ANSI Lumens vs Color Light Output:
The Debate between LCD and DLP
September 19, 2013
You may have noticed that several projector makers now publish a controversial new specification known as Color Light Output (CLO), along with the traditional ANSI Lumen ratings on their spec sheets.
Why do we need two different ways to measure a projector's brightness? In this article we'll explore how Color Light Output differs from ANSI lumens, and what it means to you as the projector buyer.
Does Color Light Output (CLO) matter?
Advocates of the new CLO spec argue that since three-chip projectors and single-chip projectors create white and color values differently, the ANSI lumen spec is not a valid way to compare their brightness. Certainly, if a 3LCD projector and a DLP projector both measure 3000 ANSI lumens (which is a measure of white brightness only), that means they can both project a white test pattern of equal brightness. But, they point out, who watches a white test pattern? Isn't it more important to know how bright projectors are when displaying full color images? And though the 3LCD and DLP both produce 3000 ANSI lumens of white, the color images on the DLP projector will often be dimmer than they are on the 3LCD. The CLO spec, it is argued, takes color brightness into account and gives buyers more info about the projectors they are evaluating.
"Not so fast, there bub," say those who object to the CLO spec. Though CLO measures color brightness, it does not take into account color accuracy. In order to get the highest ANSI lumen and CLO ratings out of a 3LCD projector, you must run all three chips wide open. And if the UHP lamp behind them has a green bias as they typically do, then the white light on the screen will have a green tint. So you may have a projector rated at 3000 ANSI lumens of white light and 3000 lumens of Color Light Output, but the picture looks bad anyway because the color balance is way off. Since the CLO spec does not address color balance, it gives buyers nothing new or important about the projectors they are evaluating.
Meanwhile, DLP engineers can compensate for green lamp bias with a larger red filter, so the white light from DLP is often a cleaner, more neutral white than you'd normally get from a 3LCD projector with all chips wide open. And if you try to calibrate out the green bias on the 3LCD its lumen output drops, sometimes a lot, and the ANSI lumen and CLO specs may no longer be relevant. So how does the introduction of a new brightness spec that is flawed in the same way the ANSI lumen spec is solve anyone's problem?
The thing is, people on both sides of the CLO debate have excellent points. But it is much easier to understand the controversy when you can actually see the differences in actual images rather than just talk about the concepts. So that's what we will show you in this article.
Definitions: ANSI Lumens vs Color Light Output
ANSI lumens: The ANSI lumen spec measures the brightest white that the projector can produce. It is measured by taking meter readings on a projected 100% white test pattern. The number you derive from the readings, say 3000 ANSI lumens, is the maximum brightness of white that the projector is capable of.
Color Light Output ("CLO"): The CLO method is similar, but instead of using a 100% white test pattern, one uses red, green and blue test patterns instead. Separate meter readings are taken for red, green, and blue, then added together. This time if we end up with 3000 lumens, that is the maximum brightness of color that the projector is capable of. So it is called color brightness, or color light output.
Why do different technologies create color differently?
All three-chip projectors, whether DLP, 3LCD, or LCoS, have three independent color channels for red, green, and blue. In order to produce white, all three color channels are turned fully on. So by definition, maximum color light output and maximum white light output are always the same. If a projector measures 3000 ANSI lumens, it will also produce 3000 lumens of Color Light Output.
However, on most single-chip DLP projectors made for business or classroom use, color is defined with a spinning color wheel that has not only red, green, and blue filters, but also a white filter and perhaps some secondary filters like cyan or yellow. The white filter allows white light from the lamp to bypass the color filters and boost the brilliance of the image. By doing this you end up with a very bright white as measured by the ANSI lumen method. But if you measure red, green, and blue independently and add them up like the CLO spec calls for, they don't add up to the total white value. So on a single-chip DLP projector with a white filter in the wheel, the CLO number would be less, often much less, than the ANSI lumen number. Since there is no marketing advantage to the makers of single-chip DLP projectors to quote CLO numbers, they never do.
The big question is...what difference does it make?