Answer: F. 30-60°.
Answer analysis: When discussing the emitting angle of conventional SMD LED lamp beads, we need to consider their design and application scenarios. SMD LEDs are widely used in various lighting and indication fields due to their small size, easy installation and high efficiency.
Now, let's analyze them one by one according to the given options:
A: 15-20°: This range is usually associated with highly concentrated LED lamp beads and may be suitable for special applications that require precise pointing or high brightness, but it is not a typical emitting angle for conventional SMD LEDs.
B: 90°: A single angle value may not be enough to accurately describe the emitting characteristics of SMD LEDs, because their emitting angles may vary depending on the design and packaging method. However, 90° as a specific value may not be the most common emitting angle range.
C: 90-120°: This range is relatively wide and may be suitable for some SMD LEDs that require medium to wide light distribution. However, it may not be a universal emitting angle for all conventional SMD LEDs.
D: 120°: Although the emission angle of some SMD LEDs may be close to 120°, this single value may not be sufficient to fully describe the emission characteristics of conventional SMD LEDs.
E: 120-140°: This range is more inclined to the large-angle emission range, which may not be the typical emission angle of conventional SMD LEDs unless there are special considerations in the design.
F: 30-60°: This range is relatively moderate, and considering that SMD LEDs are usually used in occasions that require a certain light distribution but do not require too wide lighting, this range may be more in line with the common emission angle of conventional SMD LEDs. It can provide a certain light concentration and ensure a certain light distribution range.
In summary, although the emission angle of SMD LEDs may vary depending on the specific design and application, based on general LED lamp bead design experience and optical principles, we can reasonably infer that option F (30-60°) is a more reasonable range of the emission angle of conventional SMD LED lamp beads.
Therefore, the correct answer is F: 30-60°.