How do you define perceived energy for an image?

From: kiki (lunaliu3_at_yahoo.com)
Date: 11/29/04 

Date: Mon, 29 Nov 2004 00:14:38 -0800

HI all,

Suppose I have an image, after upsampling and some other processing, I made
a high resolution (say 16x16 larger size) version of that image...

but the R, G, B values have been changed a lot, and new image looked darker,
but I want to make they perceived to be the same -- ie. their perceived
energy should be the same? So how to define some a term and then apply to
these two images to make them look same, (say several meters away for the
16x16 larger version?)

Thanks a lot!