## 2 Materials and Methods
### Time Sequence of the Irradiation-Acquisition Interleaved Imaging  
The time sequence of interleaving the irradiation and data acquisition for imaging the delayed luminescence is illustrated in Figure 4. Each cycle contained 4 phases of time: (1) irradiation, (2) dark time, (3) acquisition, and (4) post-operation. The four phases were set for the following functional needs of the proposed approach, which were also dictated by the instrument's configurability. Figure 4: The sequence of interleaving the irradiation and acquisition for repeating the delayed luminescence to be acquired over incrementally increased time of duration of acquisition. The irradiation time was fixed at 1 s. The dark time was fixed at 20 ms. The acquisition time ranged from 1 us to 1000 s with 10 equal increments over each decade. The post-operation took approximately 10 s. The total sequence in a non-stop mode takes \(\sim\)2.0 h to complete. Figure 3: Top row: the schematic of the master sample holder and the ways of placing samples in the sample holder. Bottom row: The actual samples in the sample holder that can be bottom-loaded to the sample chamber shown in Figure 2. (**A**) Yeast clusters containing 1–16 grains placed in small cells at a sample platform. (**B**). Dense clusters of yeast placed on top of a sample platform. (**C**) A solid scattering phantom was placed amid the dense clusters of yeasts to produce a laterally heterogeneous sample. The arrows mark the corresponding positions or sample-types between a place in the schematic of the top panel and a spot in the photograph of the bottom panel. 1. The time of irradiation was the duration of switching on the fiber light delivery via the fiber switch. The fiber-delivered irradiation was switched on for 1 s. The duration of 1 s was determined after experimenting with the time of irradiation of yeasts from as short as 1 ms to as long as 100 s. The 1 s duration provided a good balance between adequate signal-to-noise ratio and non-saturated irradiation, as will be shown in one section of the results. 2. The dark time was the time lapse between switching off the fiber light delivery and starting the data acquisition of the CCD camera. A dark time of 20 ms was experimented to be the minimal duration needed to overcome the response lag of the fiber switch to ensure that there was no residual fiber-delivered light to overwhelm the camera once the acquisition of delayed luminescence started. With this dark time, however, there is still a long-lived background afterglow of the fiber-chamber environment to negotiate, as will be revealed in the results. 3. The time of acquisition was the exposure time of the CCD imager after dark time. The following numbers were chosen as the acquisition times: 1 to 9 us at a step of 1 us, 10 to 90 us at a step of 10 us, 100 to 900 us at a step of 100 us,1 to 9 ms at a step of 1 ms, 10 to 90 ms at a step of 10 ms, 10 to 900 ms at a step of 100 ms, 1 to 9 s at a step of 1 s, 10 to 90 s at a step of 10 s, 100 to 900 s at a step of 100 s, and 1000 s. Please note that 1 us was the minimal time of exposure that was configurable for the imager. Additionally, even though the computer user interface of the imager would allow gating operation, the saving function did not allow an autonomous depository of the consecutive imagery that could be acquired by separate but continuous gating of the time sequence of delayed luminescence. In other words, the configurability of the CCD imager interface limited the imaging approach that could be used to obtain the time trace of the delayed luminescence or the time trace of the integration of the delayed luminescence. 4. The post-operation time was the time needed to manually operate data saving and re-engaging the fiber switch for the next sequence of a longer time of acquisition than the one completed after the same period of 1 s of irradiation.