measuring lens thickness

[Daniel Cunningham]

Hi:

I heard a rumour that for executives and progressives, when determining lens thickness, the distance and the add power are added together. If this produces a plus number, then the lens is treated as a plus lens when determining thickness.

For example: distance Rx is -1.00 D.
the add power is +2.00 D.
the net reading power is +1.00 D.
this lens should be treated as a plus lens when determining thickness so I would be measuring edge thickness not centre thickness.

Is this true?

Daniel

[chip anderson]

Lens thickness should be measured at the optical center of the distance power, period.

[lensgrinder]

The thickness should be measured at the MRP, which can be the OC, but not always.

[MarcE]

I think he is pointing out an aspect that needs to be considered when surfacing a lens. If in this case, you assume that this is a minus lens, when in fact it is a plus lens at the bottom edge, you could have surfaced it too thin on the bottom temporal and nasal edges to cut a groove.

in this case a 1.5mm CT poly lens could work out to a 2.2mm ET at the distance. But this same lens would be about 1.0mm ET at the bottom corners. Too thin to groove, for example.

[Refractingoptician.com]

,,

[chip anderson]

Can only tell you this for sure but in contact lenses we always calculate the thickness for the lowest minus (highest plus) power when the lens has cylinder or is concentric bifocal (we still measure at optical center, or in a few rare situations geometric center).

Chip

[gemstone]

I certainly hope no one here thinks for example if the Rx is -.50 distance with a 2.50 add, that you can grind a 2.0 center, or 1.0 for poly. I think that is the essence of the question here. You must consider the add power when grinding executives and progressives. Daniel's rumor is pretty dead on, as with most rumors.

You grind based on the center thickness all the time and measure it there as well. However if the add power is strong enought to overpower the minus in the Rx, it must be considered. That's pretty basic explanation as why you sometimes can't grind a 1.0 center, for example on a progressive. It often gets much more complicated than that , with strong cyls and other things that influence the Rx.

[lensgrinder]

Well after re-reading the question I will offer another explanation.
To determine the thickness required you need to take into account the base curve of the add power. For example if you pull a 6.00 D base with a 2.00 add then to determine the correct CT you need to add the 6.00 base to the 2.00 add and use 8.00 D to determine your sag and then determine your center thickness needed to produce whatever edge thickness you are looking for.
So, for example a -1.00 D lens with a +2.00 add pulled on a 6.00 base in 1.5 index in a 50mm eye with a 1.5mm edge will have to have a center thickness of roughly:

ha Harry I used it again, I think we are even!
sag_front = 5.7mm

sag_back = 4.5mm
5.7 - 4.5 + 1.5 = 2.2mm center
Now the top of this lens will not have a 1.5mm edge because it is a 6.00 on top. So your edge in the upper portion will be roughly:

sag_front = 3.8mm
4.5 - 3.8 + 2.2 = 2.9mm edge at top

[HarryChiling]

touché Brent ☻☺:) -

[Darryl Meister]

Brent has hit it pretty close with his analogy. Unlike "rotationally-symmetrical" lenses, such as single vision and contact lenses, the center thickness of progressive lenses varies significantly with Add power since the surface steepens toward the bottom of the lens blank.

A good prism-thinning calculation routine will consider the distance prescription, the Add power, the fitting measurements, and the frame shape. The routine will attempt to minimize the thickness difference between the upper and lower edges of the lens using vertical prism (typically, this requires base down prism, but base up prism may also be used in some cases). Prism-thinning can be applied to minus lens powers as well, though you generally see more of a benefit in plus prescriptions.

See this article on prism-thinning for more information.

[shanbaum]

The thickness should be measured at the MRP, which can be the OC, but not always.

Make that "PRP" - Prism Reference Point. "MRP" has been used in various ways over the years, making its meaning ambiguous. The VCA's Lens Data Standard defines a couple of preferable terms: Prism Reference Point (the point on the lens where the prescribed prism should be measured) and Layout Reference Point (the feature on the lens that is used for positioning the lens for surfacing and/or finishing). In the case of finishing a single vision lens, the LRP is the PRP. For PAL's, the LRP is the fitting cross; for other multifocals, it's the segment.

I'll look for the relevant section of the standard and post it.

[lensgrinder]

Shanbaum it is listed in ANSI as the PRP in section 5.2.1(1999 standard) I am not sure of the section in 2005. I had said the MRP because it is a term that everyone can associate with, but as you said it has become ambiguous.

[HarryChiling]

The standard that shanbaum refers to can be downloaded here:

http://www.visionsite.org/s_vision/p...scStandard.pdf

sections 2.1.16 refer to reference points on the lens.