How do you calculate base curve? Simple Rule for estimating lens base curve on a MINUS POWER lens For minus power lenses take half of the spherical equivalent and add 4 diopters to that. For example a -4.00 sphere power you would use -2.00 (half of the SE) + 4D, which would give you a 2BC. ie Rx -4.00sph -> [ (-4.00 x 0.50)+ 4.00D] = 2.00BC.
How do you find the base curve of a power curve?
The more minus the power (less plus power to minus power) the lower plus the base curve will become. A high minus Rx may even have a base curve of +0.25 or even 0.00. To do that you can use Vogel’s Rule which is this: Another common method is to simply use a base curve chart.
How do you choose the right base curve chart?
You will find charts have overlapping ranges so you must be careful and be sure you are choosing the curve that is closest to that +6.00. A simple base curve chart might look like this: BUT it also falls between -1.50 and -5.75 and that BC is closer to our +6.00 so we choose +4.25 NOT +2.25.
What are base curve sizes?
This is the inside curve measurement of your lenses. Some brands offer only one base curve size, which is more common among brands of soft contact lenses. People who wear hard contact lenses need more choices when it comes to base curve sizes, because of the inflexible, rigid nature of these lenses.
How to calculate the base curve of a lens?
Simple Rule for estimating lens base curve on a MINUS POWER lens For minus power lenses take half of the spherical equivalent and add 4 diopters to that. For example a -4.00 sphere power you would use -2.00 (half of the SE) + 4D, which would give you a 2BC. ie Rx -4.00sph -> [ (-4.00 x 0.50)+ 4.00D] = 2.00BC.
How do you calculate base curve for contacts?
Simple Rule for estimating lens base curve on a PLUS POWER lensFor plus power use the spherical equivalent (SE) and add 4.00 diopters to that. For example, if you have an Rx of + 2.00 sphere, the base curve for the lens will be approximately 6.00.Rx +2.00Sph -> [+2.00 +4.00D] = 6.00BC.
Is my base curve 8.5 or 9?
Studies show that a single base curve of 8.4mm managed a “good or better” fit in approximately 90% of individuals,1 and base curves of 8.4mm and 8.6mm together encompassed 98% of individuals....LENSBASE CURVES AVAILABLE (mm)DIAMETERS AVAILABLE (mm)88.714.298.714.0108.714.2118.5, 9.014.27 more rows•Sep 1, 2017
What is the base of a curve?
In optical theory it is said that the base curve is, “The curve from which all other curves are measured.” In modern lens design we can think of the base curve as always being the front surface of the lens.
How do you find the base curve in K reading?
For example, if the K reading is 7.50mm (45.00D) and the HVID is 11.6mm, then the actual arc length of the cornea is 13.26mm. If you wanted a lens that was 0.3mm flatter and 5.0mm longer arc length, the lens order would be: 7.50mm + 0.3mm = 7.8mm base curve for the ordered lens and 13.26mm + 5.0mm = 18.26mm.
Is there a big difference between 8.4 and 8.8 base curve?
The 8.4mm base curve is still the likely best fit for the majority of eyes. In instances when the 8.4mm lens is too steep, the 8.8mm lens allows a flatter option. This is more likely needed in smaller eyes, and possibly in some very flat corneas.
How do I choose a base curve for soft contact lenses?
Base Curve Many lens fabricators suggest starting with a base curve radius (BCR) that is 4.00D to 5.00D flatter than flat K. Another method is to trial-and-error fit by selecting the middle BCR available. Most soft lens designs feature three BCRs: Steep, median and flat.
What does 6 base curve mean?
A 6-base curve is a medium base with a minimal curve. If you have a high prescription, the flatter base curve will accommodate the stronger correction. However, the lower the base curve, the more light will be let in through the sides of your sunglasses.
How do you calculate base curve for RGP contact lenses?
Base curve = 0.95 * 34.82 D = 33.07 D and then round up or down to the nearest whole diopter to arrive at the following final base curve to use for a contact lens over-refraction: Base curve = 33.00 D (actual measured base curve is 32.95 D)
What is base curve and cross curve?
curve, base (BC) 1. The shallower principal meridian of a toroidal surface of a toric lens. The other meridian of the toroidal surface which has the maximum power is called the cross curve.
Where is the prescription base curve?
Learn the abbreviations used in a prescription: OS - for left eye. OD - for right eye. BC - for base curve.
What is base curve on eye prescription?
The base curve refers to the degree of curvature of the contact lens, or how closely it fits against the eyeball. A lower BC, like 8.40, means the lens is more curved and will fit snugly against the eye. Higher BCs like 8.70 indicate a flatter lens.
How do you determine the base curve and diameter of contact lenses?
Generally, your eye doctor will use a keratometer to measure the curve of your cornea, which is the front surface of the eyes – where contacts rest. These numbers help to determine the lens diameter and base curve that appear on your contact lenses prescription.
What is the shape of the base curve of a lens?
In modern lens design that curve is always plus (+) and has a convex shape. The corresponding curves that create the actual lens power (bend light) are placed on the back of the lens, are minus (-) and have a concave shape.
How to find the spherical equivalent of RX?
The formula to determine the spherical equivalent of any Rx is: Rx sphere power added to 1/2 the cylinder power. If the Rx is a sphere like -2.50 then you work with the -2.50. Spherical equivalents are also used in other formulas and when working with contact lenses.
Can a computer determine the base curve of a lens?
1) In an upcoming lesson you will learn about free-form lens design. Only a computer can determine the correct base curve on a lens designed using free-form technology. In fact most labs today will not allow you to change or request a base curve at all.
What is your base curve?
The base curve is the measurement of the inside curve of your contact lenses. As part of your contact lens exam, your optometrist will do a Keratometer reading. This assessment helps the optometrist measure the shape of your cornea.
What happens if the base curve size is wrong?
If you realize that you’ve gotten contact lenses with an incorrect base curve, you may feel it. Lenses with the wrong base curve feel like they won’t settle right on your eyes. If the lens is too curved for example, it may just slide around in your eye. Every time you blink, it may shift its position.
What is the base curve of a contact lens?
The base curve is a number from 8.0 to 10.0 on your prescription. Look at the last number on your prescription, which indicates the diameter of the lens. When you blink your eyes, your contact lenses move over the limbus, which is the part of your eye where the cornea meets the sclera.
Do contact lens prescriptions have a base curve?
These base curve values are similar to shoe sizes: With some brands, you may wear a different size. If your contact lens prescription does not have a base curve number, that brand only offers one base curve size.
What is the effect of the base curve on the eye?
THE MAGNIFYING EFFECT OF BASE CURVE. The radius of curvature of a lens' front curve, aka its base curve, along with the lens' thickness, help to determine its overall cosmetic and image magnifying on the eye. The steeper the front/base curve and the greater the lens thickness, the greater the magnifying effect.
How to reduce thickness?
Today, one way to reduce thickness is a higher index of refraction material. By using higher index’s greater light bending power, flatter curves can now be used. This reduces plate height and helps overcome problems of increased thickness, weight, magnification and glazing difficulty.
