What does the True Base of Blank means?

[copland]

What does the True Base of Blank means?

Do the true base of front surface of lenses have same Index in different index of refraction?

[shanbaum]

What does the True Base of Blank means?

Do the true base of front surface of lenses have same Index in different index of refraction?

You probably meant to write "Does the true base of front surface of lenses have same power in different index of refraction?" to which the answer is no.

One would like to be able to say that the "true base" or "true curve" of a lens is the actual surface power of the lens. That is calculated by dividing the index of refraction minus one by the radius of curvature of the surface in meters.

Unfortunately, our industry has done something quite odd, for a long time now, in referring to curves of lenses in diopters calculated using a "shop index" of 1.530. That makes some sense when referring to the surfaces of grinding laps - after all, they're not transparent, and so have no index of refraction - but in the case of lenses, it's just confusing. I suppose it saves having to keep lots of lens clocks on hand.

So, in the U.S., and to some extent worldwide, you'll see published lens "true curves" computed in the "wrong" index - an index (usually 1.53) other than that of the material out of which the lens is made, in which case, the "true curve" is not the surface power of the lens. But if you know the index used to calculate the stated dioptric curve, you can easliy calculate the radius of curvature, and therefore, the power of the surface in any index (most importantly, that of the material out of which the lens is actually made).

[chip anderson]

How difficult would it be for lens clocks to have a multi indicated dial that showed "base curves" for different indexes, or even for them to be digital with buttons for index?
Come on, catch up instrument makers. The world has changed.

Chip

[copland]

Thanks for your anwser.

I just want to know what's means if a semi-finished blank marked base 4.50, the material index is 1.502. According your reply, the Base 4.50 index is a "shop index" of 1.530, Is it right? and how to caculate the true base in 1.502? what about the 1.67? the index of front Base of 1.67 lenses is the" shop index" 1.530?

[shanbaum]

Thanks for your anwser.

I just want to know what's means if a semi-finished blank marked base 4.50, the material index is 1.502. According your reply, the Base 4.50 index is a "shop index" of 1.530, Is it right? and how to caculate the true base in 1.502? what about the 1.67? the index of front Base of 1.67 lenses is the" shop index" 1.530?

The "4.50" doesn't necessarily tell you anything much at all - the "marked" curves are categorical (that is, a blank marked "4.50 base" from a particular product line is merely distinguished thereby from the other available bases). That 4.50 base would have an actual or "true" front curve that could be almost anything in the vicinity of four and one-half diopters. In fact, the least likely true front curve for such a lens would be (speaking solely from experience)... 4.5.

That 4.50 base lens will have a true curve that may or may not be printed on the box. You may have to obtain information from the provider of the lens as to what the true front is, and in what index the true front is calculated. There is insufficient uniformity of practice in this area (that is, how the information is communicated in printed materials or on lens blank boxes) to allow anyone to depend on a common practice - which is why, when we developed a standard for communicating such information from manufacturers to lab software systems, we specified that the true curve must be expressed as a radius of curvature, which unlike a dioptric curve, or a sag (two common ways of quantifying curvature), is an independent value.

Converting a curve from one index to another is trivial - simply divide the "target" index (less one) by the "current" index (less one). If you have a CR-39 lens with a front curve of 4.59 calculated in 1.530 diopters, and you want to know the actual surface power (which would be the curve in 1.498 diopters), multiply 4.59 by 498 divided by 530.

[copland]

When designing the lenses of different Index materials, Do they use the same index of 1.530 to design the front curve, so as to measure the front diopter curve using the present diopter meter?

[Darryl Meister]

When designing lenses in materials with a refractive index other than 1.530, the curvature of the front surface may still be specified using a 1.530-based value, although the software must adjust this curvature as necessary in order to obtain the desired optical performance in the actual refractive index of the lens material.