Specific information about how sensors can be used to measure things about a person, organized approximately by relevant sensor, sensor technical considerations kept elsewhere
Color sensor for allat stuff…
14 channel 16 bit multipurpose spectral light sensor 11 channels in visible spectrum 1 near infrared 1 clear channel 1 flicker detection channel
requires an “Achromatic bulk diffusor”
- smth to stabilize performance
- placed above the device aperture
- some requirements as to specs; check datasheet
What to measure?
- Heart rate
- Blood oxygen saturation (SpO2): Photodiodes, combined with red and infrared LEDs, measure blood oxygen levels—critical for assessing respiratory function and overall cardiovascular health. This is now a staple in many advanced smartwatches.
- Heart rate variability (HRV), stress, recovery: By tracking changes in the interval between heartbeats via the photoplethysmogram (PPG), photodiodes can offer insights into stress, recovery, and cardiovascular fitness.
also consider: blood alcohol content anemia glucose monitoring sleep staging
alternate LED pulses! (less noise)
HR (heart rate)
heart rate is how many times your heart beats per minute. pulse rate is how many times per minute arteries expand because of heart beating. usually same but occasionally medical complications.
2 ways to measure HR
- Electrical - electrocardiography
- Optical - photoplethysmography
- use infrared light to see the expansion of arteries (pulse rate) general - use green light
- hemoglobin absorbs green light well
- green light (wavelength ~525-560nm) doesn’t penetrate deeply ⇒ surface capillaries (clearest pulse signals)
Need: green LED
HRV (heart rate variability)
measuring changes in time between heart rate (ie 60bpm doesn’t mean 1 beat every second, it could be 0.9, 1.2, 1.1,…) (RR interval — not respiratory rate — Time between consecutive heartbeats (R waves) )
measure of autonomic nervous system which tells ie stress levels, body’s capacity for recovery, overall health/resilience
High HRV - flexible & adapting well Low HRV - signs of stress, fatigue, dehydration, illness - “take care of yourself” flag
How to calculate?
High-res RR intervals from PPG
How to represent? - rMSSD
- Root Mean Square of Successive Differences (RMSSD): This measure calculates the square root of the mean of the squares of the successive differences between adjacent NNs. RMSSD is a good indicator of short-term components of HRV and is particularly sensitive to changes in parasympathetic nervous system activity. (src) collect RR intervals find differences from the RR interval right before it square differences average them take square root measured in milliseconds (ms)
for long term trends:
- measure during deep sleep or in the morning before anything short term (stress tracking, etc): *flag - not sure on power consumption
- look for clean windows to measure - low movement, stable HR, PPG signal quality
- if conditions are not met, do not measure
- measure 30-60s
- stress detection - if previous windows within past 2 hours suggest higher stress then do smth
- limit stress detection to once per few hours
Need: continuous HR data for period of measurement
SpO2 (blood oxygen saturation)
- % of oxygen saturated hemoglobin in blood stream
- “oxyhemoglobin”
- 95+% = healthy, 90-94% = watch out, <90% = danger generally measured using pulse oximeter
- shines red & infrared light through skin
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HbO2 (oxygenated hemoglobin) absorbs more infrared light & less red light
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Hb (deoxygenated hemoglobin) absorbs less infrared & more red light compare how much light of each wavelength is absorbed = relative ratio of HbO2 to Hb = SpO2
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\frac{\text{infrared light}}{\text{red light}} = \text{blood O2 reading}
- uses pulsing of blood (during heartbeats) to filter out static tissues (skin, bone) reflective pulse oximetry - photodetector (diode) & LED on same side. photodiode relies on backscattering of light - lower accuracy than pulse oxiometer devices with LED and photodetector on different sides but enough to capture trends **only works on radial artery (wont work on other parts of wrist)** need to program the LEDs based on skin tone, etc hardware requirements (???): - if pulse oxiometer method, red & IR - photodiode - motion detection to capture SpO2 when not in motion (similar to HRV? ## Stress - **acute stress typically causing a rapid decrease in peripheral skin temperature** due to blood flow being redirected to the core for fight-or-flight responses high stress = lower outside skin temp bc lower blood flow ## Anemia What is Anemia? - blood cells have reduced capacity to carry oxygen when measuring SpO2 - that is % of saturation, not amt of cells carrying blood (ie you only have 20 cells that are fully saturated w o2, = high SpO2, but you really need 200; the reading doesnt capture that) SpO2 - oxygenated vs. deoxygenated hemoglobin, Anemia - **overall absorption signature** that correlates with hemoglobin concentration > #### **Here are some situations when the SpO2 monitoring via your smartwatch can fall short.** [source](https://www.myhealthyapple.com/apple-watch-blood-oxygen-vs-other-smartwatches/) In order to judge a patient’s condition, ideally, you would like to have blood oxygen saturation expressed as a percentage of the total hemoglobin that is saturated with oxygen. Under many circumstances, that is the reading you get from pulse oximeters. However, if the patient has a large amount of non- functional hemoglobin, the reading is not accurate. Several situations can lead to such large amounts of non-functioning hemoglobin. Carbon monoxide poisoning and even heavy smoking increase the amount of carboxyhemoglobin, non-functioning hemoglobin. So if you are a heavy smoker, chances are that your SpO2 readings obtained via any of these leading smartwatches may not be correct. The same holds for users who suffer from Anemia. Damage to red blood cells may cause anemia, a lack of red blood cells, and thus hemoglobin in the blood. An anemic patient may not have enough functioning hemoglobin in the blood to oxygenate the tissues. The small amount of functioning hemoglobin in the blood may be well saturated with oxygen, so the patient may have a normal SpO2 reading, but the patient may not have enough oxygen going to the tissues. The human body reduces the heat lost by the skin by constricting the peripheral blood vessels. So if you are performing an on-demand SpO2 reading in colder temperatures, you will have to account for the inaccuracy in the readings. ## Sleep Apnea - SpO2 drops >5% (rough) that lasts for >15s measure the changes in the respiratory rate (RR) from PPG - irregular breathing will change PPG waveform