In macroscopic imaging, painted color targets are often used to calibrate and characterize cameras and scanners. For image capture under magnification, such as with a loupe or microscope, determining the color ground truth becomes more challenging because such targets are spatially non-uniform (rough) under magnification. An interference coating, color mirror target is more advantageous to use under magnification compared to a traditional painted target because mirrors are smooth under magnification. Photolithographic techniques can be used to create small and very advanced color mirror arrays so that many colors can be captured in a single image, no matter the magnification. The small arrays will have a similar reflected spectral performance when compared to a larger sample from the same fabrication cycle. Sedgewick, “Improving Color Consistency, Color Integrity and Consequent Speed in Reading Slides,” DataColor, 2014, 28 Apr. 2014, which is hereby incorporated by reference in its entirety. Photolithography is not compatible with paint. Paint consists of many sized particles that do not create homogeneous color under magnification and there can be shadowing or contamination that changes the color in localized regions. Color mirrors are fabricated under vacuum on smooth, specular surfaces reducing the probability of particulate contamination. The mirror's specular surface allows one to image their colors with only a small fraction of the light necessary compared to painted samples, allowing for shorter exposure times. Paint is susceptible to environmental conditions and will fade over time; color mirrors do not fade. Interference coating designs can create spectral reflectance profiles of any shape imaginable for not only the visible wavelength region, but also the ultraviolet and infrared regions, allowing one target to be used with a variety of camera sensors. They can also be used to create highly chromatic color mirrors. This ability expands the color gamut of a target, which is useful when imaging objects, such as bird feathers or butterfly wings. This expansion is an additional advantage compared with painted samples. There are many coatings incorporating color mirrors and metals such as automotive finishes and nail polish where the expanded target is more appropriate for instrument calibration and verification. Lastly, interference coatings can be used to create an ideal training target for spectral reflectance reconstruction in multispectral imaging applications reducing the sample number significantly from those using paint. N. Tsumura, H. Sato, T. Hasegawa, H. Haneishi, Y. Miyaki, “Limitation of color samples for spectral estimation from sensor responses in fine art painting,” Optical Review, 6, pp. 57-61 (1999) which is hereby incorporated by reference in its entirety. One issue with these color mirrors is their spectral sensitivity to Increasing incidence angle. The spectral reflectance will shift to lower wavelengths at higher Incidence angles. Fortunately, there are design techniques that can be utilized to minimize the Angular sensitivities of interference coatings. J. D. T. Kruschwitz, “designing non-polarized High reflecting coatings within immersed high-index media,” in optical interference coatings, Page tub3; optical society of america (2001); j. D. T. Kruschwitz, r. S. Berns, “non-Polarizing color mirrors on a high reflecting metal base,” appl. Opt. 53, no. 16, pp. 3448-3453 (2014), which is hereby incorporated by reference in its entirety.
For transmission microscopy, an array of colored gels or interference bandpass filters has been used successfully as color targets. Sedgewick, “Improving Color Consistency, Color Integrity and Consequent Speed in Reading Slides,” DataColor, 2014, 28 Apr. 2014; Y. Yagi, “Color standardization and optimization in whole slide imaging,” Diagn Pathol, 6, Suppl 1:S15 (2011), which is hereby incorporated by reference in its entirety. These filters are not as convenient to use in an off-axis reflection system because they are flat and specular, and these microscopy systems are typically used to image diffuse reflected color. Light sources that are placed at 45° incident to a flat, specular sample will have their light reflect off the surface at the opposite 45° never entering the microscope lens.
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