Accurate pupil measurement in Laser Vision Correction
Editor's note: Many of the terms and concepts referred to in this article may not be familiar to the casual reader. Please refer to the encyclopedia and previous articles by The Lone Dog for further information.

Numerous books on laser vision correction (LVC) have been written for public consumption, many of which are quite good. Almost all of them emphasize the importance of the size of your pupils for 1) planning surgery, and 2) gauging your risk of side effects from the surgery. In LVC spin, a “side effect” is generally something you see after surgery that you didn’t see before, and you would rather you didn’t see at all. The most frequent side effects are glare or halos around headlights and streetlights when driving at night. One identifiable cause of glare and halos is an effective optical zone that is smaller than your pupils.

What’s an optical zone (OZ)? What is the pupil, anyway? For necessary background, look up these terms, and also “maximum dark pupil diameter”, in The Lone Dog’s Pupil Primer for Casualty Prevention. This article is going to move on from the Primer and concentrate on empowering you, Lou “Big Pupil” Schmo. Knowledge is power. And by knowing what your surgeon should do in order to measure your MDPD, you can make sure that he does do it. Actually, the LD’s approach would be to say nothing, and if you aren’t satisfied with the pupil examination, walk.

Two separate requirements exist:

#1: The surgeon must elicit your MDPD.

#2: He must measure it accurately.

The LD suspects that screw-ups occur about equally between the two requirements, but the refractive surgery industry (RSI) is paying no attention whatsoever to #1. They have gizmos and whizmos to measure your pupils via light amplification, infrared, infrared-video, yada-yada, to two decimal places if you want. Although these devices often have not been proven to measure pupils accurately to even one decimal place, (a) ophthalmologists love toys, (b) the RSI is all about a really big toy (the excimer laser), and (c) faith is implicit. There is the small issue of the human operator, but we’ll deal with that later.


#1: Whaddaya do in the dark?

Or: How dark is dark enough in the Chamber of Pupil Measurement?

Intuitively, the goal is to figure out just how big your pupils might ordinarily get under natural circumstances. Probably almost everyone reading this article has had their pupils dilated with eye drops. That isn’t natural. For first-world types, the #1 activity you can perform alone in the dark for a fairly long time is driving. At night, the average urban roadway is filled with what are called “point” light sources: headlights, taillights, streetlights. These are fairly intense isolated lights that are a principle source of glare, arcs, starbursts, and halos (GASH) post-LVC. It doesn’t take a rocket scientist or a vision fanatic to detect and be bothered by GASH. Bad GASH can stop you from driving at night, period. Since a not-insignificant number of post-LVC patients will get GASH, the RSI hastens to state that 1) it is a known side effect, 2) it is due to something they don’t have under control just yet called spherical aberration, 3) therefore, it is NOT due to surgeon or excimer laser screw-up, so 4) quit whining.

However, get Joe Surgeon too drunk to prevaricate, and he will tell you that to minimize the chance of inflicting (currently unfixable) GASH on some poor slob, ideally you would make the optical zone larger than the night-driving-pupil-diameter (generally equal to the maximum dark pupil diameter, unless you drive with your cab light on).

Side note: It just so happens that the fine folks at VISX have figured out that many people with MDPDs greater than 6.0 mm will still be GASH-free road warriors if a 6.5 mm optical zone blended out to 8.0 mm is used on a VISX laser. The LD tends to believe them on this score, because some elementary optical modeling shows that a mismatch between OZ and MDPD will be less symptomatic when it starts further away from the center of the pupil. However, Joe (when drunk) will still admit that the risk of GASH increases as the gap between the OZ and the MDPD increases, particularly for higher prescriptions. Also, deciding whether GASH is a reasonable trade-off for what you hope to get from LVC is a highly individual decision. So you, as an LVC candidate and potential LVC casualty, have a right to be informed of your very own, personal risk of GASH after surgery. In order to counsel you appropriately, Joe has to get your pupil measurements right. This means that he has to use a pupil-measuring protocol that is designed to get your pupils to be “night-driving” size or larger.

Let me restate: Joe’s job is not to “measure your pupils in the dark”. Joe’s responsibility is to measure your pupils when they are the largest that they are likely to get short of coma. Pupils get bigger in the dark. The longer you are in the dark, the bigger they get, up to a point. This is known as dark adaptation. Unfortunately for lots of LASIK casualties, the RSI has effectively ignored many decades of meticulous experimentation on pupil physiology and dark adaptation [see The Pupil: Anatomy, Physiology, and Clinical Applications by Irene E. Loewenfeld — text 1590 pages, bibliography and index 633 pages]. What they particularly don’t care to be reminded is that the median MDPD for typical LVC-age people is about 7 mm, which is kind of a stretch for their lasers. They also don’t pay much attention to the fact that there’s nothing like sleep to shrink your pupils, and you don’t actually have to be asleep, your body just has to be shutting down. Stick Lou or Louise Schmo in a low-light environment for 5 minutes or more, unattended, no interaction, nothing left but to recite the LVC mantra (noglarenoglarenoglarenoglare) . . . and a part of the brain called the reticular activating system (RAS) starts to power down. The LD can’t resist mentioning the RAS because it makes him sound medically educated. Anyway, your brain is in drift state, your RAS is turning off the juice, and your pupils are shrinking. Your eyes are still dark-adapted, the RAS is a separate issue.

BAM, back into the Chamber walks Joe Surgeon, who flashes light into your dark-adapted eyes when he opens the door, startling the living daylights out of you because you have been almost asleep, although not yet drooling. Without further ado, he measures your pupils with a device. And the answer is . . . 6.0 mm. You are safe! We’ll make that OZ 6.5 mm with a nice big blend zone, and it’s no worries (or not very many anyway) on the GASH score. Sign here.

Wait a minute. What Tips for Dark Adaptation can be gleaned from all that dusty research? In general, 1) the ambient Chamber illumination should probably be less than 5 lux to simulate night driving (this equals ‘way too dim to read without squinting’), 2) you should stay in there for a minimum of 3-5 minutes, depending on how much light you were exposed to before you were pushed into the Chamber, 3) you should not stay in there all by yourself for "too long", because your RAS starts to power down, 4) nobody should enter the Chamber during adaptation and testing, because a flash of light in your eyes, no matter how brief, may cause constriction of your pupils to an unknown degree for an unknown duration, 5) you can’t try to read (the consent form, say) to keep yourself awake, because looking at up close stuff constricts your pupils.

The LD has had many a pupil measurement in the process of researching this article and has noted the following faux pas:

  1. Failure to dark-adapt you at all

  2. Dark-adapting at a room illumination that is too high

  3. Dark-adapting in a dim room while you are facing the projected visual acuity chart from a short distance away

  4. Dark-adapting you for a few minutes immediately after you have had lots of very bright lights shined in your eyes

  5. Dark-adapting for an insufficient amount of time, period

  6. Failing to prevent your urge to focus up-close during the actual measurement

  7. Failing to prevent your urge to fall asleep

  8. (this is the big one) Tucking you away in the Chamber of Pupil Measurement, getting you nicely dark-adapted but not somnolent, and then having the technician or the doc walk back in the room from a brightly illuminated corridor

The LD can hear the protests already from refractive surgeons: “How else are we supposed to get back in there? That light isn’t very bright, and I slide through sideways . . . plus, my patients are facing away from the door. What, I should sit in there with them myself for five minutes? Answer questions?”

To which the LD queries: Dear Joe, have you demonstrated to your own satisfaction that your rocket-like re-entry has no affect whatsoever on the MDPD? Done some studies on yourself, your staff, your attorney (barrister)? There are scads of research out there that show that the pupils of dark-adapted eyes are very responsive to even brief, dim illumination — the type of light that would not cause a pupil size decrease if your eyes weren’t dark-adapted. But they are. That’s the point.


#2: The next flat tire: devices and end-users for pupil size measurement

Or: I paid $14,000 for it — of course it’s accurate.

Let’s start with one of the LD’s favorite examples of misplaced faith in technology, the Procyon video infrared digital pupillographer ( This is a gizmo that does just what the name says: takes a continuous infrared video picture of your pupils (both at once), records a still picture every tenth of a second, averages 10 readings, and measures them to 3 digits. To make it extra smart-seeming, the Procyon reports pupil size in microns, e.g. Lou’s pupils are 798 microns (or 7.98 mm for plebes). The patient places his/her face into this mask-thing that looks a lot like what the radar operators looked into in old submarine movies. You have to bend forward to do it, and it isn’t super comfortable. The interior is a homogeneous gray surface designed to be visually "bland" so that you don’t have any stimulus for up-close focus. The operator watches your pupils in action on a monitor on the opposite side of the instrument.

Procyon’s developers are marketing it hard lately as an “objective, operator insensitive” method of pupil measurement that has the advantage of “hard-copy output”. That is, the computer does the measuring, the technician is unlikely to screw it up, and it prints out a permanent record for the chart. It sounds just wonderful. Docs who buy this believe they are buying a med-mal safety net [“Pupillometers meet refractive surgery niche.” EyeWorld: February 2003]. The LD in all innocence stumbled across two serious Procyon end-user problems.

First, the unit might be located in a general testing area with average illumination, or even in a corridor down the middle of the clinic waiting area (the LD having personally encountered the latter). So your pupils may not be dark-adapted at all when you are first plopped into the chair on your side of the machine. In order to get you to the end point of dark-adaptation — where the Procyon is measuring your MDPD as opposed to just some pupil size in the dark — you might have to sit face-to-mask for 10+ minutes. Go ahead.

Second, the featurelessness of the interior view confuses your brain (OK, it confused the LD’s brain). In a desperate effort to find something to look at, erratic up-close focusing occurs, which is visible to the technician on the screen as a lot of bouncing up and down of your pupil size. The alternative second problem is that you were tired to begin with, and staring into grey nothing for 10 minutes causes RAS power-down. Since the “scotopic” (really dark) internal illumination level on the Procyon is 0.04 lux, the biological temptation to snooze may be irresistible.

Anyway, in the LD’s humble opinion, the Procyon unit is just hardware, and like any inanimate object its output quality will depend on decisions made by humans: how to dark-adapt patients, what to watch for on the screen to indicate the patient is at his MDPD, etc. So if you — Lou — are grappling with pupil measurements that disagree (Joe Doc gets 543 microns on his Procyon, Jay Doc gets 6.0 mm some other way), and the Procyon values are smaller, do not assume the Procyon reading is right because it contains a computer and costs more.

The Colvard Pupillometer is probably the most frequently used electrical appliance for dark pupil measurement ( At $1750, it is about nine times cheaper than the Procyon. I give points to old man Colvard for recognizing that something better than a knotted string was needed (the son actually built the thing). This unit looks like a miniature radar detector for eyeballs. The operator plops it against your face (one eye at a time), and sighting down the barrel, can see your pupil with "amplified" light. There is an internal millimeter scale that is positioned over the pupil. The LD has been Colvarded several times, and has encountered the following constellation of . . . ah . . . operator errors:

  1. Failure to dark-adapt in any rational fashion

  2. Instructing the LD to look at the technician’s shoulder, which causes up-close focusing and pupil constriction, and also causes the eyes to move downward so that the midline of the pupil is not easily centered on the scale (unless the tech hunches, then you look down more, then he hunches more, etc.)

  3. Instructing the LD to look at the technician’s ear, which avoids the hunching but still causes up-close focusing

  4. Taking a reading while the LD was facing directly into the illuminated projector chart, which was shining into the eye not being measured

  5. Failing to notice rapid, large changes in pupil diameter which the LD was deliberately inducing by focusing back and forth from the far end of the room to some body part on the technician (having practiced in a mirror first)

  6. Failing to scold the LD for being totally uncooperative and deliberately looking all over the place, causing the pupil being tested to whiz around inside the pupillometer and almost never line up with the scale

More than one Joe Surgeon was perfectly prepared to develop a surgical plan based on pupil readings that the LD had made every personal effort to screw up, in completely obvious ways. The goal here is not to get an answer; it is to get the right answer to the question “how large is the MDPD of Lou ‘Big Pupil’ Schmo?”

So you still want to have LVC, huh? The LD says, go ahead, it’s a great procedure for millions of people. The problem with shopping for LVC is that consumers (known as “patients” in other fields of medicine) don’t have the slightest idea how to distinguish quality from schlock. This is exacerbated by the fact that happy consumers will tell you about it but unhappy consumers (who want their vision back more than their money back) often don’t say much. So if you go by word of mouth, you may be unduly influenced by the minority of Joe’s patients who actually had appropriate treatment. But now you are an empowered consumer who knows a lot about “best practices” for pupil measurement. This is an important test, and if Joe and/or his staff and/or his co-managing optometrist buddies blow it, don’t give them any second chances.

One last word on research and Lou: I’ve said that the median MDPD for normal, alert humans in the 20-40 year old range is about 7 mm. I’m deliberately being a little vague on the decimal to avoid dog-kicking arguments with irate representatives of the RSI. The point is that about half of this article’s younger readers have MDPDs that are larger than 7 mm. From the same data we can tell that fewer than 10% of ordinary humans will have MDPDs smaller than 5.5 mm. The LD suggests that 1) you ask to know your pupil diameter to one decimal place for each eye separately, and 2) you consider the following responses based on the number you are given:

  1. 7.0 mm or greater: How many patients with pupils this big you have treated? What optical zone* are you planning to use on me? Is there some reason why you cannot use an optical zone larger than my pupils** to treat me? What is the gap between the optical zone and my pupils going to be? What does that mean about my risk of GASH? How is my risk of GASH influenced by other factors such as the prescription you are trying to correct?
  2. *Do not get snookered by a quote of an overall treatment size which is bigger than your MDPD, but which is really composed of an optical zone smaller than your MDPD plus a blend zone.

    **See several previous comments on special issues related to the VISX laser and pupils larger than 6.0 mm.

  3. 6.5 mm: Same as above. Depending on your faith in Joe, consider a re-measurement by a different individual and/or different technique prior to the surgery.

  4. 6.0 mm: Are you sure? Same questions as above. Definitely require a re-measurement.

  5. 5.5 mm: Are you really really sure or are you . . . (pick one or more)
    a. lazy
    b. sloppy
    c. uninformed about the normal population distribution of MDPD
    d. overly dependent on co-managing optometrists for revenue and unwilling to contradict them and refuse to operate on their referred patients

  6. 5.0 mm or less: So long, and thanks for the free consultation!


Personal note from the LD to all the LVC surgeons out there: Your patients are going to live with your decisions every waking moment for the rest of their lives. Can you give them an extra 5 minutes of yours?


Copyright June 11, 2003 by The Lone Dog. All rights reserved.
No portion of this article may be duplicated in any format without permission from the Author.

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