Table 6 Light budget for measurement of specular reflections at 30° angle of incidence.

From: Mid-IR standoff measurement of ageing-related spectroscopic changes in bitumen in the 6 µm (1700 cm−1) region. Part 1: Measurement strategy and instrument design principles

Parameter

Proportion retained at each stage

Light power remaining/W

Comment

Light emitted from laser, after collimation

100%

3.5 × 10–4

Modulated emission of 1mW peak-peak: RMS value for sine-wave modulation

Light retained in beam-combining optics

86%

3.0 × 10–4

Using reflective optics with reflectivity ~ 95% for 3 reflections

Light retained in specular reflection

∥ 3.2%

⊥ 7.1%

∥ 9.7 × 10–6

⊥ 2.2 × 10–5

Mean (low level) reflectivity of bitumen

Change in reflected power for a change in A of 0.01AU

2.3%

∥ 2.2 × 10–7

⊥ 5.0 × 10–7

Equation (3)

Noise equivalent power (NEP) of detector

 

2.8 × 10–10

D* 5 × 108 cm Hz1/2 W−1, 2 mm × 2 mm48, integration period 0.5 s

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