According to the Merriam Webster dictionary, emissivity is

: the relative power of a surface to emit heat by radiation : the ratio of the radiant energy emitted by a surface to that emitted by a black body at the same temperature

So, you could think of it this way: every material or object emits radiant heat or infrared radiation at its own specific rate. The specific rate at which it emits thermal energy depends on the type of material it is made of, its surface properties, and the temperature of the object itself.

Because an infrared thermometer is measuring the infrared emissions of the object to determine temperature, the rate at which the object being measured emits infrared radiation must be factored into the thermometer’s temperature reading in order for it to be accurate.

Emissivity essentially compares the thermal radiation of a material to that of a perfect emitter, a ‘black body’. This black body has an emissivity of 1.0 and is used as the reference point for the perfect emitter.  That is, the ‘black body’ emits the full spectrum of infrared radiant energy when measured.

Emissivity is also closely related to absorption and reflection of light. All light falling onto the black body (incident light), is absorbed by the black body and the entire infrared component radiated back, no incident light is being reflected.

Materials that are not perfect emitters reflect a portion of incident light, and radiate less infrared light than a perfect black body emitter. An object with a polished metal surface, for instance, emits far less infrared radiation than the same object painted with matte black paint.

Infrared thermometers measure surface temperatures of objects by sensing the infrared radiation emitted by the object’s surface and then making a direct correlation between the amount of radiant energy being sensed and the surface temperature of that object. But because the amount of radiant energy being radiated depends on the emissivity of the object’s surface,  the temperature reading of the thermometer must be corrected for emissivity to be accurate.

The emissivity scale ranges from a relative value of zero to one. One represents the perfect black body emitter, while zero represents zero thermal radiance.

Most organic objects fall close to 0.95 on the emissivity scale and for this reason many IR thermometers are pre-set at an emissivity of 0.95.  However, objects with much lower emissivity ratings, such as polished metal, will give false temperature readings if the emissivity setting is not adjusted before the temperature reading is taken.

You might be tricked into assuming that emissivity is easy to determine simply by looking at an object. However, emissivity can be quite tricky to determine optically.  Most people might assume, for example, that shiny metal and ice have similar emissivity ratios; or that asphalt would have an emissivity close to that of the perfect black body used to develop the emissivity scale.  In fact, ice has an emissivity rating of 0.97, while shiny metals have emissivity ratings closer to 0.1 or even lower, and asphalt has an emissivity of 0.88, lower than ice; and certainly far from the 1.0 of the perfect black body emitter.

There is another consideration when dealing with emissivity values and temperatures, which is that rates of thermal radiation vary at different temperatures. While this variation is generally not significant, in some materials it is pronounced enough that it must be taken into consideration when determining temperature using an infrared thermometer.

The solar and energy conservation industries have been developing new materials which take advantage of high absorbency (like the black body emitter) for solar absorption but, unlike the black body have very low emissivity. This provides a material that is an excellent solar receptor but which holds rather than releases the collected solar energy.

Emissivity of some common materials (only use these values as a guideline, we cannot guarantee their accuracy):

Aluminum Foil 0.04
Aluminum anodized 0.90
Asphalt 0.93
Black Body, matte 1.00
Brick, red 0.75 – 0.93
Chrome, polished 0.05
Concrete 0.85 – 0.94
Copper, polished 0.02 – 0.05
Copper, oxidized 0.87
Fabric 0.87 – 0.98
Galvanized Pipe 0.46
Glass 0.92 – 0.95
Granite 0.45
Gypsum 0.85
Ice 0.97
Iron, polished 0.14 – 0.38
Iron, rusted red 0.61
Limestone 0.92
Marble, polished 0.89 – 0.92
Marble, white 0.95
Paper, white 0.68
Plaster, rough 0.89
Plastic 0.84 – 0.95
Quartz Glass 0.93
Rubber, black 0.95
Sand 0.90
Skin, human 0.98
Snow 0.80
Soil 0.92 – 0.95
Silver, polished 0.02
Silver, oxidized 0.04
Snow 0.80-0.90
Steel, oxidized 0.75
Steel, polished 0.07
Tape, electrical, black 0.97
Tile 0.97
Water 0.95 – 0.98
Wood 0.86 – 0.90