Technology Article for optometric and ophthalmics
A Quick Guidance of Slit Lamps
|S260S: 3 Steps||10X, 16X||S350S: 3 Steps||10X, 16X||6X, 10X, 16X||6X, 10X, 16X|
|S280S: 3 Steps||S360S: 3 Steps||25X, 40X||25X, 40X|
|S260: 5 Steps||S350: 5 Steps|
|S280: 5 Steps||S360: 5 Steps|
|Halogen Bulb||Halogen Bulb||Halogen Bulb||Halogen Bulb||Halogen Bulb||Halogen Bulb|
|LED Bulb||LED Bulb||Beam Spliter||LED Bulb|
PRISM RX FLAT TOPS
When a flat top bifocal lens Rx is ordered with prescribed prism... (flat top bifocal or a progressive for that matter), the amount of prescribed prism is placed in front of the pupil, in line with the visual axis. The optical center (OC) is moved to create the correct amount of prism for the way in which the patient should wear their lenses. The fitting height of the segment is ordered as measured without changes.
THE 1,2, 3 OF PRISM
Here's some basics about prism in lenses
1. Eyewear with prism, means that the optical centers of the lenses do not occur at the PD. For example, if the left eye has 2 prism diopters Base Out prescribed, the point on the lens that is 2 Base Out is located at the PD.
2. As a result, in eyewear where there is prescribed prism, we find the prescribed prism values at the PD.
3. Using a manual lensmeter, the reticle is your key to locating prism.
4. Using the target, the sphere and cylinder lines, place the center of the target (where the center of the sphere and cylinder lines cross) at the center of the reticle. That is the point of no prism, the optical center. See the illustration to the right.
5. Next, move the center of the target to the point of prescribed prism and dot the lens. Voila, that's the point from which the PD is measured.
So, if there is prescribed prism in both eyes, the place dotted on each lens at the point of prescribed prism should be equal to the patient's PD.
MEASURING VERTEX DISTANCE (PUPILLOMETER)
USING A PUPILLOMETER
This method sometimes works if the pupillometer can get close enough to the eye so the image is not too blurred. To help, pull back the forehead bar.
Set the distance dial to infinity. Move the thumb slides to the temporal most direction, look at the value in mm (will be about 38.5mm).
Holding the Pupillometer at the patient's right side, view through the left window and place the hairline at the front of the cornea. Next, slide the hairline to the back of the lens. Read the new mm value, the difference is the vertex distance.
Repeat the procedure for the left eye using the right window.
MEASURING VERTEX DISTANCE (MM RULER)
When wearing glasses, the distance from the front of the cornea to the back surface of a lens is called the Vertex Distance. Why is this measurement important?
Prescription lenses in eyeglass frames have different effective powers at different distances, tilts and wrap angles from the eye. That means that the same prescription on two different people, one wearing lenses farther than the other would actually have different powers. Lenses become more plus (less minus) when moved away from the eye (increasing the vertex) and less plus (more minus) when closer. That explains why telling a progressive or bifocal wearer to slide their glasses to the tip of their nose provides a stronger add to read the phone book or a medicine bottle.
TO MEASURE VERTEX DISTANCE - Using a mm ruler, measure from the back of the lens to the front of the cornea
If you can’t see the back of the lens - measure from the front of the lens to the front of the cornea (then subtract lens thickness).