what is the lighting term <150% THD?

Answer 1

THD refers to "Total Harmonic Distortion".

In lighting, there is concern over the use of different ballasts to create "dimmers'. Some are less costly than others (to produce), but may cause more problems when installed (by creating harmonics).

A typical light fixture (incandescent or otherwise) introduces a relatively low level of harmonics to the power system ... somewhat less than 20 percent, usually. However, as the light is "dimmed", that harmonic content can rise quite high - often above 160 percent.

Giving a limit of "less than 150 percent THD", as you've suggested in your question is basically allowing a lot of garbage to be fed back onto the system.