LED-based lighting and display systems are becoming increasingly popular due to their low cost, flexibility, and efficiency. Measuring the light and color output of LEDs is, therefore, becoming more important as their performance is compared to and evaluated against traditional technologies. In addition, inherent performance variations from device to device must be understood and controlled.
In measuring LED and LED-based systems, the selection of the measurement method and system will be a function of measurement objectives and, most likely, will be adapted to the specific nature of the LEDs. Measurement objectives span informing or assessing a LED or LED-based system design, characterizing a light source, evaluating the source for acceptance testing or quality control, or modifying and controlling performance.
Standards and recommendations are important guides for what measurement quantities are significant and how they might be measured, especially when characterizing LEDs for use in lighting systems. For incoming or outgoing quality inspection, a set of indicator quantities will most likely suffice, simplifying the measurements needed. In all cases, it is important that the measurement relate to human perception of light and color.
When compared to other light sources, LEDs have several interesting characteristics. First, placement of the LED die in the package can significantly affect the direction that light is output. Second, when LEDs are initially turned on they require some time — perhaps minutes — to stabilize. Third, they are inherently narrowband light sources, so the creation of white light requires some form of color mixing. Fourth, the output of LEDs will vary, non-linearly, with current. And finally, LEDs become less efficient over time due to electron depletion, and so “age” in use, diminishing in brightness over time. All of these factors should be taken into account when measuring LEDs.
Describing LED Brightness and Color
The performance of LEDs, and light sources in general, can be described in terms of the angular distribution of their output power as a function of wavelength. Direct description of output power as a function of wavelength is referred to as radiometric. To describe brightness and color as perceived by the human eye, this spectral power distribution is weighted according to how the human eye perceives different wavelengths and integrated to provide a photometric (brightness as perceived by the human eye) or colorimetric (perceived color) description.
LED Brightness: Brightness is measured as luminous intensity, which is the weighted (according to human perception) power emitted by a light source in a given direction per unit solid angle.
Luminous intensity is described in units of candela (cd). A related quantity is luminance, which is the luminous intensity per unit area emitted in a particular direction. The units of luminance are candela per square meter (cd/m2), often referred to as a “nit”.
LED Color: The color of a source, again in a particular direction, is described in terms of a color space, the most common of which is the CIE 1931 color space. Here color is defined in terms of XYZ coordinates - or tristimulus values, X, Y, and Z - where the Y coordinate is brightness of the source and the chromaticity parameters are derived from X, Y, and Z.
Two other quantities describing the color of a source are often useful. First, its Correlated Color Temperature (CCT), which is technically the color temperature of a black body radiator that most closely matches the source. CCT is measured in Kelvin (K). Less technically, higher CCT (>5000°K) are “cooler” (more bluish) and lower CCT (<3000°K) are “warmer” (more yellowish). Second, the color-rendering index (CRI) of a light source is a measure of the accuracy of the color appearance of illuminated objects when compared to illumination by natural (ideal) light. CRI can be derived from the spectral measurements of a light source. Unfortunately, CRI as a descriptive quantity is problematic. It provides indicative information, but is known to be inaccurate in some cases; newer approaches to defining CRI are being actively researched.
For a complete description of an LED system, these radiometric, photometric, or colorimetric quantities need to be described as a function of angle relative to the source.
This creates three interesting measurement considerations. First, why measure at all? Second, what angular granularity is required? And third, is the source measured as a point source or an extended source?
The question of why to measure at all is more than a philosophical one. If the measurements are being done to characterize a source for modeling purposes, it can be argued that a theoretical model will suffice. In general, the potential complexity of a light source as a system and the potential for error in theoretical understanding are sufficient to make a measurement preferred, as it is a real description of the source. Of course, for evaluation, inspection, or control, having the real data specific to a device is necessary as the measurement is the point.
For some applications, a point measurement from one viewing angle or a single integrated measurement will suffice. But for most applications, the variation of luminance and color with angle is an important attribute of the device, especially for LEDs given manufacturing variations on where the die is placed in the package – a slight variation can result in significant change in the distribution. There are two kinds of angular measurements: those in which all the desired output light is captured and measured and those where the angular data is sampled on a grid. In the latter case, the spacing between angular measurements can be determined based on the anticipated continuity and rate of change of distribution.
Near and Far-Field Measurements
For some applications (such as evaluating the brightness of an LED indicator light), the source can be treated as a point source. For other applications, such as characterizing an LED or LED luminaire for optical design, the source should be treated as an extended source — meaning one that has physical extent and so has spatial variation in light output from point to point on the source. In this case, the source needs to be measured in a way that yields this more detailed distribution. Measuring the light as a point source yields a far-field measurement. Measuring the light source as an extended source yields a near-field model of the source.
In application, the near-field model is expressed for optical design purposes as a ray set. The quality and usefulness of the ray set will be a function of both the number of rays in the set and how the rays are statistically sampled based on the near-field measurements of the source. Common statistical sampling methods range from simple Monte Carlo sampling to importance sampling. Importance sampling weights rays according to the brightness of the point on the source where they are emitted rather than just selecting starting points with equal weighting.
A near-field model can be extrapolated to a far-field model, but the reverse is not true. This is because the far-field model is a limiting case of the near-field model with a collapsed light source. A rule of thumb for the boundary between the near and far-field regions for optical devices is about 10X the largest dimension of the source. For an LED, this would be a few centimeters, but for a luminaire this might be tens of meters. Beyond this range the far-field model and the near-field model will give essentially the same results.