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