Focus variation


Focus variation is a method to calculate a sharp image and to measure the depth with an optics with limited depth of field.

Algorithm

The algorithm work as follows:
  1. at first images with difference focus are captured. This is done by moving the sample or the optics in relation to each other.
  2. then for each position the focus over each plane is calculated
  3. the plane with the best focus is used to get a sharp image. the corresponding depth gives the depth at this position-

    Optics

Focus variation requires an optics with very little depth of field. This can be realized if a microscopy like optics and a microscope objective is used. These objectives have a high numerical aperture which gives a small depth of field.

Usage

The use of this method is for optical surface metrology and coordinate-measuring machine. This means measuring form, waviness and roughness on samples. With optimized hardware and software components a lateral resolution of 500 nm and a vertical resolution of several nm can be reached.

Advantages and disadvantages

Advantages:
Disadvantages:
committee is working on a new series of ISO standards that is called the ISO 25178 series. The part 6 document describes the available methods for roughness measurement. Focus variation is one of the described methods.