- #1

roam

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I want to know what exactly is the

From the definitions:

When light is incident on a material, by finding the fraction that gets reflected and transmitted, we can find the

$$\underbrace{R}_{\text{reflectance}}+\underbrace{A}_{\text{absorptance}}+\underbrace{T}_{\text{transmittance}}=1$$

$$\text{Absorbance}=\log_{10}\left(1/T\right)$$

For a given sample, I used a spectrophotometer to find both quantities. But the curves appear to give contradicting information about my sample:

Here is the problem:

The

I believe that these quantities are providing different information about the sample. But what exactly is the difference?

Any explanation would be greatly appreciated.

*physical significance*of absorptance versus absorbance. I have not come across a reference that clearly explains the distinction between the two quantities.From the definitions:

When light is incident on a material, by finding the fraction that gets reflected and transmitted, we can find the

*absorptance*— the fraction that is absorbed:$$\underbrace{R}_{\text{reflectance}}+\underbrace{A}_{\text{absorptance}}+\underbrace{T}_{\text{transmittance}}=1$$

*Absorbance*(also called optical density) follows from the Beer-Lambert law and is written:$$\text{Absorbance}=\log_{10}\left(1/T\right)$$

For a given sample, I used a spectrophotometer to find both quantities. But the curves appear to give contradicting information about my sample:

Here is the problem:

The

*absorptance*curve above shows that in the red (~680 nm) and in the blue-violet (<450 nm) regions we have approximately the same level of absorption. But the*absorbance*curve shows that the absorption is*much*greater in the blue-violet than in the red. These can't both be true.I believe that these quantities are providing different information about the sample. But what exactly is the difference?

Any explanation would be greatly appreciated.