# SRPhotoplethysmogramOpticalSample A data sample from the photoplethysmogram (PPG) optical sensor. ``` class SRPhotoplethysmogramOpticalSample ``` ## Overview To get the PPG waveform, use the [`normalizedReflectance`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/normalizedReflectance-9aidm) property. Apple Watch uses an arrangement of emitters and photodiodes to measure the PPG waveform. ### Interpret second-generation PPG sensors data Second-generation optical heart sensors, in Apple Watch 4, 5, SE 3 and SE models, use green or infrared (IR) LED lights paired with light-sensitive photodiodes.  The active photodiode’s indices show which photodiode or combination of photodiodes the Apple Watch uses. The light-sensitive photodiodes have different arrangements between the Apple Watch models:   For the second-generation sensors, the mapping of the optical heart sensor (SE, Series 4 and 5) emitter is: |Emitter index|LED arrangement | |-------------|--------------------| |0 |0, Infrared (940 nm)| |1 |1, Infrared (940 nm)| |2 |2, Green (525 nm) | |3 |3, Green (525 nm) | |4 |4, Green (525 nm) | |5 |5, Green (525 nm) | ### Interpret third-generation PPG sensors Third-generation optical heart sensor, in Apple Watch Series 6 and later, plus Apple Watch Ultra and Ultra 2, use an additional red LED with a different arrangement of emitters and photodiodes than the second-generation PPG sensors. The active photodiodes indices show which photodiode or combination of photodiodes the Apple Watch uses.  For the third-generation sensors, the mapping of the optical heart sensor (Series 6, 7, 8, 9, Ultra, and Ultra 2) emitter is: |Emitter index|LED arrangement | |-------------|--------------------| |0 |0, Infrared (850 nm)| |1 |1, Infrared (850 nm)| |2 |2, Infrared (850 nm)| |3 |3, Infrared (850 nm)| |4 |4, Infrared (940 nm)| |5 |5, Red (660 nm) | |6 |6, Red (660 nm) | |7 |7, Red (660 nm) | |8 |8, Red (660 nm) | |9 |9, Green (525 nm) | |10 |10, Green (525 nm) | |11 |11, Green (525 nm) | |12 |12, Green (525 nm) | ## Topics ### Accessing optical data [`emitter`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/emitter) The index of the LED in use during the sample. [`activePhotodiodeIndexes`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/activePhotodiodeIndexes) The set of photodiodes in use during the sample. [`signalIdentifier`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/signalIdentifier) The identifier for a signal that photodiodes and emitters produce. [`nominalWavelength`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/nominalWavelength) The wavelength in nanometers that the emitter produces while operating at a specific temperature. [`effectiveWavelength`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/effectiveWavelength) A temperature-compensated wavelength estimate in nanometers that the emitter produces. [`samplingFrequency`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/samplingFrequency) The sampling frequency of the photoplethysmogram (PPG) data in hertz. [`nanosecondsSinceStart`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/nanosecondsSinceStart) The time in nanoseconds since the system turns on the sensor. [`conditions`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/conditions) The sensor context or conditions that may affect the sample. [`Condition`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/Condition) The conditions that may occur when recording photoplethysmogram optical data. [`noiseTerms`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/noiseTerms-swift.property) The mathematical terms for computing the noise in this sample. [`NoiseTerms`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/NoiseTerms-swift.struct) The mathematical terms that you use to compute the photoplethysmogram (PPG) noise. [`normalizedReflectance`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/normalizedReflectance-15f2k) [`whiteNoise`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/whiteNoise) An estimate of the white noise of the sensor. [`pinkNoise`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/pinkNoise) An estimate of the pink noise of the sensor. [`backgroundNoise`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/backgroundNoise) An estimated timeseries of ambient noise intrusion. [`backgroundNoiseOffset`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/backgroundNoiseOffset) The white noise variance estimate in the background noise signal. [`normalizedReflectance`](/documentation/SensorKit/SRPhotoplethysmogramOpticalSample/normalizedReflectance-9aidm) The photoplethysmogram waveform in normalized units. ## Relationships ### Conforms To [`NSSecureCoding`](/documentation/Foundation/NSSecureCoding) [`CustomDebugStringConvertible`](/documentation/Swift/CustomDebugStringConvertible) [`CustomStringConvertible`](/documentation/Swift/CustomStringConvertible) [`NSObjectProtocol`](/documentation/ObjectiveC/NSObjectProtocol) [`NSCopying`](/documentation/Foundation/NSCopying) [`NSCoding`](/documentation/Foundation/NSCoding) [`Hashable`](/documentation/Swift/Hashable) [`SendableMetatype`](/documentation/Swift/SendableMetatype) [`Sendable`](/documentation/Swift/Sendable) [`CVarArg`](/documentation/Swift/CVarArg) [`Equatable`](/documentation/Swift/Equatable) ### Inherits From [`NSObject-swift.class`](/documentation/ObjectiveC/NSObject-swift.class) --- Copyright © 2026 Apple Inc. 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