Introduction
The term "dynamic range" refers to the ratio between the darkly shaded and lightly shaded areas of a transparency. In an image scanner, it refers to the degree to which the device is able to capture and reproduce nuances in transparencies of varying optical densities, such as film, slides or negatives.
In its Technical Guidelines for Digitizing Cultural Heritage Materials, the Federal Agencies Digital Guidelines Initiative specifies minimum dynamic ranges for varying levels of image quality, from 3.5 for basic quality to 4.0 for the highest level. Dynamic ranges approaching 4.0 are often difficult to achieve in many scanners, and thus it is important to understand what techniques can improve dynamic range and to what degree these techniques are successful.
Some image scanners have a multi-exposure feature that, when used with the appropriate software, increases the exposure range when scanning transparencies. This is done by making passes at lower exposure levels to capture nuances in brighter sections, then additional passes at higher exposure levels to capture nuances in darker sections. These details are combined to produce a single HDR image.
Unlike a smartphone, the multi-exposure feature on a scanner takes a significant amount of time to complete subsequent passes, which can slow down workflow.
This publication focusses on how scanning techniques and other variables affect dynamic range. The research involved testing various features, using two types of transparency scanners and two of the most common scanning software applications capable of using the multi-exposure feature.
- To what degree does the multi-exposure feature on a scanner improve dynamic range?
- How is dynamic range affected by other common scanning settings?
- Is the additional investment in equipment, software and workflow time worth the effort?
Methodology
The ISO 21550:2004 protocol was followed to measure the dynamic range of two different scanners, using two different image scanning software applications for two different parameters: multi-exposure and infrared dust and scratch removal.
In all cases, the dynamic range test target ST-53-1 was used. The following scanners were tested:
- A flatbed scanner with a backlit lid, hereafter referred to as the flatbed scanner
- A slide and film scanner, hereafter referred to as the slide scanner
The following scanning settings were tested:
- A multi-exposure feature with a yes or no setting
- An infrared dust and scratch feature with a yes or no setting
Combined with two commercially available third-party image scanning software applications, the above combinations yielded a total of 16 tests, with each test comprising 10 trial scans.
Test protocol
Scanning
For each test, the following protocol was used:
- Set environmental conditions: dust-free environment, room temperature 23-24°C, relative humidity 35-40%
- Prepare the hardware: clean film tray and scanner surface
- Prepare the test target: clean ST-53-1 target and load onto scanner
- Prepare the software: set multi-exposure, infrared features, resolution, bit depth and file format
- Produce trial scans: 10 trials per test as per ISO 21550:2004
Colour sampling
GIMP version 2.10.12 was used to sample RGB values from each greyscale patch. Nearly 13,000 colour values were recorded over the course of this experiment.
Interpretation of data
Dynamic range is the ratio between Dmin and Dmax discernible by a scanner. Mean luminance is used to determine both values as per ISO 21550:2004.
Observations
| Test | Scanner used | Scanning software used | Use of multi-exposure | Use of infrared scanning | Dynamic range |
|---|---|---|---|---|---|
| 1 | Flatbed | 1 | No | No | 3.03 |
| 2 | Flatbed | 1 | No | Yes | 3.65 |
| 3 | Flatbed | 1 | Yes | No | 3.42 |
| 4 | Flatbed | 1 | Yes | Yes | 3.88 |
| 5 | Flatbed | 2 | No | No | 3.57 |
| 6 | Flatbed | 2 | No | Yes | 3.47 |
| 7 | Flatbed | 2 | Yes | No | 3.65 |
| 8 | Flatbed | 2 | Yes | Yes | 3.65 |
| 9 | Slide | 1 | No | No | 3.16 |
| 10 | Slide | 1 | No | Yes | 3.16 |
| 11 | Slide | 1 | Yes | No | 3.53 |
| 12 | Slide | 1 | Yes | Yes | 3.74 |
| 13 | Slide | 2 | No | No | 3.29 |
| 14 | Slide | 2 | No | Yes | 3.27 |
| 15 | Slide | 2 | Yes | No | 3.31 |
| 16 | Slide | 2 | Yes | Yes | 3.58 |
Discussion
Observed effects of multi-exposure scanning on dynamic range
On average, dynamic range increased by 0.27 when a multi-exposure feature was used. Multi-exposure scanning increased dynamic range regardless of the hardware, software or infrared settings used.
Observed effects of infrared scanning on dynamic range
Dynamic range increased by an average of 0.18 when using infrared scanning. When infrared scanning was combined with multi-exposure scanning, the average dynamic range improved from 3.26 to 3.71, a 0.45 increase.
Conclusion
Multi-exposure scanning increases dynamic range, often by a full level of quality specified by FADGI technical guidelines. Infrared scanning also appears to improve dynamic range by reducing noise levels. Using both features is likely to be justified in most workflows.
Acknowledgements
- Ember Lundgren, Royal BC Museum and Archives
- Cassandra Tavukciyan, Canadian War Museum
- Chloé Lucas, Chloé Lucas Conservation
- Bruce Covington, Document Imaging Solutions Center, Public Services and Procurement Canada
Glossary
- Dynamic range
- The degree to which a scanning device is able to capture and reproduce nuances in transparencies of varying optical densities.
- Multi-exposure scanning
- A process whereby an image scanner takes multiple scans at different light exposures to produce a composite image.
- Dmax
- A numeric value expressing a scanner's ability to detect image information in the darkest sections of an image.
- Dmin
- A numeric value expressing a scanner's ability to detect image information in the lightest sections of an image.
Bibliography
- Federal Agencies Digital Guidelines Initiative. Technical Guidelines for Digitizing Cultural Heritage Materials, 3rd ed. Washington, D.C.: FADGI, May 2023.
- International Organization for Standardization. ISO 21550:2004, Photography — Electronic scanners for photographic images — Dynamic range measurements , 2004.