# Optical Forums > Ophthalmic Optics >  Result of lens thickness w/ prism

## cvbs

Hello all,

  I am looking for a calculator (similar to "thickness calculation" on OptiCampus.com)that will allow me to figure out what the edge thickness of a lens would be with x amount of prism put in. This is more for the 180 axis so I can give correct answer to patient as to "how thick will my lens be?"

  If by chance this calculator would allow for changes in lens material .... THAT WOULD BE GREAT!!

t.i.a.
cvbs

----------


## cvbs

Hi All,

  As to the word "Calculator" .... I don't mean a stand alone calculator ... I am referring to a program that I can use on my computer or... if not that how about a formula or combination of formulas that I can combine in a spreadsheet like xl to calculate my answer?  

Any suggestions????  

Hope this qualifier will help you understand what I need.

t.i.a.

----------


## humbleABOCtx

Advice first...*KEEP IT SIMPLE* w. most pt's. :Cool: 
Try and keep the Pt's PD and the overall Frame Pd mm as close to poss.
*most Rx's 5mm or more diff in dec. .....Choose a diff.frame!
*High Rx's try to get 0-2mm in dec...otherwise Choose a diff.frame!!!!
of course pay attention to "B" measurement to accom. rx needs (bif,PAL,etc..pt preference...)

However if you get the Analytical Engineer... :Nerd: 
give this a go..

*(MBS/2)2 x Compensated Lens Power* 
*______________________________      +  1.5       =  mm thickness*
*2000 x (Index of Refraction-1)*

MBS(minimum blank size)
Compensated Lens Power/Prentice Rule (quick math method: Sphere + 1/2cyl)
Index of Refraction (Calculation done 4 u.just plug in)
1.Cr-39 1.498 (996)
2.Poly 1.586 (1172)
3. HI 1.60 (1200) or 1.67 (1340)
 FOR Prism I haven't tried yet....but adjust the "Compensated Lens Power" calculation with the added diopters of prism

The unknown variable of course is _"What is the generator setting at?" !!!_
careful what you promise the pt down to the mm....the lab has tolerance's that is well with guidelines. If you promise 1.1 thickness w. prem HIP and the lab makes 1.3 or 1.4 thickness on a -9.00rx...well you'll have everyone upset w.you--the pt...the lab....so careful the #'s you give to pt.... esp the engineers that ask for your PD stick at dispense....

*TRY THIS TOOL!!!* _Essilor_ has a 
LENS CENTER-EDGE THICKNESS-WEIGHT COMPARISON chart (w/rx avail too) 
it shows several diff Rx's (+and-). The frame the pt. chooses you can hold over the lens (est.OC on frame though) and show how much edge thickness there will be. This is a great tool for the visualist out there. 
Hope this helps. :bbg:

----------


## cvbs

*(MBS/2)2 x Compensated Lens Power* 
*______________________________      +  1.5       =  mm thickness*
*2000 x (Index of Refraction-1)

*humbleABOCtx,  Thanks for the response.... and YES, we will not get tht deep w/ the engineers!!:hammer:we ALL know how much fun they can be :cry: 

Your *(MBS/2)2 x Compensated Lens Power..* is that to mean 50 eye size /2 =25 and then that result is ADDED OR MULTIPLIED by 2xCompnesated Lens?

Secondly when you say add the prism to the compensated power ... is that if I have 6d out and the power is -6, make it 12?

Just trying to follow the logic here.

Thanks again for the help.

How about the rest of the brain power out here... is there any other suggestions / ideas ?

and just for query ... I was wondering why this hasn't been asked for before??? :Confused:

----------


## humbleABOCtx

MBS---whole diff equation..sorry I assumed it was a given.

[(Frame PD - Pat.PD) x 2] + LD = MBS (minimum blank size lens needed)
 then plug in the rest of the #'s you have


(Frame Pd = A + DBL)

----------


## cvbs

Thanks... my bad.... once mbs portion of the formula is completed...  that result is ADDED?? OR MULTIPLIED?? by 2xCompnesated Lens?

----------


## HarryChiling

> Thanks... my bad.... once mbs portion of the formula is completed... that result is ADDED?? OR MULTIPLIED?? by 2xCompnesated Lens?


The 2 is an exponent.

The compensated lens power he mentions should be the power on the 180 meridian if that's what your after. Then you can use the MBS to determine the thickness due to prism.

_t=base to apex difference in mm (additional base thickness)_
_d=lens diameter (MBS from above equation)_
_P=prism_
_n=index of material_

*t=[d*P/100(n-1)]*

keep in mind that prism thickness is additive in plus powered lenses but must be subtracted from teh minus power lenses while still keepign adequate thickness for the center.  Since your only interested in the 180 then the horizontal component of the prism shoudl be used in thsi formula.

----------


## cvbs

Thanks HarryChilling ...  	 		 		*(MBS/2)2 x Compensated Lens Power* if The 2 is an exponent, do you mean multiply the compensated power by itself or just take the result of (mbs/2) times 2 times the compensated lens power.

much appreciated.  


cvbs

----------


## humbleABOCtx

MBS divided by 2       then squared       then multiple  the   compensated lens power
 then divide that by Index 
                                              then add 1.5  you have your est. thickness

----------


## HarryChiling

Here you go:

http://www.onlineopticianry.com/thickness.php

It's not an highly accurate form, but it'll get you there and it uses a neat google visualization Only gives you the thickness on the 180 so it ignores any vertical prism. Don't say I never gave ya nothin'.;)

Here's the source code (still hand coded using notepad) free to use just credit me with a comment tag:


```

 <?php$sph = $_POST["sph"];$cyl = $_POST["cyl"];$axis = $_POST["axis"];$prism = $_POST["prism"];$base = $_POST["base"];$pd = $_POST["pd"];$a = $_POST["asize"];$dbl = $_POST["dblsize"];$index = $_POST["index"];$minthick = $_POST["minthick"];$angsin = pow(sin(deg2rad($axis)),2);$power = $sph + ($cyl * $angsin);$mbs = $a + 2*((($a + $dbl)/2) - $pd) + 1;$dec = 10 * $prism / $power;if ($power<0 && $base<=1) { $dec *= (-1);}if ($power>0 && $base<=1) { $dec *= (-1);}$modmbs = $mbs + $dec;$thick1 = (pow(($modmbs/2),2) * $power) / (2000 * ($index - 1));$thick1 = round($thick1*100)/100;if ($thick1<0) { $thick = (-1)*$thick1 + $minthick;}else { $thick = $thick1 + $minthick;} ?><html> <head> <!-- Created by Harry Chilinguerian ABOC-AC, NCLC-AC free to use as long as this disclaimer is included --><style type="text/css">#chart_div {position: absolute; left: 450px; top: 150px;}#frm_div {position: absolute; left: 50px; top: 20px;}#dir_div {position: absolute; left: 450px; top: 50px; width: 400px;}</style><script type="text/javascript" src="http://www.google.com/jsapi"></script> <script type="text/javascript"> google.load("visualization", "1", {packages:["gauge"]}); google.setOnLoadCallback(drawChart); function drawChart() {  var data = new google.visualization.DataTable();  data.addColumn('string', 'Label');  data.addColumn('number', 'Value');  data.addRows(1);  data.setValue(0, 0, 'Thick');  data.setValue(0, 1, <?php echo $thick; ?>);  var chart = new google.visualization.Gauge(document.getElementById('chart_div'));  var options = {  min: 0,  max: 10,  width: 1200,   height: 361,   redFrom: 0,   redTo: 1,   yellowFrom: 1,   yellowTo: 2,  greenFrom: 2,  greenTo: 5,  majorTicks: ['0','1','2','3','4','5','6','7','8','9','10'],   minorTicks: 2};  chart.draw(data, options); } </script> </head> <body> <div id="chart_div"></div> <div id="frm_div"><table><form action="<?php echo $PHP_SELF; ?>" method="post"><tr><td colspan="2"><h1>Lens Thickness</h1></td></tr><tr><td>&nbsp;</td><td><h3>Prescription Data</h3></td></tr><!-- <tr><td>Eye</td><td><input type="radio"  name="side" checked /> OD <input type="radio" name="side" /> OS</td></tr> --><tr><td>Sphere Power</td><td><input type="text" name="sph" value="<?php echo $sph; ?>" /></td></tr><tr><td>Cylinder Power</td><td><input type="text" name="cyl" value="<?php echo $cyl; ?>" /></td></tr><tr><td>Axis</td><td><input type="text" name="axis" value="<?php echo $axis; ?>" /></td></tr><tr><td>Horizontal Prism</td><td><input type="text" name="prism" value="<?php echo $prism; ?>" /></td></tr><tr><td>Prism Direction</td><td><input type="radio"  name="base" value="1" checked /> IN <input type="radio" name="base" value="0" /> OUT</td></tr><tr><td>&nbsp;</td><td><h3>Biometric Data</h3></td></tr><tr><td>Pupil Distance (monocular)</td><td><input type="text" name="pd" value="<?php echo $pd; ?>" /></td></tr><tr><td>&nbsp;</td><td><h3>Frame Data</h3></td></tr><tr><td>Frame A</td><td><input type="text" name="asize" value="<?php echo $a; ?>" /></td></tr><tr><td>Frame Dbl</td><td><input type="text" name="dblsize" value="<?php echo $dbl; ?>" /></td></tr><tr><td>&nbsp;</td><td><h3>Lens Data</h3></td></tr><tr><td>Index</td><td> <select name="index" />  <option value="1.499" selected="selected">1.49 Hard Resin</option>  <option value="1.523">1.53 Crown Glass</option>  <option value="1.529">1.53 Trivex</option>  <option value="1.537">1.54 Spectralite</option>  <option value="1.555">1.56 Mid-Index</option>  <option value="1.558">1.56 Ormex</option>  <option value="1.586">1.59 Polycarbonate</option>  <option value="1.600">1.60 Finalite</option>  <option value="1.661">1.67 MR-10</option>  <option value="1.700">1.70 EYRY</option>  <option value="1.732">1.74 MR-174</option> </select></td></tr><tr><td>Minimum Thickness</td><td><input type="text" name="minthick" value="<?php echo $minthick; ?>" /></td></tr><tr><td>&nbsp;</td><td><input type="submit" value="Calculate Thickness" /></td></tr></form></table></div><div id="dir_div"><font size="2">If the guage points in the red the minimum thickness is too thin, if the guage points in the yellow the minimum thickness may be too thin depending on the material, if the guage is in the green this is the optimal thickness, if the guage is in the white you may want to consider changeing the material.</font></div></body> </html>

```

----------


## cvbs

Harry.... this is very nice.  Thank you..... could you put in the following specs and see if you get what I was getting.  Sph.+2.25 cyl -1.00 axis 65    5 out prism    27.5 p.d.   polycarb lens    min 1.5 thickness ( I am assuming you mean edge w/ plus lens)

tia

----------


## cvbs

Harry .... I forgot frame size ....48-18

thanks

----------


## HarryChiling

> Harry.... this is very nice. Thank you..... could you put in the following specs and see if you get what I was getting. Sph.+2.25 cyl -1.00 axis 65 5 out prism 27.5 p.d. polycarb lens min 1.5 thickness ( I am assuming you mean edge w/ plus lens)
> 
> tia


I made the program using decentration of prism to compensate the minimum blank size before figureing thickness, I did it this way to avoid too much logic in the programming so that I can quickly whip something up, it's obvious that it has it's failures in the lower powers, I will go back to the drawing board and do it the right way, no shortcuts.  I'll post it when it's complete.

----------

