Density for the dense...

[Uncle Fester]

Am I correct that the lower the density the thicker the lens?

A patient is adamant that trivex is thinner than poly. Using the opticampus thickness calculator I got 54-18 frame 62 PD -4.00 poly 1.5 CT at 5.2mm Trivex 5.6mm at 180. This is because of the density?

[Darryl Meister]

At the same minimum thickness, polycarbonate will be thinner than Trivex, since the index of refraction is higher. Density really only affects weight.

[HarryChiling]

Density Index

Density has no relation to thickness. Density is a unit of weight to volume. Thickness is going to be determined by what the material can be surfaced too, (center thickness), aspherics or curves, index, and decentration.

[For-Life]

Density is how close the particles are. For instance, a piece of styrofoam has a very low density where a brick has a much higher one. It is defined as Mass and Volume. So you take two objects with the same volume, the one that is heavier is denser.

[jrumbaug]

Sometimes you will hear people talk about the SPECIFIC GRAVITY of the material, that is about the same, or is the same as density.

If memory serves me right, Trivex has a lower specific gravity ( density ) than poly, but it also has a lower index of refraction. so it may be a lighter weight material, but you may need to use more of it to get the same rx.

The subject of CENTER THICKNESS is just as important or more so. If one lens has a 2.0 center and the other a 1.5, that may be a bigger difference than specific gravity or index of refraction.

Jim Rumbaugh

[MarcE]

You may have thought that the higher density material has more light bending power because there is more mass in a specific volume. This is true of glass, but only as a by-product of the refractive index. It's not the density that is causing the refractive power of the material. It is only the refractive index. Poly will be thinner - everything else being equal.

At a -4.00. Trivex will be noticably thicker. That is the high-end of the Rx range for us using Trivex.

[Uncle Fester]

Thanks for the quick responses.

I should have said I know index of refraction determines thickness:hammer:
and does density also have an effect.

Thanks for clarifying density. The first and only formula they taught me in high school chemistry was density is mass divided by volume. This was the first day of class as I recall. I asked the teacher after class if the math got harder. He said "much". A few days later I had 3rd and 4th period off. :bbg:

[Robert Martellaro]

Trivex has a lower specific gravity ( density ) than poly, but it also has a lower index of refraction.

1.11g/cm3 compared to 1.21 for poly, n= 1.53 and 1.59 respectively.

so it may be a lighter weight material, but you may need to use more of it to get the same rx.

Jim Rumbaugh

Right, although Trivex's extemely low density allows it to be the lightweight king across an extremely wide range of powers compared to any other material/index of refraction lens available in the US.

It's interesting to note that when the index of refraction increases, the specific gravity increases, and/or the Abbe number decreases. However, I want my cake and eat it too: 1.90 index, abbe above 50, and a density below 1.40. Realistically, we'll probably never see more than two out of three.

(Doh!- two out of three excluding the abbe/density combo.)

[Darryl Meister]

With glass lenses, the increase in density with increased refractive index could be come quite significant, resulting in much heavier lenses. This was primarily due to the addition of heavier metal elements, such as lead, to the glass composition in order to increase refractive index.

With plastics, on the other hand, the change in density is relatively small between most materials. Mid-index acrylics, polycarbonate, and even most high-index polyurethanes (e.g., 1.6 high-index, 1.67 high-index, and so on) are all within 10% of standard hard resin in terms of density. Even Trivex is only 0.2 grams per cubic-centimeter lighter than standard hard resin. The primary reduction in weight is due to the reduction in thickness that occurs because of the higher refractive index and the thinner minimum center.