Testing LED Applications Print E-mail
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Written by Mark Lau   
Friday, 30 March 2012 10:28

A more reliable method to classical analog measurement of voltage and frequency is discussed.

LEDs are seldom used singularly. When one or more LEDs are assembled in an array, the contrast in color shades or light intensity between the LEDs is noticeable by the human eye. In certain applications, it becomes necessary to inspect the LED assembly for consistency in color and light intensity.

An LED is a semiconductor junction diode, and its performance seldom deteriorates and deviates over its life. During manufacture, an LED is tested as a semiconductor device for color, luminosity and electrical characteristics either individually or randomly sampled within a batch. LEDs are then sorted to part numbers according to the test results.

Semiconductor test inspects for the LED’s emission of color and luminosity, as specified in the product datasheet. The important criterion in all LED applications, however, is the light received at the intended target: for instance, road surface for a car’s LED headlamp. Between the emitter and receiver is a medium that can be air, water, etc. Light intensity, and to a lesser extent color, will suffer a loss when transmitted through a medium (Figure 1). Thus the measurement of light at the receiver will not be identical, nor exceed the specification stated in the LED’s datasheet.

The lowest cost of inspecting an LED’s application is during its assembly on a board, immediately after it is soldered. At this stage, an in-circuit tester is commonly used to detect assembly defects. LED inspection can be incorporated into ICT using external light sensor(s) installed in ICT fixtures. Yet LED inspection at ICT has been hampered by poor return on investment because of lengthy test times and inconsistent test results for color and intensity.

The current technology of LED inspection at ICT relies on using two analog ICT measurement attributes: voltage and frequency. This methodology is susceptible to conversion errors twice: once when converting color/intensity to voltage or frequency and second, interpreting the voltage or frequency to the equivalent color or intensity.

A more reliable method is to reproduce the color measurement immediately, preferably as a unit of wavelength in nanometers. The measurement in nanometers can then be compared against the LED’s datasheet color specification to ensure the correct LED has been assembled and its performance matches its specification. Figure 2 shows wavelengths associated with colors in the visible spectrum. A lower and upper measurement limit can then be set in the ICT test program to detect an LED performing outside the test limits, with an appropriate diagnostic message for the test and repair operator. A narrower measurement limit for an array of identical LEDs will reduce the occurrence of contrasting color shades.

White light has no wavelength. It is the result of combining the primary colors red, green and blue (RGB). RGB color palettes can form white by setting R=255, G=255 and B=255. The LED sensor can detect white light by breaking it down into each RGB component and comparing each primary color to 255.

ICT measurement for LED intensity can be made more challenging than color, especially if an additional transfer medium like optic fiber is introduced in the LED test setup. The LED’s intensity will suffer two losses, through air and the fiber optic media, putting a premium on the sensitivity of the light sensor to detect intensity differences. The test setup coupled with the double conversion errors, previously explained when using voltage or frequency as measurement units, will make the intensity measurement highly unreliable.

The LED intensity at the light sensor can be influenced by three additional elements: dispersion angle of LED; distance of the LED from the light sensor, and ambient light. Every LED is encapsulated with a lens that determines the viewing angle (specified as dispersion angle in the LED datasheet). Light from the LED is not uniform when viewed from different angles. A narrow dispersion angle will concentrate the light in a tight beam. Figure 4 illustrates how the LED lens influences the dispersion and the distribution of color and intensity at the target. When the light sensor is placed farther away from the LED, the additional distance increases the transmission loss, putting a premium on the sensitivity of the light sensor. In such situations, a poorer quality light sensor will produce inconsistent measurements. Unreliable results are costly, as they will require another verification process.

Ambient light will dilute the LED intensity at the target. It is imperative the environment in the ICT fixture is made conducive for light sensors to produce consistent and accurate measurements. It is not unusual to find the light sensors housed in a darkened enclosure to eliminate noise from ambient light and improve the consistency of the measurements. Alternatively, light sensors can be individually fitted with a shroud tube to shield ambient light and focus the LED beam directly on the light sensor.

While the LED color can be represented in units of nanometer, the light intensity should be measured as the amount of energy on the surface area of the target. The unit of measure will not be identical to datasheets specifying the emission of LED luminosity in miniCandelas (mcd). The light intensity at the target will be weaker after passing through the transmission medium. A reasonable unit of measure is microWatt per centimeter square (µW/cm2), with energy represented in watts and surface area in cm2.

The intensity of the LED is just as important as the color. Two identical LEDs of the same color but of different intensities can easily be distinguished by the human eye. In critical LED applications like medical devices, a dimmer LED can misrepresent the status of the device. Similarly a stricter measurement limit for LED intensity will prevent such incidents.

The two main criteria in judging the performance of LED applications are color accuracy and intensity consistency. In ICT, tight measurement windows for LED color and intensity are consistently achievable if the light sensors are repeatedly accurate and reliably sensitive to discern minor variations. Inferior light sensors will produce more false calls within a narrow measurement window, reducing the productivity of LED test at ICT, and consequently poorer ROI.

Mark Lau is in product marketing, Measurement Systems Division at Agilent Technologies (agilent.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Friday, 30 March 2012 16:34


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