Recently, we've been hearing reports from Twitter and Reddit that the Apple Watch's plethysmograph sensor plays not-so-nicely with wrist or arm sleeve tattoos. The ink pigmentation interferes with the sensor's ability to read your heart rate — and with it, the Watch's ability to assess whether or not it's maintaining skin contact.

After some brief tests, we're inclined to agree with those early reports — if your tattoo happens to be a solid, darker color. This is has to do with the way Apple measures your heart rate. Let's break it down.

The science behind Apple's sensors

Apple's support article on the Watch's heart rate sensor explains a fair amount about its inner workings, including how it measures your pulse:

Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it's less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate. The heart rate sensor can also use infrared light. This mode is what Apple Watch uses when it measures your heart rate every 10 minutes. However, if the infrared system isn't providing an adequate reading, Apple Watch switches to the green LEDs. In addition, the heart rate sensor is designed to compensate for low signal levels by increasing both LED brightness and sampling rate.

In short, Apple uses various spectrums of light to track the blood flow through your skin. Anything that reduces that light's reflectiveness — ink pigmentation within your skin, for example — can interfere with that sensor.

For those wondering: natural skin pigmentation doesn't block light the same way artificial ink pigment or even scar tissue does, so you shouldn't run into a problem if your skin is naturally darker.

The Watch and tattoos

So does the Watch run into problems with wrist tattoos? Yes and no. I spent an hour today testing the Watch's sensor reading on multiple tattoo colors, and have indeed managed to replicate some of the issues Reddit and Twitter users were having.

For disclosure: We tested the Watch's sensors against tattooed and non-tattooed sections on both the wrist and elsewhere on the body. On non-tattooed non-wrist sections, the sensors gave identical readings as when also tested on the wrist; on tattooed sections, sensor readings varied wildly depending on colors and shading.

Dark, solid colors seem to give the sensor the most trouble — our tests on solid black and red initially produced heart rate misreadings of up to 196 BPM before failing to read skin contact entirely. Tests on lighter tattoo colors including purple, yellow, and orange produced slightly elevated heart misreads of 80 BPM (compared to 69 BPM on the wearer's non-tattooed wrist), but otherwise did not appear to interfere with skin contact registration.