Prescription Safety Glasses: Understanding SPH, CYL and Axis
Ordering safety glasses for the workshop? Understand your prescription numbers, recognize equivalent cylinder notation and keep optical correction separate from impact protection.
Your prescription describes lens power. Protective eyewear must also be appropriate for the hazards at your workstation.
What SPH, CYL and Axis Mean on a Glasses Prescription
Read the right and left eye separately. SPH and CYL are measured in diopters (D); axis is an angle, not another lens power.
| Field | Meaning | What to check |
|---|---|---|
| SPH · Sphere | The spherical lens power. Minus generally indicates short-sightedness; plus indicates long-sightedness. | Keep the sign and decimal places. PL or plano means zero sphere. |
| CYL · Cylinder | The cylinder power used to correct astigmatism. It can be written in plus or minus notation. | Do not drop the sign or treat CYL as an optional extra when it is prescribed. |
| Axis | The orientation of the cylinder correction, usually written from 1° to 180°. | Keep it with the CYL for the same eye. A larger angle does not mean stronger correction. |
For the prescription terminology, see the Association of Optometrists’ prescription explanation.
Plus and Minus Cylinder: Different Numbers, Same Correction
A supplier’s order form may use a different cylinder convention from your prescription. The numbers can look different while describing the same correction. This rewriting is called cylinder transposition.
Illustrative values for one eye, shown as SPH / CYL × axis.
Add the signed powers
New SPH: +2.00 + (+1.00) = +3.00 D.
Reverse the cylinder sign
New CYL: +1.00 becomes −1.00 D.
Shift the axis by 90°
New axis: 90° becomes 180°. Keep the result within 1–180°.
Method: Specsavers’ Foundations in Dispensing, Chapter 6, printed page 74.
A Correct Prescription Does Not Establish Eye Protection
In a machine shop, matching the prescription is only one part of selecting eyewear. Ordinary prescription glasses should not be assumed to provide the protection required for flying chips or other workplace hazards.
Prescription built in
Protective eyewear can incorporate your prescription. Confirm the suitability of the complete frame-and-lens assembly with the supplier and your workplace safety lead.
Protection over glasses
Another option is suitable eye protection worn over prescription glasses, without disturbing the correct position of either pair.
For US workplaces, OSHA requires side protection where flying objects are a hazard. Its eye-protection rule also specifies compliance criteria referencing ANSI standards. SPH, CYL and axis do not demonstrate that a product meets those criteria.
Source: OSHA 29 CFR 1910.133, especially (a)(1)–(3) and (b). Requirements outside the US may differ.
Before You Order Prescription Safety Glasses
- Send the complete prescription. Use the original document from your eye-care provider rather than copying only the three fields in this guide. Include any additional prescribed information.
- Resolve a form mismatch with the supplier. If a sign or axis value does not fit the order form, ask the dispensing optician to check it. Do not change just the CYL sign to make the form accept it.
- Describe the job. Tell the supplier and safety lead what tasks you perform and what other protective equipment you wear. Ask them to confirm the required protection and fit.
- Keep the order confirmation. Compare the eye labels and prescription details with the submitted document. Refer discrepancies to the supplier before using the eyewear at work.
This guide explains prescription notation. It is not an eye examination, a personalized prescription or a workplace hazard assessment.