What is the measurement that documents a patient's current spectacle prescription called?

Lensometry is the correct term for the measurement that documents a patient's current spectacle prescription. This process involves using a device called a lensometer to assess the prescription of eyeglasses by determining the power of each lens in the spectacles. It provides precise measurements of the curvature and focal length of the lenses, allowing for accurate documentation of the prescription.

Refraction, while related, refers to a different process where an eye care professional tests a patient’s vision to determine the appropriate prescription for glasses or contact lenses based on how light is refracted through the eye. Visual acuity measures the clarity or sharpness of vision but does not document the actual prescription itself. Optometry is the field of healthcare related to eye examinations and vision care but does not directly refer to the measurement process for spectacles.

Thus, lensometry is specifically focused on documenting the existing spectacle prescription, making it the most appropriate answer.