An irradiator is developed using two UVA wavelength ranges of SMD LEDs as a curing light source. This module has dimensions of 545×111×300 mm³ and is equipped with a TIR bar-shaped lens made of PDMS silicone resin. The developed irradiator offers high uniformity, with 89% in the centerline of the horizontal axis direction, for two different wavelength ranges of 365 nm and 385 nm. The radiation intensity from the light source module shows highly directional characteristics, and the irradiator provides a maximum irradiance of 1,634 mW/cm² at a working distance of 50 mm. During the initial 5 minutes of operation, the irradiance experiences a rapid decrease. However, this issue is addressed by optimizing the LED’s current reduction characteristics and managing the Transistor’s temperature rise in the constant current circuit. After continuous operation for approximately 60 minutes. The highest temperature, near the central part of the irradiating surface, reaches 69.7℃, while the lowest temperature, near the edges, is 41.1℃.

A total internal reflection (TIR) linear lens of size 190 (W) × 5 (D) × 2.1 (H) ㎣ has a directivity of 25° and was made of a polydimethysiloxane (PDMS) silicone resin with a refractive index of 1.4 and a transmittance of 93% at 365 nm UV wavelength. A light source with a size of 190×25.5 ㎟ was fabricated by installing a TIR linear lens on a chip on board (COB) type LED module mounted with a 1.1×1.1 ㎟ size UV LED. The optical characteristics of the light source showed a maximum irradiation density of 3,840 mW/㎠ at a working distance of 5 mm and a high uniformity of 91.6% over a 150×25 ㎟ irradiation area. The thermal characteristics of the light source were measured at a supply current of 500 mA. The saturation temperature was reached after 30 min of operation, and measured to be 95℃.

A high directivity TIR (total internal reflection) lens in the UV-A region was designed using a silicone resin, and a UV light source module with a maximum irradiation density of 150 mW/cm² was fabricated. The beam angle of the TIR lens was designed to be 8.04° and the maximum diameter of the TIR lens was ø13.5. A silicone resin having a UV transmittance of 93% and a refractive index of 1.4 at a wavelength of 365 nm was used, and the lens was manufactured using an aluminum mold, from which silicone could be easily released. The module was fabricated in a metal printed circuit board of COB (chip on board) type using a 0.75×0.75 mm² UV chip. A jig was used to adjust the focal length between lens and chip and to fix the position of the lens. The optical characteristics such as illumination distributions of the lens and module were designed using ‘LightTools’ optical simulation software. The heat dissipation system was designed to use a forced-air cooling method using a heat-sink and fan.

The improvement of irradiation intensity and irradiation uniformity is essential for large area and high power UVA light source application. In this study, large number of chips bonded by micro soldering technique were driven by low current, and current limiting diodes were configured to supply constant current to parallel circuits consisting of large number of series strings. The dimension of light source module circuit board was 350 × 90 mm2 and 16,650 numbers of 385 nm flip chip LEDs were used with a configuration of 90 parallel and 185 series strings. The space between LEDs in parallel and series strings were maintained at 1.9 mm and 1.0 mm distance, respectively. The size of the flip chip was 750 × 750 μm2 were used with contact pads of 260 × 669 μm2 size, and SAC (96.5 Sn/3.0 Ag/0.5 Cu) solder was used for flip chip bonding. The fabricated light source module with 7.5 m A supply current showed temperature rise of 66℃, whereas irradiation was measured to be 300 mW/cm2. Inaddition, 0.23% variation of the constant current in each series string was demonstrated.

Photoemission is a process in which photons are converted into free electrons. Photocathodesare the typical materials for the process. They emit electrons when a light is irradiated upon. Thetraditional method of manufacturing photocathodes is complicated, requires specialized equipment, and islimited very small sized samples. Cs3Sb photocathode was formed on a substrate in   atmosphericconditions. The photocathode formation was a gas phase reaction with the substrate. Vacuum deviceswere made to test electron emission characteristics of the formed photocathode. Visible light ofwavelength 475 nm was used for the primary light source. The results showed high current density andlong term stability of the photoelectron emission.