Low dispersion glass

Low-dispersion glass (LD glass) is a type of glass with low dispersion. Crown glass is an example of a relatively inexpensive low-dispersion glass.

Special low dispersion glass (SLD glass) and extraordinary low-dispersion glass (ELD glass) are glasses with yet lower dispersion (and yet higher price). Other glasses in this class are extra-low-dispersion glass (ED glass), and ultra-low-dispersion glass (UL glass).

Low-dispersion glasses are particularly used to reduce chromatic aberration, most often used in achromatic doublets. The positive element is made of a low-dispersion glass, the negative element from a high-dispersion glass. To counteract the effect of the negative lens, the positive lens has to be thicker. Achromatic doublets therefore have higher thickness and weight than the equivalent non-chromatic-corrected single lenses.

In comparison to telephoto lenses, shorter focal length objectives benefit less from low-dispersion elements, as their chief problem is spherical aberration instead of chromatic aberration. The spherical aberration introduced by the LD elements can be corrected with aspheric lens elements. The increased sharpness provided by SLD elements allows using lower f-numbers and therefore faster shutter speed. This is critical, e.g., in sports photography and wildlife photography. The shallow depth of field provided by a telephoto lens also allows the subject of the photography to stand out better against the background.

Low-dispersion glasses are also employed in handling ultrashort pulses of light, in e.g. mode-locked lasers, to prevent pulse broadening by group velocity dispersion in the optical elements.

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