Analog circuit for an autofocus microscope system

Abstract

An analog circuit for an autofocus microscope system measures a degree of focus of an object directly from the video signal of a microscope CCD camera. The circuit then returns an index to a host computer for the purpose of adjusting the position of the microscope's objective lens to bring the object in focus. Best focus is found by comparing indices at several different vertical positions. The criterion adopted for determining the degree of focus is derived from the energy distribution of the video signal spectrum. The high frequency energy of the video spectrum is a maximum at best focus and as the optics defocus, the distribution shifts to lower frequencies. Low cost, real time autofocus is achieved with the analog circuitry of this invention, replacing more expensive dedicated, real time image processing hardware.

1. In an autofocus system having a microscope, a source of an image signal representing a magnified image produced by the microscope, and an automatic microscope focus control that focuses the microscope in response to a focus index signal, a circuit for producing the focus index signal, comprising:
 a filter for providing a filtered signal representing predetermined frequencies in the image signal;  
 a transient remover connected to the filter for removing scanning artifacts from the filtered signal;  
 a squaring circuit connected to the transient remover for squaring magnitudes of frequency components of the filtered signal; and  
 an index integrator connected to the squaring circuit for producing a focus index signal representing focus of the microscope by integrating squared magnitudes of the frequency components of the filtered signal.  
 
     
     
       2. The circuit of  claim 1 , further including an illumination integrator for producing an average illumination signal in response to the image signal. 
     
     
       3. The circuit of  claim 2 , wherein the focus control includes a processor for producing a focus position signal representing a focus function of the microscope in response to the focus index and the average illumination signal. 
     
     
       4. The circuit of  claim 1  further including a diode connected to an output of the index integrator for preventing the focus index signal from decaying. 
     
     
       5. The circuit of  claim 3  further including:
 a first diode connected to an output of the index integrator for preventing the focus index signal from decaying; and  
 a second diode connected to the output of the illumination integrator for preventing the average illumination signal from decaying.  
 
     
     
       6. The circuit of  claim 1 , wherein the transient remover applies a one-dimensional window to the filter signal. 
     
     
       7. The circuit of  claim 1 , wherein the transient remover applies a two-dimensional window to the filter signal. 
     
     
       8. The circuit of  claim 1 , wherein the image signal is a video signal. 
     
     
       9. The circuit of  claim 8 , wherein the scanning artifacts include ends of a scan line of the video signal. 
     
     
       10. The circuit of  claim 1 , wherein the filter has a transfer characteristic substantially equal to an optical transfer characteristic of the microscope. 
     
     
       11. An autofocus system for producing a focus index signal in response to an image signal representing an image produced by an imaging device having an automatic focus control that focuses the device in response to the focus index signal, comprising:
 a filter for providing a filtered signal representing predetermined frequencies in the image signal;  
 a transient remover connected to the filter for removing scanning artifacts from the filtered signal;  
 a squaring circuit connected to the transient remover for squaring magnitudes of frequency components of the filtered signal; and  
 an index integrator connected to the squaring circuit for producing a focus index signal representing focus of the device by integrating squared magnitudes of the frequency components of the filtered signal.  
 
     
     
       12. The system of  claim 11 , further including an illumination integrator for producing an average illumination signal in response to the image signal. 
     
     
       13. The system of  claim 12 , wherein the focus control includes a processor for producing a focus position signal representing a focus function of the device in response to the focus index and the average illumination signal. 
     
     
       14. The system of  claim 13  further including:
 a first diode connected to an output of the index integrator for preventing the focus index signal from decaying; and  
 a second diode connected to the output of the illumination integrator for preventing the average illumination signal from decaying.  
 
     
     
       15. The system of  claim 11  further including a diode connected to an output of the index integrator for preventing the focus index signal from decaying. 
     
     
       16. The system of  claim 11 , wherein the transient remover applies a one-dimensional window to the filter signal. 
     
     
       17. The system of  claim 11 , wherein the transient remover applies a two-dimensional window to the filter signal. 
     
     
       18. The system of  claim 11 , wherein the image signal is a video signal. 
     
     
       19. The system of  claim 18 , wherein the scanning artifacts include ends of a scan line of the video signal. 
     
     
       20. The system of  claim 11 , wherein the filter has a transfer characteristic substantially equal to an optical transfer characteristic of the device. 
     
     
       21. A method of controlling a focus of a microscope, comprising:
   scanning an image to generate an image signal representing an image magnified by the microscope;        filtering the image signal to pass the upper half of frequencies of the image signal to produce a filtered signal;        removing scanning artifacts from the filtered signal;        squaring magnitudes of frequency components of the filtered signal;        integrating the squared magnitudes of frequency components of the filtered signal; and        processing the filtered signal to produce a focus index signal for controlling the focus of the microscope.     
     
     
       22. The method of  claim 21 , further comprising producing an average illumination signal based on the image signal. 
     
     
       23. The method of  claim 22 , further comprising producing a focus position signal based on the average illumination signal and the focus index signal, the focus position signal representing a focus function of the microscope. 
     
     
       24. A circuit for controlling a focus of a microscope using an image signal representing an image being produced by the microscope, the circuit comprising:
   a filter for passing the upper half of frequencies of the image signal to produce a filtered signal;        a transient remover connected to the filter for removing scanning artifacts from the filtered signal;        a squaring circuit for squaring magnitudes of frequency components of the filtered signal; and        an index integrator for integrating the squared magnitudes of frequency components of the filtered signal to produce a focus index signal for controlling the focus of the microscope.     
     
     
       25. The circuit of  claim 24 , further comprising an illumination integrator for producing an average illumination signal representing an average illumination of the image signal. 
     
     
       26. The circuit of  claim 25 , further comprising a processor for producing a focus position signal based on the focus index signal and the average illumination signal. 
     
     
       27. A method of controlling a focus of a microscope, comprising:
   acquiring an image signal representing an image magnified by the microscope;        filtering the image signal to pass substantially the upper half of frequencies of the image signal to produce a filtered signal;        removing scanning artifacts from the filtered signal;        squaring magnitudes of frequency components of the filtered signal;        integrating the squared magnitudes of frequency components of the filtered signal; and        processing the filtered signal to produce a focus index signal for controlling the focus of the microscope.     
     
     
       28. An apparatus for controlling a focus of a microscope, comprising:
   a scanner for providing an image signal representing an image being produced by the microscope;        a filter for passing substantially the upper half of frequencies of the image signal to produce a filtered signal;        a transient remover connected to the filter for removing scanning artifacts from the filtered signal;        a squaring circuit for squaring magnitudes of frequency components of the filtered signal; and        an index integrator for integrating the squared magnitudes of frequency components of the filtered signal to produce a focus index signal for controlling the focus of the microscope.