Saturday, October 7, 2017

Can a light be gray?

I follow Quora. I am not saying I am proud of that, but at least I will admit it. I look for questions in my topic of expertise, which is to say, color. And of course, my motives for answering the questions are purely altruistic. Answering the questions is part of my crusade to overcome the general public's ignorance of that noblest of the Sciences, Color Science.

Or maybe I just like to hear myself talk.

Regardless of the reasons, here is a question that I recently answered. My answer has been embellished a bit for this blog post -- mostly to make me sound more important. But I also added some cool pictures.

Is there gray light?

This is an interesting question! I have some experiments that will answer the question.

First experiment

If you connect a white LED to a variable power supply, and gradually turn the voltage down, it will always appear white, even though gray is somewhere between full white and black. I show that in the image below. This is a white LED (color temperature of maybe 6500K) run at 2.4V. This is about as low as it will go without flickering out. The camera clearly sees it as white.

There is no way to suppress the whiteness of this LED

Don't have a variable power supply? You can buy a white LED flashlight and leave it on until the batteries almost run down to nuthin. The LEDs will still be white.

So, first answer: No, there ain't no such thing as gray light. 

Second experiment

But if your arrange those white LEDs into a matrix and call that matrix a computer screen, then you can dim a portion of those white LEDs and get gray. Yes, Virginia, there is gray light, and it's what's coming at you when you look at the image below! Contrary to the guy who wrote about the first experiment, you can make gray light with white LEDs.

You're looking at gray light, right this very minute!!

(I should clarify... some, but not all, computer displays use white LEDs as a backlight behind filters. The idea here is that in principle, you could make a computer display with white LEDs, and you could display gray on that screen.)

Third experiment

Turn your entire computer screen into “gray” (RGB values of 128, for example), and turn out the lights in your room. After a few minutes, you will perceive the screen as white.

I am totally dumbfounded by how white my gray screen looks
Or maybe just dumb? 

Why did that happen? Gray is not a perceivable color all by itself. To see it, you need a white to reference it to.

In the first experiment, the white LED is not putting out a huge amount of light, but the light from the white LED is all coming from a small point. This means that the intensity at that point is very, very high, and likely much brighter than anything else in your field of view.

In the second experiment, I didn’t say this, but it is likely that there are some pixels on your computer screen that are close to white (RGB=255), so the area with RGB=128 will appear gray in comparison. In the third experiment, the only white reference that you have is the computer screen itself, so once your eyes have accommodated to the new lighting, the computer screen will be perceived as white.

Fourth experiment

I came up with a startling way to demonstrate this idea that "gray is perceived only in comparison to a reference white". Brilliant idea, really. I used the same set up I did for that first cool picture of a white LED. But in this case, I used two LEDs, wired in series. Note that I had to crank up the voltage to 4.8V. Electricity passes through each of the LEDs, so in principle they produce the same amount of total light.

The difference between the two LEDs is that the one on the right doesn't have a clear plastic bubble -- the plastic bubble on the LED on the right is a translucent white. The one on the right is a diffuse LED. The light from the diffuse LED is about the same, but it is spread out over a larger area, and not focused, so the amount of light hitting my eye is much less.

My camera saw the diffuse LED as being somewhat dimmer than the one on the left. Maybe from the picture you would call this a gray LED? My eyes looked at the two white LEDs and saw the one on the right as being gray. Honest to God, it was emitting gray light. My eyes saw the LED on the left, and used that as the white reference. The fact that I was drinking heavily during this fourth experiment is largely irrelevant.

Today's special - gray LEDs

So, I can definitively say that "gray" light exists, since I built a system with both a white and a gray LED. I'm sure if I would have introduced this a few weeks ago, I would have gotten an early morning call from Mr. Nobel about some sort of prize. Well, maybe next year. I will try to look surprised.


This blog has to have a moral. It was a bit hard for me to set up an experiment that demonstrated the emission of gray light. Why? Light, when taken in isolation, can never be gray. We only see gray when it is viewed in contrast to another brighter, whiter color.