Where Do You Put the Measuring Stick?

As president of IEM LED Lighting Technologies, Dr. M. Nisa Khan consults in the solid-state lighting industry and educates consumers about LED lighting. She has a bachelor’s degree in physics and mathematics, and master’s and Ph.D. degrees in electrical engineering. Email her at nisa.khan@iem-led.com

Lighting, like architecture, is a wide-reaching field. And, like architecture, it involves science and art, and perhaps other aspects of human perception and experience that are less known or understood. But, relatively speaking, the scientific part of lighting or illumination has been fairly well established at the basic level. It’s best to first recognize that lighting science primarily involves three fields:
– Physical science (physics);
– Physiological science (the makeup of the human eye); and
– Psychology (the interaction of the eye with the human brain).

The latter two fields are subject to human variations and thus lack precision. However, the first — physics — is exact and has been a critical part of lighting science since the humans began to understand it. Physics and its accompanying mathematics produce exact science, and thus, this aspect of lighting has no exception.

Why physics is important
By and large, current lighting-industry professionals aren’t physicists or electrical engineers (EEs), although physicists and EEs have certainly contributed to lighting science. Currently, however, few, if any, work on lighting or illumination. Some experts say that physicists or EEs aren’t very comfortable with the human variation of lighting science and, therefore, seek other engagements. Also, until recently, lighting designers have worked primarily with homogeneous and substantially omni-directional, light-distribution systems — those produced by traditional lamps and, therefore, simplified understandings of light sources and measurements were sufficient.
However, as LED lighting, which bears unconventional source characteristics, continues to make market strides, it becomes even more essential to establish a clear, comprehensive and rigorous treatment of illumination physics and metrology, as well as to develop engineering tools to better design and evaluate various luminaires.

Illumination-related measurements
Although lighting physics is exact by nature, its lighting-industry understanding and implementation remain inadequate, particularly for the handling of LED-based lamps and luminaires. Lighting comprehension involves analyzing and evaluating light sources and their cast illumination. For practical reasons, the fields that involve light have separated into illumination and display (other related fields include vision, machine vision and imaging sciences).

The first, illumination, typically constitutes the lighting industry.
 

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Signage, however, is a subfield within the display arena. And, although related, the different fields have developed almost independently — each focusing on slightly different physical parameters and measurement principles. But, interestingly, they all involve measuring light properties from certain sources, or illuminated objects, either directly or indirectly.
Primarily, the lighting industry relies on measuring light flux or power (quantified by units of lumen) that falls onto a surface. This quantity, known as illuminance, is measured in units of lumen/square-meter or other equivalent units. Often, this measurement doesn’t involve measuring any light-source property directly. Rather, it involves measuring secondary light from an illuminated surface.
Lighting designers use this measurement and, sometimes, a set of such measurements, to ensure that a desired illumination in space is achieved. They also perform calculations to design luminaires or "shades" around a light source using "luminous intensity" data that’s calculated from measured, goniometric illuminance, or luminance data, for the light source of interest. (Luminous-intensity data of well-known light sources and luminaires are catalogued in the IES or other similar libraries.)

Although luminous intensity, defined by luminous power contained with a unit of solid angle, is a primary lighting parameter that’s essential for calculating light distributions from lamps, it can only be calculated, but not measured.

Why can’t it be measured? A light detector (including the human eye) always measures lumen per square meter by nature. It’s described as measuring the quantity "illuminance". But, when it’s directly aimed towards a light source, the measurement also corresponds to the light power contained in a finite volume, which is defined by the light cone between the source and the detector.
This process is described as measuring the quantity "luminance" (assuming the light source is small, uniform, diffused and sufficiently distant from the detector).

Under particular conditions, both illuminance and luminance are measured by the same physical experiment, but they describe two different types of "intensities" or flux densities.

Intensity, here, is the light power contained within some bounded space.
 

Illuminance is lumens contained within a surface defined by square meter or square foot.
Luminance is lumens contained within a conical volume defined by a solid angle that has been truncated at a particular distance. In both cases, a physical measurement is possible because the light power is contained within a finite amount of space.

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However, luminous intensity, as discussed earlier, is light power defined within a solid angle that extends to an arbitrary length. It cannot be physically measured because the experimentalist would then ask: "Where do you put the measuring stick?"

This subtle realization, however simple, isn’t common in the lighting industry, and many professionals, including academics, treat luminous intensity as a fundamental photometric parameter — without realizing that it cannot be physically measured. In fact, many academics vigorously object when they don’t see luminous intensity as a parameter in the lighting-measurement table.

The importance of luminous intensity
Luminous intensity, although not physically measured, remains an important lighting parameter. In the early days, the physicists that pioneered optics and light introduced this concept because light perception or detection by the human eye occurs, by nature, from a particular direction. Thus the luminous intensity concept, i.e., the amount of light contained in a solid angle and thus providing flux density in a particular direction, is very important. Nevertheless, because it cannot be physically measured, it’s not a photometric parameter belonging to a measurement table.

In contrast, illuminance and luminance are photometric parameters because each can be measured; they are not always unique because – want the comma? in a given system (of light source and illumination region), a specific relation can be drawn between the two.

The luminous-intensity distribution (LID) calculated from 3-D, goniometric illuminance or luminance measurements is useful to design luminaires for general-purpose lighting.

Automotive headlamp design benefited from LID information utilization. With the present interest in LED lighting, the 3-D LID information is especially useful in depicting why LED lamps are directional). Such goniometric measurements, described in my book, “Understanding LED Illumination” (require time and effort because high-precision instruments are required to capture light power data at small angular increments over 3-D space covering the full solid angle range. Further, the experimentalist must use certain cautions and satisfy a set of conditions to ensure reliable data.

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The light-intensity distribution data is helpful for determining how well a sign will illuminate when an array of LED modules is placed behind the sign face. Such calculations can help a sign designer decide on the number of LED modules, the spacing between them, and the height between the illuminated surface and the modules. These, along with the correct luminance specification of LED modules, will also ensure manufacturers produce signs — and automotive taillights — with glare-free illumination.

In conclusion, it’s time to look at lighting and its metrology under different illumination, where proper distinction is made between measured and calculated parameters, and where luminance is a mandatory specification for LED-based systems, so that professionals can design and evaluate high-quality lighting products.