Optical Path Principle of DLP Near-Infrared Spectroscopy Modules

Introduction

Spectroscopy is a powerful technique for identifying and characterizing physical materials by analyzing the absorption or emission changes of a sample at different wavelengths of light. A spectrometer performs detection by measuring the absorption changes of light by a material. The NIR-M-RX series is a complete near-infrared spectrometer module that enables the design of high-performance, cost-effective portable NIR spectrometers for applications in food, pharmaceutical, oil and gas, medical, security and other emerging industries, delivering field, laboratory-grade detection performance. The module includes the DLP2010NIR digital micromirror device, the DLPC150 digital controller and the DLPA2005 integrated power-management component.

The new DLP2010NIR digital micromirror device (DMD) is optimized for the 700 to 2500 nm wavelength range. The NIR-M-RX series is a typical application example of this technology, with an operating wavelength range of 900 to 2400 nm.

DLP-based spectrometers use the DMD for wavelength selection and a single-point detector in place of a conventional linear-array detector, as shown in Figure 1-1. By sequentially scanning the DMD columns (turning on specific pixel columns), light of specific wavelengths is directed to the detector and captured. For details, refer to the DLP spectrometer design considerations.

Technical Advantages

• Compared with linear arrays using very small pixels, higher performance can be achieved by using a single-point 1 mm large detector.
• A lower-cost system is achieved by using a single-element detector and low-cost optical components. The high-resolution digital micromirror device (DMD) can generate custom patterns to compensate for optical distortion of individual systems.
• The enhanced signal capture comes not only from the DMD's wider sensing range compared with conventional technology, but also from the use of fast, flexible and programmable patterns and spectral filters.
  • Through programmable patterns, the DLP spectrometer can:
  • Adjust the detector light intensity by controlling the number of pixels in a column.
  • Change the system resolution by controlling the column width.
  • Use a set of Hadamard patterns, each capturing light of multiple wavelengths. The individual wavelengths are then obtained through a decoding procedure. Each pattern turns on 50% of the DMD pixels at a time, directing more signal into the detector than the column scanning shown in Figure 1-1.
  • Use custom spectral filters to select specific target wavelengths.

Optical Path Composition

The DLP NIRscan Nano EVM is a complete near-infrared spectrometer solution based on DLP technology. The module includes the following core components:

Optical Engine

A near-infrared spectrometer optical engine optimized for the 900-1700 nm wavelength range:

• Illumination system
• Dual integrated infrared lamp reflection illumination module (lens-end tungsten lamp)
• Lamp power: 1.4 W (typical)

• Beam intersection angle: 40°, intersection point about 0.75 mm from the sapphire window (tolerance ±0.25 mm)

• Incident optics

• Input slit: 1.8 mm × 0.025 mm (25 μm)
• Collimating lens

• 885 nm long-pass filter

• Dispersion and modulation system

• Reflective diffraction grating
• Focusing lens

• DLP2010NIR digital micromirror device (DMD)

  • Array specification: 0.2-inch WVGA, 854 × 480 orthogonal pixels
  • Pixel size: 5.4 μm × 5.4 μm
  • Near-infrared optimized coating
  • Micromirror tilt angle: ±17°

• Detection system

• Light collection optics
• Single-pixel InGaAs uncooled detector (Hamamatsu G12180-010A, 1 mm photosensitive diameter)

Optical Engine Working Principle

The optical engine adopts a post-dispersion configuration with a removable reflectance sample module:

1. Sample illumination: The dual lens-end lamps illuminate the sample at a specific angle to avoid specular reflection entering the system while collecting diffuse reflected light.
2. Signal acquisition: The sample's diffuse reflected light is converged by the collection lens and enters the optical engine through the input slit.
3. Collimation and filtering: After collimation, the beam passes through the 885 nm long-pass filter to remove short-wavelength stray light.
4. Dispersive imaging: After grating dispersion, the different wavelength components form a horizontally dispersed image of the slit on the DMD surface through the focusing lens (900 nm to 1700 nm corresponding to the two ends of the DMD).
5. Wavelength selection: By programmatically controlling the DMD column micromirror states (+17° on / -17° off), specific wavelengths are selectively reflected to the detector.
6. Signal detection: The light of the selected wavelengths is focused by the light-collection optical system onto the single-pixel InGaAs detector to complete photoelectric conversion.

Note: To accommodate mechanical tolerances, the slit image has a 10% underfill in the DMD dispersion direction and an overfill in the orthogonal direction, with an actual effective imaging area of 1.69 mm × 0.025 mm.

Warning: Opening or disassembling the optical engine without authorization will void the warranty and may cause contamination and misalignment of optical components, requiring return to the factory for recalibration.

Mechanical Specifications

• Dimensions: 62 mm (length) × 58 mm (width) × 36 mm (height)
• Test temperature range: 0°C to 50°C (operating temperature 25°C)

Electronic Subsystem

Consists of four functional circuit boards:

1. Microcontroller Board

The system main control unit, with core components including:

2. DLP Controller Board

3. Detector Board

4. DMD Board

• Core device: DLP2010NIR digital micromirror device
• Works together with the DLPC150 controller to achieve high-speed spatial light modulation

System Specification Overview

Figure 1-2 DLP NIRscan Nano system block diagram

Figure 1-3 Optical engine optical path diagram

Figure 1-4 Illumination module top view

Figure 1-5 Mechanical dimension drawing