Editor’s Notes: This is from a discussion on the COG (Concours Owners Group) list and is reprinted with permission of the author; Paul Pencikowski
Jim wrote… “You’re right, it really is a subjective thing. I find that light with more yellow in its spectrum provides more contrast than light with more blue. Especially when it comes to illuminating natural objects, eg. critters, rocks, etc. With more blue, man made objects seem to jump out sooner and brighter, but natural objects seem to fade. Ah don’ know fer shur, but there’s got to be a good reason shooters were amber glasses and not blue.”
First off, Jim, it is *not* “a subjective thing” although *you are 100% correct in your observations*. For instance PIAA “white” really is an improvement and here’s why…
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 more-light-on-target and therefore your resolution improves slightly. *Together* this means YOU SEE A LOT BETTER!
Incandescent is yesterday’s news.
Point: 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. For example among 55w (power-consumption) halogen bulbs, lumens-output can vary significantly among manufacturers.
Yes, very simplistic, and I hope it gets the points across. Riders have often stated they installed a PIAA white bulb and (paraphrase) “saw things better at night”… OF COURSE! Resolution and Contrast Ratio went up measurably. There are people who say the latest, high-quality (expensive) “really white” bulbs are just an “optical delusion and don’t really work”; these people simply do not understand the science.
A short explanation of amber glasses: “blue blockers”…
Blue light, on passing through any lens (a prism in a laboratory, the cornea in your eye) diffracts blue at a far different rate than other colors. So much so that as the reds, greens, and yellows can be focused *simultaneously* on the optic nerve, blue cannot. So everything we see (even those with 20/20) is “slightly out of focus” if there is any blue light in the received image. So “knocking out the blue” makes a slight color-shift in exchange for a *major improvement* in resolution (“clarity of focus”).
P.S. 20/20 just means “very good uncorrected vision for humans”. 20/10 is rare but excellent vision (usually means “farsighted” and these people usually need reading glasses. 20/10 means “sees at 20 feet what a normal person can only see at 10 feet”). Hawks, in comparison, (*gross oversimplification here, for illustrative purposes*) see at about 20/.1 or better.