To ensure that both donor and acceptor proteins maintain their characteristics as part of the BRET fusion constructs, we measured their spectral properties in the 460–720 nm range by bioluminescence imaging of cells using an IVIS-200 imaging system equipped with 20-nm bandwidth filters. Fusion of the donor and acceptor proteins induces no significant change to the spectral signature of these proteins (Fig. 2B and Fig. S1). As anticipated, the emission maximum of cells expressing BRET6 was red-shifted to 640 nm (emission of TurboFP) relative to the 540-nm RLuc8.6 signal (Fig. 2B). The BRET ratios for BRET4.1 (0.55 ± 0.02), BRET5 (0.59 ± 0.04), and BRET 6 (0.58 ± 0.02) (Fig. 2C), indicated similar energy transfer efficiencies. BRET6.1 (0.78 ± 0.04) showed slightly higher BRET efficiency than BRET6 but comparable with that of previously reported BRET3 (0.79 ± 0.01). In contrast, the use of CLZ-v (BRET3.1) provided a slightly lower BRET ratio (0.74 ± 0.02) than BRET3 in accordance with lower spectral resolution. Additionally, the calculated BRET ratios were independent of cell number in the given experimental setting, showing the ratiometric characteristic of the measurements (Fig. 2D). These results revealed that BRET6 and BRET6.1 systems hold the greatest potential for in vivo application because of their high BRET efficiencies, unprecedented red wavelength emission (640 nm), and high spectral resolution (100 and 65 nm, respectively).