Occasionally, the doctor may ask you to identify/verify the prescription in a pair of eyeglasses (New patient exams/Rx checks). the lensometer is the instrument used to read eyeglasses prescriptions. Here is a breakdown of the parts of a lensometer and how to read the prescription in eyeglasses:

Parts

Eyepiece: the part of the instrument which the user looks into.

Lens holder: a spring loaded arm that holds the lens securely for reading and marking

Lens table: adjustable straight metal shelf on which the frame rests and which ensures the axis reading is correct.

Power drum: the large wheel on the side of the lensometer used to determine lens power. It is marked with power ranging from -20 to +20 diopters

Axis drum: the large wheel on the back of the lensometer marked from 0 to 180, used to determine the axis of astigmatism.

the following are the markings you must identify as you look through the eye piece:

Reticle: A prism scale on which mires are focused and which are marked with a series of concentric circles. The reticle is used to center the lens for marking or verification of power.

Mires: Lines of light projected through the reticle. There are two sets of mires: three thin lines and three thick lines.

How to use a lensometer

1) Determine if the specs are single vision or PAL.

-Hold the specs one foot away from you with a straight line in the background (the edge of a table or counter works well), and move them up and down. If the line bends as you move the specs up and down, the specs are a PAL. If the line stays straight, the specs are single vision.

2) Focus the eyepiece before inserting specs into the lensometer. If this is not done, the final prescription you record could be incorrect.

-Set the power drum to (0)

-Rotate the eyepiece clockwise all the way. Then, look through the eyepiece and rotate it counter-clockwise until both sets of mires become clear and focused.

3) Rest both lenses flush with the lens table with the temples pointing away from you. Move the right lens into position first. If the lens is single vision, adjust the table to read from the center of the lens; if the lens is PAL, adjust the table to read from the top of the lens. Fix the lens in place with the lens holder.

4) Rotate the power drum all the way toward you until you reach +20. Look into the eyepiece and rotate the power drum away from you until either both sets of mires come into focus at the same time or a single set of mires comes into focus.

-If both sets of mires come into focus at the same time then the lens is spherical, without astigmatism. Read the measurement on the power drum and round to the nearest quarter diopter. This is the spherical value for the lens.

Both sets of mires in focus at the same time

-If only a single set of mires comes into focus (or none at all) then the lens has astigmatism correction (see step 5)

5) To measure and astigmatism lens:

clear but fractured mires, rotate the axis dial until they are unbroken.

-Rotate the power drum and axis dial until one set of mires comes into focus with unbroken lines (if the lines are clear but fractured then continue to rotate the axis dial until they are unbroken). Note the power on the power drum.

-Rotate only the power drum until the other set of mires comes into focus (if you started with thin lines then find the thick). Note the power on the power drum.

-Record whichever of these two numbers is more “plus” as the sphere of the lens (if the first set of mires came into focus at +2.00 and the second at -1.00 then the lens sphere is +2.00).

-Subtract the sphere from the second number and record the difference as the cylinder of the lens. (using the previous example, -1.00 - (+2.00) = -3.00)

-Finally, go back to the sphere power and observe the mires. If the thin set of lines are in focus and unbroken then record the number on the axis dial as the axis. If the thick set of lines are in focus then rotate the axis dial 90 degrees until they turn into thin lines, then record this number as the axis power of the lines.

6) Repeat the same steps for the left lens!

7) Determining ADD power

-If the lenses are PAL, the ADD power can usually be seen as a watermark on the lens that can be observed by holding it up to a light. Record this number.

-If the lenses are a bifocal or trifocal then after measuring the sphere/astig power in the lens hold the lenses on the lensometer so that you are reading through the bifocal portion of the lens and again bring the thin lines into focus. Note the power on the power drum and subtract the already recorded sphere power from it (if the sphere for the lens was measured at +2.00 and the thin lines come into focus through the bifocal at +4.00, then (+4.00) - (+2.00) = +2.00. Record the difference as the ADD power.

Here are a couple videos that discuss how to use a lensometer using a slightly different method.