Why don't we need a new basic unit to measure the perception of sound as happens with light?

Question

Light power it's measured in Watts/stereoradian but this is not sufficient to measure the human perception of light so it's "invented" the candela, which has to do with the “light sensitivity function”.

So it happens with sound. We perceive sounds with same power but different frequency as if they had different intensity. So it’s invented the loudness or the dBA which consists on adding or subtracting dB to the real intensity so we obtain loudness.

The difference between this two facts is for mi very big: light has to be measured with a new basic unit (candela) but sound can be measured just adding or subtracting to the real measurement (in dB’s) a factor given in a chart (in dB’s too) obtaining dBA which is not actually a new unit.

My question(s) (at last!) is(are):

Couldn’t be the light measured just with a correction factor as it’s done with sound so we didn’t have to use the candela?

Could be defined a new basic unit to express the loudness?

Answer 1

Interesting and non-trivial thought. Got me thinking.

On first impression I would have to say you are probably correct. One could measure visual light intensity just the same way as one measures loudness.

I think the closest you get to this concept is the visual luminosity scale of astronomers. One "mag" is a a logarithmic step of approx. 2.512 which is nothing but the fifth root of 100!

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

I believe this is completely analogous to dB in acoustics and it is probably rooted just as much in the logarithmic brightness scale of the human eye and visual cortex as the logarithmic dB "units" are based on the logarithmic response of the human ear.

What do you think?

Answer 2

We don't need a new one. We already have one, called loudness (see first reference). The decibel is just a convenient logarithmic scale. A factor of 2 in power is about 3dB. The loudness scale is weighted by human auditory perception, just as the candela is weighted by human visual perception. The two are indeed analogous.