Most people arrive at vision correction with one number in mind: their prescription. It is the number on the glasses receipt, the number the optometrist reads out, the number typed into every online calculator. It is also, on its own, close to useless for deciding whether you can have SMILE, LASIK, or an ICL.
The screening exam exists to answer a different question. Not "how short-sighted are you" but "what is the physical state of this particular cornea, and which procedures can it support for the next several decades". That is a structural question, and it takes measurements a prescription does not contain.
Corneal thickness, and why it sets the ceiling
Every laser procedure removes or reshapes corneal tissue. How much tissue you have to begin with, and how much must remain untouched afterwards, sets a hard limit on how much correction is possible. Two people with an identical prescription can get different answers here: one has a cornea thick enough for several options, the other has a cornea that rules out some of them.
This is measured directly, not estimated. Thickness is mapped across the whole cornea rather than at a single central point, because the pattern matters as much as the number.
Corneal shape and the search for irregularity
Alongside thickness, the exam maps the curvature of the front and back surfaces. What the examiner is looking for is asymmetry — a cornea that is steepening somewhere it should not be. Certain patterns suggest a cornea that would respond poorly to tissue removal, and those patterns are the single most common reason a candidate is redirected from laser surgery toward an implantable lens instead.
This is one of the reasons remote screening from photographs or a faxed prescription cannot substitute for the exam. The information simply is not in those documents.
Pupil size, measured in the dark
Your pupil widens in low light, and the treated zone on the cornea has an edge. If your pupil in darkness is wider than the optical zone the treatment can comfortably cover, the odds of night-time glare and halos rise. Measuring this properly requires a darkened room and a purpose-built device, which is why it is done at the clinic rather than estimated from your description of night driving.
A large dark-adapted pupil does not automatically exclude anyone. It changes which procedure and which treatment parameters make sense.
Tear film and the dry eye question
Refractive surgery temporarily disrupts the corneal nerves that regulate tearing, so eyes that are already dry tend to feel drier during recovery. The exam looks at tear volume and tear film stability before surgery rather than after, because the sensible move is often to treat the dryness first and operate later. Flap-free procedures disturb fewer nerves than flap-based ones, which is part of why this measurement influences the recommendation rather than just the aftercare plan.
Prescription stability over time
A prescription that is still moving is a prescription that will keep moving after surgery. This is why bringing your old glasses or contact lens records matters more than most patients expect — two or three years of readings say something a single current measurement cannot. If your numbers have shifted recently, the honest answer may be to wait.
There is a related instruction that catches many international patients off guard: soft contact lenses need to be out for a period before the exam, and rigid lenses for considerably longer. Lenses reshape the cornea slightly, and measuring a reshaped cornea produces a map of the lens rather than of your eye. Arriving in Seoul having worn lenses on the flight can mean repeating the exam.
What the exam produces
At the end, the result is not a yes or a no. It is a list: which procedures are open, which are closed, and what the trade-offs are between the ones that remain. Some patients arrive expecting SMILE and leave with an ICL recommendation. Some arrive expecting to be told they are impossible cases and find several options available.
For patients flying from North America, this is why we ask for records in advance. A preliminary review of your prescription history and any existing corneal maps will not replace the exam, but it does tell us whether the trip is likely to be worthwhile and how many days to plan for. If you would like that preliminary review, send what you have and our international coordination team will tell you what is missing.