Focimeter / Lensmeter Use & design

[eyeboy]

I had a thought today and I'm trying very hard, to remember the principles of focimeter/lensmeter design but since i studied this 20 years ago i need some help!

Focimeter light is projected on the backside of the lens - passes through the lens - and then power read in the eyepiece.

This would then mean in basic terms that the Front Vertex Power is being measured not the BVP. So what does the focimeter do to correct this or have I (and every optical proffesional i have met) being using it the wrong way round? (sides pointing down towards the bench)

Thanks for your help in advance

[Chris Ryser]

turn the lens around, minus side towards the bottom is the usual way.

[eyeboy]

turn the lens around, minus side towards the bottom is the usual way.

Sorry but that doesn't really answer my question regards FVP versus BVP!

[Diane]

When the concave/ocular surface of the lens is on the lens stop (greatest distance from the eye), you would be reading back vertex power. I just made some very simple drawings. Hope they help.

Diane

[Diane]

Looks like only the BVP picture was inserted in above post.

Diane

[Darryl Meister]

Focimeter light is projected on the backside of the lens - passes through the lens - and then power read in the eyepiece... This would then mean in basic terms that the Front Vertex Power is being measured not the BVP

Remember that, although you are observing the spectacle lens from the front, you are viewing an image at optical infinity produced by the spectacle lens with a real object (the focimeter target) located at the back focal length of the lens.

Due to the reversibility of light rays, however, this is equivalent to bringing an object at optical infinity to a real image focus at the back focal length of the lens, when considered in reverse. That is, the object distance behind the lens required to produce an image at infinity is also equal to the image distance behind the lens produced by an object at infinity.

Best regards,
Darryl

[optical24/7]

I follow what you're saying Darryl. But then why do we need to reverse a stronger + lens (read concave side toward the eyepiece) to verify the correct add power? Is it simply thickness induced magnification?

[MakeOptics]

I follow what you're saying Darryl. But then why do we need to reverse a stronger + lens (read concave side toward the eyepiece) to verify the correct add power? Is it simply thickness induced magnification?

The add power is the difference between the front vertex powers. Your reading will be a little off when comparing the difference between the back vertex powers.

[MakeOptics]

In a pair of glasses rays of light from optical infinity are focused through an ophthalmic lens and our cornea to produce a sharp image on the retina.

In a lensometer the image on the retina, from the scenario above, is replaced by a light bulb. This means this bulb has a vergence, that travels through the back of a lens and through the lensometers eyepiece to come out with a neutral vergence which our eye sees as focused mires.

[Wes]

The extra thickness adds to the vertex distance of the seg, displacing it farther away than the focimeter is calibrated for. A plus lens moved farther away (increasing vertex) is effectively stronger. While the extra thickness of the lens will induce extra magnification and increase power, this is already evident in the carrier itself, and is not the root of this particular issue.

Additional info: a pair of lenses with equal front and back curves but differing thicknesses will have different magnification and power and is accounted for in surfacing programs and calculations.

[Darryl Meister]

But then why do we need to reverse a stronger + lens (read concave side toward the eyepiece) to verify the correct add power? Is it simply thickness induced magnification?

Basically. For traditional, semi-finished multifocal lenses, the add power is defined as the difference in surface power between the segment and the distance (or major) portion. However, when measuring the vertex power of the lens, the thickness and opposite surface curve of the lens will influence the vergence of light passing through the lens.

The only way to isolate the difference in surface power between the segment and the major portion, when the segment is on the front, is to measure the front vertex power. This ensures that the same vergence from the opposite surface (that is, the single back surface) reaches both the segment and the major portion, so that any measured difference in vertex power is due strictly to the difference in surface power between the segment and the major portion.

In reality, the near vision power actually experienced by the spectacle wearer through the multifocal segment isn't equal to the front vertex power or to the back vertex power. The near back vertex power F_NEAR is given by:



where w is the working distance in centimeters, t is the center thickness in meters, n is the refractive index, F_A is the surface power of the segment, and F₂ is the back surface power. The lens manufacturer generally has no real control over this power, since it varies with lens thickness, so only the difference in surface powers is evaluated.

That said, for higher add powers in which the range of clear vision (or depth of field) through the bifocal segment is limited to the focal length of the bifocal segment, the difference between the front vertex powers of the segment and major portion becomes exactly equal to the spectacle lens add power required by the wearer, which also makes the front vertex power a convenient measurement of add power.

You can verify this by noting that F_A - 100/w in the expression aboves becomes equal to the front curve F₁ of the major portion when the working distance is equal to focal length associated with the add power, since F_A = F₁ + A, where A is the desired add power. This means that the entire equation for F_NEAR reduces to the normal back vertex power equation.

Of course, eyecare professionals are generally not exposed to much of this kind of information nowadays, so the reasons for using the front vertex power notation have become lost to antiquity...


Best regards,
Darryl

[optical24/7]

Thanks for the explanation guys!

[MakeOptics]

Of course, eyecare professionals are generally not exposed to much of this kind of information nowadays, so the reasons for using the front vertex power notation have become lost to antiquity...


Best regards,
Darryl

Not lost to antiquity, just reserved for the connoisseur. If you would like a copy of the original contact me. I also have copies of schematics for slit lamps, refractors, radiuscopes, etc. It's nice to have an intimate knowledge of the inner workings of the equipment. I had thought of creating a steam punk version of a lot of the equipment used in an average optical, fun thought.