The accurate digitization of film using high-resolution digital cameras, especially historic positive and negative film, presents a difficult challenge for cultural-heritage imaging. Approaches used for reflecting materials—e.g., profiling using color targets—are difficult to apply to transparent materials due to a paucity of film-specific targets, measurement challenges of small patch sizes, and the inadequacy of these targets for historical films and negatives. Research was carried out to design, construct, and verify a new transmission target. Simulation was used to select 80 filters, optimized from a 476-filter set of absorption filters with criteria including colorimetric performance for the 80 filters and four validation spectral datasets, color gamut, and spectral diversity. A prototype target was constructed, measured, and imaged. All criteria were met. Future research will refine the target and validate its performance using independent targets and color-challenging photographs.
An imaging process is described which captures spectral transmittance for transmissive media. The specific application is positive and negative large-format film. The system is based on a ten channel LED backlight source and a monochrome camera. The LED source sequentially back-illuminated reference targets and film samples, with an image captured for each LED channel. From the measured data and images of reference targets, a model was developed to predict spectral transmittance. With that model, the 10 images of a sample were combined to a single 31-band spectral image. Spectral images can be used to calculate colorimetric data for each pixel. These colorimetric results show that the system produces good colorimetric predictions when compared to the most relevant FADGI guidelines. Some improvement is required for the spectral model particularly in the red region.