[sphelps]

While I applaud your skills of posting diagrams exactly how is posting diagrams of bulbs running with different ballasts relevant to bulb life? I also obtain the skills to post diagrams from articles however I will post ones more relevant to the discussion.

First we can look at the same bulb at 0 months and 17 months:

While the AVERAGE spectrum or CCT will shift in one direction the actual plot does not actually shift but rather decrease in certain areas while increasing in others resulting in a new CCT. Typically the intensity of the blue will decrease but depending on the ballast you use the other areas of the spectrum will actually increase maintaining your total PAR but changing the overall color of the bulb.

Here we can see PAR readings of the same type of bulb over time

Clearly shown is a steady par reading but a decrease in CCT

So we can pretty clearly conclude

If the change in spectral output is not a big concern, these lamps can be used for much longer periods of time

So yes the CCT of halide bulbs will change from a result of a change in intensity of different wavelengths but this doesn't mean lower par. Results will vary for different bulbs but proper selection will prevent overly negative results.

The reason you don't see the change in CCT with LEDs is simple. The spectrum is much more defined and concentrated over a smaller area. The intensity simply decreases over time and since it lacks the other wavelengths there is no increase in other sections keeping CCT constant.


So once again the change in PAR is more directly connected to the change of intensity of each wavelength that makes up the Photosynthetically Active Radiation (PAR) and not necessarily the overall change of the Correlated Color Temperature (CCT).

Also there is more to PAR than just red and blue

Photosynthetically Usable Radiation, or PUR, is that portion of Photosynthetically Active Radiation (PAR) that is actually absorbed by photosynthetic pigments. Photopigments found in zooxanthellae of corals include chlorophyll a, chlorophyll c2 and peridinin. Typically, these pigments as a group absorb violet, blue and portions of the green wavelengths, as well as most red wavelengths. Since zooxanthellae can 'tweak' pigment ratios in response to light intensity and/or spectrum (within reason!), we can only generalize the wavelengths associated with PUR - for our purposes, PUR for corals and other animals containing zooxanthellae includes bandwidths of 400-550 nm (violet, blue and some green) and 620-700 nm (red).

http://www.advancedaquarist.com/2009/3/review
http://www.advancedaquarist.com/2008/7/aafeature1
http://www.advancedaquarist.com/2006/8/review2