Area Scanner with «IR-Clean»

Technical Overview

First of all, the device is a film scanner for mounted 35mm slides. The microcontroller based hardware supports an additional infrared scan used for digital dust and scratch removal. For the hardware and software process, we have submitted the application for the word mark «IR-Clean» to the German Patent and Trademark Office. There are two types of series scanners:


area scanner
Fig. 1: Area Scanner with Sony α7R III

Within a 9-month project, we developed the prototype of an area scanner, which was funded by the Bavarian program for the promotion of technology-oriented start-ups (BayTOU). In the course of development, we at Rasch-Diascan decided to combine the Sony α7R III system camera with a specialized industrial lens. Sony is the world's largest manufacturer of image sensors, which are built into many brands in the consumer market and in industrial cameras. It took another 12 months for the software-based process of «IR-Clean» to start with the market launch of an innovative scanning service. The camera scanner developed on this basis has the following technical features:


Laser Auto Focus

Laser Sensor
Fig. 3: LaserSensor for Autofocus
Laser Sensor
Fig. 2: Laser Sensor for Autofocus

When a 35mm slide film is projected onto a full-frame sensor, the corresponding magnification ratio is 1:1. In this case, the calculated depth of field (DOF) is between 0.3 mm and 1.0 mm, depending on the aperture. To meet these specifications an autofocus is required but it is not available in case of a reproduction lens. Our in-house developed solution is a laser autofocus, which is able to detect and correct the deviation of the slide film from the focusing plane with slide mounts of different thicknesses with an accuracy of 10 µm.

Depth of Field in Comparison

USAF-1951 Target
Fig. 4: Slide with Line Pairs “USAF-1951 Target”

The resolution of a slide scanner can be measured using a so-called USAF-1951 target. The slide for the USAF-1951 resolution test contains line pairs of known line widths, which correspond to a certain resolution. Only the small red square on the left is of interest here, which is shown on the right as an enlarged section.


The following images give an impression of the depth of field by showing the results of an increasing defocus aberration. The optical system of the camera scanner works with a magnification ratio of about 0.94 compared to 1.25 for the Nikon LS-5000 ED (Coolscan). According to the mathematical relationship, the result confirms the larger depth of field at a lower magnification.

Line Pairs USAF-1951
Fig. 5: Area Scanner in Focus
Line Pairs USAF-1951
Fig. 6: Defocus Deviation 0.3 mm
Line Pairs USAF-1951
Fig. 7: Defocus Deviation 0.5 mm

Line Pairs USAF-1951
Fig. 8: Nikon LS-5000 ED in Focus
Line Pairs USAF-1951
Fig. 9: Defocus Deviation 0.3 mm
Line Pairs USAF-1951
Fig. 10: Defocus Deviation 0.5 mm

The resolution of the scans is around 5,800 x 3,800 pixels after cropping the black slide-mount-border and is thus clearly above the nominal resolution of 4000 dpi. However, the quality of slide scans cannot be reduced to a single value. More important is the effective resolution, which depends on high-quality hardware and on the workflow as well.

Hardware-Based Restoration with «IR-Clean»

Comparable to an X-ray, infrared light can pass through the colour emulsion of slides, while dust and scratches appear as dark structures in the infrared image. This information can be interpreted by «IR-Clean» to localize and remove defects accurately without affecting the original details or sharpness of the image. Infrared cleaning is by far the most reliable and efficient cleaning method to date. In contrast to software-based dust and scratch removal systems, all infrared cleaning solutions are hardware-based and were previously only available for selected line scanners. The following video explains how «IR-Clean» works:

 

The following video gives an impression of the hardware-technical process when digitising a single slide including infrared scan. The belt-driven actuator moves the laser sensor to a measuring position behind the slide. The sensor detects the position of the slide film, which depends on the thickness of the slide mount. After focusing by the lead-driven actuator, a scan with visible light and another scan with infrared radiation are carried out automatically.

With «IR-Clean» we have designed our own innovative solution, which uses the hardware of a cost-effective area scanner. «IR-Clean» can process all color slide films including Kodachrome fully automated, a unique feature, which sets this technology apart from similar products. The workflow is optimised for high quality post-processing. For this reason, always three files are provided per slide:

 

28.07.2022, Helmut Rasch

 

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