Optical pyrometry reference point: In brightness (optical) pyrometry, high-temperature measurements are commonly referenced above the metal fixed point at 1063°C, known as the __________ point.

Introduction / Context:
Optical pyrometers measure high temperatures by comparing the brightness of a target to that of a calibrated filament at a selected wavelength. Calibration often uses internationally agreed fixed points—melting/freezing temperatures of pure metals—providing reproducible references. This question asks you to identify the fixed point at 1063°C used in classic pyrometry.

Given Data / Assumptions:

• Fixed-point metal at 1063°C (approximately).
• Application context: optical/brightness pyrometry.
• We assume standard ITS-90 fixed points used in calibration chains.

Concept / Approach:
The melting point of gold is 1063°C (more precisely 1064.18°C under ITS-90), well known as the “gold point.” Optical pyrometers traditionally reference temperatures at or above the gold point because targets exhibit visible incandescence and brightness techniques are accurate in this range.

Step-by-Step Solution:

Verification / Alternative check:
Calibration tables for brightness pyrometers list the gold point as a primary fixed point above which optical methods are mainstream.

Why Other Options Are Wrong:

• Antimony, silver, nickel, tin: Their fixed points do not align with 1063°C; silver is ~962°C, nickel is ~1455°C.

Common Pitfalls:
Confusing the silver point (~962°C) with gold; remember the mnemonic “Ag ~ 962°C, Au ~ 1063°C.”

Final Answer:
Gold