“Believe none of what you hear and half of what you see.” —Benjamin Franklin
When I was in college, I had a crush on a pretty girl from my hometown. One evening, I looked up her number in the phone book and called. A soft, feminine voice picked up. I said hello, told her who I was, and after some small talk, I nervously asked if she would like to go to a concert. She said yes. It was music to my ears.
A few days before the show, I ran into her on campus. “I’m looking forward to the show this weekend,” I said. Came her reply; “What show?” Confused, I recounted the telephone conversation we had the week before. But she had no recollection of it. Furthermore, she said, she already had a date for the concert. The music in my ears was replaced by confused silence.
I ran back to my house and looked up the phone number again. I quickly dialed the number and another pretty, but different voice picked up. Her telephone greeting was simply three Greek letters. It was a sorority house. I had been duped. Somewhere, some sorority girl was laughing so hard that she probably split her designer jeans. Ha, ha.
From that lesson I learned two things; I’m not particularly fond of sorority humor, and you can’t always believe what you hear. To this day, I try not to take anything I hear at face value.
One of the things you often hear when lighting and video techs get together is that double stacking two video projectors doesn’t double the brightness of the display. Is that the truth or another vicious rumor started by maniacal sorority girls?
As easy as this is to prove or disprove, it’s a bit surprising that there are so many questions about this. The basic premise behind it is that footcandles are not additive. Intuitively, it goes against the most fundamental law of the universe, that energy is conserved. After all, if you stack 100 footcandles on top of 100 footcandles and you get anything but 200 footcandles, where do the missing footcandles go?
The answer is that light is additive. You can prove it by taking any two lighting instruments, measuring each one individually, and then stacking them and measuring the result. You should get the sum of the two.
As a lighting guy, I didn’t have to think too long before concluding that the double stacking projectors should double the brightness. As a part-time imaginary scientist, I had to test my hypothesis. The only light sources to which I happen to have convenient access at the time were two Chroma-Q Color Block 2 LED heads. So I broke out my Minolta T-10 Illuminance Meter and the LED fixtures. I turned one on in full white light and focused it on the wall about 12 feet away. I measured 165 lux. I then turned that head off and checked the other; it also measured 165 lux. When I turned them both on, I reasoned, I should read 330 lux. But I was wrong! I actually measured 285 lux.
I was perplexed. So I did what any good lighting professional would do – I called someone smarter than me. Since I’m a product of the Texas public school system, I could pretty much randomly pick someone out of the phone book and they would probably be smarter than me, provided it wasn’t one of my classmates. But in this case I called my friend and neighbor Mike Wood (www.mikewoodconsulting.com). If Mike doesn’t know the answer, it’s probably not a solvable problem.
After a short conversation, I realized that there’s a lot more going on than meets the eye (or the meter). Since LEDs are driven by pulses of current, they are constantly flashing on and off at a very high rate. Depending on the sample window of the illuminance meter, the timing of pulses, and the pulse width, the results of the meter reading will vary. If, for example, the pulses are completely out of sync with each other and the “on” times never overlap, it’s possible that the illuminance would never exceed that of a single fixture. If, on the other hand, the pulses were completely synched, then the meter could read double the illuminance of a single fixture depending on the sampling period of the meter. The odds are that the LEDs will be somewhat out of sync but the pulses would overlap to a degree, leading to a reading somewhere between that of a single fixture and twice that of a single fixture. The same holds true of discharge lamps that are powered by magnetic ballast power supplies. (Electronic power supplies have a higher frequency than the meter sampling rate and the meter is more likely to read more accurately.)
Now consider the variables in a typical video projector. An LCD projector uses a discharge lamp and a switch-mode power supply with a relatively high frequency, typically in the range of tens to hundreds of kilohertz. A DLP projector also uses a discharge lamp and a switch-mode power supply, but the DMD switches the light on and off very quickly. Now throw in the sampling window of the illuminance meter and all of these factors could result in a reading anywhere between the illuminance of a single projector to twice the illuminance of a single projector.
Perhaps a better question might be, could you even tell if a surface was lit twice as bright as another? Probably not. The human eye is a marvelously complex instrument. It sees not by absolute measure, but by evaluating contrast. If you stack two light sources it’s very difficult to distinguish with the naked eye how much brighter it is than a single source. When I worked for a lighting manufacturer, R&D was constantly trying to squeeze more light out of new fixtures, but I could never see the 50 percent increases we were constantly being told about. The only way to really tell is to use an illuminance meter, and most video people (or lighting people for that matter) don’t carry them around. They’re not cheap, and what you see and perceive is what really matters.
What ever happened to the pretty girl? I’m glad you asked. I seldom ran into her for the rest of my college career and I never worked up the nerve to ask her out again. About ten or twelve years after we left college, I saw her in a grocery store in north Austin. She was pushing a shopping cart with a baby in the seat and two young children following closely behind. After some small talk she told me that she was happily married to a guy with whom we both went to high school and who happens to be an audio engineer. I told you she was pretty, but I never said she was smart.
Thanks to Jeff Monner of Talking Tree Creative for suggesting the topic and re-opening an old wound. -ed.
