Additional sensors integrated into the Watch include an accelerometer, gyroscope, and barometer, which are used to determine device orientation, user movement, and altitude. The back of all Apple Watches is equipped with a Heart Rate Monitor, which projects infrared and green light from light-emitting diodes (LEDs) onto the user's skin and photodiodes measure the varying amount of light reflected. Because blood absorbs green light and reflects red light, the amounts of each type of reflected light are compared to determine heart rate. The Watch adjusts the sampling rate and LED brightness as needed. Starting with the Series 4, Apple added electrical sensors to the Digital Crown and back, allowing the Watch to take electrocardiogram (ECG) readings; the device won FDA clearance in October 2018, becoming the first consumer device capable of taking an ECG. A blood oxygen monitor was added with the Series 6 in 2020, albeit as a "wellness" device not capable of diagnosing a medical condition. The blood oxygen monitor added red LEDs to the back, allowing the watch to determine oxygen levels by measuring blood color. The Watch SE reverted to the capabilities of the Series 3, dropping the electrical sensors and blood oxygen monitor.