Answer: F, 30-60°.
Answer analysis: When considering the emitting angle of LED direct-insert concave lamp beads, we need to realize that this design may affect the propagation and distribution of light. The concave design may mean that there is a certain depression on the surface of the lamp bead, which may change the emission direction and angular distribution of the light.
Now, let's evaluate 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 applications that require precise pointing or high brightness. However, the concave design does not necessarily lead to such a small emitting angle unless there are additional optical elements to further concentrate the light.
B: 90°: It is difficult for a single angle value to accurately describe the emitting characteristics of LED direct-insert concave lamp beads, because their emitting angles may vary depending on the design and packaging method.
C: 90-120°: This range is relatively wide and may be suitable for some applications that require medium to wide light distribution. The concave design may affect the scattering of light, but it does not necessarily lead to such a wide emitting angle unless the design itself is designed to achieve wide-angle lighting.
D: 120°: Similarly, a single angle value may not be enough to accurately describe the luminous characteristics of the concave lamp beads.
E: 120-140°: This range is more inclined to the large-angle luminous range, which may not be the typical luminous angle of the direct-insert concave lamp beads, unless there are special considerations in the design.
F: 30-60°: This range is relatively moderate, and considering that the concave design may have a certain focusing or scattering effect on the light, this range may be more in line with the common luminous angle of LED direct-insert concave lamp beads. It can provide a certain degree of light concentration and ensure a certain range of light distribution.
It should be noted that since the specific form and depth of the concave design may be different, the luminous angle may also vary. However, 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 luminous angles for LED direct-insert concave lamp beads.
Therefore, the correct answer is F: 30-60°.