Produced by: Light Modifiers Rental
Written by: Camrin Petramale & Neil Adamson
Part 1: Illumination Distribution Graphs
Manufacturers love comparing their lights to industry standards – especially when it comes to claims about output. Unfortunately, these claims come from each company’s own standards and lack uniformity across the industry. Knowing this, are these claims enough to gain a true understanding of a fixture’s characteristics?
The "output" of a light is derived from the luminous flux (the amount of light energy created [ie, intensity]), and the distribution of that light energy (Figure 1) The problem is while two lights may create the same amount of light, if one concentrates all of that light into a small area, and another distributes it evenly across a space, each fixture will produce significantly different light levels when measured at the same distance (Figure 2).
There are different factors that impact how a fixture’s light is distributed, and those factors will also influence other characteristics such as falloff (the rate at which the illumination decreases over distance), and quality (the degree of definition in a light’s shadow). These are important elements to consider when choosing a light, but we first want to focus on output in a better, more practical context.
Understanding planar illumination distribution (the concentration of light emitted in the range of an angular span across a planar surface area.) helps add that practical context. While some manufacturers will offer ameasurement of their fixture’s planar illumination distribution, they each use different scales and methods, making comparisons between different fixtures unreliable. We decided to create a test that would level the playing field across all light fixtures, and add important context to properly and practically answer the question of light output across distribution curves.
Why Illumination Distribution (curves) Matters:
Even minute changes can have a large effect on the final image. As creative technicians, we need to preserve our vision, and need a proper understanding on the characteristics of light in order to achieve that. We have more variety of light manufactures and sources than ever before, and we need to fully understand the characteristics to use them most effectively in the correct scenarios.
ANATOMY OF AN ILLUMINATION DISTRIBUTION GRAPH
3 Characteristics of an Illumination Distribution Curve:
1. Beam Spread: How wide is the “Beam Angle”
2. Edge Definition: What is the ratio of slope angle to beam angle? The greater the ratio, the more defined the beam edge is.
3. Uniformity: How smooth is the curve? Does it have multiple Spill and Edge Peaks? Is there a visible spike across a narrow angle? Is
there a relatively even illumination level across the beam and field area boundaries? The Beam Angle?
Our goal was to create a reference document that accurately, uniformly, and consistently compares planar illumination distribution across fixtures.
Placing a light 5m from a measurement plane, we gathered 15 separate, angle-targeted readings from the center of the beam down the plane to the right along the horizontal axis. While a more comprehensive test would include the vertical axis as well, most of the light fixtures had circular front elements, meaning that their beam spread should be relatively similar along both the horizontal and vertical axes. It is important to note that these curves do not apply to the vertical distribution of illuminance on fixtures with square and rectangular front elements, only to their horizontal distribution.
Although manufacturers perform a similar test to convey illumination distribution, there are three key differences in our method.
Rather than measuring isometrically (at an equal distance from the center of the light for anywhere from 180º-60º), we choose to measure along a planar surface because this yields the most practical results for usage in the industry (shooting through diffusion, bouncing, washing cyc walls, etc). This meant that not only did the angle change with each measurement, but the overall distance from the source did as well.
Manufacturers measure with candelas because this makes calculating the output at any distance possible. However, that is only applicable if the light is a true point source – which no manufactured lights are. This means that, according to manufacturer data, any indication of how a light will perform when used in a real-world environment is misleading and inaccurate. We measured all sources at a 5 meters planar distance. We chose 5 meters because at this distance, the falloff rates for the lights tested have roughly evened out to give a more equitable representation of intensities. Increasing the distance beyond 5 meters would have introduced spill and ambient bounce, skewing the readings.
Manufacturers publish their data using “relative intensity” along the y-axis of their graphs because they are referring to the change in output for that light only, so it can be based off of the peak intensity of that specific fixture, and used to calculate at any distance from the source. As we mentioned, those calculations will not match actual measurements, and relative intensity does not allow for the comparison of illumination curves between two or more lights. By us measuring illumination rather than intensity, maintaining scale uniformity among all graphs, and setting the y-axis to a logarithmic scale*, any fixture’s graph can be placed beside another to give an accurate comparison of the planar illumination distribution across fixtures.