I guess the fuss is more from the manufactures then ? Does the Light types effect the PAR levels then ? I notice in one of your PFK articles you mentioned levels of 30-75 ?
Different tubes types will have varying PAR levels but the difference is really not worth worrying about in the home aquarium environment.
Tubes that are labelled "plant growth" tend to be rather pink/purple. These may have marginally higher photosynthetically usable radiation (PUR) per watt (different to PAR) due to their peaks in the red and blue parts of the spectrum i.e. 400-550nm (violet, blue) and 620-700nm (red). The Arcadia Plant-Pro and older Original Tropical tubes are good examples.
Some plant tubes have a very high red peak with almost no blue at all i.e. Dennerle Special-Plant and Hagen Flora Glo (3000K, 2700K respectively). I find using these tubes exclusively produces a gawdy/too warm rendition, but it's a matter of taste.
Have you seen the pinned topic at the top of this sub-forum? -
It doesn't go into PAR but gives a good indication of the differences you can expect to see in terms of aesthetics from different Kelvin-rated tubes.
Any of the combinations would grow plants effectively, which is why I always suggest choosing lamps from an aesthetic and cost perspective.
When I get the time I would like to test out different lamps to compare PAR readings, but until then I can rest assured that any lamp I choose will grow plants effectively providing all other aspects are ok i.e. nutrients and circulation.
For the record I prefer 6500K to 9000K because it tends to really pop the greens, so looks great in most modern planted tanks. Much lower than that and the rendition is too warm/yellow for my taste. Much higher and it's too cool/blue. This is why a combination is also popular, as inferred by Tom in his recent post.
One of the drawbacks to lighting =>6500K is that most lamps tend not to bring out reds too well. A good example of this is the ADA NA-Green lamp. They are 8000K, and are great for green and blues, but their red rendition is very poor.
Relatively expensive lamps with more phosphor coatings can help, as they can peak in red, green and blue parts of the spectrum. These are known as full-spectrum lamps and are usually around 6500K, which explains why you may have made the association between planted tanks and 6500K. The less expensive full-spectrum tubes are usually tri-phosphor. I think the most I've heard of is five-coated phosphor tubes. You will pay a premium for these. Whether or not their PAR or PUR is higher, I do not know. The only example of these I know of is the Giesemann Powerchrome Midday 6000K T5.
LED is another story that goes beyond the scope of this thread.