Thin film coatings play a very important part in any optical system. An uncoated glass surface will reflect or transmit a percentage of the light depending on its refractive index and the angle of incidence.
Thin film coatings change the amount of light reflected or transmitted through the optics. These are very thin layers (of the order of Angstroms) of materials coated on to the substrate, hence the term "Thin Films". These can be single layers or multiple layers coated on top of one another to achieve the required reflection / transmission. Oxides, Fluorides, metals are used for this purpose. The evaporation has to be carried out in special coating machines developed for this purpose and under high vacuum. The whole process of coating is quite complicated and requires continual development. The coatings are designed using special software and then stabilized on the coating machine to get performance as per the design. Astro Optics has a two "box coater" units from Leybold, Germany, equipped with turbo molecular pump, e-beam evaporator, thermal evaporator, radiation heaters and Ion source. Starting with coating designs and processes purchased from Leybold in 1988, we have learnt and developed all coatings in-house over the years. We design our coatings using "Essential Macleod" from Thin Film Center, USA. With years of experience in this field, we have a vast range of coatings, mainly developed as per customer requirements. All the coatings are checked for performance on our Perkin Elmer spectrophotometers.
We are able to do coatings in the wavelength range 190-2500nm. Our coatings can withstand adhesion / abrasion tests as per MIL 675C / MIL 13830B. The cleaning of the optics prior to coating is done in Class 10,000 clean room area and the final checking and putting the pieces in the coating jigs is done on class 100 clean room tables. Each piece is individually inspected for defects prior to coating.
All glass surfaces have some reflection depending on their refractive index. N-BK7 glass which is most widely used, has a refractive index of about 1.51 in the visible region of light, and reflects an average of 4.25% of the light incident on it. Materials with higher index reflect a greater percentage of light. Anti-reflection coatings (or ARC's) are used to reduce the reflection from the surface of the optics and thus increase the transmission. These coatings can be of single layer or multi-layer depending on the material to be coated and application.
Some applications require the substrate to be coated so that large amount of the incident light or a specific amount is reflected. These are high reflection or partial reflection coatings. These can be either metal coated or dielectric coated.
Beam splitters are used to split the incident beam in two parts. These are used at an oblique incidence, usually 45°. These can be designed for either of the two polarization states or for the mean polarization. The beam is split in required R / T ratio.