FLD (Fluorite Low Dispersion?) glass is the highest level low
µ, low dispersion glass available with extremely high light
Nikon calls this Extra-low Dispersion (ED) and Super ED glass. Sigma calls it FLD glass.
FLD glass helps correct chromatic aberrations, or optical color
defects, caused when different light wavelengths do not converge at the
same point after passing through optical glass.
Calcium fluorite (CaF2) crystals were once used to correct this problem in telephoto lenses, but the substance cracked easily and was sensitive to temperature changes.
The optical performance of FLD equals low dispersion fluorite, at an affordable price.
It corrects for residual chromatic aberration (secondary spectrum) which cannot be corrected by ordinary optical glass and ensures high definition and high contrast images, even at max. aperture.
Also the density of FLD glass is lower than traditional optical glass. Makes large aperture lenses lighter.