Optical Pyrometer Temperature Measurement

PrayogSetu — Interactive Experiment Module

System Ready
Objective: Understand the construction, working principle, and disappearing filament principle of an Optical Pyrometer for non-contact high-temperature measurement.

Virtual Experiment Setup

Electric Furnace Thermal Radiation Objective Lens Eyepiece Optical Pyrometer ---°C PYROMETER READING Temperature Display Hot Metal Object ▼

Eyepiece View — Brightness Comparison

Furnace OFF

Start the furnace, then adjust the lamp brightness until the filament disappears against the glowing object.

Experiment Controls

Furnace Controls

Construction View

Lamp Brightness Control

0% 0% 100%

Furnace Temperature Setting

600°C 900°C 1500°C

Live Readings

🌡️
Object Temperature 25°C
💡
Filament Brightness 0%
📏
Measured Temperature ---

Experiment Status

⏸
Furnace OFF. Click Start Furnace to begin the experiment.
0%
Sr. No. Object Temperature (°C) Lamp Brightness (%) Filament Status Observation
No observations recorded yet. Start the experiment to begin.

🔧 Construction

An Optical Pyrometer consists of:

  • Objective Lens — Focuses thermal radiation from the hot object onto the filament plane.
  • Red Filter — Passes only a narrow band of wavelengths (~0.65 μm, red light) for monochromatic comparison.
  • Filament Lamp — A tungsten filament whose brightness is adjustable via a rheostat.
  • Eyepiece Lens — Allows the observer to view the superimposed image of the hot object and the filament.
  • Battery & Variable Resistor — Controls the current through the filament, thereby controlling its brightness.
  • Ammeter (calibrated in °C) — The current at the match point corresponds to a specific temperature.

⚡ Working Principle

The Optical Pyrometer works on the principle of brightness comparison (Planck's radiation law).

  1. The hot object emits thermal radiation proportional to its temperature.
  2. The objective lens focuses this radiation to form an image on the filament plane.
  3. The red filter ensures only a single wavelength is compared.
  4. The observer adjusts the filament current until the filament brightness matches the object brightness.
  5. At the match point, the ammeter reading (calibrated in °C) gives the temperature.

Range: 700°C to 4000°C (typically used above 700°C)

Accuracy: ±5°C to ±10°C depending on conditions

🎯 Disappearing Filament Principle

This is the core measurement principle of the optical pyrometer:

Filament Too Dim

When the filament temperature is lower than the object temperature, the filament appears as a dark line against the bright background of the hot object.

Filament Too Bright

When the filament temperature is higher than the object temperature, the filament appears as a bright line against the relatively dimmer background.

Filament Matched ✓

When the filament temperature equals the object temperature, the filament disappears — it becomes indistinguishable from the background. The ammeter reading at this point gives the measured temperature.

✅ Advantages

  • Non-contact measurement — no physical contact with the hot object required.
  • Can measure very high temperatures (up to 4000°C).
  • Suitable for measuring temperature of moving objects.
  • No heat loss due to conduction.
  • Simple construction and operation.
  • Good accuracy at high temperatures.