Optical Pyrometer

The filament and the target must appear at the same focus, or you cannot compare them.

1. Fit an objective lens at one end of a blackened tube, focusing the target onto a plane inside.
2. Mount a small tungsten filament lamp at exactly that plane, so the filament sits IN the focused image.
3. Add an eyepiece behind it, focused on the filament.
4. Look through and confirm you see the filament sharply, superimposed on a sharp image of whatever the objective is pointed at.

Both must be sharp together. If the filament is in focus and the target is not, you are comparing a bright wire against a blur and the match point becomes vague. Getting the two into one plane is the whole optical design, and it is why the instrument has a fixed working distance range.

Filament current is the measurement. Everything else is optics.

1. Power the filament from an adjustable supply through a fine potentiometer.
2. Put an ammeter in series so the current is read precisely.
3. Turn the knob and watch the filament go from black through dull red to bright yellow.
4. Note the current at several visible colours.

The dial reads current and is LABELLED in temperature. There is no thermal sensor anywhere in the instrument — the scale is a calibration, established once by pointing the pyrometer at sources of known temperature. That is the same arrangement as the airspeed indicator in the pitot blueprint: a pressure gauge with a velocity scale printed on it.

Comparing brightness across different colours is unreliable. Remove the colour.

1. Fit a deep red filter between the filament and the eyepiece.
2. Look again: both the filament and the target now appear in the same narrow band of red.
3. Compare the ease of judging the match with and without the filter.

Without the filter you are comparing an orange filament against a yellow furnace and guessing. With it, both are red and only their BRIGHTNESS differs, which the eye judges far better than it judges colour. There is a second reason too: the physics relating brightness to temperature depends on wavelength, so the calibration is only valid if the instrument always looks at the same wavelength. The filter enforces that.

The instrument assumes the target radiates like a perfect black body. Real surfaces do not.

1. Point it at a kiln with a known thermocouple reading and set your scale against it.
2. Now heat two objects to the same temperature: one dull black oxidised steel, one bright polished steel.
3. Read both with the pyrometer.
4. Compare against a thermocouple touching each.

Ludwig Holborn and Ferdinand Kurlbaum published the disappearing-filament pyrometer in 1901 at the Physikalisch-Technische Reichsanstalt, the German national standards laboratory. Earlier optical pyrometers compared brightness against a standard lamp side by side; putting the filament INTO the image so it visually disappears is what made the judgement easy and the instrument practical.

Null methods are the most accurate kind of measurement, and this is a fine example. You are not asking the observer to judge how bright something is — a hopeless task — but only whether two things are the SAME brightness, which the eye does extremely well. The Wheatstone bridge in the resistance thermometer works identically: it does not measure resistance, it detects when a difference is zero. Whenever you can turn a measurement into the detection of a null, do it.

Where it sits in the chain: Wedgwood's pyrometer read a permanent effect after the fact. The thermocouple gave a live signal but had to be inserted into the process and eventually degrades there. The optical pyrometer removes contact entirely, so it can read a steel ladle, a glass furnace or a filament in a vacuum tube — anything visible and hot enough to glow.

Its honest limits: it only works above roughly 700 °C, where objects glow visibly; it depends on emissivity, which is often unknown; smoke, dust and dirty windows between instrument and target all read low; and the classic version depends on an observer's eye, so two operators may differ slightly. Modern radiation pyrometers replace the eye with a detector but inherit the emissivity problem completely.