What is the measured value?
The measured value is the dioptric power in the major reference point of a lens and is measured in the focimeter using a defined measuring technique.
What is the prescribed power?
The prescribed or wearing power is the dioptric power experienced by the user in the major reference point in a defined wearing situation. When computing a lens, the aim is to ensure that the wearing power corresponds to the value measured in refraction.
In bifocals, trifocals and progressive lenses from ZEISS the measured near power differs from the prescribed power for two reasons:
A trial lens (TL) displays a lower centre thickness than a finished spectacle lens (SL) with the same vertex power. As the diagram shows, the principal planes of the spectacle lens are closer to the convex side of the lens than those of the trial lens. This results in different back focal lengths (f’TL < f’SL ), while the back vertex focal lengths are identical (f’v TL = f’v SL ). As the reciprocal of the focal length f’ equals the equivalent power F and the reciprocal of the back vertex focal length f’v equals the back vertex power F’v, the trial lens and the spectacle lens have different equivalent powers F, but the same back vertex powers F’v.
If an object at a reading distance of 40 cm is sharply imaged through a trial lens with F’v = + 5.00 D , the same object is imaged by a spectacle lens displaying the same dioptric power and with the same corneal vertex distance behind the retina. The lens power is too weak for the wearer.
To ensure that the finished spectacle lens has the same reading power as the trial lens during refraction, the required correction value is added. The lens then has more positive back vertex power. In the above example, a lens with F’v = + 5.25 D would be required.
For bifocal, trifocal and progressive lenses from ZEISS, the correction value is calculated for a standardised situation of use and is automatically implemented in the lens.
No correction value is added for single vision reading lenses. Instead. It must be taken into account by the optician during refraction.
Situation during Refraction
The patient has sharp vision at the required reading distance when looking through the trial lens (here + 5.00 D); image O’ is generated on the retina.
Two Situations with Spectacle Lenses without Taking the Wearing Power into account
Without the correction value, the patient’s vision is unsharp when looking through the lens (here + 5.00 D) at the required reading distance; image O’ is generated behind the retina.
Without the correction value, the wearer has to increase the reading distance to see sharply (power here + 5.00 D); image O’ is located on the retina.
Needless to say, neither of these possibilities is the aim of the refraction procedure. The wearer's dioptric power must therefore be corrected.
The dioptric power of a lens is always dependent on the ray path for which it was determined. In bifocal, trifocal and progressive lenses in particular, the ray path during use of the lenses for near vision differs considerably from the measuring ray path. It is for this reason that a different dioptric power is measured in the focimeter from that effective in front of the patient’s eye.
In the focimeter the dioptric power F’v is obtained using a parallel bundle of light. This means the focimeter measures the back vertex focal length for an infinitely distant object. The lens is arranged on the focimeter mount in such a way that the principal ray of the measuring ray bundle leaves the lens at an angle of 90°.
During use of the lenses for near vision, however, a different ray path is present:
Here, a divergent ray bundle emerges from a near object. In near vision, prismatic effects in the near reference point of the lens cause a deviation of the centrally imaging principal ray, with the result that the principal ray is not perpendicular to the back surface of the lens. The dioptric power in front of the eye is not the same as that measured in the focimeter. ZEISS lenses are produced so that the prescribed power is actually present in front of the eye. The power measured in the focimeter is specified on the lens packet.
Due to the oblique ray bundle, additional astigmatic error occurs in near vision and must be corrected. This correction value is also taken into account in all bifocal, trifocal and progressive lenses from ZEISS.
The measured near power of all ZEISS multifocal lenses is corrected as follows:
Thanks to the above corrections, the wearer experiences exactly the power which he found to be best during refraction. All bifocal, trifocal and progressive lenses from ZEISS are therefore true to prescription.
The Rx power for near vision is specified for a standardised situation of use. For additions to + 2.50 D, the measured powers of ZEISS lenses are calculated for a distance of 380 mm. For additions over 2.75 D, the working distance becomes smaller, making a calculation necessary for each addition (object distance [mm] = 1000/Add.).
The lens packets for bifocal, trifocal and progressive lenses from ZEISS specify both the prescribed power and the measured power.
The sole purpose of the specified measured power for near vision is to allow the lens to be checked in the focimeter. If the same value is measured with the focimeter as that given on the lens packet, the prescribed reading powers will be effective for the standardised situation of use during wear.
The measured values are specified as the powers in the two principal meridians and as the axis of the first principal meridian.
Example of the measured and Rx powers specified on the lens packet
|+ 4.00||+ 2.00||180||+ 3.00|
|Measured powers||1st principal meridian||2nd principal meridian||Axis|
|Distance||+ 4.00||+ 6.00||180|
|Near||+ 7.26||+ 9.40||1|
The measured value in the major reference point for distance vision is measured in the semicircle (distance measuring circle) above the distance centring cross.
The measured value for near vision can be checked in the lower measuring circle.
Measuring bifocals and trifocals
The dioptric power in the major reference points for distance and near vision are measured in the same way as for single vision lenses. The measured reading power is checked in the major reference point for near vision which is located 5 mm below the segment top in bifocal lenses.
No distinction between the measured and Rx powers is necessary for the dioptric distance power of a lens. Although the geometry of the trial lens differs from that of the spectacle lens, the imaging conditions for distance vision are the same: despite the different positions of the principal planes, a distant object point is always imaged in the far point MR of the eye. The lens therefore has the correct dioptric power for use. The measured and Rx powers are approximately the same. For distance vision, the lens power experienced
during wear corresponds to the power measured in the focimeter.
A distinction is also made between measured and Rx powers in prismatic lenses from ZEISS. One reason for this is the different geometry of prismatic trial lenses and prismatic spectacle lenses. Another is the different ray paths present during measurement in the focimeter and during actual wear.
In prismatic prescriptions, the prism should not be generated by decentration under any circumstances, as this results in considerably poorer visual quality than that obtained with lenses ordered with prism.