MadSci Network: Biophysics

Re: Why is it that it is tough to focus on blueish LED-style Christmas lights?

Date: Sat Mar 20 09:42:06 2010
Posted By: Kenneth Mitton, Associate Professor of Biomedical Sciences
Area of science: Biophysics
ID: 1260838515.Bp

The blue light problem is something that many companies with lighted or neon signs 
inadvertently use for marking their stores; they are hard to focus on and to me,
annoying! The 
blue Xmass tree lights are the same! Drives everyone crazy. Well, drives me
nuts. Maybe you as 
well? The problem is simple, because the human eye cannot focus a clear image
for that light. 
This is due to a problem called "chromatic aberration". A lens, even one in a
camera or 
magnifying glass, will gather light reflected by an object and focus it at a
certain focal distance 
after it passes through the lens. If you take Lens, and white piece of paper,
you may notice that 
you can focus the image of your classroom windows (on a sunny day) onto the
paper. You have 
to get the magnifying lens to just the right distance from the paper. (note if
you use a 
magnifying glass the windows will look upside down on the paper) That is the
focal length of 
that lens, where the image comes to a sharp focus. Light coming from one part of
the window is 
focused by the entire lens; some light hits near the outer edge of the lens and
is focused onto 
the paper. Other light enters near the center of the lens and is also focused on
the paper. When 
the image is in focus, and sharp, the light hitting the edge of the lens is
focussed to the same 
point as the light hitting the center of the lens. Thus, you can see a nice
small image of your 
classroom windows. Most high quality man-made lenses, and the lens in your eye,
are able to 
focus all of these different colors of light to the same focal length, so the
image seems very 
sharp. Most lenses, also including the eye's lens, cannot focus blue light
hitting the edge of the 
lens and blue light hitting near the center of the lens to the same spot. This
makes the parts of 
the image in the blue spectrum look fuzzy and never sharp. That is why some
company signs 
and blue Xmass tree lights always look annoying and fuzzy. If you take your
magnifying glass 
and look at some magazine pictures, you can notice that the images you see a the
edge of the 
lens start to have red or blue edges, but they look ok in the center of the
image. You may see 
the same effect looking in a microscope. The objects in the middle look their
natural color, and 
if you move them to the edge of the microscope field of view, you see a colored
edge forming on 
them. This is due to the chromatic aberration. Man made lenses tend to suffer
from this in the far 
red and far blue visible light ranges. The human eye focuses light using your
cornea, the 
aqueous humor and lens of the eye. Light is focused on your retina. Human eye
optics do better 
than man made lenses; they focus the reds just fine, but they still have trouble
with the blues. 
That is why blue lights will never look focused, and that kind of blue is a
foolish choice for store 
signs at night. It is also a foolish choice for cell-phone or car dashboard
night lighting for the 
same reason. Blue light is also why Blue-blocking (yellow) glasses reduce glare
and make it 
easier to see on a sunny bright day. They have yellow lenses, block the blue
light, taking out the 
poorly focussed blue part of the scene, so everything looks sharper. Many Winter
Biathletes (x-country ski and target shooting) wear yellow glasses. Their
targets look sharper, 
and more in focus when they are target shooting during their events. I used to
wear clip on 
yellow lenses when flying aircraft many years ago for the same reason. It made
is much easier to 
spot other planes in the airspace around me, and increases safety. You can find
diagrams of the 
eye at

Kenneth Mitton, PhD
Associate Professor of Biomedical Sciences
Eye Research Institute
Oakland University
Rochester, Michigan

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