title: Focus variation
text: # Focus variation
  
  Focus variation is a method used to sharpen images and to measure surface irregularities by means of optics with limited depth of field.
  
  ## Algorithm
  
  The algorithm works as follows:
  
  * At first images with difference focus are captured. This is done by moving the sample or the optics in relation to each other.
  * Then for each position the focus over each plane is calculated
  * 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 (limitation of wavelength of light) and a vertical resolution of several nm can be reached.
  
  ## Advantages and disadvantages
  
  Advantages:
  
  * Can be used on samples with steep flanks. This is because a ring light can be used to extend the illumination aperture
  * Can deliver color information
  * Can measure on rough surfaces
  
  Disadvantages:
  
  * Can not be used if the surface of the sample does not give structure in the image. This means it can not be used for wafers and glass
  
  ## Standardisation
  
  The ISO committee is working on a new series of ISO standards, called the ISO 25178 series. The 6-part document describes the available methods for roughness measurement. Focus variation is one of the described methods.
  
  