Image Processing for Hybrid Processes II – Digitization with Scanner

Although the technological obsolescence is nowadays a serious constrain for a suited digitization, specially with high end level scanners, I will explain some hints and tips useful for the current medium and high level scanners still available.

The first question to be aware about the scanner performance is the actual possibilities to penetrate a given optical density and the range of optical densities available in an unique scanning. It is common to find maximum optical densities up to 4.0OD in the device specifications. This upper limit would be very useful with some old glass plate negatives, often over-exposed and/or over-developed. But the reality is that the useful dynamic range (DR) of a given device is more restricted in between this scale from 0.0OD to 4.0OD.

Beyond this claim of capabilities, the most important way to be concious of the actual device performance is to try it under a suitable test target as the Stouffer Transmission Step Wedge T4110 .

At this point, it is very difficult to advice about the particular steps to follow because of the diversity of options available en each scanner software. Assumed we are dealing with B&W negatives or transparencies, a general list of recommendations would be:

• Scan the Stouffer T4110 test target  with the settings available for Negative B&W film. Repeat the scanning now with the settings for Color transparencies (avoid Color Negative). Compare if the resulting DR is different with the two proposed settings and choose the setting providing the wider one. Use always the better color depth available, 16bit if possible.
• Remember that in order to convert test target steps in DR expressed as Exposure Values (EV) or f/stops, with the Stouffer T4110 each three steps equals one EV or f/stop. In any case and no matter which is the test target employed, the equivalence is always 1EV = 0.3OD.
• After decide if the B&W or the Color setting is the best, try specific fine tuning tools, if they are available in the scanner settings, in order to find even a better option affecting the total resulting DR.

When the best option has been determined, it is time to analyse the linearity of the resulting digital file. A lack in linearity will affect the tone reproduction of all the B&W negatives or positives scanned with these settings. To do that, it is necessary a software to measure the pixel gray value average for each test step and a suitable software where to plot a graph. The sequence with Adobe Photoshop would be:

• Prepare a spreadsheet file with a column of values representing the test target step numbers (1 to 41) or its corresponding OD (0.05 to 4.05).
• With Adobe Photoshop Selection Tool, trace a rectangular selection into the first gray step.
• Apply the Filter > Blur > Average command into the active selection.
• Read out the resulting averaged gray value with the Info Palette.
• Note the gray value and write it in the first cell of at right of the previously written column of values in the spreadsheet.
• Repeat the operation for all the steps of the test target image.
• In a third column at right, calculate the Base 10 Logarithm of the corresponding Gray Values with the formula <=Log10(cell)> or the text indicated in the given spreadsheet to calculate the Base 10 Logarithm⁽¹⁾.
• Select the first and the third columns of values in the spreadsheet and plot a graph (x, y). The resulting plot should be a straight line. Any departure of this straight line indicate a device and/or settings lacking in linearity.
• If there is a lack in linearity, derive from the plot an inverse curve. This inverse curve can be used in the Curves Tool of the scanner software to correct the lack in linearity. Saving the curve, if the software permits it, allows the Correction Curve be recalled each time the scanner is used with the same setting and type of originals to scan.

This procedure is essentially the determination of the device Opto-Electronic Conversion Function (OECF). The OECF provides information about both DR and Response Linearity. With the actual DR of a given device and reading the OD range of our originals on film, it is very easy to decide about the reliability of the chosen device for a given task.

If the necessary DR of our negatives is not covered by the scanner OECF, there is the possibility to find out in the scanner software some special setting allowing for the regulation of the lamp intensity or the exposure during the scanning operation. This is not a common feature but it was present in some scanner software.

A second option for the gray values measurement is the use of the software ImageJ (https://imagej.nih.gov/ij/). This software is free, open source and available both for Windows and MacOS. In its tools palette there are a lot of selection, averaging and measuring tools with the possibility to export the resulting measurements to an spreadsheet format.

In any case, the digitization possibilities with scanner are a bit restricted related with those offered today by the digitization with high end cameras and that will be treated in another post. Another problem with the scanner is the relative long time of irradiation necessary to acquire the digital file, specially with high spatial resolutions. This can be very harmful for delicate films in bad conservation condition. Finally, some transmission scanners are not designed to host glass plates and are restricted to thin and flexible film. Some models, allowing for separate the scanner lid from its hinges, permit the scan of glass plate putting gently the retired lid over the glass plate.

(1) As the test target steps follow a density scale and density is a logarithm related to exposure or transmitance, we cannot expect a straight line plot when this scale is plotted facing to a Gray Value progression, because the latest are a function of transmitance. Then, the simplest way to correctly plot the two magnitudes is to convert OD in Transmitance, or conversely, maintaining the OD scale and converting Gray Values in its own logarithm. The latest conversion is very easy with any spreadsheet.