About a year ago, my trusty clock radio died. I replaced it with a cheap digital clock that had fairly large numbers in a blue LED display. I hated that damn clock. Had enormous difficulty reading it from across the room. Finally, the buttons on the thing stopped working–whole clock was poorly made, I think. So I replaced it with another clock radio, with much smaller numbers in a red LED display. The red numbers are approximately half the size of the blue ones, yet I can read them easily from across the room, and even while walking down the hall. As far as I can tell, my vision hasn’t changed in the last year.
Why would it be so much easier to see the smaller, red LED numbers as opposed to the larger blue LED numbers?
I asked my Dad about this (he’s is/was a physicist).
The wavelength for the blue LED is such that the human eye has trouble resolving it. We had to chuck out our blue LED alarm clock for exactly this reason - or some reason it was uncomfotable to look at in the dark.
I will google to see if I can find confirmation but I’m sure a Doper Physicist will be able to explain better.
Physicists don’t typically deal with retinal efficiency. You’d probably want a Doper Opthalmologist to explain the ability of the eye to perceive different wavelengths of light.
Actually, Optical Physicists do it all the time. the average eye response to wavelength is even used as one of the three standard “colors” of the CIE Chromaticity Diagram. The eye’s response is pretty basic to the branch of optics called Colorimetry
The human eye exhibits a fairly large chromatic aberration, meaning that when red is in focus, blue is out of focus. This might help to explain why blue is harder to resolve
As one of the resident physicists (though not one of the best ones) I offer two points:
Blue focuses differently, though this isn’t such a problem unless you are trying to focus red and blue together. A point source of purple light illustrates this nicely - your eye generally focuses to make the red sharp and leaves the blue as a haze around it. But if it’s isolated blue things against a dark background this isn’t so important.
Much more significant to the OP, you don’t have the retinal ability to resolve blue very well. It isn’t the sensitivity, it’s the density of blue receptors or how they are wired. This makes blue text on a black background hard to read, and yellow text on a white background hard to read (in these two cases it’s only in the blue that letters and background differ).
Speaking empirically blue is perceived to be (in my experience) brighter (and more irritating) than red or green in a clock LED format. Blue looks cool in the store, but in a dark room it’s a whole different story.
I don’t believe these are correct – if you only have blue LEDs in your field of view, you’re going to be trying to focus on blue, and not on other colors, so the chromatic aberration is beside the point.
And the “density of blue receptors and how they are wired” is what determines the sensitivity to blue, so the sensitivity is the point. Look at the plot of sensitivity on the page I link to – it’s way down for blue, which is why you have a harder time seeing it. In a similar way, the sensitivity id greater=st for green, and if you look at greed and red laser pointers of the same output the green one looks much brighter. This is entirely a function of the sensitivity.
The OP didn’t specify whether this was in the dark or not.
Since he said “walking down the hall” I assumed that he meant with the lights on, in which case chromatic aberration might play a significant role. If it’s in the dark, the LEDs certainly emit enough light to be visible, so it’s not likely to be a sensitivity issue.
The difficulty is when the lights are off–like when I want to glance at the clock while lying in bed. The blue numbers on that clock were damn near impossible to read without much squinting.
With the red numbers, it doesn’t matter–lights on, lights off, walking/lying down/whatever…much smaller numbers, much easier to read.
On a totally dissimilar note I got one that works as an iPod dock, and I just use my phone as an alarm clock.
This one is white, and I swear even at the lowest brightness I could almost read from the light of those damned things. I actually prop something in front of the display so that I can get the room dark.
Have you been tested for tritanopia? I haven’t obtained a formal diagnosis yet, but I’m clearly seeing blue colors differently than normal. On a single hard-copy test plate I couldn’t read the number in the dots. There are variations of ‘blue color-blindness’, so I don’t know what I have exactly, or how much perception of the color I’m lacking, but what you describe would apply to me.
If this is the case, I would expect near-sighted to have problems with one of either red or blue, and far-sighted people to have problems with the other.
FWIW, I’m a little near-sighted, and have noticed problems focusing blue lights at night.
Anecdotal; One of the chains common in malls (Spencer Gifts maybe?) for a long time had a bright bluish-purple electric sign over their door. I could never get my eyes to focus on it properly. Now, I have one of the very common semi-color-blindness sort of things, so maybe that was adding to it.
