What can we use to show that sunspots are cooler than the rest of the Sun's photosphere?

Question

Quizplus · Accepted Answer

Sunspots appear darker than the surrounding areas of the Sun's photosphere because they are cooler. Wien's law relates the temperature of an object to the peak wavelength of its emitted radiation, and the Stefan-Boltzmann law relates the temperature of an object to its total emitted energy per unit surface area. By observing the spectrum and intensity of light from sunspots and comparing it to the rest of the photosphere, these laws can be used to deduce that sunspots are indeed cooler.

What Can We Use to Show That Sunspots Are Cooler