The Business of Contact Lenses

Avoiding Common Slit Lamp Traps

The Business of Contact Lenses

Avoiding Common Slit Lamp Traps


As an adjunct professor, I have externs in my practice all the time. We are constantly working to change them from those who know about optometry into optometrists—it isn’t always easy. For example, when using a slit lamp to evaluate scleral lens clearance, it is easy to fall into three traps: parallax, fluorescence quenching, and “the horizon effect.”

Parallax Effect

The parallax effect can occur by not looking at a true cross section of the cornea and the lens. When setting up the light column, the plane of light projection must be normal to the apex of the cornea. Further, the ocular must be normal to the light column. So, how do we set up this alignment? First, have the patient look 45° temporally. Second, rotate your light column 45° temporally as well so that the light is shining directly at the center of the cornea. Third, rotate the oculars so they are 90° from your light column.

The light source needs to be set up in a direct illumination optic section on high illumination. Increase your magnification so that visualizing the small distances can be more readily affected. When you are not in this configuration, you might over- or underestimate the clearance; just like looking through the air into water, the tears and the lens material have different refractive indexes. Therefore, there is a slight difference between the light refraction through the lens and the tear layer, and the relationship between the relative thicknesses varies just a bit; this subtle variation can throw off your estimate of the clearance.

Fluorescence Quenching

In our case, the quenching effect has to do with using sodium fluorescein. For all fluorescence, there are ways of reducing the fluorescence of a particular substrate, one of which is a static quenching. Depending on the concentration of the fluorescein, there is a thickness of the fluorescing substrate—in this case, the fluorescein-impregnated tear film—that causes a marked reduction in the fluorescence that might mislead you into thinking that clearance has disappeared.

The way to avoid this phenomenon is to use a white light source. The green fluorescein-impregnated tear film is still visible, but because you are not exciting fluorescence, you avoid the quenching effect.

The Horizon Effect

The final slit lamp trap is the horizon effect. This is the most obvious and the least talked about effect. If you set up your slit lamp correctly, but are not disciplined in your survey of the clearance across the lens optic, you will witness this effect if you do not move the light column and microscope in unison.

When observing a boat as it sails over the horizon, the boat appears to sink into the ocean, with the hull disappearing from the bottom up before the sails disappear. That is because the boat has sailed beyond the observing apex relative to the observing height.

The same thing can happen at the slit lamp. Remember that a slit lamp traverses along the axle and cogs in a non-arc fashion. If you don’t keep realigning your system, the observing point will be in front or behind the corneal observation plane.

And, just like in the boat scenario in which it seems to “sink,” the clearance will disappear from posterior-to-anterior, causing you to underestimate your clearance. If you don’t believe me, you can easily create this phenomenon the next time you look at a scleral lens through the slit lamp. CLS

Dr. Newman has been in private practice in Dallas since 1986 specializing in vision rehabilitation through contact lenses as well as corneal disease management, optometric medicine, and refractive surgery. He is a Diplomate in the AAO and a consultant or advisor to Alcon, Allergan, AMO, B+L, EyePrintPro, GPLI, Johnson & Johnson Vision Care, SynergEyes, TruForm Optics, and Zeiss Optics. Contact him at