How can you measure the absorbance of infrared by different surfaces?

To measure the absorbency of infrared set up an infrared heater with two metal plates on both sides. The inner side of the one plate is painted with shiny metallic paint, and the other with black matt paint. And the outside of the plates has a drawing pin attached using Vaseline. Now switch on the heater and start timing. The temperature of the metal plates will increase as they absorb infrared. Record the time it takes for the Vaseline to melt and the drawing pins to drop off. Finally, the drawing pin will fall off the matt black plate first because matt black surfaces absorb more infrared than shiny metallic surfaces. Therefore black matt surface better absorbs infrared than the shiny metallic surface that tends to reflect infrared.