The purpose of the study was to compare the transmission of light radiation in the ultraviolet to infrared wavelength range in three dentin thicknesses. This experimental laboratory study with a descriptive-analytical approach was carried out on 15 tooth specimens with a thickness of 300, 600, and 1000 µm and five samples from each section after disinfection. Ultraviolet, visible, and infrared wavelengths were irradiated from above (corono-apical) and below (apico-coronal).
The penetration of light radiation through dentin specimens depends on tissue's optical properties, chromophores such as melanin, hemoglobin, and water, the technical specifications of the device, and the operation method [37, 41]. Furthermore, another study considered the increase in transfer rate in enamel to dentin with differences in refractive indices coefficients related to light extensions .
Gutknecht et al. studied the antibacterial effect of 445 nm blue diode laser in root canal dentin on Enterococcus faecalis in human teeth at three thicknesses of 300, 500 and 1000 µm, and showed the effectiveness of blue diode laser in the appropriate radiation parameter .
For this reason, we employed three dentin thicknesses to investigate the kinetic of absorption and light transmission in the aforementioned wavelengths.
Dogandzhiyska et al.  measured the absorption and penetration of light in the wavelength range 350-1000 nm on dentin specimens with a thickness of 1 mm prepared from above the roof of the pulp chamber perpendicular to the longitudinal axis of the tooth. They reported that the highest tissue uptake in radiation was observed in the blue spectral region and the lowest in the infrared spectral range. Therefore, the lower the absorption of light with a specific wavelength to the dentin, the greater its transmission.
In contrast, in our study, dentin samples were used with three different thicknesses of 300, 600, and 1000 µm in a broader light spectrum of 190 to 1100 nm. The samples were cut transversely only from the chamber pulp's roof perpendicular to the longitudinal axis of the tooth for matching. Nonetheless, our study indicated that the transmission and absorption of the wavelength spectrum of Ultraviolet range was significantly different with visible and infrared light wavelengths (p >.001).
Moreover, some studies have evaluated the optical properties of the diffusion of light through the dentin and enamel. Zijp et al.  in a theoretical model, irradiated 16 µm thin layers of dentin cut parallel to the tubules with He-Ne laser light at various angles to determine the light transmission in the vertical and parallel directions to the dentin tubules. They concluded that dentin tubules are directly involved in the scattering of light on this tissue. When the light was applied parallel to the dentin tubule, the dentin's transmission was more intense. On the other hand, light perpendicular to the dentin tubule axis further reduced its intensity (referring to dentin's anisotropic properties). These data suggest that transcendental transmission of laser light occurs, at least partly, through dentin tubules.
Masayuki O. et al. (2010) conducted a study entitled “Transmission and passage of laser light through dentin” using a laser with a wavelength of 805 ± 20 nm and on the one-millimeter disks of dentin, half of which had been prepared perpendicular to the longitudinal axis of the tooth and half parallel to that. The study found that the laser beam transmission in sections perpendicular to the dentinal tubules was two and a half times the sections prepared parallel to tubules .
In another study, Vaarkamp et al. prepared various sections and examined 15 enamel and 15 dentin specimens with parallel and perpendicular incisions of the proximal teeth with a thickness of approximately 0.03 ± 0.85 mm and the light transmission using the He-Ne laser light source (633 nm) by changing the radiation angle. Their study showed the ability to transmit light through dentinal tubules with a high refractive index and is based on the internal reflection through peri-tubular dentin .
In this study, to simulate the clinic conditions of radiations during dental treatments like endodontics and bleaching, the beams were radiated in both the corono-apically and apico-coronally directions due to inconsistency in the existing dentin tubules. The thickness of the discs could, however, eliminate the anisotropic properties of the specimens in some extent, so dentin discs with thicknesses of 0.3, 0.6, and 1 mm were used. Nevertheless, the results could not be directly referred to the findings obtained through radiating the beams to dentin discs from proximal area of the teeth or thicker dentin discs [42, 43].
In our study, two factors are considered important to justify the weaker association between the permeability of the dentin and the attenuation and distribution of light. First, the dentin discs were cut from the upper part pulp chamber and the aperture of dentin tubules is wider at this distance. The second and main factor relates to the direction of radiation which was along the dentin tubules from corono-apical or apico-coronal sides. The results of this study indicated that the applied radiation protocol based on which the angle of radiation and dentin cross section were matched from one sample to another did not interfere with trans-dentinal light transmission.
There is a possibility that the temporary difference in the number and diameter of the tubules could interfere in the transmission of light, but no study has yet reported the significant effects of these factors on light distribution through the structure of dentin. Therefore, it might be assumed that the number and diameter of tubules could assist the light distribution, though this could not be statistically significant. For this reason, there was no direct relationship between dentin permeability and the attenuation of trans-dentinal light. Moreover, it seems that washing the surface of dentin with EDTA before the permeability experiment was important as the open apertures of tubules facilitate light transmission. Smear layer, which consists of mineral debris and is the outcome of cutting and rinsing, could reduce 86% of the dentin’s permeability .
In another study, Kienle et al. provided dentin discs with 1-3 mm thickness from the upper part of the pulp chamber. Also, they investigated the HeNe laser light transmission with 633 nm wavelength through the inter-tubular dentin which mainly consisted of collagen fibers and hydroxyapatite crystals. The amount of light in dentin discs cleaned with EDTA reduced by 2% (from 61–59%) in comparison with the discs covered by smear layer. This result shows that, under the experimented conditions and parameters of LED, the existence of smear layer does not have a remarkable role per se in the reduction of trans-dentinal light transmission. This finding was confirmed in the present study as the attenuation of light was not influenced after changing the direction of radiation despite the removal of smear layer formed on the two sides (pulpal and occlusal) of the disc, indicating the capability of light distribution through dentin tubules.