Thursday, August 26, 2010

Relative Luminance vs Current

To measure the light emitted by the green LED I took a series of three pictures for each value of current using an EV bracketing of plus or minus 0.3 EV and adjusted the exposure so that the spot meter indicated that the exposure of the LED was at about +1.0 EV. The image was reduced to B&W and the pixel values were decompressed. The total "luminance", L, for each image was determined by summing the values for each pixel and the three bracketed luminances for each current setting were interpolated to find an EV correction which set log L equal to a central value of 6.8.

The EV for each current was computed from the exposure settings of the central image and the adjustments were made to get the EV which "equalized" the exposure. One can manipulate the formula for the EV to show that the relative flux, φ, is equal to 2 raised to the power of the EV.

The plot of the relative flux vs the current in mA proved to be linear on a log-log plot and the slope and intercept were computed.

So we have found the relative amount of light emitted by the LED as a function of the current in mA. Conversely, for a given relative flux we can compute the current required and use a milliammeter to control the LED light output.

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