The National Institute of Standards and Technology (NIST), Biophysical and Biomedical Measurement Group (BBMG) within the Microsystems and Nanotechnology Division of the Physical Measurement Laboratory (PML) requires an advanced spectral photodetection system for incorporation into NIST's unique and world-class Serial Flow Cytometer for high temporal and spectral resolution optical measurements of biological micro and nanoparticles. This will enable accomplishment of our mission of developing microscale measurement science and technology applied to characterization of biological systems. Flow cytometry is a technology that rapidly analyzes properties of thousands of single cells or particles per second as they flow in a fluid stream. A laser incident on the fluid stream interrogates the cells, which can be labeled with fluorescent markers to enable the quantification of specific cell markers and classification of different cell types within populations. NIST develops a Serial Flow Cytometer (SFC) that uses multiple light sources to repeat optical measurements of single cells to provide uncertainty quantification (UQ). UQ is a valuable outcome that enables optimization of flow cytometers and modeling to improve confidence in flow cytometry measurements and instrument comparability. NIST requires a complete, multichannel and multispectral photodetector system as listed below in the Scope and specified below in Minimum Requirements. The system will integrate directly into NIST's Serial Flow Cytometer to perform sensitive, dynamic, and hyperspectral fluorescence measurements of rapidly flowing micro and nanoparticles including cells, gene therapy nanoparticles, and fluorescence calibration particles. Incorporation of the system within NIST instruments requires compatible connections to optical lines and routing of voltage signals to NIST recording devices. The advanced photodetector system is critical to achieve NIST project missions as it will enable the R&D team to build generalizable, multimodal cytometric tools with world-class deep spectral probing to advance Critical and Emerging Technologies (CETs) in Biotechnology.