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This study provides an estimation and methodology update on previous paper that studies the effect of having a second major in undergraduate on future earnings. Using 2019 National Survey of College Graduates (NSCG) data and Propensity Score Matching (PSM) method, I find that double majoring increases earnings by around 3% for the general population, and this earnings premium is more remarkable for people under the age of 40, which amounts to about 4%. While the proportion of double majors in the population drops from over 20% in 2003 to slightly above 13% in 2019, the returns to double majoring increase from around 2.5% since 2003. I also compare results from OLS regressions and PSM and argue that PSM can relax some of the parametric assumptions imposed by OLS regressions and hence reduce misspecification and extrapolation bias from OLS regressions, which previous literature on this topic relies on.

The construction of a smart city puts forward new requirements for lighting systems, such as variable color temperature adapting to environment and low energy consumption. We introduce a variable color temperature laser lighting system that produces uniform light with minimum energy. The color temperature is controlled by tri-color RGB diode lasers, and uniform lighting is achieved by microlens arrays. Tri-color diode lasers with wavelengths of 650, 556, and 450 nm are used as the lighting sources, and the white light laser output is achieved by combining the three beams. The color temperature is controlled by changing the power ratio of each lighting source. Finally, the homogenization of laser energy is regulated by the microlens arrays, and the energy uniformity reaches 91.1%. Moreover, we do an experiment to compare LED street lighting and laser street lighting, finding that the street lighting system with this design can increase the energy utilization rate by 113.33%, and the color temperature of the car headlamps with this design can be changed according to the environment. Therefore, this laser lighting system is an effective solution for modern smart lighting systems and energy saving, which have vast application.