Being the overly analytical type of engineer that I am, I looked up the specifications of the family of Sylvania bulbs
:link: and I was surprised that the visible light output (luminous flux) in Lumens is exactly the same for the SilverStar Ultra, SilverStar, XtraVision and Standard halogen bulbs. Raw light output is called radiant flux and is a measure of the total electromagnetic radiation that a light emits. However, since the human eye has a limited spectral sensitivity range, i.e. ultraviolet and near infrared light is imperceptible, luminous flux is the radiant flux with each element of the spectral band weighted by the sensitivity of the eye to that band. Therefore luminous flux is the measure of a lights output that is visible to the human eye. Since luminous flux is by definition a measure of the perceived brightness of light source by the human eye, how can one light be any brighter that another when they are all rated at the same luminous flux?
I was starting to think that it was all just marketing BS, however based on antidotal evidence (i.e. complaints to the NHTSB about blue lights causing more glare) I thought there must be a reason that everyone seems to believe the higher effective operating temperature (i.e. whiter lights) appear brighter. So I talked to one of our optical science experts who pointed out that in low light conditions the cone cells are nonfunctional and our vision relies exclusively on the rod cells. Rod cells are most sensitive to wavelengths around 498 nm (blue-green light) and are insensitive to wavelengths longer than about 640 nm (red). This is called Scotopic
:link: vision as opposed to Photopic vision and it occurs at light levels between 10[SUP]-2[/SUP] to 10[SUP]-6[/SUP] cd/m[SUP]2[/SUP]. Technically, I believe when our headlights are on, our eyes are operating in Mesopic vision (10[SUP]-2[/SUP] to 1 cd/m[SUP]2[/SUP]), which is a combination of Scotopic and Photopic vision, because in true Scotopic vision we have no ability to discern colors, but the sensitivity shift towards the blue end of the spectrum still occurs, just to a lesser magnitude. This explains why whiter or bluer lights appear brighter. Lumens are based on the Photopic luminosity function, however in Mesopic vision our eyes see based on the Scotopic luminosity function which favors the blue end of the spectrum.
In addition, since the Department of Transportation (DOT) limits the Lumen output of lights, whiter lights or lights with more output in the blue end of the spectrum can have higher radiant flux and still comply with the DOT limits in Lumens due to the Photopic weighting function. In other words there really is a Scientific reason that the pure white, higher effective operating temperature lights, like the Sylvania SilverStar Ultra appear to be brighter than the standard “Yellow” halogen bulbs. The downside of the SilverStar Ultra and SilverStar is the life of the low beam filament is only 327 hours compared to 1500 hours for the Standard halogen bulbs (22% of the standard bulb life) and the high beam filament is 75 hours versus 250 hours for the standard (30% of the standard life). I also noted that the claimed life of the SilverStar and the SilverStar Ultra is the same, so the only advantage of the SilverStar over the SilverStar Ultra appears to be the price. On the other hand the benefit of the Ultra over the SilverStar should be negligible based on the difference in effective operating temperature.
However, once you get into the very blue and violet portion of the spectrum, you have gone past the optimum point and the light output will appear less bright. Therefore, the blue and violet lights are just for looks.
I’ve ordered and received the SilverStar Ultra’s so I’ll be installing them any day.