Lighting spectrum refers to a band of colors, like those of a rainbow, produced by the separation of components of light by the different degrees of refraction according to wavelength and which emit an electromagnetic spectrum appropriate for photosynthesis.
In general, different spectrums of light trigger different hormonal / chemical responses from the plant and its photosynthesis; lighting with a blue hue induces vegetative growth while red induces flowering.
When talking about light intensity, there are several metrics that people (mostly lighting manufacturers) use to reference spectrum and intensity, most of these are irrelevant to plant growth / response.
Watts are a measurement of electricity and electricity doesn’t promote photosynthesis.
Lumens – This is a number used to describe how the light appears to the human eye and has no baring on photosynthesis.
PAR or Photosynthetically Active Radiation simply refers to a type of light emitted and does not measure any amount of photosynthesis.
The acronyms that we are interested in however are PPF and PPFD. PPF or Photosynthetic Photon Flux tells us how much light is emitted by the fixture but doesn’t tell us how many photons per second are hitting a given area of the plant(s). PPFD or Photosynthetic Photon Flux Density is the metric which measures exactly how much light is actually touching a specific part of the plant(s), per second. PPFD is shown as μmol/m2/s on PAR meters.