Lens Power range limited by base curve

Written by Optician Club on . Posted in Freeform Rx Lenses, Optician Tools, Progressive Lenses

Manufacturers typically produce a series of semi-finished lens blanks, each with its own base curve. This "base curve series" is a system of lens blanks that increases incrementally in surface power (e.g., +0.50 D, +2.00 D, +4.00 D, and so on). Each base curve in the series is used for producing a small range of prescriptions, as specified by the manufacturer. Consequently, the more base curves available in the series, the broader the prescription range of the product. Manufacturers make base curve selection charts available that provide the recommended prescription ranges for each base curve in the series, let's check this lens power range limited by base curve.
 
A Typical Base Curve Selection Chart
Power Range
Base Curve
+8.00 D to +4.75 D
10.00 D
+2.25 D to +4.50 D
8.00 D
+2.00 D to -2.00 D
6.00 D
-2.25 D to -4.00 D
4.00 D
-4.25 D to -7.00 D
2.50 D
-7.25 D to -12.00 D
0.50 D
 
The base curve of a lens may affect certain aspects of vision, such as distortion and magnification, and wearers may notice perceptual differences between lenses with different base curves. Consequently, some practitioners may specify "match base curves" on a new prescription. Some feel that these perceptual differences should be minimized by employing the same base curves when the wearer obtains new eyewear. This would conceivably make it easier for particularly sensitive wearers to "adapt" to their new eyewear. It's not proper to request a lab to provide over -6.00 with Base 8 front curve.
 

The “best form” or "corrected curve" shape of a lens is one that provides as close to the the same vision at center and in the periphery as possible, as the eye moves normally behind the lens. For finished lenses, manufacturers choose both front and back curves; for semi-finished lenses, a recommended series of front curves are provided usually in a base curve chart.

Lens power range limited by base curve

 

The choice of base curve influences a number of optical and mechanical factors, which are important in both traditional and free-form lens design.

This is one of most important fact to determine which type of lenses should be used for a specific prescription, check more about   Advantages and disadvantages of FreeForm.


Optical Factors Associated with Base Curves:
1. Off-center vision quality
2. Magnification
3. Geometric distortion

Flatter lenses generally compromise off-center vision quality in conventional lens designs using spherical base curves. Aspheric and optically optimized free-form lens designs are not subject to the same optical limitations, since the design can correct oblique astigmatism using local changes in curvature. However, as the base curve deviates farther and farther away from the "ideal" best form base curve, the asphericity or optical optimization required also increases. And the lens design then becomes more sensitive to factors such as lens tilt.

Flatter lenses generally produce less magnification through the center of the lens but more distortion (pin-cushion in plus powers, barrel in minus powers), which is caused by a change in magnification away from the center of the lens. Fortunately, the visual system will often "adapt" to these magnification effects within a week or so, since the brain essentially "remaps" the new visual input based upon the expected visual input and other sensory input. Aniseikonia can also become an issue in the presence of significant anisometropia.

Check more about Mechanical Factors is Associated with  Lens Thickness and Base Curves

 

 

Segment Height Measurements

Written by Optician Club on . Posted in Optician Tools, Progressive Lenses

Eyecare Segment Height Measurements


Precise segment and progressive fitting heights, first time, teach patients that you understood their vision needs as well as considered the height in their old glasses, their posture, work, leisure and driving needs. Ask questions to discover what the old glasses didn’t do well because the height was off. One height will meet most but can’t work for all needs so learn the activities for which the glasses will be used most often or where the most comfort is required. Counsel that another pair of glasses, for those other tasks would be better. For example, a pair of progressives for all day wear deliver vision at all needed distances but probably won’t work as well for reading in bed. A pair of single vision readers or near variable focus lenses would work better.A Pantoscopic tilt of about 10 degrees is preferred, especially for progressives.

To measure multifocal fitting heights, sit opposite the patient, adjust the frame so that it is straight and comfortable for the wearer, which is the same procedure when you take monocular PD. Ask the patient to put on the glasses where they are comfortable wearing them. Then, with your eyes at the same height as the patients dot the fitting height with a marking pen. For bifocals and trifocals, the starting points are top of lower lid for bifocals and top of lower pupil margin for trifocals. For progressives it is pupil center. Next, remove the glasses and draw a straight line (about an inch) across the dot. Place the pen against the table edge, dot against the pen and slide the glasses right and left. See the illustration.

Segment Height

To measure the right height, two things are needed. First, the optician's eyes must be at the same height as the patients. If not, the resulting ink dot will be too low if the optician's eyes are lower. The reverse is also true – sit higher and the dot will be too high. It's simple geometry. Look at this illustration. While it's a bit exaggerated, see where the optician's line of sight crosse the patient's lens. Too low gets a dot that is too low, too high, fitting cross is too high.

Second, once you have a dot for reading seated, ask the patient to stand verify that the dot marked is also OK for walking around.

 Ask the patient to put the glasses back on and ask them to stand. From the side view their line of sight so that for progressives they are looking through the line, for bifocals and trifocals above it. For bifocals and trifocals, hand the patient a reading card and as they look down, it is easy to see if they look below the line. Check it again while sitting. In bifocals and trifocals, it can also be confirmed by placing a piece of scotch tape across the line. The tape should be completely in the way for reading and out of the way for distance and walking. Watch a patient’s posture as they walk with you around the office. Adjust segment height as needed. Also, a patient’s posture changes as they age so take that into consideration. Never assume that the previous height will be good for the new glasses.