Colour brightness: What it is, and why it's important

As you may have noticed, there's a (relatively) new spec for projectors in town, which is referred to as colour brightness, or colour light output, or simply CLO. Whatever you call it, this spec has an excellent pedigree as part of the Information Display Measurements Standard version 1.03 (IDMS 1) dated 1 June 2012.

IDMS 1 was developed by the International Committee for Display Metrology (ICDM), which is part of the Society for Information Display (SID), in cooperation with the Video Electronics Standards Association (VESA). All of which makes it a pretty definitive standard.

As you may also have noticed, Epson recently began touting its LCD projectors as having three times brighter colours than "leading competitive projectors." The claim also applies more broadly to any projector using 3LCD technology, which means a light engine with three LCD chips, with one each for the red, green, and blue primary colours.

The competition Epson is referring to is the universe of DLP projectors that use a single DLP chip (which I'll refer to simply as DLP projectors). The claim is based on the typical difference in colour light output between a 3LCD projector and a DLP projector that both have equal brightness in terms of ANSI lumens, which measures white light output. The problem with the claim is that although it's both true and meaningful, it's also easy to misunderstand.

Why three times as bright isn't three times as bright

The first issue is that brightness is actually the wrong word, although almost everyone uses it this way (and I'll continue to use it through the rest of this discussion). Technically, however, brightness refers strictly to perception, so how bright something is means how bright it looks. The intensity of light that's coming out of the projector is more properly called illuminance.

The difference matters because the perception of brightness is logarithmic. Triple the illuminance of a projector, and the same size image will look brighter, but nowhere near three times as bright. So what Epson is actually claiming is that its 3LCD projectors typically deliver colour with three times the illuminance of DLP projectors with the same ANSI lumen rating, not three times the brightness in the technically correct definition of brightness.

A second semantic issue is that you can also use brightness as a term for describing colour quality. In fact, the hue-saturation-brightness colour model uses brightness as one of the three parameters for describing any given colour, with a vibrant yellow, for example, qualifying as brighter than a less vibrant version of the same yellow hue.

What makes this second issue particularly problematic is that a projector's colour brightness actually does tell you something about its colour quality. However, the relationship between the colour brightness and colour quality is complicated. Because of that, I will ignore issues of colour quality for the rest of this discussion, and cover them separately. What I'll concentrate on here is the difference in colour brightness measurements between 3LCD and DLP projectors.

Where brighter colours come from

The reason for the difference in colour brightness between the two technologies is easy to understand. Projectors with 3LCD technology simply add red, green, and blue together to create white. So if you measure the lumens for red, green, and blue separately, and then add them up, the total will be the same as the industry-wide ANSI lumen measurement for white.

In contrast, DLP projectors create colours by projecting one colour at a time, in sequence. The vast majority use a rotating colour wheel, shining light through colour panels on the wheel. Almost all boost their brightness for white light – which is what ANSI lumens measures – by adding one or more additional panels beyond red, green, and blue, typically using some combination of white (a clear panel), cyan, and yellow.

If you measure the brightness for red, green, and blue with these projectors, and then add up the measurements, the total will be less than the measurement for white by however much the additional panel or panels add to the total ANSI lumen measurement.

Different DLP projectors use different colour wheels, with a variety of both arrangements of colour panels and proportion of colour panel colours they use to create various colours on screen. Even with a given projector, the proportions change depending on the colour mode you're using, which is why colours look different with different colour modes.

This means that the ratio between white light output and colour light output will vary from one DLP projector to another, and even from one colour mode preset to another on the same projector. In one set of tests with business projectors in the 2000 to 3500 lumen range that the 3LCD Group provided, the actual level of colour light output as a percentage of white light output for any given DLP projector ranged from about 20 to 60 per cent. My own tests have produced an even larger range, with colour light output running from about 20 per cent to well above 80 per cent of the white light output for any given projector.

On average, however, the 3LCD Group says the ratio between white brightness and colour brightness is about three to one, compared with one to one for 3LCD technology. All of which is the basis for Epson's claim of offering three times brighter colour levels.

A real-world example

To get an actual example of the difference between an LCD and DLP projector with the same ANSI lumen rating, I tested one of each. Both were rated at 3000 lumens, and both came in a little higher on my tests, at 3087 lumens for the LCD projector and 3198 lumens for the DLP projector. For projectors in the 3000 lumens range, the roughly 110 point difference is far too little to be noticeable. In fact, it's within the error range for the tests.

The difference in colour brightness, however, was huge, with the LCD projector delivering essentially the same number for colour brightness as for white brightness, and the DLP projector coming in at just 680 lumens, or about 21 per cent of its measurement for white light.

I also measured the brightness for both in the best colour quality mode for each, using Cinema mode for one and the equivalent Movie mode for the other. Both projectors came in at about the same brightness for white light for these modes too, at about 2000 lumens, and here again the LCD projector offered the same results for both white brightness and colour brightness. The DLP projector did relatively better on colour brightness compared to its brightest mode, but still came in at a much lower number, with colour brightness at 794 lumens, or 39 per cent of its white brightness for Movie mode.

The practical difference in brightness

What these results confirm is that, as a practical matter, if you're showing primarily black and white images like word processing documents or spreadsheets, DLP and LCD projectors with the same ANSI lumen measurements will both be equally bright. For colour images however, whether PowerPoint slides or photos, the LCD projector will be brighter.

To prove the point, I set up the two projectors side by side and took photos, shown below, of some of the DisplayMate images we use for testing as well as some additional images. In each case, I've also included two versions of each photo, with the original in colour followed by the same photo modified in Photoshop to remove the colour information. The second version makes it easy to focus specifically on brightness, without your sense of brightness being affected by colour.

Keep in mind that because camera sensors don't see brightness or colour quite the way the human eye does, and because colours and brightness can also change somewhat depending on the computer monitor you're using, the third generation images as you see them aren't quite the same as the originals that show on the projector screen.

In particular, on my computer screen at least, white shows as a touch brighter for the DLP projector, even though I measured the two as essentially equal brightness. That said, the photos are close enough to the originals to be reasonably good representations of what the originals look like.

The first set of photos below shows both black text on a white background, and white text on a black background.

As you can immediately see in the second version of the photo, the image from the DLP projector on the left is easily as bright as the image from the LCD projector on the right. In fact, the DLP projector's image is a little brighter in this case, primarily because the white characters on black are thicker with the DLP projector than with LCD projector, and the black characters on white are thinner, making the overall image brighter.

The photos below show what happens with a colour image.

Here again, there's little brightness difference for white, but a substantial difference for each of the colours. On the other hand, also note that if you compare the relative brightness for the red, green, and blue areas, the DLP projector is a lot more than one fifth as bright as the LCD projector to the human eye, even though it's colour brightness is only 21 per cent as much.

These differences in image brightness are a major part of the reason why colour brightness is worth knowing about when you're trying to determine a projector's brightness level, which in turn determines how big an image you can use in a given level of ambient light. The other major reason colour brightness matters is because of the issue I mentioned earlier, namely that colour brightness can also tell you something about colour quality. To my mind that's actually far more important than the question of brightness as such, but I'll save that for a separate discussion. Stay tuned.