Bolometer

What is the definition of a bolometer?

A bolometer is a device that detects and measures the power or heat of incident electromagnetic radiation of RF or microwave energy using a temperature-sensitive sensitive element. Temperature-sensitive sensitive elements commonly employed include thermistors and barretters, whose resistance varies with temperature. It can monitor microwave energy at low and medium power levels.

Who invented the bolometer?

An American astronomer named Samuel Pierpont Langley devised a sensitive apparatus to measure heat and electromagnetic radiation in 1878. This device is known as a bolometer, and it is used to measure microwave radiation at low and medium power levels. It uses a heating substance and a temperature-resistant element to measure the power of electromagnetic radiation.

Bolometers have a higher energy resolution, are efficient and are very sensitive while they can operate at room temperature, avoiding the need for refrigeration. Low power is measured in the range of 0.1 mW to 10 mW, while medium power is measured in the range of 10 mW to 1 W.

Bolometer Principle

Bolometers are temperature coefficients that are either positive or negative depending on the type of temperature resistive sensing device used. The diagram of the bolometer will be in the shape of a bridge, with one arm containing a temperature-dependent resistor for measuring the power of microwave radiation. When the bridge circuit is unbalanced, it utilises differential amplifiers and oscillators to balance it.

The bolometer bridge circuit employs two types of temperature-sensitive resistors. They are:

Thermistor: This semiconductor material has a negative temperature coefficient and inversely proportional resistance to temperature.

Barretter: A barretter is an extremely thin metal wire with a positive temperature coefficient and resistance that is proportional to temperature.

A bolometer has a thermal reservoir and an absorbing element, a thin metal sheet. A thermal connection connects the thermal reservoir with the absorbent components. When electromagnetic radiation strikes an absorbing element, it raises the temperature over the thermal reservoir, resulting in a high temperature that exceeds the absorbed power. The intrinsic thermal time constant, which is the same as the absorbing element’s thermal conductance and heat capacity of the absorbing element and thermal reservoir, determines the instrument’s speed. A thermometer can be used to measure the change in temperature or resistance of absorbing materials.

So, let ‘P’ be the incident power absorbed by the absorber and ‘C’ and ‘T’ be the thermal mass’s heat capacity and temperature, respectively.

The thermal connection that links the thermal reservoir to a thermal mass is the thermal conductance G.

ΔT = P/G is the formula for temperature rise.

A thermometer, a temperature-dependent resistor, senses temperature changes based on the power P. 

P = C/G gives the thermal intrinsic time constant

The working of a bolometer

A bolometer uses a temperature-sensitive resistor to detect microwave power at medium and low levels. It monitors low power in the 0.1 mW to 10 mW range and medium power in the 10 mW to 1 W range. 

A colorimetric approach is a heat transfer technique for determining the power of microwave radiation. It operates on the idea of temperature change caused by a change in the absorber’s resistance, which is monitored by a thermistor or a barretter. It is a temperature-dependent material made of two leads and a semiconductor material bead. It might be a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC) since it employs a thermistor or barretter to assess temperature change.

The bolometer circuit is intended as a bridge circuit. It features a temperature-sensitive resistive element. This element detects and measures the power of incoming microwave radiation.

When RF energy or microwave energy is delivered to the bridge circuit, the heat created alters the temperature resistive element’s resistance and absorbs the radiation or power that is to be measured. The thermistor or barretter can measure the change in resistance caused by heat.

When a thermistor is employed as a resistive element, the resistance lowers as the temperature rises. As a result, the bolometer will have a negative temperature coefficient (NTC).

When a barretter is employed, the resistance rises as the temperature rises. Thus, the bolometer has a positive temperature coefficient (PTC).

To balance the bridge, oscillators and differential amplifiers are utilised to vibrate the circuit. Because of the shift in resistance, the bridge becomes imbalanced. The power will be absorbed to balance the bridge circuit by the resistive element.

Also, to balance the bridge, we can change and alter the DC bias. By absorbing power, microwave radiation raises the temperature and modifies the resistance of the element. In this way, the resistance change may cause the bridge circuit to become unbalanced.

The oscillator output is reduced by the imbalance bridge circuit, and the oscillator output is balanced again. The cold resistance causes the bridge to become imbalanced. The oscillator increases the output power, and the absorbed power by the resistive element may be measured with a Voltmeter. By evaluating the three requirements, we can measure the change in resistance to power:

• Bolometer’s impulse load

• Bolometer’s constant load

• Thermistor load

With this, minute variations in resistance may be detected using a bolometer, and absorbed power can be easily quantified.

Conclusion

A bolometer detects microwave power at medium and low levels by using a temperature-sensitive resistor. When RF or microwave energy is provided to the bridge circuit, the heat generated changes the resistance of the temperature resistive element, absorbing the power or radiation to be measured. Oscillators and differential amplifiers are used to vibrate the circuit to balance the bridge. A bolometer may be used to detect minute differences in resistance, and absorbed power can be easily calculated.