An eyeglass prescription is written in a standardized format with standardized notation so it can be interpreted worldwide. Let’s look at an example and break it down: On the DISTANCE section of the Prescription note the numbers Sphere: -2.00 Cylinder: -1.00 Axis: 180.
- The first number (-2.00) tells us the spherical refractive error (farsightedness or nearsightedness). In this case, because there is a minus sign in front of the 2.00, this patient is nearsighted. A plus sign would indicate farsightedness.
- The second number (-1.00) is the astigmatism. If there is no astigmatism, we generally write the letters DS or SPH after the first number to let the optician know that we didn’t just forget to write in the astigmatism.
- The final number (180) is the direction (AXIS) of the astigmatism. Astigmatism, a football-shaped eye, can be measured in any direction around the clock. We use the numbers from 90 to 180 to indicate the orientation of the football shape.
There may be additional numbers in a glasses prescription. For instance, if the basic prescription is followed by a small number with a superscript (1^) it indicates prism correction. There may be more than one set of prism numbers for each eye.
Lastly, there can also be numbers in the ADD area denoting the amount of near reading strength needed for reading (bifocal or progressive). They usually range from +0.75 to +3.00, depending on age and visual need.
You might also see the letters OD and OS in front of a prescription let us know which eye each string of numbers is for. OD (Oculus Dexter) stands for right eye and OS (Oculus Sinister) for left eye, while OU (Oculus Uterque, Oculus Unitas or Oculus Uniter) means both eyes.
Prescription Abbreviations
SPH = SPHERE - The amount of long or short sightedness. This value specifies the optical power of a lens in diopters. It always has a plus (+) or a minus (-) sign in front or above it. The lens power for the distance correction will be minus while the correction for near distance and reading will be plus power. There will always be a value for sphere, even if it is just zero (written as 0.00, Plano or P). If the vision in one eye is very poor, the word 'balance' or 'bal' may be written.
CYL = CYLINDER - This is the measure for astigmatism. Again, there is always a plus (+) or minus (-) sign present. There may not be a value of cylinder in your prescription.
AXIS - This will only appear if there is a value for cylinder, as it relates to the cylindrical lens and denotes a reference point that indicates the direction of the power of the cylindrical lens. It will be a number between 1 and 180. It may or may not be written with a degree 'o' symbol after the number. In some prescriptions, the axis value is indicated on a semi- circular scale. This should be read to the nearest 5 degrees.
PRISM is a box on the rx prescription form that is not normally filled in. Occasionally, when the two eyes are not properly aligned and looking directly at the same thing, PRISM can be ground into the lenses in order to re-align them. This can occur with strabismus (ie - an eye turn) or in situations where the eyes are properly aligned but are under a tremendous amount of strain in order to keep them aligned. The value under the 'prism' heading denotes the strength of the prism.
BASE - rotation of the prism that is simply specified as 'base in' (where 'in' means towards the nose)or 'base out' or 'base up' or 'base down'. Only these four positions exist, however, orientations between these four positions can be specified by using combinations of horizontal and vertical prisms in the same lens.
PD = PUPILLARY DISTANCE - This measurement is used to accurately center the lenses in the frame that you have selected. PD is the distance from the center of the pupil (black circle) in one eye to the center of the pupil in the other eye. It is called Binocular PD. In cases where the pupils are not equidistant from the nose, a monocular PD is required. Monocular Pupil Distance is measured from the centerline of the nose to each pupil. In such a case you will have different values for the right eye (OD PD) and the left eye (OS PD). Another reason for more than one Pupil Distance is the need to correct both distance and near vision. In this case the prescription will state Near PD and Dist PD. The Near PD measurement is shorter than the Distance PD by 2 to 5 mm because the eyes turn in at near distance.
PD measurement is a must for all RX orders for it insures that your prescription lenses in your glasses are alligned properly in front of the eyes.
Reading Portion of the Eyewear Prescription
ADD = ADD POWER (Sometimes referred to as SEG POWER as well)- It is additional plus power in the lower portion of a multifocal lens (progressive, bifocal, or trifocal) that is required for near vision.
SEG HT = SEGMENT HEIGHT - is the distance from the lowest portion of the eyewear to the top horizontal line of the Segment.You will need to know the Segment Height for the frame you chose. You might need to take the frame to the optician for him to measure the Seg. Height for this particular kind of frame to fit properly. If you are thinking of ordering sunglasses with progressive or bifocal lenses please take a look at our trial program.
PLEASE NOTE, IF YOUR DOCTOR SKIPPED ANY OF THE MEASUREMENTS MENTIONED ABOVE (WITH THE EXCEPTION OF PD AND SEG HT), PLEASE LEAVE IT BLANK WHEN SUBMITTING YOUR ORDER.