## 1 Introduction  
Holography [1, 2] is a technique utilizing interference of light to record a complex amplitude distribution of an object wave. The recorded information is called a \"hologram\". A three-dimensional (3D) image is reconstructed from the hologram by utilizing the diffraction of light. Holography can be used to record and reconstruct a 3D image of an object or a phase distribution of a wave without having to use multiple cameras or an array of lenses. Furthermore, 3D motion-picture images of any ultrafast physical phenomenon (such as light pulse propagation in 3D space) can be recorded and reconstructed with a single-shot exposure [3, 4]. Digital holography (DH) [5, 6, 7, 8, 9] is used to record a digital hologram that contains an object wave and reconstructs both the 3D and quantitative phase images of an object by using a computer. DH can potentially be applied to the fields of not only ultrafast optical 3D imaging [10] but also microscopy [6, 11, 12], particles and flow measurements [13], quantitative phase imaging [14], lensless 3D imaging with incoherent light [15], multidimensional bio-imaging [16], multiwavelength 3D imaging [17], depth-resolved 3D imaging [18], simultaneous recording of multiple 3D images [19], and encryption [20]. Since a full space-bandwidth product of an image sensor is available, phase-shifting DH [21; 22; 23] is one way to capture an object wave. Using phase-shifting DH with multiple wavelengths, which is termed color/multiwavelength phase-shifting DH, 3D surface-shape measurements with multiwavelength phase unwrapping [24] and lensless color 3D image sensing [25; 26] have been reported. DH using red-, green-, and blue-wavelengths is usually called \"RGB digital holography\", \"three-wavelength DH\", and \"multiwavelength DH\". In this paper, we call DH with multiple wavelengths \"two/three-wavelength DH\" or \"multiwavelength DH\" because DH using two-green- and one-blue-wavelengths was simulated. In regard to multiwavelength phase-shifting DH, two types of representative implementations have been reported: temporal division [24] and space-division multiplexing [25; 26] of multiple wavelengths. In the case of temporal division, wavelength information is sequentially recorded by changing the wavelengths of light to form a hologram. Mechanical shutters or operations to turn the light sources on and off are required for selecting the recorded wavelength. Three phase-shifted holograms are required at a wavelength [27] and nine holograms are needed for three-wavelength DH. Therefore, temporal division requires much time for multiwavelength 3D imaging. In the case of space-division multiplexing, red-, green-, and blue-wavelengths are simultaneously recorded by using a color image sensor with a Bayer color-filter array. Three exposures are required to obtain a multicolor holographic image. However, both recordable wavelength bandwidth and space-bandwidth product are determined by the array and therefore spatial information and wavelength selectivity is partially sacrificed. In the case of space-division multiplexing, crosstalk between multiwavelength object waves occurs when the wavelength selectivity of the array is insufficient [28]. The field of view (FOV) and spatial resolution of the DH system are decreased by the array due to the sacrifice of the space-bandwidth product of a hologram at each wavelength. The FOV is decreased by 75% compared to phase-shifting DH with a single wavelength. In the case of color/multiwavelength digital holography, not only temporal-division [24] and space-division multiplexing [25; 26], which are generally adopted for multiwavelength imaging in an imaging system, but also spatial division [27; 28; 29], temporal frequency-division multiplexing [30; 31; 32; 33], and spatial frequency-division multiplexing [34; 35; 36] can be merged to record multiple wavelengths. In the case of general imaging systems, wavelength information is temporally or spatially separated.