New to optics?

[mannycro80]

Hi, I'm a new lab tech for a company and trying to learn as fast as I can, I'm having trouble with knowing how to id prism through a progressive lens. Why is it all over the place when looking through a lensometor and hardly never in the optical center?

i know this a basic question, but please remember I'm new and just want to learn. My trainer goes way over my head when explaining things.

[Java99]

The place to check for unwanted prism in a PAL is at the prism reference point (PRP), noted on your lens mark up chart for each manufacturer's PAL. To simplify, there's prism all over the place because of the power shift in the lens and manufacturing techniques like prism thinning. That's why you can only check it at the PRP.

[MakeOptics]

Hi, I'm a new lab tech for a company and trying to learn as fast as I can, I'm having trouble with knowing how to id prism through a progressive lens. Why is it all over the place when looking through a lensometor and hardly never in the optical center?

i know this a basic question, but please remember I'm new and just want to learn. My trainer goes way over my head when explaining things.

When you lay the progressive lens on the layout chart and mark the reference points, the fitting cross is the point at the top in the middle of the diagram, the prism reference point is the part below that fitting mark and centered between the two alignment marks one nasal (side with the nose, reserved for manufacturers marks and often material) and one temporal (the side you find the add power on). The prism reference point is where you measure prism in the glasses but all is not done yet.

Progressive lenses have a different power between the top portion of the lens and the bottom portion of the lens, this leads to different thicknesses on the top compared to the bottom. Not a very cosmetically appealing feature of a progressive lens. Manufacturers decided that if they were to incorporate "yoke prism", that prism can be used to equalize the thicknesses. Before you get to asking what is "yoke prism" that's when you use prism in the same direction in essence cancelling each other out. You might hear it refered to as "equithinning prism", "prism thinned", etc. Just know that measuring at the prism reference point gives you the total amount of prism in the lens in all directions and for whatever reason. So to measure the actual prescribed prism take the difference of the vertical prism at the prism reference point and that my friend is your prescribed prism.

Example:

Right Eye 3dwn and 1 out
Left Eye 2dwn and 1out

The two eye's share 2dwn in the same direction so the remaining prism is 1dwn in the right, which means the prescribed prism should read:

OD: 1dwn / 1out
OS: 1out

Prism thinning should read:

OU: 2PD

It is always a good idea to note the prism thinning on the patients work order, so if they come in later and need only one eye replaced the order can be placed with little complications.

(image from makeoptics.com android app, all rights reserved)

[Robert Martellaro]

Just adding to the above:

The PRP is equidistant between the Alignment Reference Marks, typically circles, triangles, or various symbols. The ARMs are 34mm apart. Draw a line between the ARMs, bisect the line perpendicularly (17mm from the ARMs), and position the lens in the lensometer so that the lines match up with the marks on the on the stop.

Using a template, the PRP is the dot right below the fitting point, typically 2mm, 4mm, or 6mm below, or coincident with the fitting point.

http://www.opticampus.com/cecourse.p...rism_thinning/

[MakeOptics]

Just adding to the above:

The PRP is equidistant between the Alignment Reference Marks, typically circles, triangles, or various symbols. The ARMs are 34mm apart. Draw a line between the ARMs, bisect the line perpendicularly (17mm from the ARMs), and position the lens in the lensometer so that the lines match up with the marks on the on the stop.

Using a template, the PRP is the dot right below the fitting point, typically 2mm, 4mm, or 6mm below, or coincident with the fitting point.

http://www.opticampus.com/cecourse.p...rism_thinning/

Awesome additions, Robert. I have a tool I use that is loaded with lenses and their parameters so that my layout chart is created on my phone or tablet which I always have either one at work with me. That way I don't have to print off charts or look up drops and fitting heights. As a bonus I have also coded in availability so materials and whether or not they are available with a photchromic option or polarized option. I also have a pupilometer function that I can use with a credit card for reference to measure. Very accurate but it requires the user to know how to properly take the photo, I am working a deskew transformational matrix into the algorithm to make it more accurate for the layperson as well add additional measured parameters, but I find that it's easier to code for someone with some knowledge then to code for someone with nothing upstairs. My next function is a contact database.

[mannycro80]

Hey thanks for the information, it will be very helpful.

Also, when it comes to tolerance charts, vertical and horizontal prism when power is at 90 or 180. Can someone explain this? Thanks again

[toddchaney]

Prism thinning can be calculated based on the following parameters:

• Is the total power of the lens in the vertical meridian plus or minus in power?
• Is the PRP of the PAL above the datum line? The datum line is a horizontal line that passes through the geometric center of the lens. For example, if the “B” (or vertical) measurement of the lens is 30mm, the datum line will be a horizontal line 15mm above the bottom of the frame.
• Will prism thinning reduce overall lens thickness?

[mannycro80]

So we are seeing powers strong up to 0.25 in our surface work today, any tips on what could be causing it? Also can Todd or someone explain that better so that an idiot like myself understand better?

[mannycro80]

The off power problem is coming from mostly poly high power minus rx's.