A fourth measurement issue to consider is the need for spatial data as opposed to a single point or spot measurement. Spot colorimeters or spectroradiometers only measure a spot on or around the source – the “spot” is actually integrated over some regular area. This may provide some useful information; it is not very efficient for measuring sources or displays when spatial information is required.
Imaging colorimeters – calibrated CCD-based cameras with CIE match photopic and color filters – overcome these issues, providing the equivalent of millions of simultaneous spotmeter readings on a spatial grid. A typical imaging colorimeter consists of a full-frame or interline CCD, photopic or colorimetric filters, and lenses. The choice of CCD will depend on the application, with the full-frame CCD preferred when there is to be no gap in the image capture and when higher dynamic range is required. The CCD may be cooled and temperature controlled to reduce noise levels. Careful electronic design and regulation of readout speed will also reduce system noise. The photopic or colorimetric filters are preferably specially designed to allow the system to match the CIE color curves for red, green, and blue. The overall system, with the lens included, is usually extensively calibrated to remove the effects of any optical aberrations or CCD variations.
While not all measurement methods applied to LEDs and LED-based systems require the use of an imaging colorimeter, these measurements can often be made more extensively or faster by using one to capture multiple arrayed measurements simultaneously. This is particularly important when measuring an extended light source, an array of LEDs, or an LED display. In each of these cases the spatial relationships are a key component of the data required to describe and understand the system.
An imaging colorimeter that only measures photopic information is referred to as an imaging photometer. For ease of description of the various measurement methods we will refer to imaging colorimeters, but an imaging photometer could also be used, with obvious limitations.
Measuring LED Devices
Measuring LED die or packaged LEDs is usually done in R&D to assess different design options or to exhaustively characterize the performance of the LED.
With Source Imaging Goniometers
Source imaging goniometers (Figure 2) are designed to very accurately measure the near-field luminance distribution of a light source. While there are a number of physical configurations possible, they virtually all move the imaging colorimeter around the light source and capture the output light distribution at the source from multiple – usually thousands – of viewing angles. This information can be stored as raw data or converted to ray sets on the fly. Either data representation is considered a near-field model.
Critical attributes describing the physical accuracy of the source imaging goniometer are captured in a parameter known as “wobble” – this is the maximum excursion of the focal point of the system from its origin as the system is moved to various measuring positions. For measuring an LED die, which can be about 0.5 mm across, this wobble should be no more than a few tens of microns (i.e., only a few per cent of the dimension of the die). Similar accuracy requirements hold when measuring packaged LEDs and any other light source.
Another critical attribute is quality of the optical system used. The system should have a sufficiently small field of view to allow enough CCD pixels to map to the surface of the light source to see any relevant fine scale detail on the LED die or device.
Source imaging goniometer near-field models are the most comprehensive representation of the luminance and color output of LED die and devices as a function of angle. These measurements, since a scan consists of thousands of images, generally require several hours to complete for a single source. Faster measurement times are possible by compromising on angular resolution, imaging optics, or the allowed error, but this is really unacceptable for LED die and device characterization. Recent advances in source imaging goniometers incorporate the simultaneous acquisition of spectral data.
With Integrating Spheres
Integrating spheres provide a means of measuring total or integrated light output of an LED. Depending on the sensors used, radiometric, spectral, photometric, or colorimetric measurements can be obtained. An integrating sphere commonly does not capture angular information relative to any of these quantities. An integrating sphere operates quite simply by putting the LED into the sphere, reflecting the light around the sphere, and measuring the integrated light at a port on the sphere.
Integrating sphere measurements have the advantage of being very fast, and it is simple to change sensors to obtain radiometric, spectral, or colorimetric measurements. These measurements can be used to evaluate or bin LEDs. The major limitation is that no angular information is obtained, so LED packages with misaligned die (resulting in a skewed directional output of light from the LED) would not be detected.
With a Photogoniometer
A photogoniometer measures the far-field distribution of an LED by using a goniometer to move a colorimeter (or spectrora-diometer) relative to the LED device. This has the advantage of allowing multiple measurement devices to be employed to vary the information obtained from the scan. The disadvantage of the photogoniometer is the length of time required to make the measurement and the complexity of achieving the required mechanical accuracy.
Because of the breadth of applications that exist for LEDs, there is a comparable breadth of radiometric, photopic and colorimetric measurement methods. These methods balance measurement time, resolution, information content, and logistics to address the needs of R&D, manufacturing, and field applications.
The most important questions to ask in measuring LEDs are: Is near-field or far-field data needed? Is angular data needed? Is an array measurement required? The answers to these questions will indicate the measurement options. In many cases the use of an imaging colorimeter is optimal because it can capture a large number of simultaneous, spatially related, measurements. It is also flexible enough to be coupled with gonio-metric systems or other optics (e.g., the imaging sphere) to measure light and color distributions quickly and with high granularity.
As advances are being made in the design and application of LEDs, so, too, are advances being made in how imaging colorimeters can be used to measure them (e.g., by integrating spectral measurements) and improve their performance (e.g., through the correction of LED video displays).