Why does LED brightness decrease with temperature?

It is true.

I did a Chemistry experiment, where we poured Liquid Nitrogen on LEDs and they would get brighter (due to lack of resistance induced by temperature).

Putting LEDs in boiling water made them dimmer.

Obviously neither of those things will (hopefully) not happen to a Move Controller, but thats why it changes in brightness.

This article offers a good explanation of how LEDs function.

The Cliff's notes version is that "LEDs typically change in brightness and color with temperature. As the temperature rises, the output decreases and the color shifts towards the higher end of the spectrum."

Some of the background as to why it increases increases in brightness: "...There are several factors that can affect the [wavelength]. One of these is temperature. As the ambient temperature rises, so to does the LED wavelength. This increase will typically be from 0.1nm/°C-0.2nm/°C depending on the type of LED used."

As the temperature decreases, so does the LED wavelength, making the LED brighter.

Any diodes have negative temperature coof. Some around 2.1mV/°C.

Output light is a power. If voltage across the LED drops 2.1mV/°C and current is constant - power goes down, light intensity too.

Why does LED brightness decrease with temperature?

In the above question

a)constant power supplying to LED, means no variations in power ok.

So In generally materials characteristics effects with temperature.

If Heat increases beyond the particular material properties,resistance increases causing oppose to flow of electrons ---when electrons decreases, Light (Brightness) gets low.

for fun only----LED using in AC Room gets very little effect than non AC room with Temp high.

Sorry i am not good yet english.