If you're in the U.S. and this happens you can and should file a HIPAA complaint. Vision care providers are bound by HIPAA just like other medical providers and with few exceptions they are required to give you a copy of your protected health information (PHI) upon request, which includes your PD. The provider may require you to make the request in writing and may charge you a reasonable fee for e.g. the time it takes a worker to send you the information, and may take up to 30 days to fulfill the request. HHS.gov has a great factsheet and FAQ here: http://www.hhs.gov/hipaa/for-professionals/privacy/guidance/...

Edit: It turns out this varies—maybe. Here's a funny thread on an opticians' forum on the topic: http://www.optiboard.com/forums/showthread.php/11990-Hipaa-q... They don't come up with much, but they sure don't like online retailers.

The Massachusetts Board of Registration in Optometry requires optometrists (not opticians) to enter PD on prescriptions at a patient's request, in which case it would be covered by HIPAA: http://www.mass.gov/ocabr/licensee/dpl-boards/op/regulations...

The New Jersey State Board of Optometrists, on the other hand, takes the opposite position: http://njpublicsafety.com/ca/faq/optfaqsII.htm#4b

In comments to the FTC the National Association of Optometrists and Opticians said it "agrees that patients and eyeglass consumers have the right to a copy of medical records, which will include the pupillary distance if that measurement has been taken by either the prescriber or the dispenser" (emphasis theirs) but takes no position on whether or not the measurement should be required. https://www.ftc.gov/system/files/documents/public_comments/2... (pp.12)

So... that settles nothing.