I have been asked the following by a student;

“Does glass have a high or low emissivity? I always assumed that it was quite low as I can see my own reflection with my camera, but tables show it having a high value, Why?”

Answer;

Glass has a high emissivity. It is often quoted with a value as high as 0.95, which is a similar value to paint. It does seem confusing to people as these two materials have such similar emissivities, and yet we more often see our reflection in glass. Both of them are reflecting about 3% of the incident energy, and yet the reflections appear quite different. The reason this happens is due to the nature of the reflections themselves. Glass tends to have mirror like reflections, whereas paint is more diffuse. This means that the reflected energy from the camera operator is reflected directly back towards the camera, and with paint the incident energy is usually scattered in all directions. While table values are often listed as high as 0.95, it does depend on the sample, and the conditions of measurement. In my experience, it is usually a bit lower than 0.95, perhaps as low as 0.8. This still makes it a high emissivity material.