These studies are mainly focused on characterizing the height of the reliefs, but not the shape. In these works, spatial frequencies between 60 and 200 L/mm are used, and the reliefs generated are approximately 0.4 µm to 1.4 µm thick, with a development step with water and a postprocessing method with UV light. The analysis of these aspects was partially performed and reported in references, using DCG films. In the case of relief elements, the depth and the form of the surface relief depends on the exposure time. In optics, when a photosensitive material is characterized, the recording of sinusoidal patterns showing different spatial frequencies is usually performed. Moreover, the properties of gelatin such as its viscosity, gel strength, softening behavior, thixotropy, and melting point can be modified by UV light, heat, chemicals, and ultrasound. Gelatin is colorless, odorless, tasteless, non-soluble in organic solvents, environmentally friendly, easy to handle and to purchase, and cost effective, in addition it displays good clarity, biocompatibility, and biodegradability, among other characteristics. Gelatin is a good candidate for a safer fabrication of surface relief optical elements therefore, there is a need to study and characterize the formation of the relief structures with gelatin films. Ĭhemicals such as photoresists are commonly deployed to fabricate relief optical elements, but their use poses a threat for the environment and for the users. Recently the relief elements were applied as host in the alignment of liquid crystals (LC) for applications such as photoswitchable gratings, birefringence gratings, beamsplitters and in polarizing gratings. The relief elements comprise among others diffraction gratings, lenses, zone plates, holograms, and computer-generated holograms (CGH). DCG plates can be made with gelatin and dichromates or with Kodak 649 F plates used as gelatin films where the silver halide has been removed with the development and fixing process.īesides DCG plates, other materials have been used to fabricate optical relief elements, such as albumen, acrylamide dry films, polyacrylamide wet films (used to record transient interference patterns), silicone, gelatin, liquid resins, and silver halide plates. The information is stored in the gelatin bulk as changes in refractive index. He found diffraction efficiencies of 33% for thin phase gratings and 95% for thick holograms. In 1968, Shankoff proposed the use of dichromated gelatin plates (DCG plates) as the material to record holograms. This process has been used in the printing industry, where one film could be used for 2000 impressions. So that when washed with an appropriate solvent, only those areas which were exposed to light through a negative remain in the supporting material, forming a relief image. The films become hard and less soluble in areas exposed to radiation. Since the 1800s, dry films of chromates and dichromates mixed with organic materials such as gelatin, fish glue, gum arabic, or shellac were known to be affected by light. The relief height increased up to seven times when papain was used. In contrast, when UV light was used, the flat surfaces were taller than the peaks. When visible light was used, the DCG areas where the Ronchi grating had transparent slits showed a flat relief and the areas where the Ronchi grating had opaque slits showed a round peak, with the peak being taller than the flat surface. For short exposure times, the reliefs showed a sinusoidal profile. For the development process with just water, it was found that when gratings were recorded using visible or UV light, the height profile inversely correlated to spatial frequencies. This enzyme is used to improve the gratings’ relief which was studied with a profilometer. Two development processes were used, one included washing the plates with just water and the other with a mixture of water and papain. Gratings with low spatial frequencies were contact-copied on the DCG (dichromated gelatin) films. Two light sources were used: a laser (λ = 468 nm) and an ultraviolet mercury-metal halide lamp. A study of photoinduced relief using dichromated gelatin films with different thickness is described in this paper. The use of surface relief structures is increasing in the field of optics.
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