Produced by: Light Modifiers Rental
Written by: DP: Camrin Petramale & Gaffer: Neil Adamson
Part 2: Illuminance
As discussed in Part 1, output is dependent on distribution as well as distance. While Part 1 focused on distribution of illumination across a space, Part 2 will focus on how much the light illuminates through a space.
When people compare the output of two lights, they usually compare the light's illumination through a space. Oftentimes, people make these comparisons using terms such as “intensity” or “output”, and while it's easy to understand what they’re talking about, it is important that throughout our testing that we use consistent and accurate terminology since we encounter these terms elsewhere, and often people use them incorrectly. For clarification, 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.
What is the difference between intensity and illuminance?
Intensity is the total potential light created by the source
Illuminance is the light falling on a subject
Since less light reaches a subject the further away it is from the source, it is important to know how much light is available at multiple distances from the source. For instance: at 3 meters, one light measures 1000 lux, while another measures 800 lux. For this test, we have looked at the different illuminance measurements for each light at multiple distances in order to get a better understanding of how each light can be used on set, and we have provided the necessary diagrams to give a larger picture of each light’s capabilities.
A note about Falloff
The phenomenon of light becoming less powerful at further distances is known as decay, or falloff. The rate at which lights falloff varies depending on optics used (lenses), as well as the size and shape of the fixture. For Part 3, we will be exploring how these factors influence the decay rate, and the different methods to calculate falloff, including when the Inverse Square Law applies, and why.
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.
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.