Information in this article applies to:
Article ID: NDZ1126 — Created: 2 Jan 2018 — Reviewed: 16 Dec 2020
Flat-field correction is an image processing technique that improves digital image quality by removing artifacts and distortions. It compensates for different illuminator levels across the field of view of the camera. An image that has been flat-fielded will exhibit a uniform look (a flat-field).
Without flat field correction, it is easy to see the individual fields in a large, stitched image (as shown in the image to the right). The individual fields, where stitched, may differ by only a few pixel values. But, the human eye can easily discern these minuscule differences.
The problems and solutions described here apply primarily to large stitched images.
Small discrepancies in the illuminator light source cause image fields to vignette. This effect is most apparent in white (empty) fields. While the pixel value at the edges is only a few counts different than values from the center of the field, the effect is quite pronounced in the final, stitched image. Proper, perfect flat field correction eliminates this effect. However, this is sometimes difficult to attain.
A number of things contribute to poor flat field correction:
The first thing to check is that your scans use an objective filter that includes flat field correction. In general, flat field correction should always be applied to the objective camera.
If using an objective filter with flat field correction does not eliminate the repeating pattern, you may wish to try the following suggestions:
The Deep Zoom viewer incorporates an interactive image filter you can use to adjust brightness, contrast, gamma, and level clipping to improve image appearance. The following suggestions may help correct flat field issues:
If you are able to successfully create a good image by adjusting the filter settings in the Deep Zoom Image Viewer, you can apply those same settings to a Deep Zoom Filter you use to create the Deep Zoom Image. You may also use those settings in your Objective Filter when scanning.
Refer to Using Level Clipping to Improve Flat Field for more information.
By design, the flat field correction is calculated for an illuminator level of 35. Significantly higher or lower illuminator levels may negatively affect the flat field correction depending on the slide scanned.