It turns out that you can measure a person's heart rate visually. Or at least sensitive electronic sensors can. Each time a heart beats, blood pressure increases and then decreases. This causes an increase and decrease in the amount of blood in the capillaries near the skin's surface. This results in a corresponding change in the amount of light that passes through the skin.
All it takes to measure these changes are a light source, and an optical sensor.
Smartphones have all the necessary requirements. The flash is a bright light. The camera is designed to capture millions of pixels of light. All that's missing is some software to put the two to a different use.
There is an app for that. Several actually. Many of them free.
The one I ran across is Instant Heart Rate by Azumo, Inc.
You simply place a finger over the camera. The flash shines onto your finger. Some amount of light passes through your skin, like when you hold a bright flashlight up to your fingers. The camera measures the changes in light and interprets it as a pulse.
The information is graphed in real-time on the screen, and your pulse rate is calculated.
Small movements can distort the reading, so it's important to hold still. Pressing hard prevents blood from flowing into your finger, so it's important just to hold your finger lightly against the camera. Other than that, it gives surprisingly accurate results. Which is very cool. And it may even be useful to someone.