handbook of modern sensors

14.6 Thermal Detectors 435
(A)
(B)
Fig. 14.25. Equivalent circuit of electrically biased bolometer (A) and a design of an optical
bolometer (B).
A basic circuit for the voltage biased bolometer application is shown in Fig.
14.25A. It consists of a bolometer (a temperature-sensitive resistor) having resistance
R, a stable reference resistor R 0 , and a bias voltage source E. The voltage V across
R 0 is the output signal of the circuit. It has the highest value when both resistors
are equal. The sensitivity of the bolometer to the incoming electromagnetic (EM)
radiation can be defined as [12]
β v =
αεZ T E
4 √ 1 + (ωτ) , (14.24)
2
where α = (dR/dT )/R is the TCR of the bolometer, ε is the surface emissivity, Z T
is the bolometer thermal resistance, which depends on its design and the supporting
structure, τ is the thermal time constant, which depends on Z T and the bolometer’s
thermal capacity, and ω is the frequency.
Because the bolometer’s temperature increase, T is
T = T − T 0 ≈ P E Z T = E2
4R Z T , (14.25)
and the resistance of a RTD bolometer can be represented by a simplification Eq.
(16.14) of Chapter 16,
R = R 0 (1 + α 0 T ), (14.26)
Eq. (14.24) can be rewritten as
β V = 1 √
2 εα R 0 Z T T
(1 + α 0 T ) [ 1 + (ωτ) 2]
(14.27)
Therefore, to improve the bolometer’s responsivity, its electrical resistance and thermal
impedance should be increased.
The bolometers were traditionally fabricated as miniature thermistors, suspended
by tiny wires. Another popular method of bolometer fabrication is the use of metal
film depositions [12,13], usually of nichrome. In many modern bolometers, a thermoresistive
thin-film material is deposited on the surface of a micromachined silicon
or a glass membrane which is supported by a silicon frame. This approach gains
popularity with the increased demand for the focal-plane array (FPA) sensors that
are required for the thermal imaging. When an application does not need a high