Mail-order eyeglasses company Warby Parker has created an iPhone app that can replace a visit to the optometrist for some customers.

Has your eyeglasses prescription expired? Want to take a vision test at home? Well, welcome! Introducing Warby Parker Prescription Check. This is our brand-spanking-new mobile refraction service that, if you are eligible, allows an eye doctor to assess your vision and provide an updated eyeglasses prescription. (Is there anything your phone can’t do?!)

You’d of course expect there to be some small-print when you’re dealing with something as important as your eye-sight, and there is …

NordVPN

As Engadget notes, there’s quite a lot of it.

You’re between 18 and 40 and live in a state that has the company’s program (NY, CA, FL and VA so far). You also need to be a current Warby Parker customer, have no history of eye disease, have had a comprehensive exam within the last five years and only need a single-vision distance prescription (no bifocals, please). Oh, and your prescription can only have a current lens strength (SPH) between 0 and -6.0 and an astigmatism value (CYL) between 0 and -2.0.

If all that applies, and the iPhone test determines that your vision hasn’t changed, then you’re good to go. Otherwise, the app will refer you for a conventional examination.

Prescription Check is a free download from iTunes.

The iPhone has been used for a wide variety of health tasks, usually with additional hardware but sometimes just with an app. These include blood pressure monitoring, mental health studies, cholesterol measurement, ECG monitoring, food analysis and even cancer diagnosis.

Health gadgets are fast becoming a major gadget category.