There is no such thing as an HID sidelight either. Blue filters actually filter out everything but blue, incidentally.
HID lamps are strange in the way the way they affect visual acuity.
As Waynne says, they do operate at the higher end (shorter wavelength) end of the visual spectrum.
What is less known is that lamps such as these, which are essentially mercury vapour discharge lamps, do NOT emit white light. (They are doped with various metal salts which do influence the wavelengths and add some of their own, but the greater reason for doping is to accelerate the time between switch on and the availability of usable light.) The spectral emission is actually comprised of a number of discrete lines, or wavelengths, of radiation, the sum total of which gives a very good approximation of the colour of white light. But it's not white light.
Sunlight is not comprised of a number of discrete wavelengths of light. It is a continuous spectrum. As is the light from an incandescent lamp. The human eye and visual part of the brain has had far more years working with sunlight than it has with any artificial light.
So, although the HID is arguably and measurably brighter, I am not sure that the actual visual acuity afford by such light is especially usable to the human visual system. I find that some HIDs can be quite fatiguing to drive by, yet the very finest of filament lighting systems are simply stunning.