## 4 Discussion  
Light emitters such as LEDs, TV and other electronics are presently used daily. Also, photodynamic therapy and fluorescence microscopy (FM) uses light to cause-effect on biological samples. Despite the widespread of FM in cell research, the effect of light on basic cell features was not well characterized yet. In this study, we evaluated the effect of blue light (485 nm) on cell parameters such as proliferation  
Figure 6: **Quantitative phase-related parameters of dying A2780 and G361 cells. ****(a)** Example of quantitative phase images for A2780 and G361 cells in different time-points under blue light dose 208 mJ/cm\({}^{2}\times 1000\) ms. Scale bar = 50 μm (**b**) Real-time changes in cell density of A2780 and G361 cells during 24 h of the experiment. (**c**) Real-time changes in cell dry mass of A2780 and G361 cells during 24 h of the experiment. (**d**) Real-time changes in Cell Dynamic Score (CDS) of A2780 and G361 cells during 24 h of the experiment. CDS shows the average intensity change of cell pixels. activity, cell motility, and cell mass in three human cancer cell lines (A2780, PC-3, G361) and one non-malignant human cell line (PNT1A) using MHM. The light doses in our study were 24 or 208 mJ/cm\({}^{2}\) and were achieved by a combination of two different times of expositions 500 and 1000 ms and specific light intensity (Figure 1). In the last 15 years, holographic microscopy hand in hand with advanced image analysis made significant progress to become a suitable technique for conducting label-free time-lapse experiments with biological specimens. Our results show that blue light 208 mJ/cm\({}^{2}\)\(\times\) 1000 ms negatively affect malignant cell motility and does not change the motility of benign PNT1A cells. [PERSON] et al. have described the positive effect of 470 nm light on the motility of T-cells. Enhanced motility may be caused by the H\({}_{2}\)O\({}_{2}\) signaling pathway [1]. Our finding follows [PERSON] et al., where authors concluded that blue light inhibits cell migration, thus invasivity of cancer cells. This may be mediated by the effect on MAPK and NF-kB activity and down-regulation of the ERK1/2 signaling pathway which leads to a decrease in protein expression of MMP-2 and MMP-9 [40]. On the other hand, [PERSON] et al. postulated that blue light gradient could have an impact on cancer cell migration. According to their study, ROS production is proportional to light exposition. Cancer lung cells A549 exposed to 473 nm blue light gradient reported higher ROS production but also higher motility driven by increasing light gradient [41]. However, this study employed a gradient of light which may have a different effect on cells as the light dose on cells varied during the exposition. It was shown that blue light generates oxidative stress [42], induce cell damage and affects the gene expression [43]. Our results also show that blue light significantly affects cell proliferation, in most cases negatively. Although some light doses such as 24 mJ/cm\({}^{2}\)\(\times\) 500 ms in A2780 and PC-3 cells showed some proliferative effects these effects showed no statistical significance. Some proliferative effects of light have previously been described. [PERSON] et al. registered a positive effect of light on the proliferation of endothelial cells and fibroblasts but those results are not comparable with our results since a low-intensity laser with wavelength 665 nm was used [44]. The higher light doses affect cell proliferation more than lower ones. The least harmful dose for our cells was 24 mJ/cm\({}^{2}\)\(\times\) 500 ms, therefore this dose can be used for fluorescence microscopy of these cells. [PERSON] et al. [45] described the reverse effect of light on cell proliferation, where blue light exposition leads to the activation of caspase-3 and PARP inducing a caspase-dependent apoptotic pathway [45]. To correctly evaluate the effect of the blue light used in our experiment, it is necessary to correlate with cell mass distribution.