On the readout of single vision glasses from a Zeiss visulens 550, I have the following:

     VISULENS 550
Date: yyyy/mm/dd hh:mm:ss
Mode: Standard

    <right>    <left>
SPH: +0.00      -0.25
CYL: -3.00      -3.00
AXS:   92*        89*
PSM: O 0.00    I 0.50
     U 0.00    D 0.00



What is the meaning of PSM and the O, U, I and D values? What is the effect of having an I value of 0.50?

I have been completely unable to find anything about this online. I have tried google, bing and yandex to search for anything containing "visulens" and "PSM", without finding anything. Google even returns zero results. Searching for "lensometer" and "PSM" provides some results, but nothing, as far as I can tell, that explains the different abbreviations. E.g. this manual: https://manualzz.com/doc/6594154/user-s-manual

PSM, O, U, I and D are mentioned, but not explained.

The manual for the Zeiss visulens 550 (https://www.zeiss.com/content/dam/med/ref_international/products/essential-line-basic-diagnostics/visulens-550/pdf/visulens550_interface_definition_rev01.pdf), that was used in my case, has no mention of these abbreviations at all.

  • 1
    Hi, welcome to Medical Sciences! Questions here are required to show results of prior research. As described in the help center and the reasons mentioned in this meta post, this demonstrates that you’ve taken the time to try to help yourself, saves us from reiterating obvious answers, and helps you get more specific and relevant answers. Please edit your question with links to or references to what you've found in your search. Otherwise your question may be closed.
    – Carey Gregory
    Commented May 23 at 20:08
  • @CareyGregory thank you for your comment. I have edited my question to show my fruitless attempts at finding this information elsewhere. I am not sure if it is because of how search engines, e.g. google, have become completely useless in recent years, but I am honestly baffled that something that seems so basic cannot be found with a simple search.
    – a20
    Commented May 23 at 21:21
  • That does seem odd, but thank you for the edits.
    – Carey Gregory
    Commented May 23 at 22:44
  • 1
    I am wondering if it is an abbreviation for "prism" as this is occasionally necessary to correct for diplopia (double vision). However, it does not seem that the device includes the axis for the cylinder (the correction for astigmatism), which is just as important as the cylinder value in terms of creating lenses that will function for someone. However, I am not an ophthalmologist, so my knowledge in this particular area is most certainly lacking.
    – RudyB
    Commented May 27 at 0:07

1 Answer 1


The abbreviations can be explained but the printout does look incomplete. There are some reasonable summaries on various optician websites.

SPH (Sphere) is the overall correction for short or long sight in each eye.

CYL (Cylinder) is the degree of astigmatism (irregular curvature) of the eye; as pointed out in the comments this would usually have an 'AXIS' value that indicates the alignment of the cylinder correction.

PSM (Prism) is a correction for misalignment of the eyes (strabismus or squint); this can be expressed as a base value with an alignment of up or down, in or out (the U, D, I, O values given in your readout). This is again slightly odd as I would expect there to be a result for each eye, not one overall number. It looks like these are more conventionally abbreviated as BI (Base In), BO (Base Out), etc which may be why your searches didn't pick anything up.

ADD (additional) gives a value for bifocal or varifocal lenses needed to give a correction for reading.

  • Thank you! I had missed adding the axis, so I edited the question to put the complete printout there. What would the effect of an incorrect 0.5 on PSM BI be (induced diplopia?). Would it be noticeable or small enough to not be an issue?
    – a20
    Commented May 27 at 11:54

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