Light intensity units in Beer-Lambert law I have always found the Beer-Lambert law written in terms of light intensity, but what are the units of that light intensity?
When related to light, some people talk about cd, or W/cm/cm... What are the right units?
If they are W/cm/cm, could the Beer-Lambert law be rewritten in terms of power?
 A: Let's look at the Beer-Lambert law:
$T = \frac{I}{I_0} = e^{-\epsilon l c}$
It gives us transmittance $T$ - the ratio of two power intensities. Now, $I$ stands for the power intensity after travelling distance $l$ and $I_0$ stands for the power intensity in the beginning.
Transmittance $T$ is unitless, so basically the law isn't written in terms of any units. However, power intensity is power divided by surface, so it's unit in SI is $\frac{\textrm{W}}{\textrm{m}^2}$.
If the beam of light travels through material with constant cross-section, you can state transmittance as a ratio $\frac{P}{P_0}$.
Also, be careful when talking about candelas ($\textrm{cd}$). It's the unit of luminous intensity, which is a different quantity than power intensity.
A: Like many questions in life, the 'most meaningful' answer depends upon the 'context' of the question. This of course means that one must frame the answer using terms which 'make sense' to the one seeking the answer.
Firstly, when we speak of 'light' in the everyday sense, we mean that part of the electromagnetic spectrum which is 'visible' to the naked human eye, such as the light from a candle or lamp, which is used to 'illimunate' the objects around us.
Light is said to 'flow' from it's source, where the total amount of light 'flowing' from the source is is called 'luminous flux' which is measured in lumens ($lm$). 
As light radiates from the source, it's ability to illuminate decreases with distance as the light is distributed over an ever increasing area. Specifically, the 'illumunance' of the light source (measured in lux) is given as the 'flux per unit area', so $lux=lm/m^2$.
Because many light sources do not distribute light evenly in all directions, the light source typically comes with some data on a polar graph showing the 'luminous intensity' in each direction, which is measured in candela ($cd$).  
Finally, engineers and architects typically calculate the intensity of light over a surface, known as 'luminance', which is measured in $cd/m^2$.
The Beer-Lambert Law is a description relating to the attenuation of light through a medium. This law looks at the 'power' in Watts, $W$ transmitted by photons, where a photon of wavelength $\lambda$ has energy $E=hc/\lambda$. The number of such photons transmitted through a medium per unit time is a measure of the light intensity. A light source emitting more than one wavelength would be described by it's spectral power distribution.
The Beer-Lambert Law only describes the optical behaviour of the spectral power distribution through a medium. It does not take into account the 'photopic spectral response' of the human eye, that is it's response to light of various frequencies (energies), typically described as a 'luminosity function', $\bar{y}(\lambda)$ which is a measure of the 'luminous flux' per Watt ($lm/W$) over the visible range of wavelengths.

