Digitizing Slides and Negatives – A Color Conundrum?

Posted by ed oboyle on Sun, Nov 14, 2010 @ 05:42 AM

In Search of Consistency

scan photo red boat on blue lakePhotographs captured on traditional slides and negatives are increasingly being reborn and reclaimed digitally for a multitude of compelling reasons.  While converting large collections of slides and negatives into high quality digital images is appealing for most – it requires color management knowledge to increase the chances for optimum results. When scanning slides and negatives, several factors impact ultimate digital image quality.  Chief among them is accurate color rendition across workflow and output devices. 

The Color Management Mission

scan photo with color calibrationCapturing slide and negative images through high-end, dedicated film scanners offers the opportunity to create highly versatile, archive-quality digital images.  But in order to achieve superior results, you must solve a basic problem of digital imaging – color definition.  It seems that despite recent leaps in imaging innovation and digital technologies, the imaging industry has yet to implement an absolute color definition to apply across all imaging devices.  So during the process of scanning slides and negative film (scan photos, view image on monitor, process in image editor, output to viewing device/printed), each step in the basic workflow is dependent upon a device that likely represents color differently.  Without forcing a standard definition of color across your workflow, you risk never attaining “consistent color”.  For example, adjusting an image based upon viewable color on a monitor risks accidentally introducing a color-cast, or worse.

Enter the discipline of color management.  Color management is a process used to interpret and reproduce colors consistently at every point in the digital workflow.  While attaining absolute “true color” may never be possible (largely due to the necessity to convert color gamut (see below)), color management seeks to adjust gamuts as closely as possible.

Some Color Basics for Digitizing Slides and Negatives

As discussed, color definition is highly device dependent.  This means that viewing the color blue on two different monitors is often visibly different.  How can this be? Quite simply it has to do with the choices the manufacturer made around its’ proprietary technology and the platforms it was designed to support. 

digitizing slides with different color spaces, color space graphic depicting RGB, sRGB, NTSC, Adobe RGBColor models are use to describe, classify and offer a method of generating colors.  The primary color models include the Additive Color Model RGB (three basic colors add to white), the Subtractive Color Model CMY(K) (the sum of the three primary colors is black), and Lab (defines exact colors mathematically).

A single device cannot display all the colors visible to the human eye.  Therefore, manufacturers typically select a sub-set of colors from a color model that meet the specific requirements of its’ device.  This subset is known as a Color Space.  Each color model typically has several color spaces.  For example, the RGB model has the sRGB, NTSC (1953), and Adobe RGB (1998) color spaces, among others.

Gamut is the range of colors a specific device can produce and is usually synonymous with color space.  In the typical negative scanning workflow, the first step is to capture the image using a film scanner, then examine the resulting image on a monitor, and ultimately print.  Given the color gamuts of the scanner, the monitor, and the printer – it may seem logical to generate a new color space that includes all common colors.  However, doing so would significantly underutilize the capabilities of each device.  Consequently, this lowest common denominator approach would discard significant information in a scenario whereby the monitor displays fewer colors than the scanner can capture, still the monitor can also generate colors the printer cannot output.  Clearly, using such a restricted color space in this example would eliminate colors that could be scanned and printed – not the intention of color management.

Such problems can be solved by using device-independent color spaces as an interface between devices.  Implementing this approach necessitates accurate transformation between color spaces, which can be enabled by utilization of ICC profiles.

Tools of the Trade: ICC Profiles and Calibration

Lacking a single, industry-wide relevant color space, the ICC (The International Color Consortium) has devised an approach to transform images between different color spaces with the least possible loss of quality.  The common standard it defined is based upon use of “ICC profiles”.  These profiles were created for input, output, and display devices.  For film scanners, an input profile transforms the scanned image into a device independent color space.  ICC profiles are used to compensate for scanner specific deviations from standard target values.  With these profiles a device can be “calibrated” or “profiled” to a standard.

ANSI standard IT8 target used for slide scanning, negative scanning color calibrationCalibrating a slide scanner means deriving scanner deviations from a standard color definition, creating a profile and using that profile to “correct” the color by loading the profile into the scanning software.  The target standard is often defined by an ANSI standard IT8 target.  For negatives scanning, a profile can be used as a means to retrieve color characteristics for a particular film emulsion.

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