LEDs are fourth generation light-sources that offer increasingly reliable operation, and overall lower cost when compared with traditional lighting technologies such as incandescent, fluorescent, high-intensity discharge (HID), high-pressure sodium (HPS), and other lighting technologies. LEDs have long been used as indicator lights in a wide range of consumer and machinery display/control products. In automotive they are moving from dashboard and interior lights to headlights, tail replacing conventional lighting both internally and externally. Other popular external applications include emergency lighting, roadway/street lighting, and runway lighting at airports.

As with any technology, in addition to advantages, there are drawbacks. Consistent manufacturing of LEDs with the same or similar optical properties is almost impossible – even for LEDs in the same batch. Brightness and color can both vary from component to component. Output is also affected by encapsulation, the efficiency of different wafer lots, and other process variables.

Testing of LEDs for light intensity/brightness and color is important to successful use in all applications.

A SPECTRAL EVOLUTION spectroradiometer measures the wavelengths of individual colors of light. In LED metrology it can measure output in lumens, watts, moles per second, footcandles, and more. A portable UV/VIS/NIR spectroradiometer provides a useful tool for quickly testing and characterizing LEDs and LED luminaires. It can test an LED for optical output in watts as a function of varying LED current, temperature, and other optical characteristics to completely characterize the LED or LED system. Important spectroradiometer capabilities for LED test applications could include:

• Fast, full spectrum UV/VIS/NIR measurements with a spectral range from 350 – 2500nm with a single scan
• Ultra-fast scanning with autoshutter, autoexposure, and auto-dark correction before each new scan – without requiring repeat optimization steps
• Fixed array detectors for reliable operation
• Lightweight and portable for lab or field use
• Standalone operation or use with a laptop or personal digital assistant (PDA)
• Bluetooth for easy communications
• Superior signal to noise ratio with faster scan times and better reflectance measurement
• NIST-traceable calibration for taking a wide range of irradiance and color measurements to fit all the application specifications

In addition, all SPECTRAL EVOLUTION systems have the following colorimetry capabilities built-in:

• CIE 1931 chromaticity diagram with RGB gamut
• Scan results displayed graphically with a cross hair overlay
• Text display of x, y, XYZ, and RGB coordinates and correlated color temperature

The ability to quickly test for brightness and chromaticity in the lab or in the field allows for rapid screening of LEDs before use and offers a method for gauging color shift over projected lifecycle, where color and hues are critical. Since LEDs tend to shift color over time, measuring color stability can be a key in many applications. It is also useful to compare the brightness and color of LEDs versus incandescent, fluorescent or halogen lamps early in the design process, before the LEDs are used as substitutes for these conventional lighting technologies.

DARWin SP Data Acquisition software provides a built-in colorimetry capability. It can be used to calculate chromaticity coordinates and correlated color temperature of a spectral dataset.The following colorimetry capabilities are built-in:

• CIE 1931 chromaticity diagram with RGB gamut
• Scan results displayed graphically with a cross hair overlay
• Text display of x, y, XYZ, and RGB coordinates and correlated color temperature

The ability to quickly test for brightness and chromaticity in the lab or in the field allows for rapid screening of LEDs before use and offers a method for gauging color shift over projected lifecycle, where color and hues are critical. Since LEDs tend to shift color over time, measuring color stability can be a key in many applications. It is also useful to compare the brightness and color of LEDs versus incandescent, fluorescent or halogen lamps early in the design process, before the LEDs are used as substitutes for these conventional lighting technologies.

The PSR+ 3500 field portable spectroradiometer can be used to measure and compare the spectral radiance of an LED (red) versus a halogen(blue) light source. This capability becomes especially important in those applications where a design engineer is considering replacing a traditional light source with an LED and needs to verify that the LED luminaire will meet or beat the critical characteristics of the halogen lamp.