Improving IronOCR Accuracy for Missing or Garbled Text

Name: IronOCR
Brand: Iron Software
Availability: InStock
Rating: 4.86 (101 reviews)

Updated: September 17, 2026

OCR accuracy varies with the input document: its resolution, text size, layout, image quality, and preprocessing configuration. A setup that works well for one document may not produce the same results for another. When IronOCR returns missing, incorrect, or garbled characters, work through the steps below.

Solution

1. Check the PDF Rasterization DPI

When loading a PDF, IronOCR rasterizes each page before running OCR. By default, LoadPdf() uses 200 DPI. For documents with small or dense text, raising the rasterization DPI can improve character recognition.

Load the PDF at 300 DPI:

using IronOcr;
var ocr = new IronTesseract();
using var input = new OcrInput();
input.LoadPdf("document.pdf", 300);
var result = ocr.Read(input);
Console.WriteLine(result.Text);

using IronOcr;
var ocr = new IronTesseract();
using var input = new OcrInput();
input.LoadPdf("document.pdf", 300);
var result = ocr.Read(input);
Console.WriteLine(result.Text);

A higher DPI gives the engine more detail, but it also increases memory usage and processing time. That makes 300 DPI a useful starting point when the default resolution falls short. If 300 DPI clears up the recognition issue, you may not need any additional image preprocessing.

2. Do Not Rely on the Overall Confidence Score Alone

An OCR result can carry a reasonable page-level confidence while still containing an incorrectly recognized character or field. A page with thousands of correctly recognized characters may still misread a single important number or identifier without noticeably lowering the overall page confidence.

Treat confidence scores as a general indicator of OCR quality, and validate critical fields separately when accuracy matters.

3. Apply Preprocessing Only When Needed

If a higher DPI does not resolve the issue, inspect the source document and apply filters that match its condition:

using IronOcr;
var ocr = new IronTesseract();
using var input = new OcrInput();
input.LoadPdf("document.pdf", 300);
input.DeNoise();
input.Deskew();
var result = ocr.Read(input);
Console.WriteLine(result.Text);

using IronOcr;
var ocr = new IronTesseract();
using var input = new OcrInput();
input.LoadPdf("document.pdf", 300);
input.DeNoise();
input.Deskew();
var result = ocr.Read(input);
Console.WriteLine(result.Text);

Common preprocessing options include:

DeNoise() for noisy or grainy images
Deskew() for tilted or incorrectly aligned scans
Binarize() for documents with poor foreground/background contrast
EnhanceResolution() for low-resolution input
Sharpen() to improve text edges

Filters modify the input image, so avoid applying every one by default; unnecessary preprocessing can reduce recognition accuracy on already clean documents. For digitally generated or high-quality PDFs, raise the rasterization DPI first before adding any preprocessing.

4. Try Advanced Scan for Difficult or Structured Documents

For more complex documents, IronOCR provides Advanced Scan methods powered by PaddleOCR. These help with noisy images, screenshots, structured documents, passports, license plates, and other inputs where the standard Tesseract-based OCR does not give satisfactory results.

using IronOcr;
var ocr = new IronTesseract();
using var input = new OcrInput();
input.LoadPdf("document.pdf", 300);
var result = ocr.ReadDocumentAdvanced(input);
Console.WriteLine(result.Text);

using IronOcr;
var ocr = new IronTesseract();
using var input = new OcrInput();
input.LoadPdf("document.pdf", 300);
var result = ocr.ReadDocumentAdvanced(input);
Console.WriteLine(result.Text);

Other specialized methods include:

ReadDocumentAdvanced()
ReadLicensePlate()
ReadPassport()
ReadPhoto()
ReadScreenShot()

Please note: Starting with IronOCR 2025.11.31, IronOcr.Extensions.AdvancedScan was combined into the main IronOcr package. On recent versions you do not need to install a separate IronOcr.Extensions.AdvancedScan package to use these methods. On older versions, refer to the documentation for the Advanced Scan package requirements for that release.

5. Weigh the Accuracy vs. Performance Trade-off

A higher DPI generally gives the OCR engine more detail, but it also demands more resources. For example:

input.LoadPdf("document.pdf", 300);

input.LoadPdf("document.pdf", 300);

This can improve recognition of small characters compared with the default 200 DPI, while also increasing:

Processing time
Memory consumption
Rasterized image size

For applications processing many documents, test different DPI values against representative samples and pick a setting that balances accuracy and performance.

Recommended Troubleshooting Order

When OCR output is inaccurate, work through it in this order:

Increase PDF rasterization DPI: start with 300 DPI.
Test the document again without unnecessary preprocessing filters.
Apply only the preprocessing filters that match the actual condition of the document.
Validate critical fields independently rather than relying on page-level confidence alone.
Try ReadDocumentAdvanced() or another Advanced Scan method for complex or structured documents.

If the issue persists, contact Iron Software Support and provide:

Your IronOCR version
Target framework
Operating system and architecture
A minimal reproduction code sample
The source document or a representative sanitized sample
The expected text and the actual OCR output

This information lets support reproduce the behavior and recommend the most appropriate configuration for your document type.

Curtis Chau

Chat with engineering team now

Technical Writer

Curtis Chau holds a Bachelor’s degree in Computer Science (Carleton University) and specializes in front-end development with expertise in Node.js, TypeScript, JavaScript, and React. Passionate about crafting intuitive and aesthetically pleasing user interfaces, Curtis enjoys working with modern frameworks and creating well-structured, visually appealing manuals.

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