Answer: F. 30-60°.
Answer analysis: When discussing the emitting angle of LED straight plug flat head lamp beads, we need to consider their design and application characteristics. Similar to straight plug round head lamp beads, the emitting angle of straight plug flat head lamp beads is also affected by factors such as their packaging method, lens or reflector cup design.
Now, let's analyze one by one according to the given options:
A: 15-20°: This range usually corresponds to high-brightness, strong focusing LED lamp beads, which may be suitable for spotlights or indicator lights for some special purposes. However, it may not be the most common emitting angle of straight plug flat head lamp beads.
B: 90°: A single angle value is not enough to fully describe the emitting characteristics of LED straight plug flat head lamp beads, because their emitting angles may vary depending on design and application.
C: 90-120°: This range covers a wider emitting angle, but it may not be a typical emitting angle of straight plug flat head lamp beads. It may be more suitable for some specially designed LED lamp beads.
D: 120°: Similarly, a single angle value may not accurately describe the luminous characteristics of the straight-plug flat-head lamp beads, although some lamp beads may be close to this angle.
E: 120-140°: This range is more inclined to the large-angle luminous range, which may not be the common luminous angle of the straight-plug flat-head lamp beads.
F: 30-60°: This range is relatively moderate and meets the luminous characteristics of many LED straight-plug flat-head lamp beads. It can provide a certain degree of concentration and ensure a certain range of light distribution, which is suitable for a variety of lighting and indication applications.
Based on the above analysis, the luminous angle of LED straight-plug flat-head lamp beads is usually within a relatively moderate range, so that good lighting effects can be provided in various applications. Therefore, option F (30-60°) best meets the common luminous angle range of LED straight-plug flat-head lamp beads.
Therefore, the correct answer is F: 30-60°.