There is no reliable mathmatical conversion of emmissivity that I trust. It is a function of the type of material, surface finish, cleanliness, etc. In practical situations, we almost always cross check an IR Thermometer against a thermocouple probe readout, and adjust emmissivity. You could do the same and create your own conversion chart, but you may be better off budgeting for another IR Thermometer where you can adjust emmissivity.

For materials of known emissivity like copper, i have used the standard value and have had good readings (~2-5% of Thermocouple readings)..

Please read on:

Emissivity - Wikipedia, the free encyclopedia

Table of emissivity The emissivity ε (radiant emittance factor) is Emissivity: Definition and Influence in Non-contact Temperature

>is there any chart available to convert one emmissivity reading to other.

No chart, but you can calculate a corrected temperature.

An IR thermometer with a fixed emissivity of 0.95 will indicate lower temperatures than reality for objects with emissivities lower than 0.95.

In most commercial products, emissivity is a arithmetical divisor or multiplier factor.

If an object has an emissivity of 01.0, then it emits IR at only 10% of that which a black body would emit at that same temperature. The IR thermometer only "sees" 10% of the energy that a black body would produce at the same temperature.

IR thermometers with an emissivity correct factor solve an algebraic equation using the stated emissivity factor, (in the example, 0.10)

10% of what (x) = "energy seen"
0.10 * x = "energy seen"
x = "energy seen"/0.10

Dividing by 0.10 is the same arithmetically as multiplying by 10.

If the 'energy seen' for a object with a 0.10 emissivity factor = 95 Deg C (if it were a black body), then 95 Deg C/0.10 = 950 Deg C for a reading from an object with a 0.10 emissivity.

The IR thermometer is already adding about 5% as its compensation for its fixed 0.95 emissivity factor.

To correct for the fixed 0.95 emissivity of the IR thermometer, the fixed correction factor, 0.95, is divided by object's emissivity to get a correction multiplier factor: 0.95/object emissivity

For example:
For a black body with a 1.0 emissivity, the correction factor is 0.95/1.0 = 0.95

For an object with a 0.95 emissivity, the correction factor is 0.95/0.95 = 1.0; no correction necessary.

For an object with a 0.50 emissivity, the correction factor is 0.95/0.50 = 1.9

For an object with a 0.13 emissivity, the correction factor is 0.95/0.13 = 7.31

An instrument using a fixed emissivity of 0.95 might not have a sensitivity needed to accurately read the lower levels of IR radiation of objects with low emissivity.