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Here is something that I determined as part of a quality control measure and creation of an algorithm for creating a new process to measure luminance.

This diagram shows the ratio of the FFL351A 3700 K 95 CRI tested here between luminous flux and brightness in a reflector at a defined operating current, from 0 to 7 A. The x-axis shows the current, the y-axis the ratio between luminous flux in lumens and brightness in lx. The labels are in German, but certainly still recognizable.
The red line is the trend for these values to get a smoothed dataset for easier comparison.

The brightness was measured in one meter, the LED sat on the large heat sink with the fan switched on, as usual from the tests.

The luxmeter can exhibit a slight non-linearity, whereby this curve looks different depending on the LED type (especially domed/domeless and the design of the LED chip). This effect also occurs with my other luxmeter, to a similar extent. It does indeed seem to be the case that the brightness does not increase one hundred percent linearly with the luminous flux, but is subject to fluctuations.

I don’t know exactly where this effect comes from. It is possible that the refractive index of the silicone changes with increasing current and thus increasing LED chip temperature Tj. The phosphor could also promote altered radiation behavior. In any case, this effect - as far as my data already allows - is less pronounced with LEDs without domes.

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