Study on the Fabrication and Characteristics of the Bandpass Filter for Slit Lamp

Abstract

Purpose: The 450nm~500nm bandpass filters for slit lamp were fabricated and the characteristics of these filters were studied. Methods: The 450nm~500nm bandpass filter was designed by combining a longpass filter and a shortpass filter, which were based on the data of the 450nm~500nm bandpass filter design and were fabricated by electron beam evaporation apparatus. SEM photographs were taken to observe the cross section of the fabricated filters, and the spectrophotometer was used to measure the transmittance of the filters. Results: The experimental values of the fabricated shortpass filter were similar to the design values, but differed from the design values at a wavelength of about 490 nm and 350 nm to 400 nm. These differences were considered to be due to the fact that as the number of layers increased, the uniformity of the thin film became poor and the refractive index of the filter was changed slightly, resulting in a large difference in the slope of the transmittance. The experimental values of the fabricated longpass filter were similar in design values and the experimental curve was slightly shifted to the right as a whole. Therefore, the experimental curve of the 450 nm~ 500nm bandpass filter was shifted a little into the design curve of bandpass filter at 450 nm wavelength due to the right shift of the experimental curve of the longpass filter and the transmittance dropped about 20% at 500 nm wavelength. From the SEM photographs, the thicknesses of the fabricated shortpass filter and longpass filter were found by observing the cross section of the filters. Conclusions: The shortpass filter was fabricated on one side of the BK7 glass and the longpass filter was fabricated on the other side, and therefore the 450nm ~ 500nm bandpass filter was fabricated. The 450nm ~ 500nm bandpass filter was to increase the contrast of the area we want to see in observing the dry of the eye and the gap between contact lens and cornea.