The Different Units of Brightness for Projectors. Lumens, Lux, Foot-Lamberts, Nits, and ANSI Lumens

BenQ

2020/04/03

As consumers become more sophisticated, and purchases of consumer electronics over the Internet increases, the need for a deeper understanding of specifications has become essential for products where there are endless variations in terms of price point, quality, and available features. For the home theater market, and projectors in particular, brightness has become one of these areas in which an understanding of the different units involved can help bring about greater clarity, resulting in more informed decisions.

With this in mind the discussion below will revolve around defining the various units used to measure brightness and then describing how best to apply these units when making your purchase. But before getting started we must add a clarification: the term “brightness” is a subjective concept and therefore not used in optics to describe the units we will be discussing below, the term that is more commonly used by experts when discussing light is “luminance”. For the sake of this discussion and to avoid confusion we will use the term “brightness” as a catchall for the various concepts involved.

What are Lumens, Lux, Foot-Lamberts, Nits, and ANSI Lumens?

Light Output (Lumens) vs. Brightness on Screen (Nits)

Choosing the Right Projector

What are Lumens, Lux, Foot-Lamberts, Nits, and ANSI Lumens?

The most commonly used units to measure light in the home theater market are: Lumens, Lux, Foot-Lamberts, Nits, and ANSI Lumens. Below are brief descriptions of what each term means.

Lumens

Lumen (lm) is the SI-based unit that measures luminous flux, in other words the total amount of light produced by a light source per unit of time. Luminous flux as a measurement is different than radiant flux because luminous flux measures only the electromagnetic waves that the human eye can see while radiant flux measures all electromagnetic waves emitted by a source.

Lux

Lux is similar to nits and ft-L in that it also measures brightness in terms of surface area, but in this case, it is defined as lumen per square meter.

Foot-Lamberts

Foot-Lamberts, or ft-L, takes the same unit of light used in defining nits, the candela, but applies it to the unit of area customary to the United States, square feet, meaning that 1 ft-L is equal to 1 candela per square foot.

Nits

Nits is a unit that measures brightness in terms of area, or in technical terms, candela (a standard unit of light equivalent to the light produced by a single candle) per square meter.

ANSI Lumens

ANSI Lumens is a unit, defined by the American National Standards Institute (ANSI), that measures the overall amount of light output by a projector, in other words the higher the lumen value for a projector the brighter the light it produces.

Light Output (Lumens) vs. Brightness on Screen (Nits)

Anyone who has looked into projectors on the market knows that lumens is the main unit of brightness that is used in a projector’s product specification. But is knowing a projector’s lumens value alone useful when making a purchase? To answer this question, one must take a deeper dive into what lumens means.

As described above lumen only measures the total light output of a projector, but this by itself is not entirely useful because it does not take into account the reality that the viewer’s eyes are not directed at the lamp itself but at the surface that the projector’s light is being projected on (e.g. a screen). And since a projector’s light reflects off a surface, the size of that surface affects the brightness of the final image. For example, the image produced by a projector that is projecting onto a 1 square meter screen is ten times brighter than if the same projector – with the same lumen value – is projecting the image onto a 10 square meter screen. Thus, simply knowing a projector’s lumen value only offers a partial picture of its brightness since a high lumen value can give a vastly different level of brightness to the eye depending on how far it is spread out.

In that sense a unit such as nits comes in handy specifically because it does take surface area into account. Nits by specifying light in terms of area allows a user to define and compare brightness as it appears on the screen, so that an image that is 300 nits is by definition brighter than an image that is 100 nits.

The Myth of High Light Source Brightness

There sometimes is a misconception among consumers that the lumen value for a projector’s light source is the same as the lumen value for the projector. In reality the light from the projector’s light source diminishes quite a bit before it is even projected, since it’s transformed by the projector’s imaging mechanism as it passes through projector. What this means is that on average the ANSI lumen value of the projector itself is usually only about 30% of the lumen value of its light source, so that a projector with a 3,000-lumen light source might actually only produce 900-lumen light. But some brands only list the brightness value of their projector’s light source so as to trick the consumer into mistaking one for the other and thinking that their projectors are brighter than they really are, which is something that consumers need to be aware of.

Choosing the Right Projector

Knowing the inadequacies of lumen values for projectors and given the fact that most projectors only list lumens in its product specifications, how is one to apply the information described above and choose the right projector when it comes to brightness?

First off you must define the brightness level that works for your situation in terms of nits. You can do so by identifying the ambient light setting of your home theater in the table below and its corresponding nit value.

Ambient Lighting and Estimated Minimum Image Brightness in Nits

NOTE: To convert the nit value to ft-L you can use the following website: https://www.translatorscafe.com/unit-converter/en/luminance/14-1/

Once you have the suitable nit value you can then match it up with the image diagonal of your screen in the table below to find out the lumen value needed for your projector.

You can then head into the projector market with this data in hand and find the right projector for your home theater.

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