Wednesday, March 01, 2006

Ch 1 Minus - The Refracting Instrument

Go Chapter 2: Introduction to Refracting - Minus

It’s a little difficult to describe the controls used for refracting because every instrument is set up differently. If possible, refer to a manual for the specific instrument that you’ll be using.
That being said, most refracting instruments have the same general concepts. We’ll discuss the location of the controls on the most popular refractors.

In our example here we have a large wheel on each side of the instrument that is used to change the sphere power. Moving the sphere wheel up makes the power become more minus or less plus. ..note how the numbers in the sphere power window are red, which traditionally denotes minus power.
On the other hand, moving the sphere wheel down makes the power more plus or less minus. …Traditionally, black numbers represent plus power.

Oftentimes the refracting instrument will also have a way to change the sphere power by larger increments. In our example here, turning this outer dial will change the sphere power in 3 diopter steps.

To change the power and the axis of astigmatism we use the two concentric knobs at the bottom of the instrument. The outer knob turns the axis. Most of the new refractors will change all the parts of the instrument that relate to the axis.
The inner knob is for adding and subtracting cylinder power. Since refractors can be minus-cyl or plus-cyl , make sure that you know what type you’re using. Usually plus cyl instruments will have black numbers in the cylinder power window while minus-cyl instruments will have red numbers.

When refining cylinder axis and power we’re going to use the controls located on this arm that swings in front of the patient’s eye. This assembly includes a special lens called the Jackson Crossed Cylinder, or JCC. This lens has a low amount of minus power at the axis represented by the red dots ..and an equal amount of plus power at the axis represented by the white dots. What makes this lens so useful is that, when you flip it,….the axes switch locations.
When the lens is back in the instrument we can flip it and show our patient what subtle differences in axis look like. We can also turn the whole unit to align with the axis of astigmatism, which allows our patient to see more and less astigmatism power, letting them decide option they prefer. We’ll discuss the details of this later in this tutorial.

There are several other controls of importance. In a monocular refraction we’re going to want to close off one eye. In our instrument, change the dial in the upper left from O (for open) to OC (for occlude) to do this.

And lastly, but very importantly, we have the controls that align the instrument. There’s usually a dial to make sure the refractor is perfectly horizontal, which can be verified using the level.
There’s a dial for adjusting the pupillary distance to ensure that the instrument’s oculars are lined up with your patient’s eyes.
Finally, there’s also a knob for adjusting the vertex distance. This is used to bring the instrument closer too or further from the patient’s forehead so you can make the lenses sit about the same distance from the patient’s eyes as their glasses lenses will.

And those are the basic controls you’ll be using. Of course, there’s a number of other controls that you may find on your refractor. These other devices are a bit beyond the scope of this tutorial, but feel free to read about them in another resource if you’re curious.


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