Automatic focusing of an optical system

Abstract

Abstract A method of automatically focusing an optical system onto an image includes deriving a measure of the contrast for each of a plurality of images obtained at different settings of the optical and selecting as the final setting that setting which corresponds to the greatest contrast, the measure of contrast being derived from the distribution of selected image elements in a grey scale. Apparatus for carrying out the method uses an image converter to produce the image of the optical system for examination by a grey level measuring device which measures the grey values of a number of image elements. The distribution of the measured grey values within a grey scale is registered in a memory and this information is used by an electrical calculating system which calculates the value of a contrast function for the particular image being examined. A control device is connected to the calculating system and searches for the maximum value of the contrast function amongst the function values obtained at various settings of the optical system to provide the final setting. BACKGROUND OF THE INVENTION This invention relates to a method of and apparatus for automatically focusing an optical system such as a microscope onto an object. In mass gynaecological health screening procedures, cell samples are taken and collected for subsequent examination by microscope. This examination which at present is carried out manually is very time consuming, requires many microscopists and is thus very expensive. It would be advantageous, therefore, if the examination of the samples could be carried out automatically, as this would reduce the time and expense involved in examining the collected cell samples and thus make it possible to conduct such mass health screening procedures more frequently. An automatic microscopy of the samples requires inter alia a method of automatically focusing the microscope. The present invention offers a solution to this particular problem and also offers a general solution to the problem of focusing an optical system onto an object. Devices for automatically focusing optical systems have been proposed previously. In these known devices the distance between the object and the lenses of the optical system is measured and thereafter the setting of the system is corrected on the basis of the measured distance. For this purpose, a plurality of sophisticated measuring methods based on, for example, optical interference, or laser distance measurements have been used. All of these methods suffer from the disadvantage that the distance is in fact measured from the surface of a slide, i.e. glass plate, covering the object, and not from the object itself. The present invention, however, performs focusing onto the object itself and, thanks to this, correct focus will be set even if the thickness of the slides vary from sample to sample. If previously known methods are used this varying thickness may often lead to an unfocussed image although the distance setting is correct. SUMMARY OF THE INVENTION It is therefore an object of the invention to provide an improved method and apparatus for automatically focusing an optical system, for example a microscope, onto an object. The present invention is based on the fact that the information content of an image of an object is a maximum when the optical system is in focus. In information theory contrast, colours etc., may be regarded as information. According to the invention, a method of automatically focusing an optical system onto an object includes deriving a measure of the contrast for each of a plurality of images obtained at different settings of the optical system, and selecting a final setting corresponding to a focussed image as the setting corresponding to the image of greatest contrast, said measure of contrast being derived from the distribution of selected image elements in a grey scale. Apparatus according to the invention comprises an image converter connected to the optical system to reproduce the optical system's image of the object, a grey value measuring device connected to the picture converter for measuring the grey value of a number of image elements appearing in the image converter's image and for arranging the grey values of said image elements in a grey scale, a memory connected to said grey value measuring device to store and count the distribution of said image elements in said grey scale at a particular setting of the optical system, an electrical circuit connected to said memory for calculating a function belonging to the following class of functions ##EQU1## where i is the grey value in a grey scale n is the number of levels of the grey scale P i is the number of picture elements having grey value i w(i) is an always positive or always negative weight function the absolute value of which has the property of being larger in or in the vicinity of the extreme value of the grey scale and smaller at its mean value and generating a signal representing the value of said function at said distribution, a control device being connected to said electrical circuit for searching for the extreme value of said function on the basis of the information given by those values which said function will attain for mutual different settings of the optical system, and a device for changing the setting of said optical system in such a direction that said extreme value of said function is reached. The present automatic focusing method and apparatus may be used with optical systems which operate with electromagnetic radiation in parts of the spectrum other than the visible, for example, in the infrared region. The method may also be used in connection with electron microscopes, such as scanning electron microscopes and transmission electron microscopes. In the case of a so-called transmission electron microscope it may happen that the contrast does not have its maximum value when the microscope is in focus. However, this problem may be obviated by selecting a final setting of the optical system which is displaced relative to the setting corresponding to maximum contrast in the image.

Inventors

• 👤 Bengt Olsen
• 👤 Jens Blomster
• 👤 Nils Buss
• 👤 Bjorn Stenkvist
• 👤 Lucien Preuss

Bibliographic Details