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82 PCB007 MAGAZINE I APRIL 2018 Among the spectrometric sensors, more ap- propriately named electromagnetic radiant en- ergy sensors, the visible light colorimeter, the ultraviolet-visible light spectrophotometer, and the infrared spectrophotometer are the most common. Only the first two are used in printed circuit processing. The colorimeter is used to analyze for cop- per in the electroless copper, acid sulfate plat- ing bath, and hydrogen peroxide-sulfuric acid etchant, and for nickel, cobalt, palladium, and chromium in their plating baths. The ultraviolet-visible spectrophotometer is now being employed because of their micro- processor control units. By selectively scan- ning wavelengths and using the first and sec- ond derivatives of the absorbance curves, these spectra can be compared to stored spec- tra and concentrations are calculated automat- ically. Many of the units can control replenish- ment and sampling directly. Analysis that can be performed in this way are determining the Au (I) and Au (III) in gold plating baths; the proprietary additives, nickel and impurities in a Watts nickel bath used for tab plating, and various copper analyses. The human can be removed, and the wave- length expanded with common UV-Visible spectrophotometers. Six common UV-Vis spec- trophotometers, sensitive from 340 nm to 1000 nm, are shown in Figure 7. Recently, with the advent of cameras in cell- phones, many DIY spectrophotometers have been designed. All work on the principle that LEDs have specific wavelengths of light and eliminate the need for expensive optical wave- length filters or monochromators gratings. A graph of the wavelength of 12 different LEDs available is shown in Figure 8, along with the popular DIY LEGO® Spectrometer used by mid - dle and high schools. The LEGO spectrophotom- eter is made from children's LEGOs and a wave- length grating cut from any used CD. The Rasp- berry Pi or Arduino camera module provides w avelength sensitivity from 400 nm to 900 nm. Design and parts are readily available online [1] . Figure 7: Six photos of common UV-Visible spectrophotometers available from laboratory supply companies.