Interesting explaination by SIM2 http://m150.sim2.com/content/led-brightness
Quote form that URL link:
"New brightness measurement concept
With the introduction of LED light source it becomes necessary to introduce a new concept of brightness that differs from the ANSI one normally used for UHP lamp projectors.
Some consideration on ANSI brightness measurement are needed:
ANSI Lumens is an engineering measurement of white light. It is a convenient relative measurement to compare lamp-based projectors with other lamp based projectors.
ANSI lumens do not accurately represent color brightness and cannot represent exactly the human eye perception of saturated colors.
It is very important to understand the concept of COLOR BRIGHTNESS.
The word “brightness” is being wrongly used in the Audio/Video market to describe how much bright a projector or a display is. In Physics, instead, another term is the correct one to evaluate it: the “luminance” (expressed in cd/m2 or Lumens)…and this is the measurable characteristic.
The brightness has actually an empirical meaning based on the human eye perception and it lacks a proper measurement unit.
It is exactly this that differs LED-based projectors form lamp-based ones.
Going into details.
Like previously said, the LED source produces richer and more saturated colors than the lamp source. Based on a particular effect called Helmholtz-Kohlrausch effect, brightness (or human perception) rises with the increasing of the color saturation.
So, comparing a LED projector with a lamp-based projector with the same “luminance” , the LED one results to be brighter to the human eyes! Based on empirical tests, brightness perception of LED-powered projectors is 25% more in respect to lamp-based ones.
In summary, to define LED projectors luminance with the same “catalog ANSI lm” as lamp projectors is a mistake because:
− the actual brightness delivered is higher, also in terms of stability over time
− the end user experience and perception of brightness is superior"
So "Helmholtz-Kohlrausch effect " is the explaination, I'll have to save that for reference!