After seeing the new Cree LED retrofit, which had a glass bulb, I got to thinking, how would it affect the light if they had used neodymium glass? Would the effect be the same as with a regular incandescent bulb? (For those of you who are familiar with Chromalux "Daylight" or GE's "Reveal" bulbs, these use neodymium glass)
Probably it is not be quite the same because LED has a different spectrum than incandescent. Filtering is not just about the overall color, it also effects the spectrum. It could potentially be used to raise the correlated color temperature without making the blue spike bigger. Yes, it would make the LED spectrum even a little more discontinuous, but nevertheless I have a feeling it might slightly
raise the CRI index. The LED phosphor already has too much yellowish frequency light, and to a much lesser extent the neodymium glass also filters out a portion of the yellowish-green, so by reducing these, it would effectively increase the relative proportions of red and blue-green frequency light in the spectrum. Basically, the net result would be equivalent to "flattening out" the yellowish spectrum hump. The bandwidth of the yellow frequency being filtered out is also relatively narrow, so it would not result in too much color shift.
I am inclined to agree with you. It would make more sense to choose the LEDs and phosphors to fit their spectral distribution targets. Using a colored filter in front of the LEDs feels like we're going backwards. I'm no expert in phosphors but I have to wonder why the blue tinted glass is needed when the LEDs themselves are abundant in blue spectral content.
Filters can potentially achieve spectrum changes that standard LED phosphors by themselves are unable to accomplish. Blue-green phosphors certainly exist, but would require violet frequency excitation.
Something else that should be mentioned is that the
amount of filtering can make a difference. For example, with neodymium glass (at least using an incandescent source), a lesser amount of filtering results in an increase in CCT, while excessive filtering results in a
decrease in CCT. This is because, while the neodymium glass readily filters out the yellow, there is also some absorption elsewhere. Once all the yellow has been completely filtered out, additional filtering just starts taking away from the parts of the blue and green. So, if the filter absorbs more than one part of the spectrum, using too much filtering can potentially result in a color change toward a different color.