Ok guys I formally worked with radiation for a living. We used a formula called the Inverse Square Law to determine our doses of radiation at a given distance. The law also works with light and other forms of electromagnetic radiation. Here is an article pulled from the web with refrences. I know wikipedia isn't the best resource but this is verified and explained pretty well. So in my words if you double the distance of your light wether it be from 1 to 2 inches or 4 to 8 feet you will receive 1/4 or 25% of the Light intensity you received at the original distance. On the other hand the opposite is also true. If you cut the distance in half you will quadruple the Light intensity from the original location. Its a significant loss when your moving your lights. This has always fascinated me and while I'm no genius this has always proved to be useful for me when setting up my lights and deciding on distances. Well I hope this helps some people. Scroll down to see the full article. This is just my dummed down summary, it took me about 6 months to understand the Inverse Square Law.
Light and other electromagnetic radiation
The
intensity (or
illuminance or
irradiance) of
light or other linear waves radiating from a
point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source; so an object (of the same size) twice as far away, receives only one-quarter the
energy (in the same time period).
More generally, the irradiance,
i.e., the intensity (or
power per unit area in the direction of
propagation), of a
spherical wavefront varies inversely with the square of the distance from the source (assuming there are no losses caused by
absorption or
scattering).
For example, the intensity of radiation from the
Sun is 9126
watts per square meter at the distance of
Mercury (0.387
AU); but only 1367 watts per square meter at the distance of
Earth (1 AU)an approximate threefold increase in distance results in an approximate ninefold decrease in intensity of radiation.
In
photography and
theatrical lighting, the inverse-square law is used to determine the "fall off" or the difference in illumination on a subject as it moves closer to or further from the light source. For quick approximations, it is enough to remember that doubling the distance reduces illumination to one quarter;[SUP]
[4][/SUP] or similarly, to halve the illumination increase the distance by a factor of 1.4 (the square root of 2), and to double illumination, reduce the distance to 0.7 (square root of 1/2). When the illuminant is not a point source, the inverse square rule is often still a useful approximation; when the size of the light source is less than one-fifth of the distance to the subject, the calculation error is less than 1%.[SUP]
[5][/SUP]
The fractional reduction in electromagnetic
fluence (Φ
for indirectly ionizing radiation with increasing distance from a point source can be calculated using the inverse-square law. Since emissions from a point source have radial directions, they intercept at a perpendicular incidence. The area of such a shell is
where
r is the radial distance from the center. The law is particularly important in diagnostic
radiography and
radiotherapy treatment planning, though this proportionality does not hold in practical situations unless source dimensions are much smaller than the distance.
References
This article incorporates
public domain material from the
General Services Administration document
"Federal Standard 1037C".
- ^ Hooke's gravitation was also not yet universal, though it approached universality more closely than previous hypotheses: See page 239 in Curtis Wilson (1989), "The Newtonian achievement in astronomy", ch.13 (pages 233274) in "Planetary astronomy from the Renaissance to the rise of astrophysics: 2A: Tycho Brahe to Newton", CUP 1989.
- ^ Newton acknowledged Wren, Hooke and Halley in this connection in the Scholium to Proposition 4 in Book 1 (in all editions): See for example the 1729 English translation of the 'Principia', at page 66.
- ^ Williams, Faller, Hill, E.; Faller, J.; Hill, H. (1971), "New Experimental Test of Coulomb's Law: A Laboratory Upper Limit on the Photon Rest Mass", Physical Review Letters 26 (12): 721724, Bibcode:1971PhRvL..26..721W,doi:10.1103/PhysRevLett.26.721
- ^ Millerson,G. (1991) Lighting for Film and Television - 3rd Edition p.27
- ^ Ryer,A. (1997) "The Light Measurement Handbook", ISBN 0-9658356-9-3 p.26
- ^ Inverse-Square law for sound
