New to me, via 9to5Mac this morning, a support document by Apple detailing how exactly the heart rate sensor in the Apple Watch measures a wearer’s heartbeat:

The heart rate sensor in Apple Watch uses what is known as photoplethysmography. This technology, while difficult to pronounce, is based on a very simple fact: 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 document also explains other conditions and factors that may influence the sensor:

Many factors can affect the performance of the Apple Watch heart rate sensor. Skin perfusion is one. A fancy way of describing how much blood flows through your skin, skin perfusion varies significantly from person to person and can also be impacted by the environment. If you’re exercising in the cold, for example, the skin perfusion in your wrist may be too low for the heart rate sensor to get a reading. Motion is another factor. Rhythmic movements, such as running or cycling, give better results compared to irregular movements, like tennis or boxing.

As with every product we make, we want as many people as possible to enjoy using Apple Watch. That’s why it’s designed with assistive technologies and features that make it easy for people with disabilities to use. Accessing them is also simple, either through Settings on the device itself or through the Apple Watch app on your iPhone.

Apple launched a new webpage for Accessibility in Watch OS last night, highlighting different accessibility features that will be available in the first version of Apple Watch. As Steven Aquino wrote last month, the Taptic Engine could play an important role for the accessibility community.

Bani McSpedden (watch editor of the Australian Financial Review and watch-next.com) has a fascinating take on Apple Watch from the perspective of a watch expert and wearer (don’t miss the video).

This point about bands stood out to me:

Even little details like strap changes have been addressed in an ingenious way. The straps on Apple’s watch – in four styles from silicon to fine leather and metal versions – slide into the case rather than loop around a spring bar, making switching from one to another as simple as pressing a recessed button.

Why traditional watchmakers haven’t come up with something like this after all these years – well, a century – is a mystery.

David Chartier made a similar argument earlier today:

When I think about the innovative ease of swapping bands and Apple’s surprisingly low prices, I won’t be surprised if the bands become a sleeper hit of the Apple Watch. These deeply personal, and now incredibly useful, devices will finally join the rest of your wardrobe in being nearly effortless to adjust and re-personalize on a daily basis. That’s going to be powerful, convenient, and appealing.

I can’t wait to see which companies will get into the business of third-party Watch bands. Fortunately, it sounds like it’ll be easy to switch between multiple bands, which should help swapping them on a daily basis. John Gruber writes:

At first, I found swapping watch bands to be a bit fiddly. I can see why Apple wasn’t allowing anyone to do so in the hands-on areas at the press events. But once you do it a few times, you get the hang of it. They really are rather easy to remove, yet they feel very secure once clicked into place. My advice: pay attention to the angle of the slot as you slide them into place.

See also: CNET’s video on swapping bands.

Steven Aquino got the opportunity to meet with Apple for a hands-on briefing of the Watch and its accessibility features:

There are two accessibility features of the Apple Watch that stand out to me: Extra Large Watch Face and Zoom. Regarding the Large Watch Face, it does what it says: shows the time in a ginormous font. It’s very well done, although for my needs, I’m confident that I can get by just as well with Large Dynamic Type. Overall, however, Large Watch Face will be great for those who need the clock to be extra big in order to read it. Likewise, the Zoom feature was helpful for me in identifying icons on the Home screen, as they’re a tad small at normal size. I found that Zoom works best for me using the digital crown, as the two-finger pan was more difficult to do using my partially-paralyzed right hand (caused by my cerebral palsy).

As he suspected, Force Touch could play an important role for people with disabilities.

Wired’s David Pierce, writing again about the design of Apple Watch:

Yet what Dye seems most fascinated by is one of the Apple Watch’s faces, called Motion, which you can set to show a flower blooming. Each time you raise your wrist, you’ll see a different color, a different flower. This is not CGI. It’s photography.

“We shot all this stuff,” Dye says, “the butterflies and the jellyfish and the flowers for the motion face, it’s all in-camera. And so the flowers were shot blooming over time. I think the longest one took us 285 hours, and over 24,000 shots.”

I never thought I’d read a story about jellyfish and say “Wow, that’s cool”.

Joanna Stern reviews the Apple Watch, with a good mix of thoughts on fashion, notifications, and fitness.

There are so many things the watch can do, so many menus and features you must spend time figuring out, that for better or worse, you end up shaping your own experience. Some may find usefulness in hailing Ubers with a tap on the wrist, or transmitting a heartbeat to a beloved. My colleague Geoffrey Fowler explored the Apple Watch as a gateway to the iPhone for many quick activities. I sought a simpler experience, turning it into a stylish watch to keep me on schedule and a workout companion to keep me moving.

Also: don’t miss her Apple Watch video. Easily the best video review you’ll watch today.