Hello Adel,
There are two threads on the forum that discuss suitable cover materials for the TMP006:
- http://e2e.ti.com/support/other_analog/temperature_sensors/f/243/t/117229.aspx
- http://e2e.ti.com/support/other_analog/temperature_sensors/f/243/t/116631.aspx
Poly-Ethylene is the cheapest option that is transparent across wavelength range of the device.
Best regards,
Abhi