method under real acquisition conditions. By accurately modelling the spectral behaviour and angle-dependent transmission of the employed bandpass filters, we ensured a close correspondence between captured and simulated reflectance. The method was evaluated on three silicone materials, beginning with simple cube geometries and later extending to a complex Einstein bust. Relative to integrating-sphere reference data, the recovered optical properties exhibit maximum absolute errors of approximately 4–10% for reduced scattering and 5–10% for absorption for the cubes, and 16–19% and 16–22%, respectively, for the bust. Forward renderings based on the recovered coefficients achieve CIE $\Delta E_{2000}$ values below 1 for the cube and below 2 for the complex geometry when compared with photographs. Additionally, we demonstrated that the approach can be applied using a common commercially available RGB camera, recovering optical parameters from each RGB channel, albeit with increased errors due to the camera’s broad spectral channels. Overall, our method enables the recovery of optical properties and the creation of accurate digital twins for objects of arbitrary shape using comparatively simple hardware, including common commercially available RGB cameras. This broadens its applicability to practical scenarios such as process monitoring and digital twinning when appearance, rather than precise material parameters, is the primary focus.

Accurate colour reproduction remains a challenge in manufactured dental restorations because tooth appearance depends on wavelength-dependent absorption and scattering, geometry, illumination, and viewing conditions. We present a measurement- and simulation-based workflow for determining 3D-printable multi-material compositions. We demonstrate our workflow by reproducing the appearance of the 16 VITA classical A1–D4 shade guide colours with a set of six Stratasys Vero and VeroUltra photopolymers. Each reference tooth was recorded in a photobox using a camera with six narrow-band spectral filters spanning 450 nm to 700 nm. A physics-based, GPU-accelerated Monte Carlo light transport simulation was then used inversely to recover the six material concentrations that minimised the CIE $\Delta E_{2000}$ colour difference between the measured and simulated appearance considering all six wavelength bands. Additionally, we compared four colour optimisation strategies for determining material compositions. We further investigated metamerism in the colour optimisation by independently the illuminant and comparing a single- and double-light-source setup. These variations produced substantial changes in the resulting colour differences. With the given material set, the resulting mixtures reproduced all 16 teeth across the shade guide with a mean colour difference of CIE $\Delta E_{2000}=2.76\pm 0.88$ over the evaluated tooth regions. Crucially, the printing materials require optical characterisation only once, after which new target teeth need only be spectrally measured to determine their printable compositions. More broadly, the framework provides a general route from spectral measurements to physically realisable multi-material reproductions, extending beyond dental applications.