Optical Properties Of Human Skin Research Articles

Relevance and utility of the in-vivo and ex-vivo optical properties of the skin reported in the literature: a review [Invited

Imaging non-invasively into the human body is currently limited by cost (MRI and CT scan), image resolution (ultrasound), exposure to ionising radiation (CT scan and X-ray), and the requirement for exogenous contrast agents (CT scan and PET scan). Optical imaging has the potential to overcome all these issues but is currently limited by imaging depth due to the scattering and absorption properties of human tissue. Skin is the first barrier encountered by light when imaging non-invasively, and therefore a clear understanding of the way that light interacts with skin is required for progress on optical medical imaging to be made. Here we present a thorough review of the optical properties of human skin measured in-vivo and compare these to the previously collated ex-vivo measurements. Both in-vivo and ex-vivo published data show high inter- and intra-publication variability making definitive answers regarding optical properties at given wavelengths challenging. Overall, variability is highest for ex-vivo absorption measurements with differences of up to 77-fold compared with 9.6-fold for the in-vivo absorption case. The impact of this variation on optical penetration depth and transport mean free path is presented and potential causes of these inconsistencies are discussed. We propose a set of experimental controls and reporting requirements for future measurements. We conclude that a robust in-vivo dataset, measured across a broad spectrum of wavelengths, is required for the development of future technologies that significantly increase the depth of optical imaging.

Optical Epidermal Mimicry from Ultraviolet to Infrared Wavelengths.

Optical tissue phantoms present substantial value for medical imaging and therapeutic applications. We have developed an epidermal tissue phantom to mimic the optical properties of human skin from the ultraviolet to the infrared region, exceeding the breadth of existing studies. An epoxy matrix is combined with melanin-mimicking polydopamine via a cost-effective fabrication strategy. Reflectance and transmittance measurements enable calculation of the wavelength-dependent complex refractive index and absorption coefficient. Results are compared with literature data to establish agreement with a real human epidermis. By analyzing emissive power at a typical skin temperature, the epidermal tissue phantom is shown to accurately mimic the radiative properties of human skin. This simple, multifunctional material represents a promising substitute for human tissue for a variety of medical and bioengineering applications.

Time-Domain Optical Coherence Tomography System for Determining The Extinction Coefficient and Group Refractive Index of Gelatin-based Skin Phantoms

Optical Coherence Tomography (OCT) is a non-invasive, non-destructive optical imaging technique that uses a low coherence interferometer to obtain real-time cross-sectional images of samples. OCT is notably used in biomedical applications including ophthalmology and dermatology. Aside from generating cross-sectional images, axial scans can also provide additional information about its optical properties such as extinction coefficient and refractive index. This study determines the extinction coefficients and group refractive indices of gelatin-based skin phantoms using a portable time-domain (TD) – OCT system. The gelatin-based skin phantoms were fabricated with varying concentrations of titanium dioxide (TiO2), while keeping the amount of both gelatin and water constant. By changing the proportion of the gelatin powder and TiO2, skin phantoms can then be fabricated to mimic various skin conditions, both pathologic and non-pathologic. Results of the study found a positive correlation of extinction coefficient and refractive index with TiO2 concentration. Thus, increasing TiO2 concentration also increases both extinction coefficient and group refractive index. The median extinction coefficient values of the phantoms ranged from 4.29 mm−1 to 8.48 mm−1. Literature showed that the epidermis can have extinction coefficients of 1.64-7.3 mm−1. For refractive indices of the fabricated phantoms, values ranged from 1.32 to 1.48, while studies on human participants showed that human skin has refractive index values of 1.34-1.56. Based on these properties, it is feasible to fabricate phantoms simulating the optical properties of human skin.

EFFECTS OF SKIN TONE AND AMBIENT LIGHTING ON ACCURACY OF CONTACTLESS MEASUREMENT OF HEART RATE AND PULSE TRANSIT TIME: AN INVESTIGATION TOWARD IMAGE-BASED BLOOD PRESSURE ESTIMATION

Abstract Objective: Image-based photoplethysmography (iPPG) using video of the skin offers potential for contactless blood pressure (BP) estimation from the pulse transit time (PTT). Optical properties of human skin and ambient lighting are likely to affect accuracy of image-based PTT (iPPT) and are investigated here. Design and method: Twelve healthy volunteers (6 female, 27 ± 6 years) with a range of skin tones (Fitzpatrick skin scores of 5 to 27) were recruited. For each subject, the experiment was performed in standard fluorescent room lighting (100 lux) and repeated at five other levels of light intensity (40–310 lux). Two-minute videos were acquired from each subject's face and palm of the left hand; simultaneous electrocardiogram (ECG) and non-invasive finger BP were also measured. In each frame, two regions of interest (ROIs) were selected from the subject's face and hand, and the average of green colour intensity for each ROI were calculated as iPPG. The iPPG signal was band-pass filtered between 0.66–3 Hz, (corresponding to a 40–180 bpm heart rate (HR) range). HR was calculated as the dominant peak in the Fourier spectrum of filtered iPPG. Beat-to-beat values of iPTT were calculated between the corresponding feet of the iPPG pulsations. Results: The absolute error between average HR (errHR) determined from ECG and that calculated from iPPG in both ROIs was significantly correlated with room lumen (RL) and skin score (SC) (errHR = – 0.08 × RL + 0.67 × SC + 12, p < 0.05). The minimum HR error was in standard room lighting (errHR = 3.9 ± 8.9 bpm). Using average iPTT in room lighting as the reference, the percentage error (iPTT%err) between reference iPTT and iPTT in other lighting conditions significantly decreased with lower lighting conditions but was not significantly changed with skin tone score (iPTT%err = -0.09 × RL + 55, p < 0.05). Conclusions: Skin tone and ambient lighting intensity affects accuracy of contactless HR measurement. Only light intensity affects accuracy of iPTT measurement. It is likely that errors in contactless BP estimation are minimised in standard room lighting conditions but increase in dimmer conditions.

On the suitability of laser-Doppler flowmetry for capturing microvascular blood flow dynamics from darkly pigmented skin

Objective: To assess the performance of laser Doppler flowmetry (LDF) in measuring blood perfusion from darkly-pigmented skin, i.e. skin with high melanin concentration. LDF provides for the noninvasive monitoring of microvascular blood flow dynamics. It has been used extensively on light-skinned subjects, i.e. on skin with low melanin concentration, in both the healthy and pathological states. Because the optical properties of human skin might affect the reliability of optically-based diagnostic equipment, the effectiveness of LDF needs to be checked and evaluated on dark skin, too, if this method is to be useful in global healthcare. Approach: Thirteen dark-skinned subjects and ten light-skinned subjects were included in the study. Microvascular blood flow dynamics was measured on both the right and left ankles using LDF with a laser diode of wavelength 780 nm. The characteristics of time-varying blood flow oscillations were investigated by wavelet analysis, nonlinear mode decomposition and wavelet phase coherence. An electrocardiogram (ECG), skin temperature, and respiratory effort were measured simultaneously with the LDF for each subject. Main results: No significant differences were observed between the groups in the mean blood perfusion (p > 0.1), or wavelet power (p > 0.6). The instantaneous heart rate (IHR), extracted from the LDF at each of the recording sites, and from the ECG, did not differ significantly between the groups (p > 0.8). Nor did the wavelet power of the IHR differ (p > 0.8) between the groups. The only significant difference found between the groups lay in left/right ankle blood flow coherence near the cardiac frequency, attributable to known ethnic physiological differences. Significance: These results indicate that high melanin concentrations in skin exert no significant influence on the ability of LDF to monitor microvascular blood flow dynamics when using a laser diode of wavelength 780 nm. Hence LDF can help in the diagnosis and exploration of the pathogenesis of diseases such as diabetes, hypertension, or malaria in darkly pigmented patients across sub-Saharan Africa.

Terahertz spectroscopy of human skin tissue models with different melanin content.

Terahertz imaging has been proposed for burns and skin cancer identification. However, the role of melanocytes, melanosomes, melanin content and distribution in determining the terahertz optical properties of human skin has not been investigated. We use terahertz time domain spectroscopy to measure the optical properties of in vitro pigmented human skin tissue models from Asian, Black, and Caucasian donors. Spectra were collected at various time intervals and used to extract the absorption coefficient and index of refraction at terahertz frequencies. Our results indicate that the degree of cell differentiation and type of donor both contribute to the measured terahertz optical properties.

In Vivo Measurement of Optical Properties of Human Skin for 450\u2013800\xa0nm and 950\u20131600\xa0nm Wavelengths

Laser or light-emitting diode therapy has been developed for medical care and skin rejuvenation. Some of these therapies are not only ineffective, but also unsafe. To address these issues, the optical properties of human skin that characterize radiation transfer to the skin have to be clearly understood. In this study, we use in-house instruments to measure the in vivo absorption and scattering coefficients for 198 Japanese participants in visible (450–800 nm) and near-infrared (950–1600 nm) region. The average values and standard deviations of the optical properties are recorded. The effects of age, gender, and body parts on these parameters are analyzed. For the scattering coefficient, the highest value is observed for the inner forearm and the lowest for the region between the thumb and forefinger; females exhibit higher values than males in the visible and some of near-infrared wavelengths; and the value decreases with the age for males in the visible region. In terms of the absorption coefficient, males exhibit higher values than females in the visible region and some of near-infrared wavelength; and the values increase with age in the visible region; the magnitude relations between the body parts differ with the wavelength.

人の皮膚の非侵襲光物性計測装置の開発

人の皮膚の非侵襲光物性計測装置の開発

Synergistic efficacy of salicylic acid with a penetration enhancer on human skin monitored by OCT and diffuse reflectance spectroscopy.

Salicylic acid (SA) has been frequently used as a facial chemical peeling agent (FCPA) in various cosmetics for facial rejuvenation and dermatological treatments in the clinic. However, there is a tradeoff between therapeutic effectiveness and possible adverse effects caused by this agent for cosmetologists. To optimize the cosmetic efficacy with minimal concentration, we proposed a chemical permeation enhancer (CPE) azone to synergistically work with SA on human skin in vivo. The optical properties of human skin after being treated with SA alone and SA combined with azone (SA@azone) were successively investigated by diffuse reflectance spectroscopy (DRS) and optical coherence tomography (OCT). Our results revealed that as the SA concentration increased, the light reflectance decreased and the absorption increased. We also found that SA@azone exhibited a synergistic effect on enhancing light penetration and OCT imaging depth. We demonstrated that the combination of DRS and OCT techniques could be used as a noninvasive, rapid and accurate measurement method to monitor the subtle changes of skin tissue after treatment with FCPA and CPE. The approach will greatly benefit the development of clinical cosmetic surgery, dermatosis diagnosis and therapeutic effect inspection in related biomedical studies.

Arrayed laser image contrast evaluation (Alice) for human body detection

Human body detection is an important research area with potential for numerous applications, including search and rescue missions, safe driving, surveillance, and security. Here, we propose and experimentally validate a novel concept named ‘arrayed laser image contrast evaluation’ (Alice) to detect the human body, based on the unique optical properties of human skin. In the Alice system, an NIR dot array laser is used for illumination, and the irradiated area is detected using a near infrared (NIR) camera. Human skin has the characteristic optical properties of relatively low light absorption and high light scattering in the NIR region. When human skin is illuminated with focused laser dots, the NIR light penetrates deeply. Light is scattered multiple times inside the skin before it is re-emitted. The light intensity distribution of the reflected light tends to be diffuse. Human skin can be easily identified using arrayed laser image contrasts, calculated from the reflected light intensity distribution. With the Alice system, an almost entirely hidden person can be successfully detected using information from even a tiny patch of skin. The Alice human body detection system thus has potential for use in a wide range of applications.

Optical Properties of Human Skin around Biological Active Points

The physical parameter for the so-called Biologically Active Points most documented in the literature is the electrical impedance. These points seem to have higher electrical conductance than their surrounding points. The stimulation of these points is made not only mechanically and electrically but also optically (laser acupuncture) and thermically (moxibustion). We evaluated by optoacoustic technique and by photothermal radiometry, some skin points along the PC acupuncture meridian in the forearm region. We found a slight relative increment in absorption in some regions around the meridian region but without a definite association with an acupoint. Moreover, the same case can be applied to a non-meridian line. In radiometric signals we did not find differences with respect to control points in a two layer model. Based in these results we can not conclude that such points are optically or thermally special.

A Biophysically‐Based Model of the Optical Properties of Skin Aging

AbstractThis paper presents a time‐varying, multi‐layered biophysically‐based model of the optical properties of human skin, suitable for simulating appearance changes due to aging. We have identified the key aspects that cause such changes, both in terms of the structure of skin and its chromophore concentrations, and rely on the extensive medical and optical tissue literature for accurate data. Our model can be expressed in terms of biophysical parameters, optical parameters commonly used in graphics and rendering (such as spectral absorption and scattering coefficients), or more intuitively with higher‐level parameters such as age, gender, skin care or skin type. It can be used with any rendering algorithm that uses diffusion profiles, and it allows to automatically simulate different types of skin at different stages of aging, avoiding the need for artistic input or costly capture processes. While the presented skin model is inspired on tissue optics studies, we also provided a simplified version valid for non‐diagnostic applications.

Motion-Resistant Remote Imaging Photoplethysmography Based on the Optical Properties of Skin

Remote imaging photoplethysmography (RIPPG) can achieve contactless monitoring of human vital signs. However, the robustness to a subject’s motion is a challenging problem for RIPPG, especially in facial video-based RIPPG. The RIPPG signal originates from the radiant intensity variation of human skin with pulses of blood and motions can modulate the radiant intensity of the skin. Based on the optical properties of human skin, we build an optical RIPPG signal model in which the origins of the RIPPG signal and motion artifacts can be clearly described. The region of interest (ROI) of the skin is regarded as a Lambertian radiator and the effect of ROI tracking is analyzed from the perspective of radiometry. By considering a digital color camera as a simple spectrometer, we propose an adaptive color difference operation between the green and red channels to reduce motion artifacts. Based on the spectral characteristics of photoplethysmography signals, we propose an adaptive bandpass filter to remove residual motion artifacts of RIPPG. We also combine ROI selection on the subject’s cheeks with speeded-up robust features points tracking to improve the RIPPG signal quality. Experimental results show that the proposed RIPPG can obtain greatly improved performance in accessing heart rates in moving subjects, compared with the state-of-the-art facial video-based RIPPG methods.

Development of Imaging Screen with Superior Feeling of Depth Inspired by Optical Properties of Human Skin

Development of Imaging Screen with Superior Feeling of Depth Inspired by Optical Properties of Human Skin

The effect of an external mechanical compression on in vivo optical properties of human skin

We have studied the influence of an external mechanical compression on diffuse reflection spectra of skin tissue under in vivo conditions. An analysis of these spectra based on the diffusion approximation of the radiation transfer theory has allowed us to find that the application of the external compression weakens absorbing and scattering properties of skin tissue. After the removal of the compression, the recovery time of the skin tissue (on the order of 1 h) considerably exceeds the stabilization time of its parameters after application of external mechanical compression (several minutes). In this case, at the initial moment of time after the removal of the compression, the fullness of blood vessels and the degree of oxygenation of blood hemoglobin in the skin tissue increase considerably compared to normal skin.

Continuous noninvasive monitoring of changes in human skin optical properties during oral intake of different sugars with optical coherence tomography.

The objective of this study was to evaluate the effects of blood glucose concentration (BGC) on in vivo human skin optical properties after oral intake of different sugars. In vivo optical properties of human skin were measured with a spectral domain optical coherence tomography (SD-OCT). Experimental results show that increase of BGC causes a decrease in the skin attenuation coefficient. And the maximum decrements in mean attenuation coefficient of skin tissue after drinking glucose, sucrose and fructose solution are 47.0%, 36.4% and 16.5% compared with that after drinking water, respectively (p < 0.05). The results also show that blood glucose levels of the forearm skin tissue are delayed compared with finger-stick blood glucose, and there are significant differences in the time delays after oral intake of different sugars. The time delay between mean attenuation coefficient and BGC after drinking glucose solution is evidently larger than that after drinking sucrose solution, and that after drinking sucrose solution is larger than that after drinking fructose solution. Our pilot studies indicate that OCT technique is capable of non-invasive, real-time, and sensitive monitoring of skin optical properties in human subjects during oral intake of different sugars.

Effects of temperature-dependent optical properties on the fluence rate and temperature of biological tissue during low-level laser therapy

The effects of temperature-dependent optical properties on the change of fluence rate and temperature distribution within biological tissues during low-level laser therapy (LLLT) were investigated by experimental and numerical methods. The fluence rate and temperature within a porcine skin were measured in vitro using an optical fiber sensor and a thermocouple, respectively, while irradiating the sample with a continuous wave laser (IPG Laser GmbH, Burbach, Germany, 1,064nm, 3.14W/cm(2)). The absorption and reduced scattering coefficients of porcine skin were estimated using an inverse adding-doubling algorithm from the total reflectance and transmittance measured with a double-integrating sphere. It was shown that the reduced scattering coefficient of porcine skin decreased significantly as the skin temperature increased within the range of 26-40°C. To incorporate the temperature dependency of tissue optical properties in the simulation, a mathematical model that adopted coupled equations for fluence rate and bioheat transfer was developed. It was shown that the predicted fluence rate and temperature by the proposed mathematical model agreed closely with the measured values of porcine skin. The calculation of human skin temperature using the developed model revealed that the skin temperature could be significantly underestimated if the temperature dependency of optical properties of human skin were ignored during LLLT simulation.

Analyzing reflectance spectra of human skin in legal medicine

Our current research in the framework of an interdisciplinary project focuses on modelling the dynamics of the hemoglobin reoxygenation process in post-mortem human skin by reflectance spectrometry. The observations of reoxygenation of hemoglobin in livores after postmortem exposure to a cold environment relate the reoxygenation to the commonly known phenomenon that the color impression of livores changes from livid to pink under low ambient temperatures. We analyze the spectra with respect to a physical model describing the optical properties of human skin, discuss the dynamics of the reoxygenation, and propose a phenomenological model for reoxygenation. For additional characterization of the reflectance spectra, the curvature of the local minimum and maximum in the investigated spectral range is considered. There is a strong correlation between the curvature of specra at a wavelength of 560 nm and the concentration of O2-Hb. The analysis is carried out via C programs, as well as MySQL database queries in Java EE, JDBC, Matlab, and Python.

Towards drug quantification in human skin with confocal Raman microscopy

Understanding the penetration behaviour of drugs into human skin is a prerequisite for the rational development and evaluation of effective dermal drug delivery.The general procedure for the acquisition of quantitative drug penetration profiles in human skin is performed by sequential segmentation and extraction. Unfortunately, this technique is destructive, laborious and lacks spatial resolution. Confocal Raman microscopy bares the potential of a chemically selective, label free and nondestructive analysis. However, the acquisition of quantitative drug depth profiles within skin by Raman microscopy is impeded by imponderable signal attenuation inside the tissue.In this study, we present a chemical semi-solid matrix system simulating the optical properties of human skin. This system serves as a skin surrogate for investigation of Raman signal attenuation under controlled conditions. Caffeine was homogeneously incorporated within the skin surrogate, and Raman intensity depth profiles were acquired. A mathematical algorithm describing the Raman signal attenuation within the surrogate was derived from these profiles. Human skin samples were incubated with caffeine, and Raman intensity depth profiles were similarly acquired. The surrogate algorithm was successfully applied to correct the drug profiles in human skin for signal attenuation. For the first time, a mathematical algorithm was established, which allows correction of Raman signal attenuation in human skin, thus facilitating reliable drug quantification in human skin by confocal Raman spectroscopy.

Optical properties of human skin

A survey of the literature is presented that provides an analysis of the optical properties of human skin, with particular regard to their applications in medicine. Included is a description of the primary interactions of light with skin and how these are commonly estimated using radiative transfer theory (RTT). This is followed by analysis of measured RTT coefficients available in the literature. Orders of magnitude differences are found within published absorption and reduced-scattering coefficients. Causes for these discrepancies are discussed in detail, including contrasts between data acquired in vitro and in vivo. An analysis of the phase functions applied in skin optics, along with the remaining optical coefficients (anisotropy factors and refractive indices) is also included. The survey concludes that further work in the field is necessary to establish a definitive range of realistic coefficients for clinically normal skin.