Drone Gimbal Filters 101: Where Should You Place Them for VIS, NIR, SWIR, and Thermal Channels?

Choosing and placing the right optical filters on a drone gimbal is crucial for capturing optimal imagery across visible (VIS), near-infrared (NIR), shortwave infrared (SWIR), and thermal channels. Here's a straightforward guide to where and why each filter type is used.

• UV/IR-Cut (Hot Mirror): Install as close to the sensor as possible, or directly after the lens. This filter blocks unwanted UV and NIR, ensuring accurate color balance and sharpness. Use when optics or sensors don't feature strong internal IR-blocking.
• Polarizer: Attach in front of the lens to reduce glare from water or roofs and intensify scene contrast. For some missions, stacking two polarizers (source and analyzer) maximizes control.
• ND Filter: Mount at the lens front to cut light intensity in bright conditions, allowing for correct exposure and professional motion blur on video.

Quick Tip: Check if your drone camera's lens or sensor already integrates a UV/IR cut filter.

• Narrow Bandpass Filter: Place immediately ahead of the sensor or in the gimbal's optical path, targeting the specific NIR or SWIR wavelength (e.g., 850, 940, 1064, 1535 nm). Tight FWHM ensures high signal-to-noise.
• Laser Notch ('Stopline') Filter: Mount in front of the sensor for protection against high-intensity threat lasers, blocking exact laser lines (e.g., 355, 532, 1064, 1550 nm).
• Dichroic Beamsplitter: Inside multi-sensor gimbals, use at 45° AOI to spectrally route VIS, NIR, SWIR or laser light to each individual sensor.

• Broadband AR Coating: Coat the gimbal's external window (germanium or sapphire) for higher transmission and fewer reflections, ensuring thermal bands pass through efficiently.
• DLC (Diamond-Like Carbon): Use as an outermost window layer for abrasion, sand, and cleaning resistance with minimal added loss.
• Hydrophobic/Oleophobic Topcoat: Add above AR for easier cleaning and resistance to rain, salt spray, and fingerprints.
• Transparent Conductive (ITO) Layer: Include in the window stack for deicing/heating or EMI protection without reducing visible clarity.

• Matte-Black Coatings: Use nanostructured, ultra-low reflectance coatings inside the gimbal to suppress glare and ghosts.
• Wide-Angle AR Coatings: Windows that see rays from wide angles (up to 50–55° AOI) must be custom AR coated to prevent off-axis reflections.

• Always specify AOI and FOV when ordering bandpass or dichroic filters, as passbands shift with angle.
• Match filter type and bandwidth to your mission: daylight (glare reduction) calls for polarizer and UV/IR cut, while NIR/SWIR needs narrow BPs and laser defense needs notch filters.
• Window stack (for thermal): substrate (Ge/Sapphire) + BBAR + DLC + hydrophobic + optional ITO.

Summary:
Correct selection and placement of gimbal filters, tailored for each spectral band, is essential for image quality, sensor safety, and reliability. Always communicate your system specifications and mission goals to your filter supplier to ensure the best results.