The present results reinforce the concept that near infrared light can transmit through human teeth to reach dental pulp, with consistently least transmission being that for 660 nm. Considering the importance of light penetration into teeth to reach the dental pulp for achieving PBM, the present results show that for visible red laser light (660 nm), tooth shade and thickness have significant effects. Moreover, the transmission of 660 nm light was found to be consistently less than for 808 nm, 904 nm, or for the LED device (700–1100 nm). These results infer that PBM treatments of the dental pulp with 660 nm light will be strongly influenced by shade and thickness, and that such factors should be considered carefully when choosing irradiation protocols.

On the other hand, the current results show that longer wavelengths in the near infrared are transmitted better, from 808 nm to 904 nm to 1100 nm. As this happens, with these longer there is less influence of shade and tooth thickness on light transmission. Clinically, this would mean that minor changes in the thickness of enamel and dentine that occur with age (e.g. due to abrasion or erosion) may not warrant changing laser parameters. Likewise, with the LED light source, there was no effect of shade or thickness, and the light source showed the strongest transmission of those tested, especially for root samples of dentine. This knowledge could simplify the use of light sources for PBM, as well as improve the efficacy of dental treatments involving light-based therapies for PBM.

The shade of teeth arises from the microstructural characteristics of enamel and dentine, especially how these influence the absorption, reflection and scatter of light [19, 20]. Both Mie and Rayleigh scattering occur within enamel, due to the presence of prisms and mineral crystals within those prisms. In contrast, dentine has a tubular microstructure, and tubules could act like waveguides for infrared light [19]. The present results reinforce this notion, with the finding that dentine transmits near infrared light much better than enamel and dentine.

A further consideration for tooth shade is that the present study used teeth without restorations and did not include teeth that have undergone endodontic treatments. The presence of various restorative and endodontic materials can cause internal staining of teeth, which could then reduce the transmission of light, as shown in past work [8]. As well as darkened tooth structure, there is also the possibility that dental materials themselves could absorb light, and that interfaces created by dental treatments (such as smear layers) could alter light transmission by causing greater scattering [8]. This is why the smear later from tooth sectioning was removed using EDTA in the present study. Further work is needed to assess how near infrared light transmission could be affected by dental treatments and their consequences.

Numerous publications have described the importance of wavelength on absorption in biological tissues [21, 22]. The present results add to these findings by showing the greater transmission of longer wavelengths in the progression from 660 nm to 1100 nm. One explanation for this may be that with increasing wavelength there is less Rayleigh scattering [19]. The LED device used had a wide wavelength range (700–1100 nm), and there is interest in using multiple wavelength devices for PBM with the aim of achieving greater PBM actions, based on the concept of activation of multiple chromophores [23]. It is already known that the penetration of light into soft tissues is enhanced by using multiple wavelengths in the range from 800 to 1000 nm [24], and the present results suggest that there a similar optical window for teeth.

It is important to consider that in the present study, each light source was delivered at 90 degrees to the sample surface in contact mode. This will place the orientation of the light source almost parallel to the dentinal tubules, which minimises the loss of energy as the light is directed to the dental pulp [17].This should help guide the light toward the pulp, despite some scatter events occurring along the journey [25].

None of the samples in the present study had dental caries. When pre-cavitation lesions of dental caries are present, an increase in light scattering may occur due to the presence of water and more surface porosity, while carious lesions in dentine may show reduced scattering of light [26, 27]. Porphyrins in dentine dental caries may absorb laser emissions, and reduce the light reaching the dental pulp [26]. Additionally, the present study did not consider structural changes in teeth due to age or traumatic injuries, such as dentine sclerosis, that also can influence the transmission of light [17, 28, 29]. There is the further caveat that the present study did not use intact teeth but rather sectioned teeth, which raises the possibility that the procedure of sectioning the tooth into halves could itself alter the optical properties of the tooth [17, 28,29,30]. Hence, some caution is needed when applying the present results to the clinical setting.

The configuration used in the present study directed the light towards the pulp chamber from buccal and lingual surfaces at the middle third of the crown, which corresponds to the position of the pulp horns [17, 28, 29], and involves the light being aimed at 90 degrees to the dentine-enamel junction [27]. The location and angulation of the light delivery tip will affect which areas of the pulp receive the highest dose of light. Further work is needed to explore how changing the position and angulation of the light delivery tip changes the effectiveness of PBM when treating an individual tooth.

Other limitations of this study include that not every possible tooth shade was included, with shades outside the range of the shade guide being excluded. Future studies could assess teeth with unusually light or dark shades outside the normal range, as well as teeth with unusual shades. Future work could also consider deciduous rather than permanent teeth, and include teeth that have been treated with restorative materials or that have undergone endodontic treatments. The present study did not include any teeth with cracks or other defects. In fact, care was taken to prevent damage during preparation of samples, and damage was checked for with magnification. In real-world clinical practice, transmission of visible red and near infrared light could be affected by fracture lines [31,32,33], as well as by other factors that affect the dentine, including dentine sclerosis due to age, traumatic injuries, or the response of the dentine to dental caries once cavitation has occurred. While the present study used a constant thickness of sample, it was not possible to control the composition of the dentine. This is an important limitation of the present work.

Further studies should also assess the impact of tooth shape and type as well as thickness. This would be relevant to dosage differences for small teeth (e.g. mandibular incisors) versus large teeth (e.g. maxillary first molars). Such studies would provide information to inform clinical protocols for PBM using diode laser or LED devices.

The results of the present study reinforce the concept that near infrared light can pass through tooth structure to reach the dental pulp. Previous work has shown that visible red light at 660–670 nm does reach the dental pulp, and can cause biostimulation of dental pulp cells when applied at 2–4 J/cm2, as shown in histological studies of extracted teeth [33, 34]. Likewise, near infrared light is transmitted well by dentine [17, 28]. It is important to consider that obstacles to light transmission will impair effective delivery of photons to target tissues. Absorption of light in tooth structure could result in photothermal changes, which are not desirable for the dental pulp. Warming the teeth is not the intended purpose when PBM is being undertaken, instead the effect should be achieved without heat or any unpleasant sensations being experienced by the patient. Based on the present results, one would predict the least heating with the light source that is transmitted the best, in this case the LED source. A recent clinical trial showed that the same LED light source used in this study when applied to healthy premolar teeth for 60 s was effective for PBM-induced analgesia, but did not cause discomfort in any subject. Under comparable exposure conditions for average power (100 mW) and energy density (17.6 J/cm2), in the same subjects and the same teeth on different days, diode lasers caused occasional discomfort (904 nm > 808 nm > 660 nm) [35]. This was attributed to photothermal changes. The association between light wavelengths and both desirable and undesirable effects should be explored further in clinical studies.

while the 904 and 808 nm lasers caused some sensations that subjects noticed.

In clinical practice, the adoption of photobiomodulation (PBM) devices by clinicians is influenced not only by their efficacy but also by factors such as cost, return on investment, training needs, radiation safety compliance, and integration into existing workflows. The findings indicate that further investigation into the dental applications of LED PBM devices is warranted, particularly due to their high transmission through dental structures, which may allow for reduced dosages and shorter treatment durations. This could enhance the value of PBM treatments, particularly for analgesia. Future clinical trials are necessary to compare different PBM devices, utilizing consistent exposure parameters for multi-wavelength LEDs and single-wavelength light sources.