Real-time autofocus scanning

1. A digital scanning apparatus comprising:

an objective lens;

a focusing sensor and an imaging sensor positioned in the optical path of the objective lens to sense a field of view of the objective lens; and

at least one processor configured to adjust a height of the objective lens during scanning of a sample by

selecting a macro-focus point on a sample, wherein the macro-focus point is within a predetermined range of both an edge of the sample and a line representing a maximum length across the sample,

scanning a single field of view that includes the macro-focus point using the focusing sensor to acquire image data that includes the macro-focus point at a plurality of image planes corresponding to a plurality of distance positions of the objective lens from the sample,

determining a contrast value for each of the plurality of image planes,

identifying a true-Z value, Z1, that indicates a distance position of the objective lens at which an image plane having a highest contrast value for the macro-focus point corresponds to an image plane of the imaging sensor,

scanning a plurality of regions along the line representing the maximum length across the sample to acquire a plurality of buffers of image data, wherein each buffer comprises a plurality of scan lines and each scan line comprises a plurality of pixels, such that each buffer comprises a number of rows of pixels equal to a number of the plurality of scan lines and a number of columns of pixels equal to a number of the plurality of pixels within each scan line,

determining a contrast ratio value for each of the plurality of buffers,

identifying a plurality of additional true-Z values, Z2-Zn, corresponding to the determined contrast ratio values for the plurality of buffers, wherein each additional true-Z value indicates, for a respective one of the plurality of buffers, a distance position of the objective lens at which the image plane of the imaging sensor has peak contrast,

storing all of the true-Z values, Z1-Zn, with respective location information, wherein the location information for Z1 indicates a location of the macro-focus point, and wherein the location information for each of Z2-Zn indicates a location of the respective one of the plurality of buffers,

determining a location of a region of the sample to be scanned,

identifying a nearest true-Z value to the region of the sample based on the determined location and the stored location information for the true-Z values, Z1-Zn,

determining a predicted-Z value for the region of the sample, based, at least in part, on the nearest true-Z value, and

adjusting a distance between the objective lens and the sample, based on the predicted-Z value, at a start of scanning the region of the sample.

2. The digital scanning apparatus of claim 1 , wherein determining a contrast ratio value for each of the plurality of buffers comprises determining an average contrast ratio value for each column of pixels in the buffer, and determining a contrast ratio value for the buffer based on the average contrast ratio value for each column of pixels in the buffer.

3. The digital scanning apparatus of claim 1 , wherein the at least one processor is further configured to, after scanning the region of the sample:

determine a true-Z value, Znew, for the region of the sample;

determine a difference between the true-Z value, Znew, for the region of the sample and the predicted-Z value for the region of the sample; and

initiate a rescan of the region of the sample when the difference exceeds a predetermined threshold.

4. The digital scanning apparatus of claim 3 , wherein the at least one processor is further configured to, after scanning a plurality of scanned regions of the sample, if the difference between the true-Z value and the predicted-Z value exceeds the predetermined threshold for a predetermined percentage of the plurality of scanned regions, initiate a rescan of the entire sample.

5. The digital scanning apparatus of claim 3 , wherein the predetermined threshold is within a range of 0.5 to 0.9 microns.

6. The digital scanning apparatus of claim 3 , wherein the at least one processor is further configured to calculate a global sample surface using all of the true-Z values, Z1-Zn, and wherein the predicted-Z value for the region of the sample is based, at least in part, on the nearest true-Z value and the global sample surface.

7. The digital scanning apparatus of claim 6 , wherein the at least one processor is further configured to, after scanning the region of the sample, recalculate the global sample surface using the true-Z value, Znew, for the scanned region of the sample.

8. The digital scanning apparatus of claim 6 , wherein the global surface is continually updated, as new true-Z values are calculated and stored, to reflect all stored true-Z values for the sample.

9. The digital scanning apparatus of claim 1 , wherein the at least one processor is further configured to calculate a local sample surface using a subset of true-Z values for buffers corresponding to regions adjacent to the region of the sample to be scanned, and wherein the predicted-Z value for the region of the sample is based, at least in part, on the nearest true-Z value and the local sample surface.

10. The digital scanning apparatus of claim 9 , wherein the local sample surface is L-shaped.

11. The digital scanning apparatus of claim 1 , wherein the plurality of regions, scanned along the line representing the maximum length across the sample, collectively form a contiguous portion of the sample that extends across the sample.

12. A method for focusing a digital scanning apparatus that comprises an objective lens, a focusing sensor and an imaging sensor positioned in the optical path of the objective lens to sense a field of view of the objective lens, and at least one processor, the method comprising, by the at least one processor of the digital scanning apparatus:

selecting a macro-focus point on a sample, wherein the macro-focus point is within a predetermined range of both an edge of the sample and a line representing a maximum length across the sample;

scanning a single field of view that includes the macro-focus point using the focusing sensor to acquire image data that includes the macro-focus point at a plurality of image planes corresponding to a plurality of distance positions of the objective lens from the sample;

determining a contrast value for each of the plurality of image planes;

identifying a true-Z value, Z1, that indicates a distance position of the objective lens at which an image plane having a highest contrast value for the macro-focus point corresponds to an image plane of the imaging sensor;

scanning a plurality of regions along the line representing the maximum length across the sample to acquire a plurality of buffers of image data, wherein each buffer comprises a plurality of scan lines and each scan line comprises a plurality of pixels, such that each buffer comprises a number of rows of pixels equal to a number of the plurality of scan lines and a number of columns of pixels equal to a number of the plurality of pixels within each scan line;

determining a contrast ratio value for each of the plurality of buffers;

identifying a plurality of additional true-Z values, Z2-Zn, corresponding to the determined contrast ratio values for the plurality of buffers, wherein each additional true-Z value indicates, for a respective one of the plurality of buffers, a distance position of the objective lens at which the image plane of the imaging sensor has peak contrast;

storing all of the true-Z values, Z1-Zn, with respective location information, wherein the location information for Z1 indicates a location of the macro-focus point, and wherein the location information for each of Z2-Zn indicates a location of the respective one of the plurality of buffers;

determining a location of a region of the sample to be scanned;

identifying a nearest true-Z value to the region of the sample based on the determined location and the stored location information for the true-Z values, Z1-Zn;

determining a predicted-Z value for the region of the sample, based, at least in part, on the nearest true-Z value; and

adjusting a distance between the objective lens and the sample, based on the predicted-Z value, at a start of scanning the region of the sample.