# simple test/measurement to quantify water opacity

I am performing some measurements on liquid surfaces and I want to quantify how these mesures depend on the liquid opacity by performing it with different concentrations of colorant. To allow reproductibility and improve the meaning of the results, I would like to measure this opacity in a standard existing scale (and not by simply saying that I put $n$ml of white paint). So my question is, do you know how can I make such a measure with no/simple instruments.

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Do you have a digital camera? – Chris Sep 8 '11 at 16:38
Yes, we have many. – benoit Sep 9 '11 at 8:12
Do you care whether the light is absorbed versus scattered by the colorant? – Steve B Sep 11 '11 at 1:59

You can get a new and pretty decent light meter for very little expense at any camera store or online. Depending on the setup of your experiment, it should be fairly simple to put a liquid sample in a clear container of a known thickness, shine a light of a known luminosity through it, and then read the light meter.

You make a tare measurement by using clear water and then scale your results accordingly (assuming no transmission to correspond to zero). If you wanted your results to be even more relevant, you could calculate the linear attenuation coefficient:

The transmitted intensity of light decays exponentially as

$$I(x) = I_0 e^{-\mu x}$$

where

$I(x)$ = The transmitted intensity as a function of thickness of material

$I_0$ = The intensity of light incident on the material

$x$ = The thickness of material through wich the light passes

$\mu$ = The attenuation coefficient of the material.

So that

$\mu = \frac{1}{x}\ln\frac{I_0}{I}$

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I'm curious as to why someone downvoted this... – David Z Sep 8 '11 at 17:09
Sometime you find these kinds of measurements expressed in terms of the "attenuation length" $l_a = 1/\mu$, and it is often necessary to express these values as a function of wavelength or frequency: $l_a = l_a(\lambda)$. – dmckee Sep 8 '11 at 18:39
Thanks AdamRedwine, that why I was looking for. I will try to borrow a light meter but I am not sure of being able to find one so if somebody has another idea, I will enjoy it. – benoit Sep 9 '11 at 8:10