Main Sensor

The sensor is the heart of the Lumen spectrometer. It converts the optical signal from the interferometer into an electrical signal usable by the acquisition chain.

Operating Principle

The sensor used in Lumen is a deuterated triglycine sulfate (DTGS) pyroelectric detector. It measures the infrared beam intensity after it passes through the sample and the Michelson interferometer.

IR Source → Interferometer → Sample → DTGS Sensor → ADC → Processing

Detection Chain

1. Optical window: potassium bromide (KBr), transparency from 2.5 to 25 µm.
2. Sensitive element: DTGS crystal, 2 mm diameter.
3. Preamplifier: low-noise transimpedance amplifier, gain of 10⁶ V/A.
4. Low-pass filter: 5 kHz cutoff for high-frequency noise rejection.

Technical Specifications

Parameter	Value
Spectral range	2.5 – 25 µm
Sensitivity (D*)	1.5 × 10⁸ cm·Hz½/W
Active area	2 mm
Max output voltage	±5 V
Noise Equivalent (NEP)	0.5 nW/Hz½
Response time	< 10 ms
Temperature range	10 – 40 °C

Calibration

Wavelength Calibration

Use a polystyrene reference film:

1. Place the film in the sample holder.
2. Acquire a spectrum with 32 scans.
3. Check the characteristic peaks at 1601 cm⁻¹, 1493 cm⁻¹, and 1028 cm⁻¹.
4. Adjust the correction factor if the deviation exceeds ±2 cm⁻¹.

Intensity Calibration

Intensity calibration is performed with a reference mirror (reflectivity > 98% in the mid-IR range):

def calibrate_intensity(raw_spectrum, reference_spectrum):
    """Calibrate the raw spectrum against the reference spectrum."""
    import numpy as np
    return raw_spectrum / reference_spectrum * 100  # % transmission

Maintenance

• Window cleaning: optical cloth + isopropanol, once a month.
• Baseline check: empty acquisition (no sample) every 50 measurements.
• Desiccant replacement: every 3 months to prevent moisture absorption by the KBr.

Electronic Interfacing

The sensor communicates with the acquisition board via a 50 Ω BNC connector. The signal is digitized by a 16-bit ADC at 100 kS/s (AD7680).

DTGS Sensor ──┬── V_out (BNC) ──→ ADC AD7680 ──→ Microcontroller
              └── GND

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