​What is Numerical Aperture (NA) and why would a neuroscientist need to know?

Someone once told me that neuroscientists are exceptional. Neuroscientists are not afraid of integrating many areas of knowledge and technology into their research and methods.

Well, I can certainly say that neuroscientists have a lot of puzzle pieces to put together to get their research going. And with Optogenetics becoming more widespread I find myself explaining a lot about fiber optics. One point I find myself explaining often is Numerical Aperture (NA).

The conversation often goes something like this.

Caller: “Will your LED work with the fibers I have implanted in our rats?”

Me: “Well, Prizmatix LEDs are designed to be used with high NA fibers. What is the NA of the fibers in your rats?

Caller: “Not sure”

Me: “Do you know what NA is?”

Caller: “Not exactly”

So here is my description of the NA of a fiber.

Picture a hose with a funnel inserted in one end. The fiber optic is the hose and the NA describes the angle of an imaginary funnel leading into the fiber optic. A high NA fiber has a wide funnel and a low NA fiber has a narrow one.

And on the output side, the light diverges out of the fiber at the same angle of the funnel going in.

The explanation: Light will only propagate down the length of a fiberoptic if it hits the wall at not-too-high an angle. A low NA fiber needs it to hit closer to perpendicular to the fiber tip and a high NA fiber can take light at a wider angle.

Does it matter?

When using a laser it does not really matter since the laser light goes straight into the fiber. But since LEDs emit light in all directions it makes a huge difference. A NA=0.5 fiber will transmit five times more light then a standard NA=0.22 fiber.

The intensity of the light at the tip of the fiber in mW/mm^2 varies only with the NA of the fiber, not the core diameter. So a 200um, and 1000um fiber will have the same intensity at the end of the fiber when coupled to an LED, even though the 1000um fiber will have 25x the amount of total power. But a 200um fiber with NA=0.63 will have 8x the light of a 200um fiber with NA=0.22. That is a big difference And in Optogenetics the intensity in mW/mm^2 is key to successful activation.

When you have a chain of fibers, the final power will be limited by the smallest NA in the chain. So it is important that all the fibers in your system have the same high NA to get the best results.

Read more about optimizing fiber optics for Optogenetics here. 

Thanks for reading, and please comment below.

Nathaniel