Hello Listers...
Somebody wrote: "...Unless you use a different technology, HID, etc,
you draw 55/60 watts on a halogen bulb, you get 55/60 watts
worth of light."
This statement is fundamentally incorrect, for two reasons. (Science lesson
follows)...
Caveat: In order to keep this post to, say less than 600 pages of
calculations (physics is like that), I have OVERSIMPLIFIED the issue, taking
care not to introduce fallacies...
A) The "watts" refers to current draw, NOT EFFICIENCY. If you used a
filament of 1/8" iron, and it drew 55 watts, it would just barely glow.
Somebody (not me) will have to do the calculations for actual "light output"
(in "lumens", or perhaps in "candela"). Different technologies yield
different outputs. Different executions of the *same* technology (ie
cheap-vs-expensive "halogens") yield different outputs. This, being
important, pales in comparison to point "B"...
B) Spectrum (frequency) Distribution of the various "colors of light"
contained within the output of the headlight. The "technology" does count
(incandescent, halogen, PIAA-white etc) and here's where the plot
thickens...
We "see" things because they reflect light. When we "see" things, there are
two major variables at work, these being (1) Resolution and (2) Contrast
Ratio. Resolution is by far the most familiar, usually being expressed as
"20/20" (20/40 etc) but more commonly as "lines-per-inch" discerned from a
grid viewed at a set distance, under *prescribed light* etc. This is
straightforward and accurate to measure. When the light falls off,
resolution falls too (we simply "can't see" the object as everything
blends-to-black).
However, the really important variable is Contrast Ratio (CR). CR is our
ability to measure "shades of gray" and for example on a Cathode Ray Tube
(CRT, like you're reading this on) it can be between 8 and 10
shades-of-gray. Again, these CRT shades are measured with a pre-set signal
injected into the amplifier, a set light output ("gain") etc. The human eye
is capable of seeing truly huge numbers of individual shades of colors on a
color-palate (like you get at a paint-store). Recall your last visit to the
eye-doc? Black letters on a white background. Maximum CR because CR is
really important in the vision-equation.
Lets explore this CR-thing.
Things are green because they reflect green light. So, if you wanted to see
as many "green things" out ahead of you as possible, you would use a green
headlight. But what if there were red things "out there" that you wanted to
see? With your green headlight, they would appear black because as
red-objects reflect only red-light, and you illuminated them with pure green
light, "no light" was reflected. The green light was ABSORBED by the red
object.
We solve this problem by using a "white headlight". Because our "white"
headlight has light from the entire visible spectrum, green objects reflect
back the GREEN COMPONENT of the light from our headlight, red things the RED
COMPONENT etc.
However, a conventional incandescent bulb, while radiating across the
visible light spectrum, IS BIASED TOWARD RED. The experiment needed to
illustrate this is simple. Turn down the voltage to your headlight (ie
"almost dead battery") and what do you see? A reddish-orange glow. Not "dim
white". Again, note the word "biased"... A conventional incandescent bulb
has "too much red" to yield "pure white" even though it may put out
equivalent "lumens" when compared to another bulb technology.
Halogen is both more efficient ("more lumens out" per watt) and is mildly
less biased toward red than conventional incandescent. PIAA "white" is more
efficient still (even more "lumens per watt" than halogen) and is noticably
biased toward even-distribution of the visible frequencies within the
radiated spectrum (even distribution = "white").
So you "see better" because with all this "whiter light", your ability to
pick out tiny differences between shades of each primary color (reds, blues,
yellows) improves. Same for "mixes" of primary colors (like "fuscia").
(Note: CRT's use as *their* primary colors red-blue-GREEN). In other words,
illuminated with PIAA white, CR improves immensely. More lumens means your
resolution improves slightly. Together this means YOU SEE A LOT BETTER!
Incandescent is yesterday's news.
Final note: There is quite a difference between reflected-light and
direct-view-light. Because of this, there are "cheap *white* bulbs" on the
market that seem to be "the real thing" but are not. They seem "whiter" when
you look *at the headlight itself*. But these cheap white-bulbs do *not*
have the frequency-distribution needed to improve CR. The *expensive-white*
bulbs _do_ produce a frequency distribution biased away-from-red hence
actually "whiter".
And again, all bulb-technologies are not created equal (e.g. 55w halogen
lumens-output can vary significantly among manufacturers).
Yes, very simplistic, and I hope it gets the points across.
To the fellow who stated he installed a PIAA white bulb and (paraphrase)
"saw things better at night"... OF COURSE YOU DID! Resolution and Contrast
Ratio went up measurably. Anybody who says the latest, high-quality
(expensive) "really white" bulbs are just an "optical delusion and don't
really work" simply does not understand the science.
Paul P.
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