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What Is A Lumen?

If you’ve ever perused the back of a video projector brochure, you have undoubtedly come across the myriad of numbers meant to impress you. Unless you have the benefit of an experienced audio-visual consultant, understanding these technical specifications is vital in comparing various models and choosing the projector that best suits your presentation needs. Yet, in many cases, manufacturers provide specifications that compare much like the proverbial apples and oranges.

The typical purpose for a large screen projection system is to present, monitor or reinforce visual information for a specific audience. Although the big screen usually won’t improve the quality of your material, it can allow dynamic video and computer images to produce a more noticeable and lasting impression. This is especially true if your projector has enough power (usable light output) to overcome the ambient lighting conditions. This rating of light output, usually at the top of the specification list, is commonly measured in Lumens or more correctly, the singular noun, lumen.

So, what exactly is a lumen? A general definition is that a lumen is a unit of measurement of the total light output from a specific source. For projection systems, it’s the total amount of light that, once passed through the optical lens system, ultimately reaches the screen.

Picture This:
Let’s step back and shed more light on this subject. Light is electromagnetic (EM) radiation detectable by the eye and ranging in wavelength from about 380 nanometers (nm) to 770nm. Its frequency ranges from 460 terahertz (THz) to 750 THz. EM radiation is a form of energy with a very wide spectrum. This includes AC electricity, audible sound, radio and television broadcasts, microwave communications and ovens, and of course, visible light.

All forms of energy have several measurable characteristics, such as frequency, intensity, modulation, direction, and coherence. Of these, let’s consider the first two: frequency, a function of time (rate or speed), and intensity, a function of amplitude (volume). Here, an automobile engine provides a good analogy. A motor gives the car a capacity for speed by its horsepower, which is measured in brake horsepower. However, it is the motor’s capacity for intensity that allows the car to pull its own weight and then some. This intensity is known as torque and is measured in pound-feet or Newton-meters.

If you are familiar with audio, consider some of its terms regarding energy. Frequency affects the pitch (note) of a sound heard whereas intensity relates to volume (loudness), commonly measured in decibels. It is the same with the measurement characteristics of light. A light source has a frequency, inversely measured as wavelength, which determines its color. The source also has an intensity, which determines its apparent brightness.

Brightness relates to luminous flux, or the amount by which visible EM radiation stimulates the sensation or perception of brightness in the eye. Unfortunately, brightness and luminance are often incorrectly equated. Brightness is a visual sensation that is perceived; luminance is an objective quantity that can be measured. It’s more appropriate to refer to brightness as a mental response. The human perception of the lumen is this sensation producing ability.

The Techie Stuff:
Technically speaking, lumen; with its Latin origin meaning “light”; is the international standard (SI) unit of measurement for luminous flux, which is the amount of visible radiation flowing through a solid angle. A lumen is equal to the luminous flux emitted per unit solid angle (one steradian) by a standard point source with a luminous intensity of one candela.

Luminous intensity, once called candlepower, is the amount of visible radiation emitted by a point source. Two hundred years ago, the “reference” candle for this was made from 0.167 troy pound of whale wax, thick enough to burn at 120 grains per hour for eight hours. The SI unit for luminous intensity is the candela, a metric base unit. This unit is important to give us a frame of reference for the lumen.

For those who must know, today, one candela is defined as the luminous intensity of a source that emits monochromatic radiation of frequency 540THz (bluish-green) with a radiant intensity of 0.001464128 W/sr. Also, luminance is the amount of visible radiation per unit area of the light source, measured in candela per square meter (or nit). Illuminance is the amount of visible radiation falling on a unit area, measured in lux (or foot-candle).

The Numbers Game:
Examining a projector’s brightness can be subjective and confusing if you don’t consider other relative factors such as optical resolution, contrast ratio, source content, ambient lighting conditions, screen surface characteristics, and viewing angles and distance.

Manufacturers’ published specifications are normally determined under ideal circumstances that yield the most favorable measurements.

Comparing light output solely by manufacturer spec sheets can be a daunting if not confusing task. In the past, these numbers were rarely based on real world performance conditions, and often the results could not be verified or reproduced. Because each manufacturer wanted to outshine the competition, rubber-ruler methodology led to the “great projector numbers game.” To help clarify the specsmanship dilemma, the American National standards Institute (ANSI) published its criterion that was intended to put everybody on a standard measuring system.

It is important to note that the process for measuring lumen output removes the screen from the equation. When you measure ANSI lumen, you point a calibrated light meter into the stream of light emitted from the projector at its highest “usable” operating resolution. Light output readings are taken at the center of nine equally sized, discrete zones on a specific size screen, in a darkened room. The results are averaged to yield the ANSI measurement. Using this method, you should be able to obtain an objective measurement of the total light output accurately and in a meaningful context.

This standard (ANSI IT7.215-1992) is designed to unite brightness, contrast and resolution specifications under a uniform measuring methodology. Even today, with some manufacturers, light output figures are published that can never be attained in the real world because of special “lab” hardware unavailable to the normal end user (e.g., special lamps, lenses or other optical components). Let the buyer beware!

Seeing Is Believing:
When accessing projected image quality, remember the old adage, “seeing is believing.” However, our eyes do not respond in a linear fashion to increased light output. For example, if light output increases by a factor of 10, our eyes do not perceive a ten-fold increase in brightness. In fact the human eye senses changes in light output on a logarithmic scale, which allows our eyes to be sensitive over an extremely wide range of light levels.

This fact makes the minor differences in projector light output arguably negligible. If you double the number of lumen, then the subjective perception will be that the “whites” get 25% brighter. The best possible measurement of projectors’ luminance is a side-by-side comparison (commonly known as a projector shoot-out). For video projectors, this is true for many other specs as well, such as resolution, contrast ratio, uniformity, true color reproduction, and so on.

Also, note that the amount of ambient light on or near the screen surface plays a significant role in how much light output may be required for any given application. Since a video projector cannot project “black” light, control over the ambient is critical to attain an acceptable contrast ratio. In reality, contrast ratio performance and screen dynamics are just as important as light output when it comes to displaying satisfactory images!

Whether you are rating a projector or any other technical device, understanding the specification terms is very helpful, if not imperative. It is as important to know the method of measuring each specification, as it is to know the unit used to define it. Since visual comparisons are always subjective, learn what to look for, know what it means, and then let your eyes be the judge. After all, it’s your image!

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