Light Absorption

Sprachauswahl/Language selection

Dieser Artikel ist auch auf Deutsch verfügbar

A service provided by

Polymer Service GmbH Merseburg
Tel.: +49 3461 30889-50 E-Mail: info@psm-merseburg.de Web: https://www.psm-merseburg.de
Our further education offers: https://www.psm-merseburg.de/weiterbildung
PSM on Wikipedia: https://de.wikipedia.org/wiki/Polymer Service Merseburg

Light absorption

Physical principles

If an electromagnetic wave hits an outer or inner boundary surface, it will partially penetrate the material ( transmission), partially bounce back at the surface or boundary surface ( reflection) and is subject to absorption in the material, whereby the intensity I₀ of the incident wave is reduced. The absorption component also implicitly contains the scattering component.

In the case of light irradiation, this is referred to as light absorption, whereby the light interacts with the medium. As a result of this effect, i.e. light absorption, an excited energetic state (kinetic energy) of the material (absorber) is created in the irradiated material. The opposite of absorption is light emission (also known as spontaneous light emission), whereby the energy of the emitting material is reduced by a defined amount. As a result of absorption, the proportion of transmission through the material is thus attenuated (Fig. 1). The attenuating components, such as reflection and scattering with absorption, are often referred to collectively as extinction.

Fig. 1:

Absorption of light by an absorber a) and intensity curve b) [1]

The LAMBERT-BEER`s law of attenuation

When light passes through an absorbent material, the initial intensity I₀ is reduced to the value I(d) due to the generation of heat. This transmission loss depends on the thickness d and the absorption coefficient α, and is described here by LAMBERT-BEER's law (named after Johann Heinrich Lambert and August Beer) (Eq. 1) in analogy to the attenuation law.

I(d)=I_{0}\cdot e^{-\alpha d}

(1)

The albedo (ratio of reflection to absorption) of the investigated material is decisive for the absorbed light component, i.e. it is the degree of reflection of materials with different spectral ranges. In practical terms, an albedo of 0 percent means that there is no reflection on the surface and 100 percent represents a material without absorption.

The appearance of plastics, such as their colour, transparency, haze or opacity, essentially depends on two properties of the incident light. On the one hand, this is absorption, through which the incident light is converted into kinetic energy in the material and thus into heat, and on the other hand, the light is deflected from its direction of incidence by scattering effects.

Determining the reflection and transmission coefficient

Absorption can only be measured by determining the reflectance and transmittance.

When evaluating the optical properties, only the energy distribution of the light is considered. As the distribution depends on the wavelength λ, it is described by spectral material characteristic values. The spectral transmittance τ(λ) is the ratio of transmitted (Φ_eλ)τ and incident spectral radiant flux Φ_eλ, which characterizes the transmittance of a material.

The absorptance α(λ) is calculated according to (Eq. 2), where (Φ_eλ)α is the total spectral radiant flux absorbed in the material.

\alpha (\lambda )={\frac {(\Phi _{e\lambda })_{\alpha }}{\Phi _{e\lambda }}}

(2)

Analogous to this procedure, the spectral reflectance p(λ) can be determined according to (Eq. 3), where (Φ_eλ)p is the total spectral radiant flux reflected at the interface of the medium.

p(\lambda )={\frac {(\Phi _{e\lambda })_{p}}{\Phi _{e\lambda }}}

(3)

The transmittance is usually measured using spectrophotometers. Most unfilled and unreinforced amorphous plastics are transparent in visible light, i.e. they have no or negligible absorption. However, if dyes, heat stabilizers or UV stabilizers are added, the transmission changes considerably. Due to their small size, these additives can usually only be detected using scanning electron microscopy (SEM) or transmission electron microscopy (TEM).

[1]	Eichler, J., Eichler, H. J.: Laser – Bauformen, Strahlführung, Anwendungen. Springer Berlin (2003) 5. Edition, p. 258 (ISBN 3-540-00376-2)
[2]	Trempler, J.: Optische Eigenschaften. In: Grellmann, W., Seidler, S. (Hrsg.): Polymer Testing. Carl Hanser Munich (2022) 3. Edition, p. 299–330 (ISBN 978-1-56990-806-8; e-book 978-1-56990-807-5; see AMK-Library under A 22)

Physical principles

The LAMBERT-BEER`s law of attenuation

Determining the reflection and transmission coefficient

See also