Patent ID: 8471565

Claim:
A method of estimating the output light flux of a light emitting diode, comprising: applying a drive current waveform to the LED over a period of time comprising a testing period; monitoring the forward voltage across (Vf) the LED during the testing period; and estimating the output light flux as a function of changes (ΔVf) in the forward voltage, wherein the drive current waveform comprises a first current portion (I high ) for current stressing the LED and a second current portion (I low ), wherein the forward voltage (Vf) is monitored during the second current portion, wherein the forward voltage (Vf) is measured at the beginning of the second current portion (I low ), after the end of the first current portion (I high ), and subsequently when the forward voltage (Vf) has stabilised during the second current portion, wherein the estimated output light flux is determined as Φ age = Φ pristine · P light_age P light_pristine wherein Φ age is the output flux after ageing, Φ pristine is the output flux before ageing, P light — age is the power supplied to the LED after ageing which results in light output rather than heating and P light — pristine is the power supplied to the LED before ageing which results in light output rather than heating, and wherein P light — age is obtained from: P light_age = P in_age - P heat_pristine · Δ ⁢ ⁢ V f_age Δ ⁢ ⁢ V f_pristine wherein P in-age the total input power, P heat — pristine is the power supplied to the LED before ageing which results in heating rather than light, ΔV f — age is the difference in measured forward voltage before and after the first current portion and after ageing, and ΔV f — pristine is the difference in measured forward voltage before and after first current portion and before ageing.