Your pupil measurements in the light and the dark is one factor that affects your suitability for a LASIK procedure. It is measured with a special device called the infrared pupillometer. This test is important because if you have a generally larger pupil size, you are at a higher risk of night vision problems such as glares and haloes. Your surgeon may make some adjustment to the parameters of your surgery in order to minimise this risk.
A cycloplegic or dilated refraction is also performed during the Lasik assessment, so that your eye doctor can get the most accurate measure of your refractive error, with minimal interference from pupillary accommodation. During the test, you will be given eye drops to fully dilate your pupils. The medicine could take up to 6 hours to wear off and during this time, your eyes will be more sensitive to light. Do bring along a pair of shades to help with that. As part of the pupillary dilation, your surgeon will also be able to fully assess the state and health of your retina. If required, some patients with retinal pathology may need to have this brought to their attention, and even require treatment prior to Lasik surgery.
In addition to the pupil dilation test, there are also various other tests necessary to determine your suitability for a LASIK and other eye procedures. Do consult your eye doctor for more information!