An optometer is a device which is used to assess a patient's vision, looking both at power and range. This device is utilized by an optometrist or ophthalmologist in a clinical setting. It may be used for a variety of purposes, and it generates numbers which can be written in the patient's chart to provide a record of the status of the patient's vision during the appointment. One useful thing about generating standard measurements is that the chart can be read by any eye care professional. A phrase like “patient's vision is poor” does not provide very much meaningful information, while measurements with an optometer provide an instant frame of reference.
One of the most common reasons to use an optometer is to fit a patient for glasses. In a vision exam, the device is used to precisely measure the refraction of the eye for the purpose of determining the appropriate lens prescription. The eye care professional is very careful during the exam to make sure that it is accurate because a lens of the wrong power can hurt the patient's eyes in addition to being not very effective for vision correction.
With a subjective optometer, the patient is shown a test object and is asked to say whether it is sharp or blurry. As the optometer is adjusted, the patient continues to provide feedback. When the object sharpens and appears as crisp as possible, the optometrist can take note of the number readings on the device and can use these to write a prescription for vision correct. However, subjective devices can be tricky to read because the eyes tend to adjust and adapt and thus the final reading may not be perfectly accurate.
An objective optometer, also known as a refractometer, measures the light reflected from the retina. This measurement may be more accurate and useful because it does not rely on self-reporting by the patient. Objective optometers are less prone to errors such as confusion by the patient, poor communication skills on the part of the patient, and so forth.
The terms “optometer” and “refractometer” are also used more generally to describe devices which measure light and refraction. These devices can be used in a variety of settings ranging from labs where reflected light is used to measure the specific gravity of samples to the field, where engineers may use an optometer to confirm that the level of light being emitted by a lamp conforms with safety guidelines.