Are there color variations among LEDs?

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Volume 7 Issue 3	May 2003

beam angle - The angle at which luminous intensity is 50 percent of the maximum intensity. bin - To sort or classify light sources (such as light emitting diodes) into groups according to their luminous intensity or color appearance. lumen maintenance - The ability of a lamp to retain its light output over time. Greater lumen maintenance means a lamp will remain brighter longer. The opposite of lumen maintenance is lumen depreciation, which represents the reduction of lumen output over time. Lamp lumen depreciation factor (LLD) is commonly used as a multiplier to the initial lumen rating in illuminance calculations to compensate for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1. conduction - The process of removing heat from an object via physical contact with other objects or materials, usually metals. convection - The process of removing heat from an object through the surrounding air. cosine distribution - A property of a light source such that its luminous intensity in a particular direction is proportional to the cosine of the angle from the normal to the source. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. heat sinking - Adding a material, usually metal, adjacent to an object in order to cool it through conduction. illumination - The process of using light to see objects at a particular location. indication - The process of using a light source as something to be seen as in signaling. junction temperature - For light emitting diodes, the temperature of the light-emitting portion of the device (see PN junction), which is inversely correlated with its light output. monochromatic - For light, consisting of a single wavelength and having a very saturated color. PN junction - For light emitting diodes, the portion of the device where positive and negative charges combine to produce light. pulse-width modulation - Operating a light source by very rapidly (faster than can be detected visually) switching it on and off to achieve intermediate values of average light output; the frequency and the duty cycle (percentage of time the source is switched on) are important parameters in the modulation. semiconductor - A material whose electrical conductivity is between that of a conductor and an insulator; the conductivity of most semiconductors is temperature dependent. spectral power distribution (SPD) - A representation of the radiant power emitted by a light source as a function of wavelength. substrate - For light emitting diodes, the material on which the devices are constructed. tri-phosphor - A mixture of three phosphors to convert ultraviolet radiation to visible light in fluorescent lamps; each of the phosphors emits light that is blue, green or red in appearance with the combination producing white light.

There are generally small differences in color among LEDs when they are first manufactured, and then drifts in color as they are operated over the long term. With mixed-color systems to create white light, the initial color is probably more consistent, but without carefully designed compensation circuits to account for the different rates of lumen maintenance for the various colors used, the appearance of the light from a mixed-color array can drift noticeably from white. With phosphor-based white LED devices, small manufacturing differences in depositing the phosphor near the semiconducting element can alter the initial appearance of the resulting light; further, differences in the degradation rates of the phosphor and the emitter can also create variations in color through time.

Manufacturers work to bin LEDs to provide batches of products that will have similar initial appearance and lumen maintenance characteristics to maintain consistent appearance. Binning LEDs for small differences can increase their cost and the resulting cost of systems using them.

How large can color variations be for acceptable use?

At present there are no standards for how large color variations can be for acceptability. Research studies for different applications provide some guidance for those applications, but the acceptable tolerances for some applications will be different from those for other applications. For example, if a lighted scene contains many different colors, such as a retail grocery display (Figure 14), the tolerance for color variations among different locations in the display would be greater than if the location to be lighted were a plain, light-colored wall (Narendran et al. 2003b).

Figure 14. Retail grocery display.

Small color variations in the illumination from one shelf to another in refrigerated display cases will be difficult to detect when the display contains colored packages as shown here.