SCHOTT XLD glasses with excellent processability and tightest tolerances for color correction in optical systems

Extremely low dispersion glasses (e.g. SCHOTT XLD glasses) play an essential role in the color correction of optical systems. Together with short flint glasses (KZFS Types) they can be used for apochromatic color correction in the visible spectrum or even for broadband color correction in combination with lanthanium crown glasses (LAK Types). Most of SCHOTT’S XLD glasses can be provided in the tightest tolerance step 0.5 (+- 0,0001 with respect to the refractive index and +-0,1% with respect to the Abbe number to the nominal catalog value). Since 2015 step 0.5 can also be guaranteed for pressings. Unfortunately the chemical composition of those glasses leads to difficulties in the processing. Therefore the glass engineer’s task is to improve the processability and the chemical resistance of these glasses while keeping their special optical position. N-FK58 XLD is an example of a new generation of XLD glasses from SCHOTT with improved processability. This presentation will discuss polishing strategies and polishing results for this glass type.