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Understanding the origins of small-scale flats of CCDs and theirwavelength-dependent variations plays an important role in high-precisionphotometric, astrometric, and shape measurements of astronomical objects. Basedon the unique flat data of 47 narrow-band filters provided by JPAS-{\itPathfinder}, we analyze the variations of small-scale flats as a function ofwavelength. We find moderate variations (from about $1.0\%$ at 390 nm to$0.3\%$ at 890 nm) of small-scale flats among different filters, increasingtowards shorter wavelengths. Small-scale flats of two filters close in centralwavelengths are strongly correlated. We then use a simple physical model toreproduce the observed variations to a precision of about $\pm 0.14\%$, byconsidering the variations of charge collection efficiencies, effective areasand thicknesses between CCD pixels. We find that the wavelength-dependentvariations of small-scale flats of the JPAS-{\it Pathfinder} camera originatefrom inhomogeneities of the quantum efficiency (particularly charge collectionefficiency) as well as the effective area and thickness of CCD pixels. Theformer dominates the variations in short wavelengths while the latter twodominate at longer wavelengths. The effects on proper flat-fielding as well ason photometric/flux calibrations for photometric/slit-less spectroscopicsurveys are discussed, particularly in blue filters/wavelengths. We also findthat different model parameters are sensitive to flats of differentwavelengths, depending on the relations between the electron absorption depth,the photon absorption length and the CCD thickness. In order to model thewavelength-dependent variations of small-scale flats, a small number (aroundten) of small-scale flats with well-selected wavelengths are sufficient toreconstruct small-scale flats in other wavelengths.

The miniJPAS Survey: A Study on Wavelength Dependence of the Photon Response Non-uniformity of the JPAS-{\it Pathfinder} Camera