re Re: Headlight Hype? or Facts

["Smith, Kirby A" <kirby.a.smith@lmco.com>]

Huw wrote in response to another lister:

The diff between a 80/100 and 55/60 bulb is only just discernable.  I think
light works a bit like sound, you have to quadruple the power to get double
the perceived effect.  85/110 is pretty much the same as 80/100, really.  A
camera's light meter wouldn't be able to tell them
apart for sure.  (or is that "fer shure"?)

- -- 
Huw is right.  The eye is logarithmic in response.  Further, there are
optical laws that can't be violated, in particular, the law of conservation
of etendue, or light throughput.  Using a higher power filament means using
a larger filament, if one wants the same lamp lifetime.  In an imaging
system, or an ideal non-imaging concentrator based system, a larger source
means a larger beamwidth (pattern), and the irradiance on the tree in the
distance will be conserved.  For higher irradiance, the lens aperture has to
be increased in proportion to the filament.

Why are Eurolights brighter - because they better focus the lamp output.
DoT lenses (I use the term advisedly) have more scatter and distortion.  Why
does Phil Payne see a brighter world with his lamp array - partly because of
more aperture, and partly because even the Eurolights are not perfect
non-imaging concentrators or focussing optical systems, and more filament
area leads to more light on the tree.  (Consider a naked lightbulb, a higher
wattage lamp provides more light because it is not imaged on the wall.)  So,
one can improve the pattern to gain irradiance at the target, or increase
wattage to improve irradiance if the imaging is not too good, but if
Eurolights had perfect imaging, or perfect beam forming, a larger filament,
higher power lamp would only make the pattern larger, not "brighter".

Now there is a trade here that has merit for some, and is also addressed on
this list.  Increasing the voltage on a given filament size will both
increase its brightness, and make the lamp a higher color temperature,
moving more of its brightness into the visable.  (Tungsten lamp output peaks
in the near infrared.)  This is why the direct-to-alternator-via-relays
harness scheme is so useful.  Note that the lamp lifetime is reduced faster
than the perceived brightness is increased, but that may be a good trade,
considering that the lamps are nearly the lowest cost replacement items on
our Audis.  Avi Meron's higher voltage alternator has merit for this
purpose.  On the other hand, I have no idea what the effect on the vehicle
electronics lifetime would be at 14.7 volts.  Perhaps the battery keeps this
from being a problem.

                .... Kirby   (Kirby A. Smith)
                              2 x 1988 90q
                          New Hampshire USA