Discussion of recent posts

[quk@sievers.com (Phil Payne)]

In message <199701210933.BAA09192@cosmoslink.net> "Meron" writes:

> They were brighter, yes, but no additional illumination of the road
> surface, 

?  Please explain this curious phenomenon.  If a bulb throws light onto the 
road, how can changing the bulb for a brighter one not result in more light 
reaching the road?  This seems to defy the laws of physics.
 
Optical systems (such as Eric's, and my Leicas) can be described in terms of 
Modulation Transfer Function.  This has (thankfully) replaced simple resolving 
power as the criterion by which lenses are judged.  Information content is 
conveyed by resolution and by contrast.  The eye's resolving power is in large 
part a function of illumination - that's why holding something under a bright 
light makes it easier to see detail.  The same is true for headlights - more 
light on a remote object makes it easier to make out the detail.
 
However - contrast is also a factor.  Remember that the lights are mounted 
_below_ the driver's eyeline.  In theory, a flat-topped beam can illuminate an 
object at a distance without illuminating the atmosphere between the _driver_ 
and the object.  A badly designed light will scatter light into this area and, 
by illuminating water and dust particles, will reduce visual contrast.  Even 
with the Euros, you can see a _damn_ sight better in fog with the low beams 
that with the high ones. 

If you install higher bulbs and _perceive_ no improvement in road illumination, 
then this is what is causing it.  A poor beam pattern is spraying light 
everywhere and your eyes are compensating.  If you put a lightmeter at road 
level, you'll see an increase.  You won't actually be able to _see_ any better, 
though.

--
 Phil Payne
 phil@sievers.com
 Committee Member, UK Audi [ur-]quattro Owners Club