effect of Height on lighting design

i.qasim

Member
Dear All,

I went through alot of sites and pdf's on internet. and everywhere the calculation for number of fixtures for a given area has no factor for height of the ceiling. which means it may be calculating for a normal height, say 3-4 m.

But the problem I am facing is, I have to increase 2m of the height of the ceiling. which means the number of light fixtures should increase.

Can someone direct me to, or provide me some information.

Thank you all.

stevenje

Senior Member
Dear All,

I went through alot of sites and pdf's on internet. and everywhere the calculation for number of fixtures for a given area has no factor for height of the ceiling. which means it may be calculating for a normal height, say 3-4 m.

But the problem I am facing is, I have to increase 2m of the height of the ceiling. which means the number of light fixtures should increase.

Can someone direct me to, or provide me some information.

Thank you all.
Goggle "Inverse Square Law of Light". This would give you a start in understanding how light and distance work together.

gar

Senior Member
150523-1113 EDT

The inverse square law applies to an isotropic radiator. In a confined space with white reflective surfaces this is not a structure that would be close to an isotropic radiator. If the light sources have reflectors and the surface area of the light source and the work area are large compared to the distance between source and receptor, then there won't be much variation in the light intensity on the recptor area with changes in that separation distance.

A 4' fluorescent fixture with a limited reflector and no parallel fixtures has a light intensity variation of 35 to 25 in going from 18" to 36". A 0.71 light intensity ratio for a distance ratio of 0.5 . Square law would predict 35 to 8.75 , or an 0.25 ratio.

An incandrescent bulb with no reflector had readings of 30 and 15 for the 1 to 2 distance change, an 0.5 light intensity ratio for an 0.5 distance ratio. But the test was not in a good free space condtion, there were some reflective surfaces.

.

GoldDigger

Moderator
Staff member
The underlying physics tells us that the inverse square law applies only to a single point source.
For a line source (approximated by a single row of properly spaced fixtures) the intensity falls off simply as the inverse of the distance.
For an infinite plane source (approximated by a grid of sources) the intensity does not fall off at all.
For an interesting side effect of this, look up Hubble's Law in astronomy.