Cult of Mac, nicely summarizing this Reddit thread:

With a variety of bands, and price tags ranging all the way from $349 – $17,000, there’s an Apple Watch for everyone. Except, possibly, the heavily tattooed.

That’s according to a new thread on Reddit which claims that several tattoo-sporting Apple Watch customers are having trouble using the device, because the wearable’s wrist-detection feature gets confused by the way in which tattoos reflect the green and infrared light emitted by the Watch.

The result? People with tattoos don’t get notifications, unless they move the Watch to an un-tattooed area, or turn off wrist detection.

Here’s the science, from further down the Reddit thread:

Oxyhemoglobin has several local peaks of absorbance which can be used for pulse oximetry: one green, one yellow, one infrared, etc. Apple uses the ones at infrared and green parts of the spectrum. Now, here’s some key facts. Melanin and ink are both equally good at absorbing frequencies over 500nm, which sadly includes the green. But, melanin’s absorbance falls down so rapidly that by the infrared end of the spectrum its hardly absorbing anything at all. That, combined with the fact that Apple adjusts the sensitivity/light level dynamically means infrared is probably black people friendly. Ink has a much more gradual fall off in absorbance, so even infrared might not work for them.

Apple’s response:

I spoke with Apple and informed them of my situation. the applecare specialist was really good, walked me through a bunch of steps to try and alleviate another issue I’m having (making calls from the watch). they are reporting to engineering and i should hear back from him this week regarding my call issue and the detection issue.

Interesting problem, good response.