Real-time autofocus focusing algorithm

A digital scanning apparatus comprising:

• a stage configured to support a sample;
• an objective lens positioned for viewing a portion of the sample, the objective lens defining an optical path;
• an imaging sensor optically coupled with the objective lens, wherein the imaging sensor comprises a plurality of imaging-sensor pixels configured to capture image data via the optical path, and wherein all of the plurality of imaging-sensor pixels are within a single first image plane that is orthogonal to the optical path;
• a focusing sensor optically coupled with the objective lens, wherein the focusing sensor comprises a plurality of focusing-sensor pixels configured to capture image data via the optical path, wherein the focusing sensor is logically tilted with respect to the optical path such that each of the plurality of focusing-sensor pixels is on a different image plane than all of the other focusing-sensor pixels, and wherein a parfocal position on the focusing sensor is parfocal with the first image plane;
• and
• a processor configured to automatically adjust a distance between the objective lens and the sample, during scanning of the sample, by
• receiving image data from each of the plurality of imaging-sensor pixels, and determining a contrast value for the image data for each of a plurality of imaging-sensor positions on the imaging sensor,
• receiving image data from each of the plurality of focusing-sensor pixels, and determining a contrast value for the image data for each of a plurality of focusing-sensor positions on the focusing sensor,
• for each of a plurality of pairs of corresponding imaging-sensor and focusing-sensor positions, calculating a contrast-value ratio of the contrast value for the image data for the focusing-sensor position in the pair to the contrast value for the image data for the corresponding imaging-sensor position in the pair,
• identifying a peak value of the calculated contrast-value ratios,
• identifying a contrast-value ratio associated with the parfocal position on the focusing sensor as a parfocal value,
• determining a distance and direction between the peak value and the parfocal value, and
• adjusting a distance between the objective lens and the sample, during scanning of the sample, in accordance with the determined distance and direction between the peak value and the parfocal value.