Absorbance is a quantitative measure expressed as a logarithmic ratio between the radiation falling upon a material and the radiation transmitted through a material.

A_\lambda = -\log_{10} \left( \frac{I_1}{I_0} \right),

where A_\lambda is the absorbance at a certain wavelength of light (\lambda), I_1 is the intensity of the radiation (light) that has passed through the material (transmitted radiation), and I_0 is the intensity of the radiation before it passes through the material (incident radiation).

Outside the field of analytical chemistry, e.g. when used in biology and the Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique, the absorbance is often defined using the natural logarithm instead of the common logarithm, i.e. as

A_\lambda = -\ln(I_1/I_0)\,

The term absorption refers to the physical process of absorbing light, while absorbance refers to the mathematical quantity. Also, absorbance does not always measure absorption: if a given sample is, for example, a dispersion, part of the incident light will in fact be scattered by the dispersed particles, and not really absorbed. However, in such cases, it is recommended that the term "attenuance" (formerly called "extinction") be used, which accounts for losses due to scattering and luminescence.[6]

See the Beer-Lambert law for a more complete discussion.

Although absorbance is properly unitless, it is sometimes reported in "Absorbance Units" or AU. However, many people, including scientific researchers, wrongly state the results from absorbance measurement experiments in terms of arbitrary units.[7]

Logarithmic vs. directly proportional measurements

The amount of light transmitted through a material diminishes exponentially as it travels through the material. Since the absorbance of a sample is measured as a logarithm, it is directly proportional to the thickness of the sample and to the concentration of the absorbing material in the sample. Some other measures related to absorption, such as transmittance, are measured as a simple ratio so they vary exponentially with thickness and concentration of the material.

Absorbance Transmittance (I_1/I_0)

0 1

0.1 0.79

0.25 0.56

0.5 0.32

0.75 0.18

0.9 0.13

1 0.1

2 0.01

3 0.001

Instrument measurement range

Any real measuring instrument has a limited range over which it can accurately measure absorbance. An instrument must be calibrated and checked against known standards if the readings are to be trusted. Many instruments will become non-linear (fail to follow the Beer-Lambert law) starting at approximately 2 AU (~1% Transmission). It is also difficult to accurately measure very small absorbance values (below 10−4) with commercially available instruments for chemical analysis. In such cases, laser-based absorption techniques can be used, since they have demonstrated detection limits that supersede those obtained by conventional non-laser-based instruments by many orders of magnitude (detections have been demonstrated all the way down to 5 10−13). The theoretical best accuracy for most commercially available non-laser-based instruments is in the range near 1 AU. The path length or concentration should then, when possible, be adjusted to achieve readings near this range.

Shade number

Some filters, notably welding glass, are rated by shade number, which is 7/3 times the absorbance plus one:[8]

shade number = \frac{7}{3}A + 1

or

shade number = \frac{7}{3}(- \log_{10} T) + 1

So, if the filter has 0.1% transmittance (0.001 transmittance, which is 3 absorbance units) the shade number would be 8.

Similar terms

Many similar terms are used to describe concepts relating to absorbance and some terms may have differing interpretation or usage in different disciplines.

Absorptance

Absorptance refers to a directly proportional ratio. Absorptance is the ratio of the radiation absorbed by a surface to that incident upon it. Total absorptance refers to absorptance measured over all wavelengths. Spectral absorptance refers to absorptance measured at a specified wavelengths.[9]

Absorptance is a simple ratio, whereas absorbance is a logarithmic ratio. This difference means that the two different measures are often used for different applications. Color and Vision Research Laboratories, Institute of Ophthalmology, UCL, explains:

Absorbance spectra are typically used to define photopigment spectra because their shape, when normalized (i.e., plotted as a fraction of the maximum absorbance), is independent of pigment optical density (pigment concentration). In contrast, the absorptance spectra, like the spectral sensitivity of the human subject, broadens as the optical density increases.[10]

Absorption factor

Same as Absorptance.

Absorptivity

In physics, the term spectral absorbance is used interchangeably with absorptivity, meaning the fraction of radiation absorbed at a given wavelength. In chemistry, absorptivity usually refers to Molar absorptivity, which is the constant \epsilon used in the Beer-Lambert law, A=\epsilon c l, where A is the absorbance, c is the concentration of the solution, and l is the path length.[11]

Mnemonic

A mnemonic to remember the difference between absorbance and absorptance is that absorbance has no t, or not t, meaning it measures all that is not transmitted.

I appreciate that very in depth response, but I mean absorption in terms of the psychological construct (i.e., the ability of an individual to become engrossed in the element of fantasy, day-dreaming, etc that they may possess in their lives). In terms of corollaries, the Fantasy facet of the Five Factor Model Openness domain provides the best general construct to explain a proximal idea.

as per Tellegen & Atkinson, 1974

Traditional methods of measuring the absorption in the bulk materials and thin films are spectrophotometric (low accuracy) and more accurate calorimetric methods. To measure the weak absorption were developed multi-pass sells with multiple reflection [ 20, 21] and intracavity methods both active and passive. The sensitivity of these methods is ~ 10-3 - 10-4 . Nevertheless, for the measurement of the absorption, for example, in thin films or small exposure of bulk samples of this sensitivity is not enough.

Laser photothermal spectroscopy is an alternative to traditional methods of absorption spectroscopy. Photothermal methods are active, are non-destructive and, in most cases, non-contact methods of research. A characteristic feature of all photothermal methods is to obtain information on the properties and composition of the medium to be analyzed on the basis of the direct detection of absorption of power by the concomitant changes in the physical and thermodynamic parameters of the medium. This immediately followed by the inherent features: 1) a method for measuring the zero (in the absence of absorption output signal is zero); 2) increase in sensitivity with increasing radiation power (up to the saturation regime of absorption); 3) the principal limitation the maximum sensitivity of the thermal fluctuations in the test environment.

Thus photothermal techniques may be implemented in two ways. In the first method, a continuous laser beam acting on the sample is periodically amplitude modulated (so called modulation photothermal spectroscopy). The second method - a pulsed photothermal spectroscopy using repetitive laser pulses. Both in the first and second object instances by absorption of laser radiation therein thermal waves are excited, causing modulation of the temperature and the detected signal. Analyzing the time dependence of the detected signal and its phase, you can get information about the various physical properties of the object, including the magnitude and dynamics of photoinduced absorption .

As noted above, photothermal spectroscopy is a modulation technique measurement when recurrent disturbance of any parameter of the test material, achieved by the action of a modulated (pulsed) laser radiation. The main advantages of its methods as compared to the conventional methods of spectroscopy due to the possibility of eliminating the background signal component unstructured and obtain the differential spectra. This is due to the fact that a detectable signal is directly connected not with the parameter to be measured and its variation, i.e. derivative of this parameter with respect to temperature. Therefore, photothermal spectra carry a particular analysis which gives an indication of the properties of matter, is not always manifested in the study by using the traditional methods of spectroscopy. Also, registering of a signal at the modulation frequency by using the synchronous detection greatly improves signal / noise ratio, which in turn improves accuracy.

Depending on the method of registration of the modulated signal of photothermal methods research matter most widely used methods of photoacoustic spectroscopy , photothermal deflection spectroscopy ( mirage effect) , thermal lens method ( thermal lens spectroscopy) , fotothermal radiometry and interferometry , as well as thermoreflection method .