Understanding IR Cameras in Behavioral Research
Most cameras—both color and monochrome—have some sensitivity to infrared (IR) light as well as to visible light. However, the way they handle IR differs:
- Color cameras typically have IR-blocking filters built-in to maintain color fidelity.
- Monochrome cameras may also have IR-blocking filters, but these are often removable (e.g., via "night mode").
- Dedicated IR cameras are optimized for IR sensitivity, but the underlying detection principle remains the same.
Common Misconception: IR Cameras and Heat
It's a common mistake to assume that IR cameras detect heat. While hot objects do emit IR, the wavelengths involved differ:
- Heat from living beings is emitted in the mid to far infrared range (3000 nm+).
- Typical behavior-tracking cameras are sensitive only to the near infrared (NIR) range—between 700 nm and 1000 nm.
- Common IR illumination wavelengths are 850 nm or 940 nm (e.g., 940 nm in the PhenoTyper).
To detect body heat, you'd need a thermographic camera, which:
- Detects mid/far IR radiation.
- Produces low-resolution, false-color images.
- Is not typically used in behavior tracking due to cost and lack of detail.
Why Use IR in Behavior Research?
In behavioral studies, IR serves as an invisible light source—a wavelength most animals (including rodents) do not perceive. This allows for unobtrusive observation, even during dark-phase cycles.
Sources of IR illumination include:
- Standalone IR illuminators mounted in observation cubicles.
- Integrated IR sources in systems like the PhenoTyper.
Using IR-Pass Filters
Sometimes, IR-pass filters are added to the camera to block visible light. This is beneficial when:
- You want to maintain image consistency during changing ambient light conditions (e.g., circadian cycles).
- You have a controlled IR-only environment.
These filters are visually identifiable by the camera lens appearing completely black (no visible light passes through).
Considerations for IR Illumination
- IR light behaves like visible light—it's reflected or absorbed by surfaces.
- Glare may occur with shiny surfaces, so you may need to adjust the camera or IR source position.
- Some materials reflect IR differently than visible light. For example:
- Corncob bedding often appears brighter in IR than to the naked eye.
- Paper-based bedding usually appears darker, though this varies by brand.
- For tracking light-colored animals, consider using dark bedding to increase contrast.
A recommended option is Purina’s Yesterday’s News (unscented), which provides a dark appearance in IR.