Prism on Digital Designs
How blocked and embedded prism is handled on a job depends upon the particular DS calculator. Prism can either be incorporated in the design that is ground onto the back surface of the lens, or it can be blocked on a prism blocker, or it can be a combination of the two. Note that, unlike a non-digital job, prism cannot be sent to the generator to be ground as part of the generating process, as what is cut onto the back surface of the blank is the progressive design.
The “Blocker Maximum Prism” field deserves explanation as it is used for the DS interface. This parameter does not specify a limit for prism to be included in a design (this is determined by the DS processor itself), rather, it operates as a Y/N switch to specify whether the lab supports blocking prism or not. If set to 0.0, then parameter PBOK=0 is sent to the DS engine, indicating that the lab will block a maximum of 0 dioptres of prism; any non-zero value will send PBOK=1, indicating that the lab can block some (or all) prism. The amount of prism to be blocked will be sent back to OMICS in the LMS file in the KPRVM parameter. (Shamir, IOT, Opto-Tech and Essilor all support this method of blocking prism in addition to prism incorporated into the design).
In the case of Epson-Seiko, they support either blocking prism or incorporating it into the design, but not both at the same time. This is controlled by a setting in the Epson-Seiko DS calculator itself. This cannot be selected on a per-job basis, and the Epson-Seiko calculator ignores the PBOK label if sent in the .LDS file, which would normally be used by the LMS to indicate whether or not blocking prism is to be used.
In addition, Epson-Seiko does not use the standard VCA labels to identify blocked prism, so the device setup screen must be set in OMICS to agree with how the Epson-Seiko calculator is set up. In the device setup for Epson-Seiko, if blocked prism is to be used, the LDPRV? = KPRV? field must be checked.
Indo always blocks prism and sends it back in the KPRV* fields, so the setting of the prism fields in OMICS has no effect for Indo.