Designing a Smartphone Spectrometer

Designing a Smartphone Spectrometer: A Guide Using Lens Design Software  

Chuong Tran

Designing an add-on spectrometer for a smartphone camera using  lens desig software such as Zemax involves a combination of optical design principles and practical considerations for miniaturization and integration. It's a tricky balance, really, between getting the optics right and making it something that can actually be attached to a phone. Here's how I'm approaching the design process for the open source project: 

1. Defining Spectrometer Specifications:

• Wavelength Range:

  • Determine the spectral range of interest (e.g., visible, near-infrared).

• Spectral Resolution:

  • Specify the desired resolution to distinguish between closely spaced wavelengths.

• Input Interface:

  • Define how light will enter the spectrometer (e.g., slit, fiber optic).

  • Consider how to couple the smartphone camera lens to the spectrometer input.

• Output Interface:

  • The output will be the dispersed spectrum projected onto the smartphone's camera sensor.

  • Ensure the spectrum fits within the sensor's active area.

• Size and Form Factor:

  • The add-on must be compact and easily attachable to a smartphone.

  • Consider the mechanical design and mounting mechanism.

• Cost:

  • Design for manufacturability and use cost-effective optical components.

2. Optical Design Considerations:

• Dispersion Element:

  • A diffraction grating is the most common dispersion element.

  • Select a grating with appropriate groove density for the desired spectral range and resolution.

  • Consider transmission or reflection gratings.

• Collimating Optics:

  • A collimating lens or mirror is needed to create a parallel beam of light before it reaches the grating.

• Focusing Optics:

  • A focusing lens or mirror is needed to project the dispersed spectrum onto the smartphone's camera sensor.

• Slit:

  • A narrow slit is often used to define the input beam and improve spectral resolution.

• Optical Aberrations:

  • Minimize optical aberrations (e.g., chromatic aberration, astigmatism) to ensure a clear and accurate spectrum.

• Stray Light:

  • Control stray light to prevent unwanted reflections and improve signal-to-noise ratio.

3. Zemax Workflow:

• System Setup:

  • Define the wavelength range, sensor size, and other system parameters in Zemax.

  • Model the smartphone camera lens as part of the system, if necessary.

• Grating Modeling:

  • Use Zemax's grating surface type to model the diffraction grating.

  • Define the grating groove density and other grating parameters.

• Lens Design:

  • Design the collimating and focusing optics using Zemax's lens design tools.

  • Optimize the lens design to minimize aberrations and achieve the desired performance.

• Merit Function:

  • Define a merit function that includes operands for:

    • Spectral resolution

    • Distortion

    • Image quality

    • System size

• Optimization:

  • Use Zemax's optimization algorithms to refine the lens design and grating parameters.

• Analysis:

  • Use Zemax's analysis tools to evaluate the spectrometer's performance:

    • Spectral resolution plots

    • Spot diagrams

    • Distortion grids

    • Ray tracing

• Tolerance Analysis:

  • Perform tolerance analysis to assess the sensitivity of the design to manufacturing variations.

• Mechanical Design:

  • Use Zemax's CAD export capabilities to transfer the optical design to a CAD program for mechanical design.

4. Practical Considerations:

• Integration with Smartphone:

  • Design a mechanical interface that allows for easy attachment and alignment of the spectrometer to the smartphone camera.

• Software Integration:

  • Develop a smartphone app to capture and process the spectral data.

  • The app should be able to calibrate the spectrometer and display the spectrum.

• Calibration:

  • Develop a calibration procedure to ensure accurate spectral measurements.

Key Zemax Tools:

• Lens Data Editor: To define optical surfaces and materials.

• Grating Surface: To model diffraction gratings.

• Merit Function Editor: To define optimization goals.

• Optimization Wizard: To automate optimization.

• Analysis Tools: To evaluate system performance.

• Tolerance Analysis: To assess manufacturing sensitivity.

• CAD Export: To transfer the design to CAD software.

By combining optical design principles with Zemax's capabilities, you can create a functional and practical add-on spectrometer for smartphone cameras.