ISO and ANSI Power tolerances vs. “Real world practice”

ISO and ANSI Power tolerances vs. ”Real World Practice”

What happens in the real world when determining “RX DP power” acceptability?

Record prescribed RX: OD: +.25 = -3.25 x 103

Measure fabricated RX:

Is sphere error > .13? Absolute difference is .12, therefore sphere power acceptable Is cyl error > .13? Absolute difference is .07 and axis is 103 as prescribed, cyl is

acceptable.

Therefore, the lens Passes for power inspection

ISO and ANSI Power tolerances vs. “Real world practice”

Shown above is the ISO 8980-1 and proposed “mounted pair” tolerance scheme, also adopted in ANSI Z80.1 -1995

It was adopted by the ANSI Z80.1 -1995 (US) its first attempt to “harmonize” with ISO Standards ANSI Z80.1 -1999 dropped each meridian, but still held on to stronger meridian (keeping it almost as complicated)

Now let’s review what Standards authors expect the

lab to do:

How does one inspect to comply with the ISO (ANSI Z80.1-1995) Table 1?

1 Record desired RX / Transpose if necessary 0.25 -3.25

Step Sphere Cyl

2 Measure lens / record reading 0.37 -3.18

3 Look up tolerance on Sphere power (Meridian 1) 0.12

4 Compute abs difference for sphere reading 0.125 Pass / Fail for sphere power error Pass

6 Look up tolerance on Cyl 0.12

7 Compute abs difference for Cyl 0.07

8 Pass / Fail for cyl Pass

9 Compute intended cylinder meridian power from sph / cyl -3.0010 Compute measured cylinder meridian power from sph / cyl -2.81

11 Look up tolerance on Cylinder Mer power (Meridian 2) 0.12

12 Compute abs difference for Cyl Mer reading 0.19

13 Pass / Fail for cyl meridian power error Fail

The lens fails ISO tolerances for a power that is not displayed on the Focimeter, but must be calculated. It is also a failure to ANSI Z80.1 because of strongest meridian error, but few labs ever apply this. It passes the common practice of tolerancing the sphere and cyl.

What would you do?

… or do what is practical and ignore the

complicated table entirely?

Use this table ?

Dick Whitney – Chair VCA LTC May 2007