I think I know the paper you are talking about.
Bottom line is you need to add a thing called a UV absorber which absorbs at 405nm (the wavelength of the light source in the ember). The amount of which would need to be determined experimentally. I speak under correction but there is probably already a UV absorber in the resin (this is typically the case). If you look up the ingredients and their role in the resin, adding more of the one which is a UV absorber will decrease your penetration depth.
If you are using the clear resin, simply using a coloured resin will decrease the ha, because the coloured pigment is absorbing. This could be something to try.
If you're using the coloured resin there may be little to no benefit of including additional absorber.
Let me know if this works out.