Yes, there are studies that examine the UV-Vis absorbance, transmittance, and reflectance properties of iron oxides, particularly black, yellow, and red oxides, which are commonly used in sunscreens as physical pigments. Iron oxides can enhance UV protection due to their ability to scatter and absorb light across the visible and near-UV spectrum, and they also add aesthetic benefits like skin-tone matching.

These pigments interact with UV and visible light differently from organic sunscreen agents:

1. Absorbance in the UV-Vis Spectrum: Iron oxides typically have strong absorbance in the visible light spectrum, giving them color (black, yellow, red). In the UV region, they provide some absorbance, which can contribute to sun protection. Black iron oxide (Fe₃O₄) often absorbs more broadly, while yellow and red iron oxides (Fe₂O₃) show variable absorbance peaks due to different energy levels in their crystal structures.

2. Reflectance Properties: Studies indicate that iron oxides can increase reflectance, especially in the visible spectrum, thus aiding in UV protection by reducing transmittance. This reflectance is dependent on the particle size and the specific oxide. For instance, red iron oxide (Fe₂O₃) is more reflective at specific wavelengths, while black iron oxide provides a broader reflection across wavelengths, which can contribute to a smoother skin appearance by reducing contrast in skin imperfections.

3. Transmittance Curves: Iron oxides generally show low transmittance in the visible range, which is why they contribute to skin tone coverage in sunscreens. However, their transmittance in the UV spectrum can vary based on particle size and coating with other materials. In some cases, transmittance curves for iron oxides show limited UV penetration, making them useful for additional physical blocking in sunscreen formulations.

For a detailed analysis, these studies often involve spectral measurements using UV-Vis spectroscopy on films or dispersions of the oxides, as well as their combination with other sunscreen agents. The combination with organic UV filters can be particularly interesting, as the scattering and absorbing properties of iron oxides complement organic filters that mainly absorb UV light, potentially resulting in broader protection with minimized white cast.

You may find valuable information by reviewing studies on the interaction of these iron oxides with organic filters in sunscreen formulations, as they often discuss the isolated and combined behaviors of these pigments, which would be essential for comparative analysis with traditional sunscreens.

Ranjith R., I'm writing to thank you for your attention. Your reply was very enlightening. Is there any article that has a graph with these curves (absorbance and reflectance) of the three iron oxides, including at least one organic filter? I would also appreciate suggestions for articles with this information. Best,