Field curvaturechart

As the MTF is a quantitative and standardized way to characterize optical systems, MTF measurements are commonly used by optical engineers and designers in order to:

For a deeper analysis, WaveView4 can also compute the Point Spread Function (PSF), wavefront aberrations and Zernike or Legendre polynomials.

Field curvatureaberration

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The MTF measurement can be processed from the wavefront map using a double fourier transform. Shack-Hartmann sensors provide the wavefront map in just in a single shot acquisition and therefore provide almost instantaneously a 2D-MTF measurement, offering access to every azimuthal angle.

Field curvatureexample

Field curvaturelens

In this article we present a MTF measurement performed with a HASO SWIR Shack-Hartmann wavefront sensor manufactured by Imagine Optic SA, a company renowned for its expertise in optical metrology instrumentation for over 25 years. It is combined with the RFLEX accessory which combines a light source and allows an easy double pass set-up.

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Note that the HASO R-FLEX instrument can also be used to  characterize large optics if combined with the LA ( Large Aperture ) accessory module.

Field curvaturephotography

On the screenshot below, we can clearly see that this optical system on Figure 2 has aberrations. Indeed, the wavefront is not uniform and the MTF curve (solid line) is not close to the diffraction limit (dotted line).

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The Modulation Transfer Function or MTF measurement is a crucial metrology technique to evaluate how an object’s contrast is captured after passing through an optical system as a function of spatial frequency. It is a critical criteria especially for imaging systems, as it quantifies the imaging quality. The MTF curve is usually compared to the ideal MTF given by a diffraction limited optical system (see Figure 1).

HASO sensor offer a measurement accuracy of  λ/100 rms which translates to a MTF calculation with an accuracy of less than 1 point (1%). Moreover, this measurement solution allows convenient implementation allowing on-axis and off-axis measurements.