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Optical Filters

Constructing photoreactors requires choosing a suitable construction material. Beside typical constraints such as chemical or temperature resistance special attention has to be paid to the optical properties of the construction material. All material between the light source and the reaction zone has to be transparent for light with wavelengths required to drive the desired reaction. This includes the reactor walls as well as cooling jackets (for both reactor and light source).

Typically, some kind of glass is used. As long as wavelengths larger than approximately 300 nm are sufficient to initiate the reaction, borosilicate glasses can be used. The optical transmissions are well suited for this wavelength range. If shorter wavelength are required, the use of quartz glasses can not be bypassed. While this material transmits irradiation down to a wavelength of approximately 170 nm and shows a very good chemical resistance, processing of quartz is much more complicated compared to borosilicate glass. The more difficult processing is the reason for much higher costs of quartz glass. As explained in the article light sources, most technically used light sources emit polychromatic light, from which usually only a certain wavelength range is beneficial for the desired reaction. Light with other wavelength might induce undesired side reactions. Hence, it is required to prevent light with undesired wavelengths from interacting with the reaction solution. This can be realized by utilizing optical filters.

Optical filters can be solid or liquid materials. A wide variety of filter materials can be purchased. Figure 1 shows an overview of the available filters for UV and visible light sold by the company Asahi Spectra. If solid filters such as colored glasses are used it has to be considered that the glasses will be heated by the absorption of the light which shall not reach the reactor. Hence, it is necessary to provide sufficient cooling. The degree of absorption of colored glasses often decreases during intense irradiation and a more frequent replacement of the filter might be required during operation of the photoreactor.

As an alternative, filter solutions of inorganic salts and/or organic compounds can be used. Circulating these solutions in the cooling system provides good heat management and an easy replacement of degraded solutions.Combinations of different filters can be used to allow transmission in small wavelength ranges only.

 LS6Figure 1: Overview over bandpass filters sold by the company Asahi Spectra.[1]

  1. Bandpass Filters for Light Sources | Asahi Spectra USA Inc., 2016, http://www. asahi-spectra.com/opticalinstrument/bandpass_filters_ls.html.

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