**Near infrared **

The shortest wavelengths of the infrared region falling just below the visible spectrum.

**Numerical aperture**

The numerical aperture (NA) of an optical fibre defines the characteristic of the fibre in terms of its acceptance of impinging light. It can be calculated and expressed as an index (a number). In optical fibre, the sine of the maximum acceptance half-angle, max, times the refractive index of the core (assuming an air-to-core interface). The larger the NA, the greater the amount of light that is accepted into the fibre for propagation to the distal end.

NA = n1 * sin(max) , where n1=1 for air

The NA for a glass-core to glass-clad interface is often derived from calculations using the equation:

NA = (n2² - n3²), where n2 is the core and n3 the cladding index.