The extinction expression is taken from Cardelli et al., ApJ, 345, 245, (1989)

A[lambda] = ( a(lambda) + b(lambda)/Rv ) * Av

Where a(lambda) and b(lambda) are seventh order polynomials depending only on lambda. The polynomials are defined peice-wise over three wavelength regions, lambda < 169nm, 169nm < lambda < 303nm and lambda > 303nm respectively.

This application allows for three different approaches to finding visual extinction (Av):

1. Simply inputting a value for Av

2. Calculating Av from column density (NH):

Av = ( NH * Rv / 5.81 )

Where NH is in units of 10^21 cm^-2'

3. Calculating Av from distance (d):

Av = 1.6 * d

Where d is in units of Kpc

The value for distance is taken from SOURCE input