Test & measurement Figure 1
different power map types, as shown below (two rays labelled 1 and 2 are shown to better illustrate the equations on the right) (Figure 1). The graph on the left illustrates the
calculation of the axial local power Psag and of the refractive local power Pref . The graph on the right shows how the tangential local
power Ptan is calculated.
AXIAL/SAGITTAL POWER MAP The sagittal or axial power map is calculated from the distances d along the normal vectors of
the wavefront to the optical axis as Psag = 1/d. Thus, the sagittal power map is only based on the local slope of the wavefront.
REFRACTIVE/FOCAL POWER MAP The refractive or focal power map is closely related to the sagittal power map. The point of intersection of the surface normal to the optical axis is determined in the same way, but the refractive power map is then calculated using
the short side f of the triangle as Pref = 1/f. The global tilt of the wavefront is always subtracted before calculating the axial and refractive power map.
INSTANTANEOUS/TANGENTIAL POWER MAP
The instantaneous or tangential power map corresponds to the local curvature of the surface in the meridional plane and is calculated using the equation:
MEASUREMENT OF A MULTIFOCAL TORIC LENS
The sagittal power map below belongs to a multifocal toric lens. The image on the left shows the power map including all aberrations. The different power in the two meridians resulting from the toric design is clearly visible. In order to better analyse the zonal structure of this lens, all non-rotational symmetric Zernike terms (result shown below on the right) are subtracted. The annular zones of the lens are now much more evident and provide a more intuitive understanding of the power distribution in each annular zone.
POLYNOMIAL- VERSUS SPOT-BASED POWER MAP ALGORITHMS SHSWorks offers two algorithm options for the power map calculation: spot-based and polynomial-based power calculation. For lenses with a smooth course of the local optical power, the polynomial-based algorithm will provide clearer results. This is because local disturbances such as dust or small defects are not represented in the power map. Instead, the polynomial-based power map shows the overall course. The desired amount of smoothing in the polynomial reconstruction can be controlled by choosing an adequate degree in reconstruction tab of SHSWorks. The quality of the reconstruction can be evaluated by the error function.
Figure 2
The polynomial-based power map is generally a good choice for lenses with continuous designs, whereas for multifocal lenses with discrete steps, the spot-based algorithms are the first choice.
Here, p corresponds to the radial coordinate, whereas R denotes the radius of curvature of the wavefront W at the respective position, as depicted in the graphs above.
LOCAL SPHERE MAP
The local sphere map is recommended for the analysis of varifocal glasses, but usually not as a representation for contact lens power maps. However, in some cases, the local sphere map can also provide interesting information about contact lenses with a continuous power transition.
Instrumentation Monthly March 2024
Figure 3
Continued on page 22... 21
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80
