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1 CONTAMINATION CONTROL IN AN EUV LIGHT SOURCE BACKGROUND [0001] Extreme ultraviolet radiation, for example, electromagnetic radiation having wavelengths 5 of around 50 nanometers (nm) or less (also sometimes referred to as soft x-rays), including radiation at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in or on substrates such as silicon wafers or in inspection processes to scan a surface of the silicon wafer or a reticle to detect defects, measure dimensions, or analyze chemical composition. Methods for generating EUV radiation include converting a target material to a plasma state. The 10 target material includes at least one element, for example, xenon, lithium or tin, with one or more emission lines in the EUV portion of the electromagnetic spectrum. The target material can be solid, liquid, or gas. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a “source” laser, for example, a CO2 laser emitting infrared light at a wavelength at or about 10,600 nm, to irradiate with one or more light pulses a target containing one or 15 more EUV line-emitting elements. The plasma is typically produced in a sealed “source vessel” which is typically within a vacuum chamber. DESCRIPTION [0002] Referring to Figs. 1A and 1B, an apparatus 100 is configured for quantifying and 20 controlling contamination in a light source 150. The apparatus 100 includes an image detector 105, an actuation system 115, and an image processing system 125 in communication with the image detector 105 and the actuation system 115. The image detector 105 is configured to obtain one or more images 106 of an optical element 155 in the light source 150. The image processing system 125 is configured to receive, from the image detector 105, a reference image of the optical element 155 and one or more 25 real-time images of the optical element 155. The image processing system 125 is configured to compare a current real-time image with the reference image to determine a contamination metric associated with the optical element 155. The actuation system 115 is configured to receive the determined contamination metric from the image processing system 125 and automatically trigger an operation in the light source 150. The operation that is triggered is based on the received 30 contamination metric and also mitigates contamination of the optical element 155. Specifically, the apparatus 100 can be configured for quantifying and controlling contamination of a surface 155s of the optical element 155. Thus, the images 106 can be taken of the surface 155s of the optical element 155. Contamination of the surface 155s can be caused by debris within the light source 150, and such debris can reduce the performance of the surface 155s and can therefore reduce the overall efficiency 35 of the light source 150. [0003] The apparatus 100 is active during a downtime mode of the light source 150. The downtime mode is depicted in Fig. 1A. While active, the image detector 105 is obtaining one or more 2 real-time images of the optical element 155, the image processing system 125 is analyzing the real- time images, and the actuation system 115 is triggering the operation in the light source 150. In the downtime mode, the light source 150 is not producing a light beam for standard use by an output apparatus 151. For example, the light source 150 can produce a stable light beam (such as a stable 5 extreme ultraviolet EUV light beam) at a very low brightness or power that would not be suitable for use by the output apparatus 151 but is bright enough to illuminate the surface 155s of the optical element 155. As another example, a secondary light source can be configured to illuminate the surface 155s of the optical element 155. [0004] Referring to Fig. 1B, the apparatus 100 is in standby during a production mode of the 10 light source 150. While in standby, the image detector 105 is not capturing images of the optical element 155, the image processing system 125 is not analyzing the images, and the actuation system 115 is...