Which instrument is commonly used to measure the tear film break-up time?

The slit lamp biomicroscope is the commonly used instrument for measuring tear film break-up time. This measurement is crucial in assessing the quality and stability of the tear film, which plays a vital role in maintaining ocular health and comfort.

Using the slit lamp, an examiner can apply a fluorescent dye to the surface of the eye, then observe the tear film under magnification. The time taken for the first break in the tear film to occur after a blink is assessed, providing insight into the tear film's stability. This test is particularly important for diagnosing dry eye conditions and understanding various ocular surface diseases.

In contrast, the other instruments mentioned serve different purposes. An ophthalmoscope is designed for examining the interior structures of the eye, including the retina. A tonometer is used to measure intraocular pressure, which is essential for glaucoma assessment. The retinoscope is primarily employed for measuring refractive error by evaluating the light reflected from the retina. Each of these instruments has its unique application, while the slit lamp biomicroscope is specifically suited for tear film evaluation.