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Federal Agencies Digitization Initiative Still Image Working Group — August 2010 - SIGNAL - Engineering Metric SFR - Spatial Frequency Response — ( ISO 12233, ISO 16067-1, ISO 16067-2, ISO 15524 ) MTF — Modulation Transfer Function definition : & spatial frequency descriptor of an imaging system's ability to maintain the relative contrast of input stimuli Resolution S Acutance Flare Evaluation Criteria (un/ts) Possible w=Secondary tertiary @=Primary failure causes Oz units: dots-per-inch, pixels-per-inch “= Poor calibration = technique » Wrong choice of units at calibration based on Luminance SFR units: { unit tess) @: Min/Max 10% spatial frequency limits of Luminance SFR | units: dpi, cycles/mm units: % SFR response ww: Min/Max 50% spatial frequency limits of Luminance SFR units: dpi, cycles/fmm - Poor (auto) focus - Poor optics - Poor choice of aperture stop ~ Mechanical vibration - Over aggressive noise control ww: Sign of SFR slope units ; positive/negative slope value - Over aggressive sharpening settings - Insufficient signal to amplify - Thinking that if a little is good then more must be better. as weighted by an appropriately chosen visual contrast function. units: TBD | - Optical performance | exceeds sampling rate urvts: (unit fess) Definition: The Definition: An imaging Definition :Amplificaton of the SFR | Definitior: An objective Definition: a skirty | Definition: The distance reciprocal of the system’s ability to resolve by means of image processing to SFR based metric that is or wide spreading | along the optical axis that ; center-to-center finely spaced detail. acueve sharper appearing images used as a correlate to of light. remains within acceptable distance between The level of spatial detail that perceived image focus. Closest adjacent can resolved in an image sharpness, pixels. The number of samples per unit distance. i - Megapixels ~ Blurred - Oversharpening ({ haloing, garish - Sharp | — Low contrast - Depth of field @ | = Dots per inch (dpi) | - Soft edges) | = Hazy - Circle of confusion 28 - Pixels per inch (ppi) | = Sharp - Snap | — Ghosting - Focus tolerance 5 « Sampling frequency | — In/Out of focus - Edgy, Sharp, Crisp | = Veiling flare - Hyperfocal distance 3 F = Spherical aberration - Edge enhancement | + Glare —) ~ Spatial detail - Unsharp masking - Integrating cavity effect (ICE) a @: The number of | @: 10% sampling efficiency | @: Max SFR gain @: Area under the SFR | @ % Flare - @: Distance along the optical axis that remains in acceptable focus units: inches, mm. - Dirty lens - Light source directed into lens - Poor quality lens ~ Stray light —— | | & i ~ Poor F-num Technical Guidelines for Digitizing Cultural Heritage Materials: Creation of Raster Image Master Files 10