Question about the brightness of light and the inverse square law.

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SUMMARY

The discussion centers on the concept of brightness and its relationship with the inverse square law, specifically how brightness diminishes with distance from a light source. Brightness is defined as a subjective perception related to luminance, measured in lumens (lm), which accounts for human eye sensitivity to different wavelengths. The correct application of the inverse square law involves comparing the ratios of spherical areas at different distances, rather than simple subtraction of distances. The formula for brightness comparison is 1/(d1^2) / 1/(d2^2), where d1 and d2 are the distances from the light source.

PREREQUISITES
  • Understanding of the inverse square law in physics
  • Familiarity with the concept of luminance and brightness
  • Knowledge of luminous flux and its measurement in lumens
  • Basic principles of geometry related to spherical areas
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  • Research the application of the inverse square law in different contexts, such as gravity and electromagnetism
  • Study the relationship between luminance and brightness in visual perception
  • Explore the calculation of luminous flux and its significance in lighting design
  • Investigate the differences between luminous flux and radiant flux in photometry
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Students of physics, lighting designers, and anyone interested in the principles of light perception and measurement.

zeromodz
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Okay, I know that the brightness of light dims by 1 / r^2. I just need a little help about brightness and so forth. What exactly is the unit of brightness ? Also, when using 1 / r^2

Say I am 5 meters away from a light bulb and find the brightness, then I step back 47 meters. Would I just find the brightness by 47 - 5 = 42.

Then take 1 / 42^2 = 1/1764

So, then if I move 42 meters away it would be 1/1764 less bright. Then it would be from my original position?
 
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Hi zero..you have the right concept, but the wrong math:

Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light.[1] In other words, brightness is the perception elicited by the luminance of a visual target. This is a subjective attribute/property of an object being observed.

http://en.wikipedia.org/wiki/Brightness

The radial lines of light, like those of gravitvy and electromagnetism from a point charge, decrease according to the area of a concentric sphere...4pi r2...

So you want the ratio of spherical areas at the two distances to determine the relative luminance...
1/(5)2/ 1/(47)2...which is NOT your calculated answer...

What would be the ratio in two dimensions? In one dimension??
 
The lumen (symbol: lm) is the SI unit of luminous flux, a measure of the power of light perceived by the human eye. Luminous flux differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light. The lumen is defined in relation to the candela by

1 lm = 1 cd·sr

That is, a light source that uniformly radiates one candela in all directions radiates a total of 4π lumens. If the source were partially covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux—only 2π lumens. The luminous intensity would still be one candela in those directions that are not obscured.
 

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