The focus of our research is on optical properties of dental hard tissues and dental materials which can be roughly divided into two adjacent fields; Color Science and Biophotonics.
Physical substances (matter) consist of discrete charge carriers called electrons and protons. Light is assumed to consist of small energy packages (photons) causing interaction with these charge carriers, exiting oscillating vibrations. Accelerated charges emit electromagnetic energy omnidirectionally in the shape of elementary waves (secondary radiation) and such excited charge carriers can either partially or completely convert the energy of an incident wave (a beam of light) into other forms of energy, for instance phonons, plasmons or magnons. This type of energy conversion is commonly referred to as absorption. The psychophysiological construction of color is related to the amount of reflected (scattered) and absorbed electromagnetic radiation by a medium.
An incident electromagnetic wave travelling through a diffusely scattering medium like tissue (i.e. dentin and enamel) with a position-dependent permittivity (anisotropy) results in multiple scattering events. Many different theories have been put forward to describe such field variations including Maxwell's, the transport and the diffusion theory. Maxwell’s equations takes into account the microstructure of the medium under consideration but is due to the complexity of tissues impossible to solve accurately, mainly because of the lack of precise knowledge of the dielectric permittivity along the propagation vector. The Radiative Transport Equation (RTE) is an approximation of Maxwell's equation, achieved through simplification by ignoring certain phenomena related to wave propagation.
By solving RTE it is possible to simulate, compare and identify tissue samples by their optical properties.
We operate our own research facility at our office in Freiburg near the University. Here we can prepare samples of dentin and enamel as well as dental materials for optical analysis.
In order to solve the Radiative Transport Equation (RTE) for a medium under consideration, reflection and transmission measurements are required to obtain accurate information pertaining to four main optical properties for a given sample:
- Index of refraction (nd)
- Scattering coefficient (µs)
- Absorption coefficient (µa)
- Phase function / anisotropy factor g
The appropriate computational method (Kubelka-Munk or Inverse Adding Doubling) are usually carried out in Matlab or Python in conjunction with Monte Carlo simulations.
A broad range of optical equipment is available including but not limited to:
- Radio Spectrometers Photoresearch PR-670 & PR-650
- UV/VIS/NIR Fibre optic spectrometers Ocean Optics USB 2000 & USB 2000+
- Bench top spectrometer Konica Minolta CM3600A
- Lap polisher Buehler Ecomet/Automet
- Slow speed saw Buehler Isomet
- Soft viewing booth Just Normlicht with 5 standard illuminants
- Labsphere general purpose Integrating sphere (4 port configuration)
- Newport general purpose Integrating sphere (4 port configuration)
- Various optical fibres, back scattering probes and collimating lenses (Ocean Optics)
- Various optical benches (Thorlabs & Newport)
- Monochromator Jobin Yvon H10 VIS
- He-Ne laser (632.8 nm) Melles Griot 05SR81
- Tunable ion-argon laser Stellar-Pro ML/1
- Xenon short arc lamp Osram XBO 75W
Deuterium light source StellarNet SL3
Hein S, ten Bosch JJ.The effect of ultraviolet induced fluorescence on visually perceived tooth color under normal light conditions. Dent Mater. 2018 May; 34(5): 819-823
Hein S, Tapia J, Bazos P. eLABor_aid: a new approach to digital shade management. Int J Esthet Dent. 2017; 12(2): 186-202.
Hein S, Tapia J, Bazos P. eLABor_aid: Ein neues Konzept für die Zahnfarbenbestimmung. Int J Esthet Dent. 2017; 12(2): 186-202.
Hein S, Tapia J, Bazos P. eLABor_aid: Ein neues Konzept für die Zahnfarbenbestimmung. Quintessenz Zahnmedizin Jahrgang 70, Ausgabe 12 Dezember 2019
Hein S, Zangl M. The use of a standardized gray reference card in dental photography to correct the effects of five commonly used diffusers on the color of 40 extracted human teeth. Int J Esthet Dent. 2016 Summer;11(2):246-59.
Devigus A, Bazos KP, Hein S. Licht in der dentalen Fotografie. Quintessenz Zahnmedizin Jahrgang 70, Ausgabe 12 Dezember 2019
Hein S, Bazos P, Tapia J, Zago-Naves L. Beyond Visible: Exploring Shade Interpretation. QDT 2014; Vol 37:199-211.
Hein S, Geller W. The Platinum Foil Technique: History, Indication, Fabrication, and Adaptation. QDT 2011; Vol 34:25-39.
Objective shade matching, communication, and reproduction by combining dental photography and numeric shade quantification. J Esthet Restor Dent. 2020 Aug 24. doi: 10.1111/jerd.12641. Online ahead of print.PMID: 32840048