Since the forward voltage varies with temperature, the output power delivered to LEDs is decreasing toward balance and may be observed if the input power variation of the LED luminaire is recorded. ![]() Short term light depreciation is mainly caused by LED’s reaching the thermal balance (Figure 1). Light degradation processĭecreasing the light output of a luminaire during operation is mainly caused by depreciation of the light source, the diminution of light transmission through the transparent screen (otherwise diffuser), the reduction of the power of the power supply and the ambient temperature conditions. Finally, the results control the LED’s supply current, through the dimming feature of the power supply in order to balance the difference in respect to the overall initial light output performance. The photometric performance, respectively, the maintenance of the luminous flux over the operating time, is analyzed in relation to the reference values in real time. Ultimately these reference values of transmittance and luminous flux are used through a mathematical process to determine the overall level of light degradation of the apparatus. Therefore the reference is a numerical value that is in a mathematical relation to the metrological value of these characteristics. The process starts by establishing a reference, respectively an equivalent value number allocated to the initial luminous flux, may also be called a preset flux, together with an equivalent value number of the light transmission through the diffuser or the optical system (transparency or transmittance) as measured by a sensor. Such a dynamic Balanced Light Output (BLO) method is maintaining the luminous flux over the lifetime of the LED luminaire, considering the real luminaire global light degradation instead of LED projected life. To include all combined effects in a CLO operation should require a different procedure otherwise may not be any longer of a preset stepping type. In reality there are many factors that require a different algorithm in terms of time rate and forward current values as to maintain a genuine constant light output. ![]() Unfortunately regardless of the CLO type procedure the method does not compensate real light output degradation of the luminaire even for same type of LEDs, otherwise even the constant light output may not be achieved. Ultimately based on a prediction supposedly applying to any LED technology and luminaire configuration. This CLO feature increases the forward current at specific time rate, disregarding overall luminaire performance. The latest generation of LED drivers is managing this problem by employing a Constant Light Output (CLO) operation. Most of the LED lighting system manufacturers are declaring luminaire lifetime based on this acceptance despite the complexity of this matter. Basically IEC 62722 tolerate replacing LED luminaire lumen maintenance with LED emitting surface as long such data is available. There are a set of standards defining how LED luminaires can be tested to determine their performance over time.Īccording to IEC 6, the LED luminaires using modules in conformity to IEC 62717 definition, may be declared the lumen maintenance value for the module as the maintenance value of the luminaire. The life of an LED luminaire is the combined effect of gradual light output degradation caused by material degradation and abrupt light output degradation mostly caused by LED components failure. The reason is the diversity of LED technologies and the particular contribution of the system components such as lens, driver, heatsink, etc., to this process. Resources | LpR Article | Engineering | Lifetime | Drivers | Method and Circuit to Maintain Constant Light Output for LED LuminairesĪccurate lifetime prediction or estimation of the light output degradation for LED luminaires is difficult. Stelian Matei, from the Photometry and Electromagnetic Compatibility Laboratory at Electromagnetica SA, proposes and explains a new method is maintaining the luminous flux over the lifetime of the LED luminaire, considering the luminaire global light degradation. However, there are many factors that require a different algorithm in terms of time rate and forward current values that are usually not taken into account. A Constant Light Output (CLO) operation is implemented in some of the latest drivers. ![]() ![]() Method and Circuit to Maintain Constant Light Output for LED Luminaires Article from LpR 66 | page 80: It is difficult to accurately predict the lifetime or the light output degradation of LED luminaires.
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