Converting Watts to Lumens: Watt Is Your Lumen Count?

Over the last few years, we’ve seen some major improvements in LED lighting technology in both color quality and output. With more and more companies producing lights, it can be challenging to decide which might fit your needs. At AbelCine, we sell and rent several different lighting technologies and do our best to help clients select the right lights for their purposes. The most common question we get on new LED lighting technology is one asking what traditional lights they compare to, or what does “wattage equivalent light” mean exactly. This article explores how to determine what your lumen count is by showing you how to convert watts to lumens.

Most of us have used traditional tungsten and HMI lighting, generally described by the wattage of power that it consumes. It’s not uncommon for someone on set to ask for an ARRI 300, meaning a 300W ARRI Fresnel light. So it’s not unusual that someone would ask us to describe an LED light in terms of what traditional light it most resembles. This is also the case when it comes to consumer LED light bulbs; almost all new LED light bulbs are described by their wattage equivalent. I just bought a 60W (equivalent) LED bulb for my home, and I chose that bulb because I generally know how much light that outputs. It’s a great reference. The same goes for production lighting. If you dig into product literature, most pro LED manufacturers compare their lights to traditional lighting, i.e., XYZ light is equivalent to a 600W Fresnel light.

These comparisons often can be misleading when it comes to professional lights, however. They may be comparing overall output, but not specific light levels and beam angles, or only incident light at a disclosed (or often undisclosed) distance, which isn’t exactly helpful when trying to draw comparisons. The good news is that most LED manufacturers publish photometric data about their lights, charts and graphs informing us how each light performs. This is very useful data, though it can be a little hard to decode. This column is all about decoding that elusive info, making sense of the terminology and hopefully helping you choose the right light for your next project.

How to Convert Watts to Lumens

If you buy an LED light bulb today, you’ll see two numbers on the box: the wattage equivalent number in big letters and a small number labeled “lumen” at the bottom. A lumen is the standard unit of luminous flux, which is a measurement for the human perceived power of light. On a light bulb, this value represents the total amount of light coming out of the bulb. The chart shows the lumen count of a traditional light bulb (below).

watts to lumens

As you can see, a standard 60W light bulb outputs 800 lumen. Most professional lights will have far greater output than a standard 60W bulb, but most manufacturers don’t publish the full lumen output of their lights, so it’s difficult to make a direct comparison. On the other hand, lumen output only tells part of the story when it comes to professional light fixtures. It doesn’t tell you how much light is actually falling at a specific distance from the source or the shape and size of that light. Light fixtures can focus and direct light in different ways, giving very different results. A soft light is very different from a focused spot light. In order to compare two lights, we have to look at how they perform at various distances. This is where that photometric data comes into play, as well as other fun terms.

Measure using Lux, not Flux

To really see how a light fixture performs, it’s important to know the amount of light that it outputs at various distances. That’s where lux (lx) comes in, which is a measurement of lumen per square meter, the total light that hits or passes through a surface. In the U.S., we aren’t so big on square meters; we prefer more imperial measurements, so generally we use foot-candles (FC).

A foot-candle, as its name implies, is the amount of light that a standard candle generates from one foot away (top). Like I said, very imperial—but it’s effectively a measurement of lumen per square foot: 1 FC equals 10.764 lux, so we generally just divide lux by 10 to get the FC equivalent value.

 

Examining Photometric Comparisons

Now that we have some terms out of the way, let’s examine some of that photometric data. Here’s a data sheet on the Litepanels Inca 6 LED Fresnel light (above). As you can see, it lists the foot-candle and lux levels at various distances. We also have details on the beam angle—15º, in this case—and the relative size of the light projection at each distance. The beam diameter defines the area of light that’s within 50% brightness of the center of the light (the brightest region). Meanwhile, the field diameter describes an area of light within 10% brightness of the center.

Not all manufacturers provide these numbers, unfortunately, and even if they do, it’s hard to compare them directly due to variations between light fixtures and the testing data they provide. We can draw some basic conclusions based on the data, however. As an example, I’ve compared several tungsten Fresnel lights to each other at a distance of 10 feet (about 3 meters), with each set to their narrowest beam angle (below). These fixtures vary in beam angle, but it should give us a general idea.

As you can see, I’ve compared several LED lights to traditional 300W fixtures from ARRI and Mole-Richardson. All data is derived from photometric data from the manufacturers. I’ll let you draw your own conclusions, but note the varying beam angles and distances: A narrower beam angle will result in a higher FC count, but that doesn’t mean it’s a brighter light. The chart only compares tungsten-balanced, focused lights, but we could make the same comparison for soft lights, daylight-balanced lights, etc.

There’s no better way to choose a light than to physically test it out. There are many aspects to a light that aren’t easily characterized in a chart, and with a simple light meter in foot-candle or lux mode, you can see how these lights actually perform.

Using a light meter like the Sekonic L-758Cine DigitalMaster in foot-candle or lux mode allows you to test your lights to determine how they will perform in your shooting conditions.

 

Comparing LED to traditional lighting: Wattage Equivalent

Comparing new LED lighting to traditional lights makes a lot of sense because it instantly gives you a point of reference, but make sure to take it with a grain of salt. There isn’t a lot of defined testing data for lighting companies to follow, so always check out the photometric data or test the light for your application. Here at AbelCine, we’ll do our best to keep comparing lights—seems like we get a new one in every week!

Andy Shipsides is the Director of Education at AbelCine. AbelCine’s training department offers practical classes for serious image-makers of all levels in professional production technology encompassing cameras, optics, audio, lighting, workflows and post solutions. Classes are available in person at their New York, Chicago and Burbank locations, as well as online. Visit training.abelcine.com.

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