I don't doubt at all that colors similar to the filter *appear* lighter, BUT- It's a contrast illusion. The similarly-colored areas appear lighter in comparison to dissimilarly-colored areas, but there is actually less light getting to the eye from all areas, even the similarly-colored regions. The filter does not increase flux regardless of color; glass absorption actually reduces throughput of the filter color slightly. I verified the issue with my light meter. In no case does the reading increase when a filter is introduced. But the effect of contrast enhancement is nonetheless incredibly useful at the eyepiece; the brain's ability to interpret a contrast enhancement as a brightening works in our favor. The same thing happens with narrowband LPR filters. Overall, the entire image is darkened slightly, even the object glowing at the filters bandpass wavelength. But since the background (analagous to non-similarly colored regions with colored filters) darkens much more, the increased contrast is interpreted by the brain as a brightening of the object. On 5/10/07, Kurt Fisher <fisherka@csolutions.net> wrote:
Here's an easy demonstration of the similar-color-lightening effect.