Prism on Digital Designs

How blocked and embedded prism is handled on a job depends upon the particular digital calculator and on the equipment being used. 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 generated by the generator, or it can be some combination of the three. Note that, unlike a non-digital job, not all generators support grinding prism on a digital job. .

The “Device max prism” field deserves explanation as it is used for the digital (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 digital engine, indicating that the lab will not block 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 Rx-Universe in the LMS file in the KPRVM parameter. Note that the digital vendor must support the use of PBOK - for some vendors, the setting for prism is controlled (or set) by the calculator, and sending PBOK has absolutely no effect on what the calculator calculates.

Prism on the Generator

Some generators permit GPRVM (generated prism) to be sent to used for digital jobs, and others do not. In general, prism will be sent to a generator (GPRVM) for a digital job only if the blocker could not block all prism requested by KPRVM, or if the calculator explicitly sends back GPRVM in the LMS file, thus explicitly asking for a certain amount of prism to be generated.

Beginning in version 6.45.00, a flag can be set on the device setup for the generator, indicating whether prism can be sent to the generator for digital jobs.

An example will help illustrate how this works.

For a sample job, the digital calculator calculates blocked prism of 6.00 and returns that value in KPRVM in the LMS file:
KPRVM=6.00

The blocker is configured to block a maximum of 4.00 prism, so KPRVM=4.00 will be sent to the blocker. This leaves 2.00 of prism that must be handled by the generator.

If the "Send GPRVM for digital jobs" is set, then the generator will receive GPRVM=2.00, and the final product will thus have 4.00 of prism blocked, and 2.00 of prism generated.

However, if the "Send GPRVM for digital jobs" is not checked, then the job will NOT be sent to the generator, because generating the lens without the required prism would ruin the lens. In this case, the device screen will show "Prsm", and as long as the device is set up to receive a status message, a message will be sent to the generator indicating that "Not all required prism was blocked". The lab may then choose to generate the lens on a generator which DOES support grinding the extra prism required.

ART Blocking

When using ART blocking, it is common to reserve some prism for the generator, and only block any remaining prism. This reduces the amount of adhesive used in the blocking process, which affects both cost of consumables and curing time.

However, when doing a digital job, where the digital vendor incorporates prism into the design on the lens, it could be preferred to not reserve any prism for the generator, and to block all prism requested by the digital vendor in the KPRVM tag in the LMS file. This can be accomplished by setting the "Do not generate prism if ART blocking" flag on the digital vendor. This allows some digital vendors to be set to not reserve prism to the generator, and others to behave like toric jobs, since not all digital vendors can or will incorporate prism into the backside of the lens.

Epson-Seiko Calculator

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 Rx-Universe 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

Indo always blocks prism and sends it back in the KPRV* fields, so the setting of the prism fields in Rx-Universe has no effect for Indo.