Biological structures located at various depths in tissue, such as blood and melanin, are challenging to perceive from digital images acquired with incoherent light sources. Use of cross-polarized light produces a low-coherence signal with latent embedded data. We demonstrate the ability to modulate visible depth of high-resolution digital image data acquired with commercially available equipment. Superficial and deep structures are visualized on a polarization-gated continuum. Conspicuity of chromophores contained in hemoglobin or melanin are increased independently through a color luminance/temperature processing approach. Qualitative visual analysis of structures distributed in skin tissue is achieved by targeting the region where red and green photosite values have equal spectral responsivity, at approximately 580nm, corresponding to a sharp spike in hemoglobin reflectance. This region contains both sufficient quantum efficiency (QE) from red and green color filter channels as well as overlapping melanin and hemoglobin signals from within skin tissue. Leveraging this overlapping tissue signal and sensor QE in the orange (580nm) region, luminance is increased or decreased according to the structure of interest. Shifting the illuminant light's properties from high to low frequencies using Kelvin temperature, polarized light is gated, allowing observation of skin structures at distinct depths. For example, superficial skin layers are separately visualized from deeper vessels or pigment. The novel viewing process may be considered independent component analysis of diffuse reflectance polarization-based optical images, or as low-coherent backscatter imaging of spatial encoded skin features on a spectral continuum. The technique has implications for an array of dermatological and tissue imaging applications.
Significance: Dermoscopes incorporate light, polarizers, and optical magnification into a handheld tool that is commonly used by dermatologists to evaluate skin findings. Diagnostic accuracy is improved when dermoscopes are used, and some major artificial intelligence (AI) projects have been accomplished using dermocopic images. Color rendering consistency and fidelity are crucial for clinical diagnostics, AI, and image processing applications.Aim: With many devices available on the market, our objective was to measure the emission spectra of various dermoscopes, compare them with other light sources, and illustrate variations in reflected colors from images of a reference sample.Approach: A spectrometer measured the spectral power distribution (SPD) produced by four dermoscope models and three alternate light sources, illustrating differences in the emission spectra. Most dermoscopes use light-emitting diodes (LEDs), which are inconsistent when compared with one another. An LED was compared with halogen, xenon-arc, and daylight sources. Images of a micro ColorChecker were acquired from several sources, and three specific colors were selected to compare in CIELAB color space. Color consistency and color fidelity measured by color rendering index (CRI) and TM-30-18 graphical vectors show variation in saturation and chroma fidelity.Results: A marked degree of variation was observed in both the emission and reflected light coming from different dermoscopes and compared with other sources. The same chromophores appeared differently depending on the light source used.Conclusions: A lack of uniform illumination resulted in inconsistent image color and likely impacted metamerism and visibility of skin chromophores in real-world settings. Artificial light in skin examinations, especially LEDs, may present challenges for the visual separation of specific colors. Attention to LEDs SPD may be important, especially as the field increases dependency on machine/computer vision.
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