TVs vs. projectors: the difference between illuminance and luminance
If you have no idea how to compare a projector's brightness output to a TV or any other emissive display, fret not; we were in the same boat when we started developing our projector test bench. What's a lumen anyway, and more importantly, how many nits (cd/m²) is it?
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The answer is a little frustrating. You can't directly convert ANSI lumens to nits since they measure two completely different things: illuminance and luminance.
A TV's brightness is measured in luminance (nits or cd/m²) because you're looking at the light source itself, which is constrained within a fixed area. Manufacturers don't use luminance for a projector, with good reason: it's a meaningless metric without context. We can measure luminance by taking a reading of the image being displayed on the projection screen, but its value will only be representative of the testing environment and equipment used. The figure to the right shows the difference between luminance and illuminance.
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ANSI lumens
The main driving factor behind the purchase of a projector over a TV is their ability to display content in larger sizes. A projector delivering a high amount of lumens is key to doing so. There's no way around it for projectors: the larger the image you display, the lower the screen brightness (luminance) since the light source is fixed no matter how small or large you make the image. It's one of the reasons that manufacturers don't rate their projectors' brightness output in terms of luminance but rather illuminance—the amount of light that falls on a surface per unit area.
The most widely used units are ANSI lumens, the standard unit for measuring light output from projectors as specified by the ANSI standard IT7.228-1997. Testing and calculating ANSI lumens is a straightforward process, allowing for a quick and efficient comparison of projectors in brightness.
To calculate ANSI lumens, one needs to use an illuminance meter, take measurements in nine predefined areas, average them, and multiply them by the screen area; it's as simple as that! The picture below demonstrates how ANSI lumens are measured:
A word of caution: it's essential to remain vigilant when a projector has its brightness quoted in lux or "LED lumens," as these values are essentially meaningless. They do not indicate how much light is projected from the lens but rather how much light is emitted at the source inside the projector before all the internal losses occur.
But wait, can ANSI lumens be converted to nits?
How do lumens translate to brightness? Simply put, there's no definitive answer without context, as the luminance in nits will be relative to the projected image size and the projection surface. It's possible to get reference values as long as the projectors are tested in the same setup, which is, fortunately, exactly what we do!
For example, our Epson Home Cinema 3800 delivers ~2300 ANSI lumens of light once calibrated. In our test environment, it gets insanely bright over a 55" projection size (~860 nits), and we still measured a solid 260 nits when projecting a 100" image. On a side note, while one could argue that 260 cd/m² is not as bright as most TVs, it's plenty bright for a projector, especially considering the impact of having such a large surface area with that brightness level.
Here's our rule-of-thumb guide about how many ANSI lumens we recommend to get an acceptable picture quality for different projection sizes and viewing environments:
| ANSI lumen output |
Dark viewing environments |
Brighter viewing environments |
| < 500 |
Small images (≤75") |
Unusable beyond very small image sizes |
| 500 - 1000 |
Standard image sizes (100 ~ 120") |
Medium image sizes (≤85") |
| 1000 - 2000 |
Large image sizes (100"+) |
Standard image sizes (100 ~ 120") |
| > 2000 |
Large image sizes (100"+) |
Large image sizes (100"+) |
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