Brightness reduction using 1.2V AA NiMh instead of 1.5v D alkaline?

[lumen aeternum]

Got some of those adapters to use AA size in a D flashlight.
I put them in a krypton 3D Maglight, and in a 3D maglight with an LED conversion.
Both seem less bright on the wall.

Does 3.6V instead of 4.5V make much difference outdoors?

What about the difference between 2.4 and 3.0 in a 2D light?
Both situations have 80% the specified voltage, but I don't know how linear voltage is for light output.

 

[xxo]

Does 3.6V instead of 4.5V make much difference outdoors?

Not really. Alkalines may be a tad brighter at first if they are very fresh, but their Voltage will soon sag below that of the NiMH's and the NiMH's will be brighter for the rest of the run because they can maintain their Voltage until they are pretty much dead.

 

[Lynx_Arc]

Actually rechargeable batteries are rated at "nominal" voltage not voltage off the charger nor after resting awhile off the charger.
Typically off the charger nimh cell are about 1.4-1.45 it varies by the charger and cell. After they rest for awhile maybe a day or two most drop to 1.38 or thereabouts. Nimh even off the charger will be lower voltage with no load.
The 2 advantages nimh have over alkaleaks is under a substantial load the alkaleaks will cave in voltage and runtime will be reduced accordingly while nimh thrives under heavier loads, even loads that would have an alkaleak cave too low to be useful.
D cell alkaleaks have a lower internal resistance than smaller alkaleaks such that it can keep up decent voltage at currents of 1A but still starts to falter at 2A and above vs even a AA nim cell. Lithium ion cells are 3.6/3.7v rated but off the charger most are between 4 to 4.2v with some even having 4.35v that are less common.
When using incans with no drivers even a 0.2V difference can be noticeable in brightness but using LEDs some of lights have drivers that either voltage is stepped down to match the LEDs required voltage.
Most incan lights the bulbs are about 1.2v/cell rated on purpose and essentially you are overdriving them at first. The difference between 2.4v and 3.0V can be very substantial with 3v being bright and white and 2.4v itself dimmer but then nimh aren't 2.4 V except under loads that would probably sink alkaleaks way down in voltage the are more like 2.7v to start off.