The reason why the UV LED lamp beads have different specifications such as 3535, 4545, 6565 is mainly to meet the needs of different application scenarios, and these numbers represent the size of the lamp beads (length x width, in millimeters). To be specific:

3535 Lamp bead: This means that the lamp bead size is 3.5mm x 3.5mm, which is a more general medium size package, suitable for applications that require a certain brightness but limited space, such as some handheld devices, small area lighting, or where dense arrangement is required. 

4545 lamp bead: the size is 4.5mm x 4.5mm, compared with 3535, its size is larger, can carry higher power chip, provide brighter light output, suitable for higher brightness and better heat dissipation application scenarios, such as indoor lighting, billboard lighting, etc.

6565 lamp bead: the size is 6.5mm x 6.5mm, which is a larger size package, can withstand higher power, provide very high brightness, suitable for remote lighting, architectural lighting, industrial lighting and other fields, at the same time, the larger surface area is also conducive to heat dissipation, to ensure the long-term stable work of LED. 

Differences in the design of different sizes of UV LED beads also affect their luminous efficiency, heat dissipation performance, cost, and installation flexibility. The user selects the appropriate size of the lamp bead according to the requirements of brightness, heat dissipation, space limitation and cost control of the specific application. For example, small devices with limited space may prefer 3535 lamps, while industrial applications requiring strong lighting effects may be more suitable for 6565 lamps.