Many medical devices and wearable health sensors work by shining light into the skin and measuring the signal that returns. These technologies are used in products ranging from pulse oximeters to fitness trackers. However, studies have shown that some optical devices can be less accurate for people with darker skin, raising concerns about health care equity and patient safety. In a recent study reported in the Journal of Biomedical Optics, researchers at the VTT Technical Research Centre of Finland addressed this problem by developing realistic skin-like test models that represent a range of skin tones and include artificial blood vessels with flowing blood-like fluid.