The subjects will go through a questionnaire session about their medical history by a designated study team member. The questionnaire data will be placed in RedCap using a subject ID that cannot be linked back to the subject. Then the subject will consent and be enrolled. Then the non-invasive data-collecting procedure will start on the following equipment.

Spatial Frequency Domain Imaging (SFDI): During the consenting period, the Modulim equipment will be calibrated and set up ready for scanning. The subject's volar aspect of the thumb along with the palm surface will be ready to target the optical camera head. Actual scanning takes less than 60 seconds. The images will be processed offline. During processing 3-5 different regions of interest (ROI) will be taken to measure the oxygen parameters such as tissue oxygen saturation (StO2), oxy-hemoglobin (HbO2), deoxy-hemoglobin (HbR), superficial hemoglobin (HbT1, sub-surface hemoglobin (HbT2). The model used is a Clarifi Modulim Transcutaneous oxygen monitoring (TCOM): Transcutaneous oxygen monitoring (TCOM or TcpO2) is a noninvasive, clinically-approved method to obtain skin oxygen levels. The method is quantitative and measures oxygen delivery to the skin from underlying tissue. Before positioning the electrode, an adhesive fixation ring will be placed on the dry skin on the volar aspect of the thumb and an electrolyte as a contact liquid will be filled in half and the probe is aligned into it by rotating clockwise to fasten it. The recording will be started and waited for the oxygen level to stabilize and a fixed value will be recorded. The model used is a Perimed PeriFlux 5000. The probe will be heated to about 45 degrees C. Although this device has other options, only the measurement of O2 will be performed using this device.

Apple Watch Oxygen Sensor: Smartwatch blood oxygen sensors also measure blood oxygen levels in the tissue. Apple Watch Series 6 introduced this new feature for monitoring blood oxygen levels using light-emitting diodes (LEDs) at the back of apple watches. A low blood oxygen level can be indicative of a serious health issue that needs immediate attention. The apple watch is equipped with green, red, and infrared LEDs that shine light onto the blood vessels in the wrist, with photodiodes measuring the amount of light reflected back. Apple's algorithms use this information to calculate the color of the blood, which is an indication of how much oxygen is in the blood. Bright red blood is well-oxygenated, while darker blood has less oxygen. This can measure blood oxygen levels between 70 and 100 percent. Most healthy people have blood oxygen levels that range from 95 to 100 percent. The apple watch sensor will be positioned on the user's preferred wrist.

Pulse Oximeter for Oxygen Monitoring: The pulse oximeter enables transcutaneous monitoring of the oxygen saturation of hemoglobin in arterial blood (StO2). Pulse oximetry is so widely prevalent in medical care that it is often regarded as a fifth vital sign\[3\]. It is important to understand how the technology functions as well as its limitations. To recognize the settings in which pulse oximeter readings of oxygen saturation (SpO2), an understanding of two basic principles of pulse oximetry is required: (i) how oxyhemoglobin (HbO2) is distinguished from deoxyhemoglobin (HbR) and (ii) how the SpO2 is calculated only from the arterial compartment of blood. Pulse oximetry is based on the principle that HbO2 and HbR differentially absorb red and near-infrared (IR) light. It is fortuitous that HbO2 and HbR have significant differences in absorption at red and near-IR light because these two wavelengths penetrate tissues well whereas blue, green, yellow, and far-IR light are significantly absorbed by non-vascular tissues and water \[3\]. HbO2 absorbs greater amounts of IR light and lower amounts of red light than does HbR; this is consistent with experience - well-oxygenated blood with its higher concentrations of HbO2 appears bright red to the eye because it scatters more red light than does HbR. On the other hand, HHR absorbs more red light and appears less red. Exploiting this difference in light absorption properties between HbO2 and HbR, pulse oximeters emit two wavelengths of light, red at 660 nm and near-IR at 940 nm from a pair of small light-emitting diodes located in one arm of the finger probe. The light that is transmitted through the finger is then detected by a photodiode on the opposite arm of the probe. In this study, the volar aspect of the thumb and index finger of the subject will be used to measure SpO2.