Marcus Thielen Discusses LED Backlighting

Recently, an old sign guy called me to complain about LED modules. He said, “We switched it on in the shop for testing after we fitted the LED modules, and all looked nice and even, all the same hue of white and the same brightness. Then, when we mounted it onsite, fitted the sign face and lit it, the patchy multiple colors looked like [expletive deleted].”

What happened?
Actually, the LEDs fooled his eyes. More precisely, human eyes are built to see a certain color, but not to distinguish different light spectra that yield the same color impression in direct view. Physically defined, “white” light means a mixture of all colors (or wavelengths) with a weighted intensity. But the human eye already imagines white light when at least two complementary, single wavelengths (blue and yellow, for example) of matching intensity are presented.
Subjectively, it’s the same light impression to the unaided eye, but physically, they’re completely different cases. In the first “white,” all colors/wavelengths are contained, whereas in the second, there are only two small ranges.
White LEDs are – partially – so efficient because they don’t cover all wavelengths. Their light comprises a blue LED semiconductor and a fluorescent material, which converts part of the blue light into the yellow-orange region. Thus, the light emitted from an LED comprises mainly blue and yellow/orange wavelengths, but virtually none in the red and few in the green region.
Which blue color and which yellow/orange is unimportant, as the human eye sees the same white in all cases (intensities were matched to obtain the same hue/color temperature), so the composition of the “white” is up to the LED manufacturer. A typical emission spectrum of two different white LEDs is shown in Fig. 1.
As the sign installer said, everything looked fine when the bare LEDs were tested. But then, the face material gets involved. Here, “white” and colored materials are two different stories, too.
White, signface material diffuses the light in order to distribute it evenly, and to minimize the “hotspot” from the tiny, LED point sources. The scattering diffusion process is usually independent of the wavelength/color of the light (except for the Mie-Tyndall effect in our atmosphere, which scatters blue light more strongly than red, but this depends on the size of the scattering particles). So the color or spectrum stays unchanged by white face materials (Photo 2).
In contrast, colored signface material additionally filters the light, which lets it pass through only specified parts of the spectrum (Fig. 2).
But, if this part of the spectrum, where the face material is transparent/translucent, isn’t represented in the light spectrum emitted by the lightsource, no light will come through the face material. An analogy: Your radio receiver will tune only to one station at a time and let the signals pass through. The FM Band contains multiple stations at the same time, but when the receiver is tuned to a channel where there’s no station, silence is the result.)

The (not) full spectrum
To calculate the resulting spectrum, physicists multiply the emission intensity by the filter’s transparency factor for each individual wavelength.
Let’s consider an extreme example. Put some red LEDs behind green plastic. The green plastic only lets light in the green part of the spectrum pass; the red LED emits only red, and no green. The result is darkness, or a very dim shade of light because the filter doesn’t completely block it.
When the “physically correct” white light shines through a filter, there will always be intensity – because the primary light comprises all wavelengths (including those where the filter is translucent).
However, if we apply the “green” filter from Fig. 2, which only lets wavelengths in the yellow-green region pass, while blocking red, blue and violet in front of a physically correct “white”, the eye will see green. If we use a “white” LED, which emits only a mix of blue and yellow, the blue is blocked and the yellow will pass. Thus, we see yellow, not green. (Photo 3).

Relevance for signs
Practically all LED and/or LED module manufacturers guarantee only a certain color temperature for white LEDs, but not a spectral distribution of the emitted light.
Not all LED chip manufacturers specify a certain spectral distribution and maximum deviation in their datasheets; mostly the spectra are called “typical”, with no binding parameters. The LED-module manufacturer, which selects the LEDs and molds them into modules – and mostly specifies a color temperature for clients, not a spectrum – wants the freedom to change the LEDs’ brand or model without notice. This creates the problem.
If the signface is white acrylic, the color will always look the same. In direct view, the color and the color temperature look the same to the human eye, because the white face material doesn’t filter the light, but only scatters it and changes the light ray’s direction.
But, if the face material will filter the light, different light spectra will yield different spectral intensities to be transmitted (Fig. 3). The color impression of two lightsources, which look exactly the same in direct view, might give completely different color impressions.
If the signshop has mixed different LED modules of the same color temperature with the specification white (and no further spectrum) in a single sign, he will be OK if the face is just white, but the result will be a patchy surface if colored face materials are used.
If the LED module manufacturer changes his LED supplier or LED type without notifying his customers (as has happened with Far East, LED-module products), and the signshop mixes those modules with different production dates, the same problem results. This is what happened in our case.
Make sure every sign is fitted only with the same batch of LED modules to guarantee the color is compatible with the face. Of course, this won’t prevent a patchy appearance if there’s incorrect LED placement or a box that’s too shallow.
The same principles apply for fluorescent lamps or neon tubing. If you have to replace anything in a box sign, replace everything at once. The other lightsources still functioning have aged, and their color and intensity have shifted. And, be sure to use only a single brand and model part.