The inverse square rule holds in free space but may not hold in an
enclosed volume. Since the cross-sectional area of a cone of light grows
with the square of the distance, a constant amount of light distributed
over the increased area would provide an intensity which was diminished
by that same ratio.
If the cross sectional area illuminated were constrained to a constant
value, then the intensity would be constant as well. To put it another
way, if you had an evacuated pipe with walls which were perfect
reflectors and shined light down it, the intensity would be constant down
the length.
One can most closely simulate that situation with the aluminized mylar
sheeting available at the same indoor gardening stores that sell halide
lamps. It provides a smooth surface with greater than 98% reflectance. Do
not use aluminum foil. It tends to crinkle on application and even the
shiny side has reflectance below 90%.
Bob Cruder - bcruder@miaco.com