Illuminance Part 3: Overview
Previously, we learned in Part 1 that intensity does not equal output and that a light fixture or luminaire influences the distribution across a space to provide more focused light, as opposed to isotropic, or omni-directional light. Not only does this affect our ability to control and use the light more efficiently light, but as we learned it Part 2, it can also influence how light from a fixture illuminates at a distance, based on the falloff. Understanding these two concepts, we can now finally start comparing illuminance with proper context.
When people compare the output of two lights, they usually compare the light's illuminance, measured in lux or foot-candles While "intensity" is the most commonly used term in this comparison, it is important to understand the difference between intensity and illuminance. Intensity refers to the total output of light and is measured in lumens. While useful throughout testing and within the right context, intensity is not something we deal with on set, at the shop, or even when determining what lights to buy or rent. What we are much more concerned with is a light’s illuminance, which is measured in lux ,or footcandles. These measurements concern the amount of light filling a fixed area (m² for lux, ft² for footcandles), which is impacted by the distance from a source.
Remember the difference between intensity and illuminance!
Intensity is the total potential light created by the source
Illuminance is the light falling on a subject
As we saw in Part 2, light spreads out as it leaves its source but, not always in the same manner. Therefore, while two lights may have the same illuminance at one distance, if their falloff rates are different, they will not measure the same illuminance at other distances. As such, just as it is important to consider beam angle differences when comparing output, it is equally as important to know how much light is available at multiple distances from the source for a complete head-to-head comparison. For this test, we have looked at the different illuminance measurements for each light at multiple distances for a comparison that factors in falloff, as well as illuminance measurements.
The test took place in a 40x40 blacked-out studio with black commando cloth laid across the level, concrete floor. To conduct the test, we only took measurements along the center beam, in order to maintain consistency. Utilizing a system of levels and laser pointers, we devised a method to ensure that all readings would follow this same path. We then tested each fixture at the following distances:
We measured each fixture from the front-most part of its aperture (knowing that some would have different virtual origin points, and used necessary formulas to correct and account for this.) The meters used for this test included: Spectra Cine IV-A Professional with dome and disk, Sekonic 858-Cine, and Sekonic C-800 Spectrometer. Multiple readings were taken for reference, and all graphed data ≤ 200,000 lux comes from either the C-800 or Spectra, which had a difference of ≤ 0.2 stops from one another. Readings > 200,000 lux were taken using the 858-Cine with recessed dome, and recorded for reference and calculation purposes. Because our graphs use a maximum of 100,000 lux, specific measurements above 200,000 lux are unnecessary to display our findings and should not be used in situations in which precise measurements are necessary.
100,000 lux or higher
Point Source Example
Illuminance (lux) at given distance (meters)
How to read the Data
We have decided to display our findings using a customized version of “false color” that uses the color spectrum to visualize the light’s falloff. Each discrete color represents one-quarter of the previous amount, conforming to the behavior of a traditional point-source doubling in distance and the Inverse Square Law.
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