The red end of the light spectrum has a wavelength that starts at about 620nm (nano metres) and goes to about 750nm. Higher than that and it turns in to non visible light called near infra-red (NIR) and infra-red (IR).
There is a lot of research happening with this end of the light spectrum due to it’s ability to potentially heal both at skin level and beneath the skin due to near infrared and infrared light having the unique ability to pass through skin and even bone with no adverse side effects.
It’s ability to do this has led to some clever advances in medicine like the simple pulse oximeter which measures the level of O2 in the blood. A pulse oximeter does this by sending both Red and NIR light through the skin (usually a finger) and measuring what gets reflected back and what gets absorbed by the blood. The level of O2 in the blood changes the depth of red colour and therefore by reflection and measurement, this gives a live and accurate reading of Oxygen content.
This recent study showed that NIR light can also be used to detect and pinpoint activity of neurons in the brain again with great accuracy.
It does this by measuring (again by reflection of NIR light) the amount in any given moment of Calcium ions in neurons. When a neuron is active it temporarily has an increase in these ions. And these ions absorb NIR light.
And due to the fact that tissue is basically transparent to NIR light.., it can be used to measure and monitor activity deeper into the brain than other methods with no intrusion expensive scanning equipment.
This potentially makes this new way of measuring much cheaper, more accessible and more accurate than what is currently in use.
Very clever.